10-year Average

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Each monthly averaged value is evaluated as the numerical average of 3-hourly values for the given month.

Units

kWh/m²/day

Note

also referred to as global horizontal radiation

Reference

SSE Methodology, Section 4, for detailed discussion of the methodology for deriving the SSE horizontal surface insolation from satellite observations.

10-year Average

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Each monthly averaged value is evaluated as the numerical average of the 3-hourly values, one per day, at the time (GMT) closest to local solar noon. The time (GMT) used is within 1.5 hours of local solar noon.

Units

kW/m²

Reference

SSE Methodology, Section 4, for detailed discussion of the methodology for deriving the SSE horizontal surface insolation from satellite observations.

10-year Average

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth when the cloud cover is less than 10%, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

kWh/m²/day

Reference

SSE Methodology, Section 4, for detailed discussion of the methodology for deriving horizontal surface insolation from satellite observations.

10-year Average

The monthly average of the number of days having an average cloud cover less than 10% during a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

days

10-year Average

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Each monthly averaged value is evaluated as the numerical average of 3-hourly values for the given month.

Minimum And Maximum Difference From Monthly Averaged Insolation

The minimum and maximum values for a given month indicate the percent difference between the year that has the least (minimum) or most (maximum) monthly averaged insolation and the 10-year monthly averaged insolation.

Units

• Monthly averaged insolation incident on horizontal surface in kWh/m²/day
• Minimum and maximum values in percent

Note

also referred to as global horizontal radiation

Reference

SSE Methodology, Section 4, for detailed discussion of the methodology for deriving surface insolation from satellite observations.

10-year Average

The monthly average amount of solar radiation incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). The horizontal diffuse radiation was evaluated using the Erbs et al. method described in SSE Methodology Section 5.1.1.

Minimum and Maximum Diffuse Radiation

The minimum and maximum values for a given month indicate the least (minimum) and most (maximum) monthly average diffuse radiation for any one year in the 10-year period.

Average Clearness Index (K)

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period (July 1983 - June 1993); (i.e. clearness index is the fraction of insolation at the top of the atmosphere which reaches the surface of the earth).

Minimum and Maximum Clearness Index (K)

The minimum and maximum values for a given month indicate the least (minimum) and most (maximum) monthly average clearness index for any one year in the 10-year period.

Units

• 10-year Average, minimum and maximum diffuse radiation values in kWh/m²/day
• K values are dimensionless

Note

no data is available when the clearness index is < 0.3 or > 0.8

Reference

SSE Methodology, Section 5.1.1

10-year Average

The monthly average amount of solar radiation incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). The horizontal diffuse radiation was evaluated using the Extended Page Method described in SSE Methodology Section 5.1.2.

Minimum and Maximum Diffuse Radiation

The minimum and maximum values for a given month indicate the least (minimum) and most (maximum) monthly average diffuse radiation for any one year in the 10-year period.

Units

kWh/m²/day

Reference

SSE Methodology, Section 5.1.2

10-year Average

The monthly average amount of solar radiation incident on a surface oriented normal to the solar radiation for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). The direct normal radiation is evaluated using the RETScreen method discussed in SSE Methodology Section 5.3.1.

Minimum And Maximum Difference From Monthly Averaged Direct Normal Radiation

The minimum and maximum values for a given month indicate the percent difference between the year that has the least (minimum) or most (maximum) monthly averaged direct normal radiation and the 10-year monthly averaged value.

Units

• Monthly averaged direct normal radiation in kWh/m²/day
• Minimum and maximum in percent

Reference

SSE Methodology, Section 5.3.1

10-year Average

The monthly average amount of solar radiation incident on a surface oriented normal to the solar radiation for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). The direct normal radiation is evaluated using the Extended Page/Empirical Staylor Hourly Method described in SSE Methodology Section 5.3.2.

Minimum And Maximum Difference From Monthly Averaged Direct Normal Radiation

The minimum and maximum values for a given month indicate the percent difference between the year that has the least (minimum) or most (maximum) monthly averaged direct normal radiation and the 10-year monthly averaged value.

Units

• Monthly averaged direct normal radiation in kWh/m²/day
• Minimum and maximum in percent

Reference

SSE Methodology, Section 5.3.2

Average@GMT

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for 3-hour intervals of GMT during the given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

kW/m²

Reference

SSE Methodology, Section 4, for detailed discussion of the methodology for deriving the SSE horizontal surface insolation from satellite observations.

