Moisture, Clouds, & Precipitation

1. Moisture, Clouds, & Precipitation Meteorology, ch. 4

2. Question of the Day • What are the three phases of matter?

3. Changes of state of water • Heat energy • Latent heat • Stored or hidden heat • Important in atmospheric processes • States of Matter • Solid, liquid, gas • Heat must be absorbed or released

4. Changes of state of water Figure 17.2

5. Humidity • Amount of water vapor in the air • Saturated air • Filled to capacity • Temperature dependent • Water vapor adds pressure (called vapor pressure) to the air

6. Measuring Humidity • Absolute Humidity • mass of water vapor/ volume of air • Mixing ratio (used by meteorologists) • Often measured in grams per kilogram • Relative humidity • Actual air water vapor content compared with Amount of water vapor needed for saturation at that temperature (and pressure) • Saturated air is 100% RH • Altered by the +/- of water vapor, change in temperature • dew point (temp. to be cooled for saturation)

7. Relative humidity changes at constant temperature Figure 17.4

8. Relative humidity changes at constant water-vapor content Figure 17.5

9. Typical daily variations in temperature and relative humidity Figure 17.6

10. Instruments used for Humidity • Two types of Hydrometer • Psychrometer - compares temperatures of wet-bulb thermometer and dry-bulb thermometer • 100% RH, both thermometers read the same temperature • The greater the difference between the thermometer readings, the lower the relative humidity • Hair hygrometer – reads the humidity directly

11. Adiabatic Temperature Change • Happens when • Air is compressed (warms) • Air expands (cools) • Rates • Dry Adiabatic (DAR) • unsaturated; 1C/100 m (5.5F/1000’) • Wet Adiabatic (WAR) • starts at condensation level (reached dew pt.) • .5-.9C/100m

12. Questions of the Day • What does it mean when air is saturated? • What is the RH reading for saturated air?

13. Adiabatic cooling of rising air Figure 17.9

14. Processes that lift air • Orographic lifting • Result can be a rain shadow desert • Frontal wedging • Cool air acts as a barrier to warm air • Fronts are part of the storm systems • Convergence • Local convection lift (thermals)

15. Processes that lift air Figures 17.10, 17.12, 17.13, & 17. 14

16. Stability of air • Types of stability • Stable air • Resists vertical displacement • Cooler than surrounding air • Denser than surrounding air • Wants to sink • No adiabatic cooling • Absolute stability occurs when the environmental lapse rate is less than the wet adiabatic rate

17. Absolute stability Figure 17.17

18. Stability of air • Stable Air • resists vertical movement • Widespread clouds; precipitation light to moderate • Unstable Air • vertical movement • Warmer, less dense than surrounding air • Continues to rise until it reaches an altitude with the same temperature

19. Types of Stability • Absolute Stability • ELR is less than the WAR • Temperature Inversions • Absolute instability • ELR > DAR • Conditional instability • ELR between DAR and WAR • Towering cumulus clouds

20. Absolute instability Figure 17.18

21. Conditional instability Figure 17.19

22. How Stability Changes • Instability • solar radiation at lower layer • heating air from below as it moves over a warm surface • upward movement of air • radiation cooling from cloud tops • Stability • radiation cooling of surface after sunset • cooling air from below as it moves over a cold surface • subsidence

23. Cloud Formation • Condensation • visible aggregate of minute droplets of water, ice crystals or both • Adiabatic cooling • Two things needed for clouds to form • air to be saturated or close to it • condensation nuclei • ground or in atmosphere (dust, smoke, salt) • hydroscopic nuclei are most effective sites

24. Cloud Classification • Based on Form • Cirrus – high, white, thin • Cumulus- globular cloud masses often associated with fair weather • Stratus – sheets or layers that cover much of the sky • Height • High: base at 6000m • Cirrus, Cirrostratus, Cirrocumulus • Middle: base between 2000-6000m • Altostratus, Altocumulus • Low: base below 2000m • Stratus, Stratocumulus, Nimbostratus • Clouds of Vertical development

25. Cirrus clouds Figure 17.21 A

26. Altostratus clouds Figure 17.21 E

27. Cumulus clouds Figure 17.21 G

29. Classification of clouds according to height and form (continued) Figure 17.20

30. Fog • Considered an atmospheric hazard • Cloud with its base at or near the ground • Most fogs form because of • Radiation cooling or mvem’t over cold surface • Radiation Fog • Advection Fog • Upslope Fog • Evaporation Fog • Steam fog • Frontal/precipitation fog

31. Precipitation • Cloud droplets • Less than 20 micrometers (0.02 millimeter) in diameter • Fall incredibly slow • Formation of precipitation • Bergeron process • Temperature in the cloud is below freezing • Ice crystals collect water vapor • Large snowflakes form and fall to the ground or melt during descent and fall as rain

32. Particle sizes involved in condensation and precipitation

33. The Bergeron process Figure 17.25

34. Precipitation • Formation of precipitation • Collision-coalescence process • Warm clouds • Large hygroscopic condensation nuclei • Large droplets form • Droplets collide with other droplets during their descent • Common in the tropics

35. The collision- coalescence process Figure 17.26

36. Precipitation • Forms of precipitation • Rain and drizzle • Rain – droplets have at least a 0.5 mm diameter • Drizzle – droplets have less than a 0.5 mm diameter • Snow – ice crystals, or aggregates of ice crystals • Sleet and glaze • Sleet • Glaze • Hail • Occurs in large cumulonimbus clouds • Rime • Freezing of supercooled fog or cloud dropplets

37. The standard rain gauge Figure 17.31