Some water in the form of invisible vapour is intermixed with the air throughout the atmosphere. It is the condensation of this vapour which gives rise to most weather phenomena: clouds, rain, snow, dew, frost and fog. There is a limit to how much water vapour the air can hold and this limit varies with temperature. When the air contains the maximum amount of vapour possible for a particular temperature, the air is said to be saturated. Warm air can hold more vapour than cold air. In general the air is not saturated, containing only a fraction of the possible water vapour.
The amount of vapour in the air can be measured in a number of ways. The humidity of a packet of air is usually denoted by the mass of vapour contained within it, or the pressure that the water vapour exerts. This is the absolute humidity of air. Relative humidity is measured by comparing the actual mass of vapour in the air to the mass of vapour in saturated air at the same temperature. For example, air at 10°C contains 9.4 g/m3 (grams per cubic metre) of water vapour when saturated. If air at this temperature contains only 4.7 g/m3 of water vapour, then the relative humidity is 50%.
When unsaturated air is cooled, relative humidity increases. Eventually it reaches a temperature at which it is saturated. Relative humidity is 100%. Further cooling leads to condensation of the excess water vapour. The temperature at which condensation sets in is called the dew point. The dew point, and other measures of humidity can be calculated from readings taken by a hygrometer. A hygrometer has two thermometers, one dry bulb or standard air temperature thermometer, and one wet bulb thermometer. The wet bulb thermometer is an ordinary thermometer which has the bulb covered with a muslin bag, kept moist via an absorbent wick dipped into water. Evaporation of water from the muslin lowers the temperature of the thermometer. The difference between wet and dry bulb temperatures is used to calculate the various measures of humidity.
All air contains water vapour of varying quantities. The dew point indicates the amount of moisture in the air. The higher the dew point, the higher the moisture content of the air at a given temperature. Conversely, the dew point of humid air will be higher than the dew point of dry air.
Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapour content) in order to reach saturation. A state of saturation exists when the air is holding the maximum amount of water vapour possible at the existing temperature and pressure.
Condensation of water vapour begins when the temperature of air is lowered to its dew point and beyond. The dew point, like other measures of humidity, can be calculated from readings taken by a hygrometer.
Dew point temperature is a measure of humidity. If air is cooled eventually enough energy will be removed for water vapor to begin to condense. Remember the water vapor was originally liquid water and to get it to evaporate you had to add energy. As long as it has sufficient energy it will remain vapor, but as you cool it at some point condensation will occur. The temperature where condensation begins is the dew point temperature. In terms of relative humidity, as the parcel of air is cooled, the relative humidity increases, when the relative humidity reaches 100%, the air parcel has cooled to the dew point temperature.
The dew-point temperature can be measured with a device called a wet-dry bulb thermometer. A wet-dry bulb thermometer is basically two thermometers placed side by side. The bulb of one is covered by a wick that is always kept wet (wet bulb) and the other is left alone (dry bulb). Just like water evaporating form a person's body right after they come out of the shower, the thermometer with the wet wick will cool as the water evaporates from the wick. The less water vapour in the air or the drier the air around us is, the more water will evaporate from the wet wick. The more water evaporates the cooler the reading on the wet bulb thermometer. The dry bulb or regular thermometer is used as a reference to give us the current air temperature. The temperature difference between the two thermometers tells us how much moisture is in the air. The greater the difference the dryer the air.
For example, a dry bulb temperature of 22 degrees Celsius and a wet bulb temperature of 10 degrees Celsius results in a difference of 12 C. In this case, the air would be considered dry. A dry bulb temperature of 22 C and a wet bulb temperature of 20 C Celsius results in a difference of only 2 degrees. In this case the air would be considered moist.
Can you explain what the dewpoint is and how it is relevant in the forecasts?
The dew point is the temperature at which water vapour condenses into liquid water. If the liquid water condenses onto objects (grass, autos, windows, an airplane wing) at a temperature above freezing, it is called dew. If it condenses onto objects at a temperature at or below freezing, it is called frost.
It is important to know the dew point because it is the temperature at which clouds form. When air rises from the ground, it cools until it reaches the dew point and then a cloud forms in the atmosphere.
What is the purpose of having a dew point in the winter?
If the temperature reached the dew point and it was still below what happens?
The purpose of having a dew point in the winter is the same as it is in the summer. We want to see how much moisture is in the air even in the winter. If the spread between the dew point and the temperature is small then the air is moist, if the spread is large, the air is dry. The more moisture the air has, the greater the risk of fog, rain or snow. If the dew point is below zero and the air temperature reaches the dew point then the precipitation will be in the form of snow.
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