Humidity, Condensation, and Clouds

Met 10 Lecture 5Dr. Craig Clements

San José State University

Circulation of water in the atmosphere

Hydrologic Cycle

Water in the atmosphereWater in the atmosphere

Definitions:

– Evaporation:

– Condensation:

– Precipitation:

Process where a liquid changes into a gasProcess where a liquid changes into a gas

Any liquid or solid water that Any liquid or solid water that falls from the atmosphere to falls from the atmosphere to the ground. (i.e. RAIN!)the ground. (i.e. RAIN!)

Process where a gas changes into a liquidProcess where a gas changes into a liquid

Water freely Water freely evaporating and evaporating and condensingcondensing

Since more water Since more water molecules are molecules are evaporating than evaporating than condensing, then condensing, then net evaporation is net evaporation is occurring.occurring.

Lid on:Lid on:

Now, evaporation Now, evaporation and condensation and condensation are equal. The air are equal. The air above water is above water is now called now called ‘saturated’.‘saturated’.

The humidity is The humidity is now 100%now 100%

CondensationCondensation

The process by which water vapor changes to a cloud droplet

Water vapor molecules may ‘stick’ to

condensation nuclei and grow (billions) to eventually form cloud droplet.

Examples of condensation nuclei include:a.a. DustDustb.b. SaltSaltc.c. SmokeSmoke

Condensation occurs primarily as temperature cools:-colder the molecules more likely they are to ‘stick’ to other molecules

Evaporation, Condensation and Saturation

saturation condensation nuclei

• In very clean air, about 10,000 condensation nucleiIn very clean air, about 10,000 condensation nucleiare typically found in one cubic centimeter of air,are typically found in one cubic centimeter of air,a volume approximately the size of your fingertip.a volume approximately the size of your fingertip.

Humidity: What is it?

Humidity refers to any one of a number of ways of specifying the amount of water vapor in the air.

We can compare the weight (mass) of the water vapor with the volume of the air in the parcel to obtain the water vapor density or absolute humidity.

We can compare the weight of the water vapor with the volume of air in the parcel with the total weight of the air and obtain the specific humidity.

Vapor Pressure

The total pressure inside an air parcel is the sum of the pressures of each individual gas…nitrogen, oxygen and water vapor.

At 1000 mb (sea level) nitrogen(78%) has a pressure of 780 mb, oxygen (21%) has a pressure of 210 mb. The partial pressure of water vapor (1%) would be 10 mb.

The number of water vapor molecules is small compared to total number of air molecules in the volume.

Actual Vapor Pressure is a good indicator of the amount of water vapor in the air.

Saturation Vapor Pressure

Saturation vapor pressure describes the amount of water vapor needed to make the air saturated at any given temperature.

Air that is saturated: number of molecules escaping the water surface = amount returning.

At higher air temperatures, it takes more water vapor to saturate the air.

Relative Humidity

The relative humidity (RH) is the ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at that particular temperature and pressure.

It is the ratio of the air’s water vapor content to its capacity:

RH =water vapor content

water vapor capacity

Relative Humidity

RH =water vapor content

water vapor capacityX 100 percent

Relative humidity is given as a percent. 50% RH means that the air contains ½ the amount required for saturation.

A change in RH can be brought about by two primary ways:

1. By changing the air’s water vapor content2. By changing the air temperature.

Diurnal Temperature and RH variation

Measuring Humidity: Sling psychrometer

A psychrometer consists of two glass thermometers with one covered with a wick (cloth) that is wet. This measures the ‘wet-bulb’ temperature. Water vapor evaporates from the wick and the bulb cools. The difference between the dry bulb and wet bulb is the wet bulb depression.

Dew Point Temperature

Dew point represents the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur.

Dew point is a good indicator of the air’s actual water vapor content.

High dew points indicate high water vapor content, low dew points = low water vapor content.

Fig. 4-8a, p. 86

Dew points

Dew and Frost

On calm, clear nights, the surface cools rapidly by what process?

Air near the ground cools to the dew point quickly, reaching saturation.

Water vapor condenses on blades of grass at the ground, forming tiny specks of water called dew.

Dew and Frost

Frost

When the dew point is below freezing (now called the frost point), frost forms which is composed of tiny ice crystals.

Water vapor changes directly into ice without becoming liquid first– called deposition.

When the air’s relative humidity reaches about 75%, some of it may begin to condense on tiny floating particles of sea salt and other substances — condensation nuclei— that are hygroscopic (“water seeking”) in that they allow water vapor to condense onto them when the relative humidity is considerably below 100%.

As water collects onto these nuclei, their size increases and the particles are large enough to scatter visible light. Haze forms.

As RH approaches 100%, the particles grow larger and eventually becoming visible to the naked eye…forming a cloud!

A cloud is a visible aggregate of tiny water droplets or ice crystals suspended in the air. Some are only found in the high atmosphere and some touch the ground.

Cirrus clouds

Cirrocumulus clouds

Altocumulus clouds

Stratocumulus clouds

Cumulus clouds. Small cumulus clouds such as these are sometimes called fair weather cumulus, or cumulus humilis

stratus clouds

Condensation trail– from an airplane.