1. How Precipitation Forms Forms of Precipitation Precipitation Measurements Intentional Weather Modification Inadvertent Weather Modification 3/04/03 Outline for Lecture 10

3. standard atmosphere 300mb  30,000 ft

4. standard atmosphere 500mb  18,000 ft

5. standard atmosphere 700mb  10,000 ft

6. standard atmosphere 850mb  5,000 ft

8. How Precipitation Forms Important Facts: -cloud droplets are tiny (20 micrometers). -many condensation nuclei are present. -tiny particles fall more slowly than large ones. -a cloud droplet’s diameter must grow ~200 times to reach a raindrop’s diameter. -to attain the volume of a rain droplet, the cloud droplet increases a million times in volume.

9. The Bergeron Process (Cold Clouds) The Collision-Coalescence Process (Warm Clouds) Precipitation Formation Mechanisms

10. The Bergeron process describes how rain or snow forms when the cloud temperature is below freezing. This process where ice crystals grow at the expense of cloud droplets is called the Ice Crystal Process. It is also named after the Norwegian researcher who discovered it (Tor Bergeron, there were others). TemperatureRH wrt * H 2 O(liq)RH wrt H 2 O(ice) 0°C100% -05°C100%105% -10°C100%110% -15°C100%115% -20°C100%121% *wrt = with respect to

11. Three important properties of water droplets: 1. Cloud droplets do not freeze at 0.°C 2.Supercooled (water in the liquid state below 0°C) water droplets will freeze immediately if agitated sufficiently or when they come in contact with freezing nuclei (a crystalline structure similar to ice) 3. The saturation vapor pressure with respect to ice is lower than the saturation vapor pressure with respect to liquid water.

12. Ice Saturation with Respect to Ice and Water Vapor pressure is the pressure due to water vapor molecules when the evaporation rate is equal to the condensation rate. Because of the crystalline structure of ice, water molecules are not able to break free from an ice surface as easily than from a water surface. Therefore, the saturation vapor pressure with respect to an ice surface would be less than the saturation vapor pressure with respect to a liquid water surface at a given temperature. Liquid Water

16. Growth Process by Bergeron Process Coexistence of supercooled water droplets and ice crystals is essential to precipitation process in the Mid latitudes. -10°C -40°C20,000 ft 3,000 ft 1,000 ft Water Droplets Supercooled water droplets and ice crystals Ice crystalstop middle lower -20°C7,000 ft

17. Gravitational Force = Frictional Force Terminal Velocity occurs when:  F = 0 (when F gravity = F friction )  · 4 / 3  · r 3 · g =  · r 2 · v ·  4 / 3  · g · r = v mass · gravity = area · velocity ·  (velocity is a function of r)

18. From: Table 5-2

19. From Table 5-A

20. As droplets fall they collide with smaller droplets and coalesce. after collecting ~1 million cloud droplets the particle is large enough to fall without evaporating. Because there are a large number of collisions needed, clouds with great vertical extent are typically produce precipitation by this process. The Process from Warm Clouds: The Collision-Coalescence Process

22. Mist drizzle rain/sleet 0.005-0.05 mm less than 0.5mm 0.5 – 5 mm Approximate size of types of Precipitation

23. Forms of Precipitation (Rain, Snow, Sleet and Glaze, Hail) Droplet size determines the type of precipitation.

24. Rain is the term for drops of water that fall from a cloud and have a diameter of 0.5 millimeter (mm). Drizzle and mist have smaller droplets. Rain mostly occurs in nimbostratus clouds and cumulonimbus clouds. These clouds are capable of producing cloudbursts. Most rain starts as snow or ice crystals; as the snow falls through the cloud it melts. Drizzle is a fine uniform water droplet with a diameter less than 0.5 mm. Rain

25. Temperature Profile for Rain

26. Snow is precipitation in the form of ice crystals (snowflakes) or more often, aggregates if ice crystals. The size and structure of the crystals is a function of the temperature at which they form. When air temperatures are cold the moisture content is very small. This results in the formation of very light fluffy snow made up of six sided ice crystals. When conditions are warmer, the ice crystals join together into larger clumps consisting interlocked aggregates of crystals. Snow

27. Temperature Profile for Snow

28. Sleet and Glaze Sleet is a wintertime phenomenon that refers to the fall of small particles of ice that are clear to translucent. Sleet forms when rain passes through a cold layer of air and freezes into ice pellets. This occurs most often in the winter when warm air is forced over a layer of cold air.

