Moisture, clouds, and precipitation

1. Earth Science Ch. 18 Moisture, clouds, and precipitation

2. 18.1 Water in the Atmosphere • What is latent heat? • A change in the state of matter without changing the temperature. • The heat that is either added to the water or removed from the water is called latent heat. • The change in the state occurs because of the change in the bonds between water molecules. • They either become weaker or stronger.

3. When bonds become weaker, you move from a high ordered state to a low ordered state. • Latent heat is added to the water. • Solid to liquid (melting) • Liquid to gas (evaporation) • Solid to gas (sublimation)

4. When bonds become stronger, you move from a low ordered state to a high ordered state. • Latent heat is released from the water. • Gas to liquid (condensation) • Liquid to solid (freezing) • Gas to solid (deposition)

5. Humidity • Humidity is the amount of water vapor in the air. • When air is holding the maximum amount of water vapor that it can hold at a given temperature, the air is saturated.

6. Relative humidity is a ratio of the amount of water vapor in the air compared to the amount of water vapor the air can hold at that temperature.

7. Lowering the air temperature causes an increase in relative humidity because cooler air cannot hold as much water vapor. • Raising the air temperature causes a decrease in relative humidity because warm air can hold more water vapor.

8. Dew Point • The dew point is the temperature to which air must be cooled to reach saturation. • If the air is cooled past this temperature then the water vapor condenses. • High dew point temperatures indicate moist air (air does not have to be cooled as much) and low dew point temperatures indicate dry air (air has to be cooled a lot).

9. Relative Humidity • Relative humidity is commonly measured by using a hygrometer. • A psychrometer is a type of hygrometer that uses 2 thermometers. • A dry bulb and a wet bulb • In order to determine relative humidity, the temperature is taken for both bulbs

10. Large differences in temperature – low relative humidity. Small differences in temperature – high relative humidity. • If the air is saturated the 2 thermometers will have identical readings.

13. 18.2 Cloud Formation • In order for clouds to form the air must be saturated. • Saturation occurs when water vapor is added or when air is cooled to its dew point. • The base of clouds is the elevation of the dew point.

14. When air is compressed the temperature rises and the air warms. • As air expands the temperature decreases and the air cools. • In both of these processes the temperature changed even though heat wasn’t added or subtracted. • These types of changes are called adiabatic changes.

15. If a parcel of air rises high enough, it will eventually cool to its dew point • Temperature decreases with altitude • The colder the air the less water vapor it can hold • It is at this point where condensation begins. • From this point on as the air rises, latent heat stored in the water vapor will be released. This causes the air to cool.

16. Processes that lift air • Orographic lifting • Occurs when mountains act as barriers to air flow and force air upward. • As air moves up a slope adiabatic cooling often generates clouds and precipitation. • As air moves down the other side of the slope the air warms. Most of the moisture has been lost.

17. Frontal wedging • When masses of air collide a front is produced. • If one of the air masses is cold and the other is warm, the warm air mass will rise over the cold air mass. • The warm air mass will be cooled below the dew point forming clouds.

18. Convergence • The lifting of air that results from air masses in the lower atmosphere flowing together. • When air flows in from more than one direction, it goes up.

19. Stability • As air rises the temperature drops due to expansion. • When the air becomes cooler than the surrounding environment it becomes denser and sinks to its original position. • Air that follows this process is called stable air.

20. Clouds that form from stable air are widespread and are not very thick. Precipitation, if any, is light and moderate. • Clouds that form from unstable air are often towering and generate thunderstorms and occasionally tornadoes.

21. Condensation • For any forms of condensation to occur, the air must be saturated. • Saturation occurs when air is cooled to its dew point. • There must be a surface for water vapor to condense on. • These surfaces are called condensation nuclei. • Some common condensation nuclei found near the surface include dust, salt, and smoke.

22. 18.3 Cloud Types and Precipitation • Clouds form by condensation when the air is saturated because it reaches its dew point. • Forms of Clouds • Cirrus • Thin clouds found high in the atmosphere • Cumulus • Rounded cloud masses • Stratus • Sheets or layers

23. Cirrus Cumulus Stratus

24. Levels of Cloud heights • High Clouds • Above 6,000 meters • Cirrocumulus • Cirrostratus • Cirrus • All high clouds are thin and are often made of ice crystals • Very little precipitation

25. Middle level clouds • 2000 to 6000 meters • Prefix alto as part of the name • Altocumulus • Altostratus

26. Low clouds • Below 2000 meters • Stratus • Stratocumulus • Nimbostratus

27. Some clouds have low bases but extend into higher altitudes • Cumulonimbus clouds are examples • These clouds are associated with rain showers and thunderstorms

29. Fog • Physically there is no difference between fog and clouds • Fogs are the result of radiational cooling or the movement of air over a cold surface • Most clouds form by adiabatic cooling

30. Fog can form by cooling or evaporation • Cooling • Warm moist air comes in contact with a cool surface • Evaporation • Cool air moves over a warm surface

31. How precipitation forms • For precipitation to form cloud droplets must grow in volume • Most of the precipitation in the middle latitudes occurs in cold clouds and forms by the Bergeron Processes: • Supercooling • Liquid water at 0°C will readily freeze if it touches a solid object called a freezing nuclei. • Supersaturated Air • If the air has greater than 100% humidity the ice crystals grow quickly and begin to fall. • Drop size increases as the ice crystals fall

32. In the lower latitudes (tropical areas) precipitation occurs by the collision – coalescence process • Condensation nuclei remove water vapor from the air • As the droplets move through the cloud they collide and coalesce (join) with other droplets

33. Forms of Precipitation • The type of precipitation that reaches earth’s surface depends on the temperature. • Rain consists of drops of water that have a diameter of at least 0.5mm. • Smaller drops of water are called drizzle. • Rain and drizzle form when the temperature is above 4°C (39°F)

34. Snow forms at temperatures below 4°C • Sleet consists of small particles of ice • Sleet forms when there is a layer of air with temperatures above freezing above a layer near the ground that has temperatures below freezing. • Freezing rain forms when rain drops become super cooled as they fall through below freezing air near the ground and turn to ice when they impact objects.

35. Hail is produced in cumulonimbus clouds • Hail stones form as ice pellets grow by collecting super cooled water droplets as they fall through a cloud. • If the ice pellets encounter an updraft, they may be carried up into the cloud again where the journey will begin again. • During each trip through the cloud another layer of ice is added.