MET 10 Lecture 6

1. MET 10 Lecture 6 Atmospheric Stability and Cloud Development Chapter 5 Dr. Craig Clements San Jose State University

2. Atmospheric Stability Hydrologic Cycle

3. Atmospheric stability We determine the stability of air by comparing the temperature of a rising parcel to that of its surroundings or ‘environment’. If the parcel is colder than its environment, it will be more dense (heavier) and tend to sink back to its original level. This condition is called stable. Stable: Air is stable when it resists upward displacement. If the parcel is warmer than its environment, it will be less dense (lighter) and will continue to rise until reaching the same temperature as the environment. This is an example of unstable air. Unstable: A lifted parcel of air will be warmer than the surrounding air and thus will continue to rise upward, away from its original position.

4. Atmospheric Lapse Rates • Adiabatic process: when a parcel of air expands and cools, or • compresses and warms, with no interchange of heat with • the outside surroundings. • As long as the air in the parcel is unsaturated (RH<100%) the • rate of adiabatic cooling or warming is constant • Dry Adiabatic Lapse Rate = 10°C /1000 m (5.5°F /1000 ft.) • If the rising air cools to its dew-point temperature, the RH • becomes 100%, condensation occurs, a cloud forms, and • latent heat is released. This added heat during condensation • offsets some of the cooling. The air now cools at a lesser rate • Moist Adiabatic Lapse Rate =6°C /1000 m (3.3°F /1000 ft.)

5. Dry Adiabatic Lapse Rate

7. Stable Atmosphere The atmosphere is stable when the environmental lapse rate is small, that is, when there is relatively small difference in temperature between surface air and the air aloft. The atmosphere becomes more stable ( it stabilizes) as the air aloft warms or surface air cools. What can cause the surface air to cool?

8. Becoming more stable

10. Unstable Atmosphere The atmosphere is unstable when the environmental lapse rate steepens, that is, when the temperature of the air drops rapidly with increasing height. The atmosphere becomes more unstable (destabilize) as the air aloft becomes colder or the surface air warms. What can cause the air to cool aloft? The surface air to warm?

11. Becoming more unstable: destabilizes

13. Convection: Stable or Unstable?

14. Two lapse rates at once!

16. Cloud Development and Stability • Most clouds form as air rises, expands, and cools. • A majority of the clouds we observe are due to the following • mechanisms: • Surface heating and free convection • Uplift along topography • Widespread ascent due to the flowing together (convergence) • of surface air. • 4. Uplift along weather fronts (different air masses).

18. Convection Air that is saturated: number of molecules escaping the water surface = amount returning. At higher air temperatures, it takes more water vapor to saturate the air.

19. Lifted by topography

20. Convergence

21. Lifting along weather fronts

23. Cumulus clouds building on a warm summer afternoon. Each cloud represents a region where thermals are rising from the surface. The clear areas between the clouds are regions where the air is sinking.

25. Orographic uplift, cloud development, and the formation of a rain shadow.

26. Clouds that form in the wave directly over the mountain are called mountain wave clouds, whereas those that form downwind of the mountain are called lee wave clouds

28. Dew point represents the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur. Dew point is a good indicator of the air’s actual water vapor content. High dew points indicate high water vapor content, low dew points = low water vapor content.

29. Cloud droplets Raindrops Condensation nuclei

30. Which of the three drops drawn here represents the real shape of a falling raindrop?

31. Fig. 4-8a, p. 86

32. Precipitation Processes • Collision-Coalescence process: • Larger cloud droplets fall faster than smaller droplets. • The larger droplets collide with smaller drops in their path. • This merging of cloud droplets by collision is called • coalescence and occurs in clouds with tops warmer • than -15°C. • Called warm clouds. • Coalescence is enhanced if the colliding droplets have • opposite electrical charges.

33. Dew and Frost On calm, clear nights, the surface cools rapidly by what process? Air near the ground cools to the dew point quickly, reaching saturation. Water vapor condenses on blades of grass at the ground, forming tiny specks of water called dew.

34. Precipitation Processes in Warm Clouds • Most important factor in the production of raindrops is the liquid water content of the cloud. • Other important factors: • The range of droplet sizes • The could thickness • The updrafts of the could • The electric charge of the droplets and electric field in the cloud.

35. Precipitation Processes in Cold Clouds • Ice-crystal process (Bergeron): • This process proposes that both ice crystals and liquid cloud • droplets must co-exist in clouds at temperatures below • freezing. • Important in area where clouds can extend upward into • regions where the air temperatures are below freezing. • These clouds are called cold clouds. • Water droplets existing at temperatures below freezing are • referred to as supercooled.

36. The distribution of ice and water in a cumulonimbus cloud.

37. Frost The ice-crystal process. The greater number of water vapor molecules around the liquid droplets causes water molecules to diffuse from the liquid drops toward the ice crystals. The ice crystals absorb the water vapor and grow larger, while the water droplets grow smaller. When the dew point is below freezing (now called the frost point), frost forms which is composed of tiny ice crystals. Water vapor changes directly into ice without becoming liquid first– called deposition.

38. Aggregation Forming snowflakes Accretion Splintering forming graupel

40. Natural seeding by cirrus clouds may form bands of precipitation downwind of a mountain chain

41. The streaks of falling precipitation that evaporate before reaching the ground are called virga. Cirrus clouds

42. The dangling white streamers of ice crystals beneath these cirrus clouds are known as fallstreaks.

43. dendrite snowflakes

44. Average Annual Snowfall over US Cumulus clouds. Small cumulus clouds such as these are sometimes called fair weather cumulus, or cumulus humilis

46. An aircraft undergoing de-icing during inclement winter weather

48. HW #4: Due Thursday October 9 • Identify cloud types for 3 days: Friday, Saturday, and Sunday would be a good choice. • List whether they are high, middle or low clouds. • List the time that you observed them. • If you can photograph them, put the photos into your • report or find similar photos on the web and include these into your report.

50. Components of a standard rain gauge.