1. Moisture, Clouds and Precipitation Standards: Concept 2: PO 14
Chapter 18
Moisture, Clouds and Precipitation
Standards: Concept 2: PO 14

2. Water’s Changes of States
The process of changing state requires that energy is transferred in the form of heat.
Evaporation – From a liquid to a gas (cooling process)
Condensation – From a gas to a liquid (releases heat)
Sublimation – From a solid to a vapor
Deposition – From a vapor to a solid (ie: frost)

3. Humidity
The general term for amount of water vapor in the air

4. Saturation When the air is holding as much water as it can
When saturated warm air can hold more water than cold air.

5. Relative Humidity
A ratio of the air’s actual water-vapor content compared with the amount of water vapor air can hold at that temperature.
Stated as a %
Lowering air temperature causes an increase in relative humidity
Raising air temperature causes a decrease in relative humidity
Increasing or decreasing water vapor with change the humidity

6. Relative Humidity Chart

7. Tools to Measure Humidity
Hygrometer
Psychrometer

8. Dew Point
The temperature to which a parcel of air would need to be cooled to reach saturation
Forms dew, fog, or clouds

9. Adiabatic Temperature Changes
Temperature changes that occur if heat is not added or subtracted
When air is expanded it cools (spreads out)
When air is compressed it warms (squished together)

10. Processes That Lift Air
Orographic Lifting
Frontal Wedging
Convergence
Localized Convective Lifting

11. Stability of Air Stable Air Unstable air
Resists vertical ( up down) movement
Remains in original position
Create Temperature Inversions
Form on cool nights
Trap pollution
Temperature increases with altitude in the troposhpere
Unstable air
Moves vertically (up down)
Tends to rise
Puffy clouds
Thunderstorms and Tornados

12. Temperature Inversion

13. Inversion Layer in Phoenix

14. Condensation Air must be saturated for condensation to occur
Water needs something to condense on called condensation nuclei
Condensation nuclei can be near the ground items such as grass or high up in the air small particles of dust, salt, smoke
When a bunch of condensed water droplets remain in the sky a cloud is produced

15. Clouds
Cirrus (curl of hair)
Cumulus (a pile)
Stratus (a layer)

16. Clouds
Nimbus means “rainy cloud”
Cumulonimbus (thunderhead)

17. Fog
A cloud with its base on or near the ground

18. Precipitation Rain (drops of water that fall to ground)
Snow (water vapor frozen into ice crystals, falling in light white flakes)
Sleet (rain that falls thru a layer of freezing air and freezes on way down)
Hail (large piece of ice falling to the ground)

19. Formation of Hail