Timothy Ballisty
Meteo 471 W
Dr. Fraser
Picture Description
The picture above is a photograph of the clouds known as cumulus humilis. Known as fair weather clouds, the cumulus clouds lived up to their name. This photograph was actually taken on a fair weather type of day. It was early November 1999 when the above picture was taken. It was a chilly day with a 40ºF to 45ºF air temperature and a slight breeze blowing from the right to the left of the photograph. However, the breeze was insignificant and only had a slight effect on the wind chill. As you can see, it was a bright, sunny day with the sun hiding behind the cumulus dotted sky every now and again. It was mid-day during the time of this picture yet the sun was not directly overhead due to the fact that the sun is located at a lower angle in the sky at this time of the year. As I looked straight up, it was very visible to me that the cumulus humilis were very shallow in height. Because of this shallowness, I could easily see the direction that these batches of clouds were taking. The cumulus were moving at a rapid rate from the northwest to southeast. Looking at the photograph in a perspective manner, the clouds were moving from the top right hand corner to the bottom left hand corner. As the day progressed and sunset approached, the cumulus humilis had passed on and/or dissipated and clear skies dominated the sky. Comparing the present day weather conditions to the day before and after, all three days were very similar in both the amount of sunshine shining down and the air temperature maxima.
Attributes
The cumulus humilis clouds have the appearance of floating cotton with a flat base and distinct yet sometimes bumpy and sharp edge, while having a lifetime anywhere from 5-40 minutes. Their color ranges from white to light gray, with bases that can be as low as 1,000 m above the ground, and have a diameter of roughly a kilometer. In the photographs shown, you can see the white tops that the cumulus humilis have as well as the light gray to almost black color bottoms. The cloud tops are usually not very high, which is the case in the pictures above and below, and designate the limit of the rising air. The relatively shallow height tops are easily seen in the two pictures as well as the flat bases which can be seen more clearly as you look into the distance of the photographs.
Formation
Fair weather cumulus are created
by buoyant masses of air, or thermals, that rise upwards
from the earth's surface. As they rise, the air
within the thermal cools and the water vapor
condenses, forming cloud droplets. The height
above the ground where this process takes
place is known as the lifting condensation level(LCL).
The location of this level can be identified
by the distinct flat cloud bases that are usually
associated with fair weather cumulus.
Is there an explanation to their flat bases?
Stable and calm conditions
must be present in the region next to the surface and there has to be no
turbulence in the atmosphere at the altitude where cumulus first formed.
This will lead to the conclusion that the only factors affecting cloud
formation will be the air temperature and the amount of water vapor in
the rising thermal. With very little or no turbulence, it is expected
that the height at which the temperature falls enough for condensation
to begin is roughly constant. This means that the base of the cloud
will start forming at a constant height above the ground which results
in the base of the cloud to form as a flat bottom surface.
Subsidence, Erosion, Dissipation
Since air is primarily rising
within the clouds themselves, there must be "subsidence", or
downward motion of air between the clouds. Subsiding
air slowly descends to replace the
warmer air that has ascended to higher levels
from below which have formed the existing clouds. In contrast, sinking
air inhibits cloud development, and as a result, clear skies are present
in areas of subsidence.
Expanding on this point, cumulus
clouds, along with many other types of clouds, in their early stages of
development have sharply defined edges, while in older clouds, their bases
and edges are more ragged in appearance. This reveals the process of cloud
erosion. Relating this point to both pictures, it seems that the
cumulus clouds in the foreground are older than the clouds in the background.
The foreground clouds tend to be wispy and ragged while the background
clouds have a more definite shape. The clouds in the foreground are
experiencing a rapid rate of evaporation at their edges. The evaporation
of cloud edges cools the air surrounding the cloud, making it more dense
and then resulting in sinking motion as explained in the paragraph above.
This is why fair weather cumulus typically have expanses of clear sky between
them. Without a continued supply of rising air into the cloud, the evaporating/erosion
process will continue, eventually resulting in the cloud's dissipation.
This process occurs mainly in the evening hours of the day. The sun's
heating effect on the ground weakens, and there is no longer anything driving
the formation of cumulus clouds. Thermals stop ascending and skies
become clear. This is exactly what happened on the day that the above
pictures were taken. The cumulus clouds were no longer in the sky
as evening came and the red sun that appeared at sunset was clearly visible.
Conclusion
The presence of cumulus
humilis in the sky usually means that the observer is experiencing a day
where the sun is shining brightly, however, it is blocked from the observer
every once in a while due to the patchy cumulus clouds. Calm winds
can also be expected, not only at the surface but also at the height of
these clouds. Calm winds at the LCL are signified by the flat bases
that cumulus humilis are associated with and which again can be seen in
the above photographs.
PICTURES Part 1 PICTURES Part 2