1. Chapter 18: Moisture, Clouds, and Precipitation
Overview

2. Water in the Atmosphere
Precipitation – any form of water that falls from a cloud
When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere

3. Water’s Change of State: Solid to Liquid
The process of changing state requires that energy is transferred in the form of heat
Latent Heat – does not produce a temperature change; stored energy

4. Water’s Change of State: Liquid to Gas
Evaporation – the process of changing a liquid to a gas
“evaporation is a cooling process” means that it takes incredible energy to evaporate water, usually it comes from whatever object it is evaporating from (i.e. you!)
Condensation – the process where water vapor changes to the liquid state
For condensation to occur, water molecules must release their stored heat energy

5. Water’s Change of State: Solid to Gas
Sublimation – the conversion of a solid directly to a gas, without passing through the liquid state
Deposition – the conversion of a vapor directly to a solid

6. Changes of State

7. Saturation
Saturated – the number of vapor molecules returning to the surface of a body of water equals the amount leaving
When saturated, warm air contains more water vapor than saturated cold air

8. Relative Humidity
Relative Humidity – ratio of the air’s actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure
Relative humidity can be changed by adding or removing water vapor from the air

9. Dew Point
Dew Point – the temperature to which a parcel of air would need to be cooled to reach saturation
High dew-point temperatures indicate moist air, and low dew-point temperatures indicate dry air

10. Measuring Humidity Hygrometer – device to measure relative humidity
One type of hygrometer, the psychrometer, consists of two identical thermometers mounted side to side

11. Air Compression and Expansion
Temperature changes that happen even though heat isn’t added or subtracted are called adiabatic temperature changes
When air is allowed to expand, it cools, and when it is compressed, it warms
Dry Adiabatic Rate – the rate of adiabatic warming or cooling in unsaturated air (1ºC/100 m)
Wet Adiabatic Rate – the rate of adiabatic temperature change in saturated air; it is always less than the dry adiabatic rate

12. Orographic Lifting
Four mechanisms that can cause air to rise are orographic lifting, frontal wedging, convergence, and localized convective lifting
Orographic Lifting – mountains acting as barriers to the flow of air, forcing the air to ascend

13. Frontal Wedging
Front – the boundary between two adjoining air masses having contrasting characteristics
The cooler denser air acts as a barrier over which the warmer, less dense air rises

14. Convergence
Whenever air in the lower atmosphere flows together, lifting results, this is called convergence
This leads to adiabatic cooling and possibly cloud formation

15. Localized Convective Lifting
On warm summer days, unequal heating of Earth’s surface may cause pockets of air (thermals) to be warmed more than the surrounding air rising into the atmosphere
The process that produces rising thermals is localized convective lifting

16. Stability
Stable air is any air which resists vertical movement due to density differences
Stable air tends to remain in its original position, while unstable air tends to rise
Air stability is determined by measuring the temperature of the atmosphere at various heights
The rate of change of air temperature with height is called the environmental lapse rate
Temperature Inversion – a layer where the temperature increases with height; the most stable conditions for air
Clouds associated with the lifting of unstable air are towering and often generate thunderstorms and tornados

18. Condensation
For any of the forms of condensation to occur, the air must be saturated
Generally, there must be a surface for water vapor to condense on
Condensation Nuclei – tiny bits of particulate matter that serve as surfaces on which water vapor condenses

19. Types of Clouds (2)
Clouds are classified on the basis of their form and height
Cirrus clouds are high, white, and thin
Cumulus clouds consist of rounded individual cloud masses, they normally have a flat base and the appearance of rising domes or towers
Stratus clouds are sheets or layers that cover much or all of the sky
There are three levels of cloud heights: high, middle, and low

20. Three cloud types make up the family of high clouds (above 6000 meters): cirrus, cirrostratus, and cirrocumulus
All high clouds are thin and white and are often made up of ice crystals
Clouds that appear in the middle range (~ m) have the prefix alto-
Middle clouds may cause infrequent light snow and drizzle
There are three members in the family of low clouds (below 2000 m): stratus, stratocumulus, and nimbostratus
Nimbostratus clouds are the main precipitation makers
Vertical development clouds have their bases in the low height range, but extend through the middle or high altitudes
Cumulonimbus may produce rain showers or thunderstorms

22. Fog
Fog is defined as a cloud with its base at or very near the ground

23. How Precipitation Forms (2)
For precipitation to form, cloud droplets must grow in volume by roughly one million times
Bergeron Process – a theory that relates to the formation of precipitation to supercooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water

24. Collision-Coalescence Process – a theory of raindrop formation in warm clouds (above 0ºC) in which large cloud droplets collide and join together with smaller droplets to form a raindrop

25. Forms of Precipitation
The type of precipitation that reaches Earth’s surface depends on the temperature profile in the lowest few kilometers of the atmosphere
Temperature profile is the way the air changes with altitude
Rain means drops of water that fall from a cloud and have a diameter of at least 0.5 mm
At very low temperatures light, fluffy snow made up of six-sided ice crystals forms
Sleet, glaze, and hail are all formed from water becoming supercooled on its trip down to the surface

26. Assignment Read Chapter 18 (pg. 504 – 522)
Do Chapter 18 Assessment #1-30 (pg ), # 1-5 pg 529