Humidity, Condensation, and Clouds Met 10 Lecture 5 Dr. Craig Clements San José State University

Circulation of water in the atmosphere Hydrologic Cycle

Water in the atmosphere Definitions: Evaporation: Condensation: Precipitation: Process where a liquid changes into a gas Process where a gas changes into a liquid Any liquid or solid water that falls from the atmosphere to the ground. (i.e. RAIN!)

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

Lid on: Now, evaporation and condensation are equal. The air above water is now called ‘saturated’. The humidity is now 100%

Condensation 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: Dust Salt Smoke So today, after these very broad introductory comments, we will spend a short time looking at the nature of energy in the E-A system, then we will look at some very basic radiation concepts (here I don’t want to get bogged down in complex detail, I just want to give you an overview of basic radiation concepts). However it is important that you have a basic understanding, because radiation is the primary energy source for driving atmospheric and ocean circulation and for all life on earth. Finally today, we’ll examine the mean annual global energy budget, concentrating especially on those significant climate leverage points. Condensation occurs primarily as temperature cools: colder the molecules more likely they are to ‘stick’ to other molecules

Evaporation, Condensation and Saturation condensation nuclei In very clean air, about 10,000 condensation nuclei are typically found in one cubic centimeter of air, 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 Figure 4.4: The water vapor content (humidity) inside this air parcel can be expressed in a number of ways.

Relative Humidity RH = water vapor content X 100 percent water vapor capacity X 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: By changing the air’s water vapor content 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.

Figure 4.8: Average surface dew-point temperatures (ºF) for (a) January and for (b) July. Dew points Fig. 4-8a, p. 86

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.