29. Temperature Profile for Sleet and Glaze

30. Hail Hail is precipitation in the form of hard, rounded pellets or irregular lumps of ice. The layers of ice accumulate as the hailstone travels up and down in a strong convective cloud. Hailstones begin as small ice pellets that grow by adding supercooled water droplets as they move through the cloud. As the ice crystal cycles up and down in the cloud the hailstones increase in size until they are forced out by a downdraft or become heavy enough to fall out.

31. This large hailstone fell from a tornadic supercell northeast of Breckenridge, TX. The stones left a path miles wide littered with three to four and one half inch stones. The hail fell from a dark cloud base to the south of a developing mesocyclone. No rain or thunder occurred in the immediate area while the stones were coming down. Hail 4”

32. Rime Rime is a deposit of ice crystals formed by the freezing of super cooled fog or cloud droplets on objects whose surface temperature is below freezing. When rime forms on trees, it covers them with ice feathers; in windy conditions only the windward surfaces will accumulate the layer of rime.

33. Table 5-4

34. Precipitation Measurements Standard Instruments: Simple rain gauge, standard rain gauge, tipping bucket gauge, weighing gauge

35. Radar Summary Good estimates of rainfall amounts can be obtain from radar summaries. Radar is less reliable for frozen precipitation. Coverage still sparse in many locations.

36. Tropical Rain Measuring Mission (TRMM) Idealized Global Precipitation Representation

37. As of late 1997, measurements of the global distribution of rainfall at the Earth's surface had uncertainties of the order of 50% and the global distribution of vertical profiles of precipitation was far less well determined. TRMM will provide the first spaceborne rain radar and microwave radiometric data that will measure the vertical distribution of precipitation over the tropics in a band between ± 35' in latitude. Such information will greatly enhance our understanding of the interactions between the sea, air and land masses which produce changes in global rainfall and climate. TRMM observations will also help improve modeling of tropical rainfall processes and their influence on global circulation leading to better predictions of rainfall and its variability at various time scales. Tropical Rainfall Measuring Mission (TRMM)

38. Intentional Weather Modification The deliberate human intervention to influence atmospheric processes that constitute the weather. Much of the motivation to attempt weather modification techniques was based on anecdotal evidence. Results were never conclusive. Hail cannons where popular in the late 1800’s, with the belief that injection of smoke particles into developing clouds could increase the ice nuclei and reduce the size of hailstones or suppress hail altogether.

39. Charles M. Hatfield (1876-1958) Lots of people in Southern California knew about Hatfield, who by then rated folk hero status. He called himself the Moisture Accelerator, but others knew him best as The Rainmaker. Since 1902 when his first experiments with chemicals and "evaporating tanks" dampened his father's ranch near Oceanside, Hatfield had pleased people in Los Angeles and farmers in the Central and San Joaquin valleys by fulfilling hundreds of their rainmaking contracts. The Moisture Accelerator!

40. 1946 Vincent J. Schafer discovered that dry ice dropped into supercooled clouds spurred the growth of ice crystals which either induced precipitation or dispersed fog or clouds. Cloud Seeding

41. In order for cloud seeding to trigger precipitation, conditions must be just right. Clouds must be present; seeding cannot create clouds. A portion of the clouds must contain supercooled water. One method assumes that the clouds are lacking in freezing nuclei and adding them will stimulate precipitation by the Bergeron process. One must be careful not to overseed as this will produce too many, too small ice crystals. Cloud Seeding

42. Precipitation is a complicated process. After fifty years of cloud seeding experimentation and study the scientific community is still lacking a full understanding of precipitation processes. Cloud Seeding

43. Frost Prevention Temperature zonation during an inversion. Temperature increases with height to the top of the inversion and then decreases. Frost protection techniques use the warmer air above the crop as a heat source.

44. http://www.atmos.umd.edu/~meto200/meto200exam1review.htm