Chapter 18: Moisture, Clouds, and Precipitation

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 (~2000-6000 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. 527-528), # 1-5 pg 529