10-year Average

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period (July 1983 - June 1993); (i.e. clearness index is the fraction of insolation at the top of the atmosphere which reaches the surface of the earth).

Minimum and Maximum

The minimum and maximum values for a given month indicate the least (minimum) and most (maximum) monthly average clearness index for any one year in the 10-year period.

Units

dimensionless

10-year Average

The monthly average zenith angle-independent expression of the insolation clearness index for a given month, averaged for that month over the 10-year period (July 1983 - June 1993); (i.e. clearness index is the fraction of insolation at the top of the atmosphere which reaches the surface of the earth).

K' = K / (1.031 * exp(-1.4 / (0.9 + 9.4 / m )) + 0.1)
where:

K' = normalized clearness index

K = clearness index

m = air mass from Kasten's pyrheliometric formula

Units

dimensionless

10-year Average

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth when the cloud cover is less than 10%, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

kWh/m²/day

10-year Average

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth when the cloud cover is less than 10% divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period (July 1983 - June 1993); (i.e. clearness index is the fraction of insolation at the top of the atmosphere which reaches the surface of the earth).

Units

dimensionless

10-year Average

The monthly averaged zenith angle-independent expression of the clear sky insolation clearness index for a given month, averaged for that month over the 10-year period (July 1983 - June 1993); (i.e. clearness index is the fraction of insolation at the top of the atmosphere which reaches the surface of the earth).

Units

dimensionless

Average

The time when the sun is due south in the northern hemisphere or due north in the southern hemisphere. The determination of monthly averaged solar noon for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).

Units

GMT time

Note

during polar winter the sun may be below the horizon at solar noon

Average

The number of hours between sunrise and sunset. The determination of monthly averaged daylight for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).

Units

hours

Note

polar daylight is 0 in winter and 24 in summer

Average

The average cosine of the angle between the sun and directly overhead during daylight hours. The determination of monthly averaged daylight average of hourly cosine solar zenith angles for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).

Average cos(Θ_Z) = {f cos^-1 (-f/g) + g[1 - (f/g)²]^½} / cos^-1(-f/g)
where:

f = sin(latitude) * sin(solar declination)

g = cos(latitude) * cos(solar declination)

Units

dimensionless

Reference

SSE Methodology, Section 5.1.2, Ref. 23: Gupta et al., 2001, The Langley Parameterized Shortwave Algorithm (LPSA) for Surface Radiation Budget Studies

Average

Approximate monthy average ratio of horizontal surface solar beam radiation to direct normal radiation. The determination of monthly averaged cosine solar zenith angle at mid-time between sunrise and solar noon for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).

cos(Θ_ZMT) = f + g[(g - f) / 2g]^½
where:

f = sin(latitude) * sin(solar declination)

g = cos(latitude) * cos(solar declination)

Units

dimensionless

Reference

SSE Methodology, Section 5.3.2

Average

The angular distance of the sun north (positive) or south (negative) of the equator. Declination varies through the year from 23.45° north to 23.45° south and reaches the minimum/maximum at the southern/northern summer solstices. The determination of monthly averaged declination for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).

Units

degrees

Average

The angle that the earth has rotated between the time of solar noon and sunset. Note that the earth rotates 15° with respect to the sun each hour. The determination of monthly averaged sunset hour angle for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).

Units

degrees

Average

The maximum vertical angle of the sun above the horizon. The determination of monthly averaged maximum solar angle relative to the horizon for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).

Units

degrees

Average

The vertical angle of the sun above the horizon. The determination of monthly averaged hourly solar angles relative to the horizon for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).

Units

degrees

Note

This information is provided to assist the user in interpreting diurnal variations in both insolation and clouds.

Average

The arc of the horizon measured clockwise from True North, to the point where a vertical circle through the sun intersects the horizon. The determination of monthly averaged hourly solar azimuth angles for each month is based on the "monthly average day" (SSE Methodology, Section 5.1.1, footnote).

Units

degrees

Note

These angles are provided as a function of GMT to assist the user in interpreting diurnal variations of insolation and clouds in this data set. If either mornings or afternoons are habitually cloudy, it may be useful to point the solar panels slightly to the east or west instead of directly south.

SSE HRZ

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Clearness Index (K)

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period.

Erbs DIF

The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk, averaged for that month over the 10-year period.

RET DNR

The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for a given month, averaged for that month over the 10-year period.

