Relative Humidity and Cloud Formation

Humidity and Cloud Formation One atmospheric factor not mentioned yet is how wet or dry the air can be. There is always water vapor in the air, which we measure as humidity.

Humidity and Cloud Formation Meteorologists, people who study weather, use a figure which they call relative humidity Relative humidity (R.H.) is the amount of moisture currently present in the air compared to the maximum amount of moisture that can evaporate at that temperature.

Humidity and Cloud Formation If the air has 40 % of all of the water vapor that could form, then the relative humidity is 40 %. When the air is totally saturated, the relative humidity is 100 %.

Humidity and Cloud Formation Warm air has more ability to evaporate liquid water into vapor. Cold air has less ability to evaporate liquid water into vapor, so it is more likely to condense water vapor into liquid water.

Dew Point and Cloud Formation If warm, moist air is cooled off suddenly, the water vapor condenses. The temperature at which the relative humidity reaches 100 % is called the dew point.

Dew Point and Cloud Formation At the dew point, water vapor condenses dramatically, forming dew on the ground…

Dew Point and Cloud Formation … or clouds and precipitation in the air.

Dew Point and Cloud Formation The Dew Point is helpful in two main ways: Low dew point usually means the air feels really dry (low humidity), while High dew point (closer to the air temperature) means high humidity. As air rises, it cools. When the air temperature drops to the dew point, clouds start to form. This altitude is called the dew point altitude.

Humidity and Cloud Formation As you go up through the atmosphere, the temperature drops off rather quickly. With a temperature of 78o and a dew point of 58o, the relative humidity on the ground may be only 40 %.

Humidity and Cloud Formation As the air rises, the temperature drops while the dew point often remains unchanged. At 1800 feet altitude, the temperature has dropped from 78o to 68o, while the dew point has remained at 58o .

Humidity and Cloud Formation At 3600 feet, the temperature (58o) has dropped down to the dew point (58o). This means the relative humidity has risen to 100 %, and clouds will start to form.

Humidity and Cloud Formation At this height (the dew point altitude), the water vapor condenses and forms clouds. Most clouds tend to have flat bottoms or bases…

Cloud Base and Dew Point Temp Clouds often have flat bottoms because water will not condense (at least not very much) below the dew point altitude. (R.H. is too low below this altitude.) 100 % R.H. The flat bottom of the clouds that forms at this altitude is called the cloud base. 40 % R.H.

Cloud Base and Dew Point Temp Clouds often have flat bottoms because water will not condense (at least not very much) below the dew point altitude. (R.H. is too low below this altitude.) 100 % R.H. Cloud Base 40 % R.H.

Cloud Condensation Nuclei

Cloud droplets are tiny: about 20 microns (mm) across Cloud droplets are tiny: about 20 microns (mm) across. Each cloud droplet is built around an even tinier grain of dust, ash or salt (0.2 mm). These secret ingredients that make clouds grow are called Cloud Condensation Nuclei (CCN).

1) Cloud Condensation Nuclei (CCN) provide the water vapor molecules in the air a surface on which to collect . 2) As the water vapor condenses on the CCN surface a cloud droplet forms. 3) It may take 1,000,000 (1 million) cloud droplets to make a rain drop!! (See next slide.)

(0.2 mm) (50 mm) (20 mm) (2000 mm)

Air Stability and Cloud Formation

Air Stability and Cloud Formation In stable air, rising warm air pockets cool faster than the surrounding air (or may be cooler than the overlying air to begin with), so they tend to resist rising. Stable air rises to the dew point altitude and then spreads out sideways to form layered clouds such as stratus and cirrus.

Stable Air = Layered Clouds Altostratus Altocumulus Stratus Cirrus

Air Stability and Cloud Formation In unstable air, rising warm air pockets cool more slowly than the surrounding air. They hit the dew point altitude just keep on rising!! Unstable air leads to clouds with a great deal of vertical development. This class of clouds is called convective clouds.

Unstable Air = Convection Clouds

Unstable Air = Convective Clouds Cumulonimbus Towering Cumulus Cumulus congestus

Fronts and Cloud Formation

Fronts and Cloud Formation Clouds also form where warm air meets cold air and rises along a front. Water vapor condenses as the warm air rises and cools, causing the temperature to quickly drop below the dew point.

Fronts and Cloud Formation This sudden increase in relative humidity often results in precipitation (rain, snow, sleet, hail, fog).

Fronts and Cloud Formation

Fronts and Cloud Formation The uplift along a cold front is sudden. This creates very unstable air and the clouds along a cold front are convective clouds such as cumulus and cumulonimbus.

Cold Front Clouds

Clouds from an Approaching Cold Front

Fronts and Cloud Formation Uplift along a warm front is more gradual, so the air is usually more stable. As a result, clouds along a warm front are more likely to be layered clouds such as stratus, cirrus, altostratus, altocumulus and stratocumulus.

Warm Front Clouds

Clouds from an Approaching Warm Front

Mountains and Cloud Formation Clouds also form when air rises as it passes over a mountain range. This is called orographic lifting. Water vapor condenses as the air temperature drops below the dew point. One spectacular variety is the lenticular cloud.

Orographic Uplift

Orographic Uplift and the Rainshadow Effect Heavy rainfall occurs on the upwind side of the mountains due to orographic lifting. As the dry air descends on the downwind (lee) side of the mountains, it becomes super dry and often creates desert conditions.