Tilt 0, Latitude-15, Latitude, Latitude+15, 90

The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Note that the differences between the Tilt 0 values and the SSE HRZ values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using the RETScreen Isotopic Diffuse Method discussed in SSE Methodology, Section 6.1.1.

OPT

The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.

OPT ANG

The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.

Units

• SSE HRZ, Erbs DIR, RET DNR, and OPT in kWh/m²/day
• Tilt angles and OPT ANG in degrees
• K is dimensionless

Reference

SSE Methodology, Section 6.1.1

SSE HRZ

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Clearness Index (K)

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period.

PAGE DIF

The monthly average amount of solar radiation incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for a given month, averaged for that month over the 10-year period.

PAGE DNR

The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for a given month, averaged for that month over the 10-year period.

Tilt 0, Latitude-15, Latitude, Latitude+15, 90

The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Note that the differences between the Tilt 0 values and the SSE HRZ values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using the Perez Non-Isotropic/Extended Page Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.2.

OPT

The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.

OPT ANG

The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.

Units

• SSE HRZ, PAGE DIR, PAGE DNR, and OPT in kWh/m²/day
• Tilt angles and OPT ANG in degrees
• K is dimensionless

Reference

SSE Methodology, Section 6.1.2

SSE HRZ

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Clearness Index (K)

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth divided by the monthly average incoming top-of-atmosphere insolation for a given month, averaged for that month over the 10-year period.

Erbs DIF

The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk, averaged for that month over the 10-year period.

RET DNR

The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for a given month, averaged for that month over the 10-year period.

Tilt 0, Latitude-15, Latitude, Latitude+15, 90

The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Note that the differences between the Tilt 0 values and the SSE HRZ values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using the Perez Non-Isotropic/Erbs et al. Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.3.

OPT

The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.

OPT ANG

The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.

Units

• SSE HRZ, Erbs DIR, RET DNR, and OPT in kWh/m²/day
• Tilt angles and OPT ANG in degrees
• K is dimensionless

Reference

SSE Methodology, Section 6.1.3

SSE MIN

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value in the 10-year period (July 1983 - June 1993).

Clearness Index (K)

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value over the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.

Erbs DIF

The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the most, or maximum, monthly averaged value over the 10-year period.

RET DNR

The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the least, or minimum, monthly averaged value in the 10-year period.

Tilt 0, Latitude-15, Latitude, Latitude+15, 90

The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the least, or minimum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MIN values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MIN in the RETScreen Isotopic Diffuse Method discussed in SSE Methodology Section 6.1.1.

OPT

The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.

OPT ANG

The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.

Units

• SSE MIN, Erbs DIR, RET DNR, and OPT in kWh/m²/day
• Tilt angles and OPT ANG in degrees
• K is dimensionless

SSE MIN

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value in the 10-year period (July 1983 - June 1993).

Clearness Index (K)

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value in the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.

PAGE DIF

The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the most, or maximum, monthly averaged value in the 10-year period.

PAGE DNR

The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the least, or minimum, monthly averaged value in the 10-year period.

Tilt 0, Latitude-15, Latitude, Latitude+15, 90

The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the least, or minimum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MIN values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MIN in the Perez Non-Isotropic/Extended Page Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.2.

OPT

The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.

OPT ANG

The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.

Units

• SSE MIN, PAGE DIR, PAGE DNR, and OPT in kWh/m²/day
• Tilt angles and OPT ANG in degrees
• K is dimensionless

SSE MIN

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value in the 10-year period (July 1983 - June 1993).

Clearness Index (K)

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the least, or minimum, monthly averaged value in the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.

Erbs DIF

The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the most, or maximum, monthly averaged value in the 10-year period.

RET DNR

The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the least, or minimum, monthly averaged value in the 10-year period.

Tilt 0, Latitude-15, Latitude, Latitude+15, 90

The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the least, or minimum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MIN values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MIN in the Perez Non-Isotropic/Erbs et al. Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.3.

OPT

The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.

OPT ANG

The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.

Units

• SSE MIN, Erbs DIR, RET DNR, and OPT in kWh/m²/day
• Tilt angles and OPT ANG in degrees
• K is dimensionless

SSE MAX

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period (July 1983 - June 1993).

Clearness Index (K)

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.

Erbs DIF

The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the least, or minimum, monthly averaged value in the 10-year period.

RET DNR

The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the most, or maximum, monthly averaged value in the 10-year period.

Tilt 0, Latitude-15, Latitude, Latitude+15, 90

The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the most, or maximum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MAX values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MAX in the RETScreen Isotopic Diffuse Method discussed in SSE Methodology Section 6.1.1.

OPT

The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.

OPT ANG

The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.

Units

• SSE MAX, Erbs DIR, RET DNR, and OPT in kWh/m²/day
• Tilt angles and OPT ANG in degrees
• K is dimensionless

SSE MAX

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period (July 1983 - June 1993).

Clearness Index (K)

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.

PAGE DIF

The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the least, or minimum, monthly averaged value in the 10-year period.

PAGE DNR

The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the most, or maximum, monthly averaged value in the 10-year period.

Tilt 0, Latitude-15, Latitude, Latitude+15, 90

The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the most, or maximum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MAX values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MAX in the Perez Non-Isotropic/Extended Page Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.2.

OPT

The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.

OPT ANG

The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.

Units

• SSE MAX, Erbs DIR, RET DNR, and OPT in kWh/m²/day
• Tilt angles and OPT ANG in degrees
• K is dimensionless

SSE MAX

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period (July 1983 - June 1993).

Clearness Index (K)

The monthly average amount of the total solar radiation incident on a horizontal surface at the surface of the earth for the year that has the most, or maximum, monthly averaged value in the 10-year period divided by the monthly average incoming top-of-atmosphere insolation for that month.

Erbs DIF

The monthly average amount of solar radiation for a given month incident on a horizontal surface at the surface of the earth under all-sky conditions with the direct radiation from the sun's beam blocked by a shadow band or tracking disk for the year that has the least, or minimum, monthly averaged value in the 10-year period.

RET DNR

The monthly average amount of direct normal radiation incident on a surface oriented normal to the solar radiation for the year that has the most, or maximum, monthly averaged value in the 10-year period.

Tilt 0, Latitude-15, Latitude, Latitude+15, 90

The monthly average amount of the total solar radiation incident on a surface tilted relative to the horizontal and pointed toward the equator for the year that has the most, or maximum, monthly averaged value in the 10-year period. Note that the differences between the Tilt 0 values and the SSE MAX values are due to approximations in the inputs and time integration inaccuracies when processing the equations and integrating over the "monthly average day" (SSE Methodology, Section 5.1.1, footnote). Total solar radiation for each tilt angle was determined using SSE MAX in the Perez Non-Isotropic/Erbs et al. Horizontal Diffuse Method discussed in SSE Methodology Section 6.1.3.

OPT

The monthly average amount of total solar radiation incident on a surface tilted at the optimum angle relative to the horizontal and pointed toward the equator.

OPT ANG

The angle relative to the horizontal for which the monthly averaged total solar radiation is a maximum.

Units

• SSE MAX, Erbs DIR, RET DNR, and OPT in kWh/m²/day
• Tilt angles and OPT ANG in degrees
• K is dimensionless

1. Minimum available insolation over a consecutive-day period (1, 3, 7, 14, or 21 days) within a particular month over the 10-year period (July 1983 - June 1993) as a % of the expected average kWh/m² value over the same consecutive-day period (%)
2. Solar radiation deficits below expected values incident on a horizontal surface over a consecutive-day period (kWh/m²)
3. Equivalent number of NO-SUN or BLACK days that must be supplied by the storage backup system (days)

These parameters are desired because of the fact that unusually cloudy conditions occurring over a number of consecutive days continually draw reserve power from batteries or some other storage device for solar systems not connected to an electrical power grid. Storage devices must be designed to withstand continuous below-average conditions in various regions of the globe. Determination of these parameters for a geographical region from ground-based observations is complex because cloud conditions are usually different every day during the below average period. The satellite-based observations in the SSE release 5 are ideally suited for determining a regional climatological average for these parameters. January conditions in Richmond, VA (37.583° N, 77.5° W) are used below to better illustrate the use of the parameters in the design of backup storage systems.

Release 5 SSE satellite-based data indicate that average daily insolation is 2.15 kWh/m² for January 10-year average cloud conditions. Clear sky values average 3.10 kWh/m². Over the 10-year period (July 1983 - June 1993), the lowest one-day insolation in January was 12.5% of the expected value (2.15 kWh/m²) or 0.269 kWh/m². This means that a battery or some other device had to supply 1.88 kWh/m² or 87% of the solar energy expected on that day. The fraction of expected daily energy that has to be provided by the storage device is sometimes referred to as an equivalent NO-SUN or BLACK day. From a storage device viewpoint, the number of NO-SUN days was 0.87 on that worse case day because 2.15 kWh/m² of energy are expected on every sunny day.

The need for additional storage capacity increases as the number of below average consecutive days increase. Again using conditions in Richmond, VA, the worse case "minimum available insolation over a consecutive-day period" for a 7 consecutive-day period where the insolation was 24.8% of expected the 7-day value of 15.05 kWh/m² (7 x 2.15 kWh/m²). This means that the storage device had to supply 11.3 kWh/m² or 75.2% of the 7-day expected energy on a continuous basis with no period for recharge. That amounts to 5.26 (7*75.2%/100 = 5.26) NO-SUN days if 2.15 kWh/m² are expected on each average sunny day. It should be noted that the worse 7-day period might have occurred in a different year than the worse one-day over the 10-year period of the SSE data.

As the consecutive-day time period gets longer, lower than average cloud fraction conditions can enter the situation, causing brighter days and some recharging of the storage device. These effects are evident in the Richmond, VA data. Using the same analyses as above, January values are as follows:

Note that the worst of the Worse Consecutive Days is the 14-day period. This suggests that a solar energy system battery or backup system should have approximately 7.5 NO-SUN days or 16.125 kWh/m² (7.5 x 2.15) of capacity if January is a critical month for operation of the solar energy system. Satellite-based day-to-day cloud variability over a historical time period can be used to design solar energy storage or backup capacity if more reliable data are not available.

Available surplus insolation over a consecutive-day period (1, 3, 7, 14, or 21 days) within a particular month over the 10-year period (July 1983 - June 1993) as a % of the expected average kWh/m² value over the same consecutive-day period (%)

10-year Average

Percent of cloud amount during daylight for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

percent

Note

a value of zero indicates clear skies and a value of 100 indicates overcast skies

Average@GMT

Percent of cloud amount during daylight at 3-hour intervals of GMT for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

percent

Note

a value of zero indicates clear skies and a value of 100 indicates overcast skies

< 10% @GMT

Percent of time the cloud amount is less than 10% (clear skies) at 3-hour intervals of GMT during daylight for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

10 - 70% @GMT

Percent of time the cloud amount is between 10 - 70% (broken-cloud skies) at 3-hour intervals of GMT during daylight for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

>= 70% @GMT

Percent of time the cloud amount is greater than or equal to 70% (near-overcast skies) at 3-hour intervals of GMT during daylight for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

percent

10-year Average

The monthly average air temperature for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth. Each monthly averaged value is evaluated as the numerical average of 3-hourly values for the given month.

Units

degrees Celsius

Reference

SSE Methodology, Section 8.1

10-year Average

The average difference between the average daily minimum and average daily maximum for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.

Units

degrees Celsius

Note

the warmest month is marked with an asterisk (*)

Reference

SSE Methodology, Section 8.1

10-year Average

The monthly average of the accumulation of degrees when the daily mean temperature is above 18 degrees Celsius over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.

Monthly Cooling Degree Days

For the days of a given month,
sum the quantity [((Tmin + Tmax) / 2) - 18]
when (Tmin + Tmax) / 2 > 18.

Units

degree days

10-year Average

The monthly average of the accumulation of degrees when the daily mean temperature is below 18 degrees Celsius over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.

Monthly Heating Degree Days

For the days of a given month,
sum the quantity [18 - (Tmin + Tmax) / 2]
when (Tmin + Tmax) / 2 < 18.

Units

degree days

Reference

SSE Methodology, Section 8.1

10-year Average

The monthly average of the accumulation of degrees when the daily mean temperature is below 10 degrees Celsius over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.

Monthly Heating Degree Days

For the days of a given month,
sum the quantity [10 - (Tmin + Tmax) / 2]
when (Tmin + Tmax) / 2 < 10.

Units

degree days

10-year Average

The monthly average of the accumulation of degrees when the daily mean temperature is below 0 degrees Celsius over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.

Monthly Heating Degree Days

For the days of a given month,
sum the quantity [0 - (Tmin + Tmax) / 2]
when (Tmin + Tmax) / 2 < 0.

Units

degree days

10-year Average

The monthly average of the earth's surface temperature for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

degrees Celsius

Note

data over oceans is sea surface temperature

Minimum

The minimum of the daily mean earth's surface temperature for a given month, averaged over the 10-year period (July 1983 - June 1993).

Maximum

The maximum of the daily mean earth's surface temperature for a given month, averaged over the 10-year period (July 1983 - June 1993).

Amplitude

One half of the difference between the 10-year average minimum and 10-year average maximum.

Units

degrees Celsius

10-year Average

The monthly average of the number of days for which the temperature falls below 0 degrees Celsius for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.

Units

days

10-year Average

The monthly average dew point temperature for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Dew point temperature values are for 10 meters above the surface of the earth.

Units

degrees Celsius

Average@GMT

The monthly average air temperature for 3-hour intervals of GMT during a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Temperature values are for 10 meters above the surface of the earth.

Units

degrees Celsius

10-year Average

The monthly average wind speed for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Wind speed values are for 50 meters above the surface of the earth. Each monthly averaged value is evaluated as the numerical average of 3-hourly values for the given month.

Minimum And Maximum Difference From Monthly Averaged Wind Speed At 50 m

The minimum and maximum values for a given month indicate the percent difference between the year that has the least (minimum) or most (maximum) monthly averaged wind speed at 50 m and the 10-year monthly averaged wind speed at 50 m.

Units

• Monthly Averaged Wind speed in meters per second (m/s)
• Minimum and maximum values in percent

Reference

SSE Methodology, Section 7

Wind Speed Ranges

The monthly average percent of time that wind speed is within the indicated range for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Wind speed values are for 50 meters above the surface of the earth.

Units

percent

Average@GMT

The monthly average wind speed for 3-hour intervals of GMT during a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Wind speed values are for 50 meters above the surface of the earth.

Units

meters per second (m/s)

10-year Average

The monthly average wind direction for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Wind direction values are for 50 meters above the surface of the earth.

Units

degrees

Notes

• measured clockwise from True North
• direction the wind is coming from

Average@GMT

The monthly average wind direction for 3-hour intervals of GMT during a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Wind direction values are for 50 meters above the surface of the earth.

Units

degrees

Notes

• measured clockwise from True North
• direction the wind is coming from

10-year Average

The monthly average wind speed for a given month, averaged for that month over the 10-year period (July 1983 - June 1993) where the wind speed was evaluated at 10 m above the surface of the earth assuming the underlying terrain is similar to that typical of airports (e.g. "airport" flat rough grass category taken from Gipe; SSE Methodology, Section 7, Ref. 13).

Units

meters per second (m/s)

Reference

SSE Methodology, Section 7

10-year Average

The percent difference between the 10-year monthly average of the wind speed at 10 m above the surface of the earth and the 10-year monthly average of the wind speed at 50 m above the surface of the earth.

Percent difference at 10 m = ((Wind Speed At 10 m - Wind Speed At 50 m) / Wind Speed At 50 m) * 100

Units

percent

10-year Average

The monthly average wind speed for a given month, averaged for that month over the 10-year period (July 1983 - June 1993) where the wind speed is evaluated using the Gipe Power Law. The wind speed at 50 m above the surface of the earth and the surface roughness exponent based on the chosen vegetation type are used in the equation:

Units

meters per second (m/s)

Reference

SSE Methodology, Section 7, Ref. 31: Gipe, 1999, Wind Energy Basics

10-year Average

The monthly average of relative humidity for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). The relative humidity is calculated from the air temperature and specific humidity at 10 meters above the surface of the earth and the surface pressure. The estimated uncertainty is 9%.

Units percent

Reference

SSE Methodology, Section 8.3

10-year Average

The monthly average of the humidity ratio for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). Humidity ratio values are for 10 meters above the surface of the earth.

Units

kg/kg

Note

also referred to as Specific Humidity

10-year Average

The monthly average of atmospheric pressure at the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

kPa

Reference

SSE Methodology, Section 8.2

10-year Average

The monthly average of the amount of moisture in the air from 20 mb to the surface for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

cm

10-year Average

The average daily rain rate based upon the total monthly averaged amount of rain for the given month divided by the number of days in the month, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

mm/day

Reference

SSE Methodology, Section 8.4

10-year Average

The monthly average amount of the total solar radiation incident on the top-of-atmosphere for a given month, averaged for that month over the 10-year period (July 1983 - June 1993).

Units

kWh/m²/day

Note

also referred to as extraterrestrial radiation (ETR)

10-year Average

The monthly average ratio of the solar energy reflected by the surface of the earth to monthly average solar energy incident on the surface of the earth for a given month, averaged for that month over the 10-year period (July 1983 - June 1993). (i.e. Fraction of insolation reflected by the surface of the earth.)

Units

dimensionless