1. Chapter 7 Clouds, Precipitation, and Weather Radar

2. Driving Question How do clouds and precipitation form?

3. Clouds A cloud is the visible product of condensation or deposition of water vapor in the atmosphere Need more than just saturation to form clouds –Need a supersaturated environment where the RH is greater than 100%

4. The Curvature Effect The curvature of a water surface affects the ability of the water molecules to vaporize The more curved a water drop is, the easier it is for water molecules to escape What this means is that in clear air there would need to be a RH ~ 340% for clouds to form –This never happens, so what else is there?

5. Nuclei Recall that that the atmosphere is composed of gases and aerosols Nuclei – naturally occurring particles that promote condensation or deposition in the atmosphere Nuclei have a radius greater than 1.0 μm - droplets grow at RH near 101%, which does occur in the atmosphere Sources: volcanoes, forest fires, pollution, soil erosion, and sea spray

6. Nuclei Cloud Condensation Nuclei (CCN) –Condensation of water vapor at temperatures above and below the freezing point of water Ice Forming Nuclei (IN) –Formation of ice crystals at temperatures well below freezing Freezing nuclei – water vapor condenses and freezes Deposition nuclei – water vapor deposits directly as ice

7. Nuclei Hygroscopic Nuclei –Special nuclei that allow clouds to form at relatively low RH’s (~70%) –Areas downwind of cities are typically cloudier and rainier Cities add heat and water vapor (increasing RH) Cities add hygroscopic nuclei – from pollution –Example: MgCl 2

8. Supercooled Water Water that cools below freezing, but does not freeze (as cold as –38.2 o F) Homogeneous Nucleation –Supercooled water drops collect on a tiny ice crystal spontaneously at a temperature less than –38.2 o F Heterogeneous Nucleation –Supercooled water drops collect on a foreign particle at a temperature less than freezing, but warmer than –38.2 o F

9. Classification of Clouds General Appearance Altitude of Cloud Base –Stratiform: high, middle, or low level –Cumuliform: clouds with vertical development Temperature –Warm cloud > 0 o C –Cold cloud at or below 0 o C Composition –Ice crystals, supercooled droplets, or water droplets

11. High Clouds Altitude: above 5000m - stratiform Temperature: -25 o C Composition: almost entirely ice crystals Appearance: –Thin and wispy –Transparent to allow sunlight through –Rarely cover the entire sky –No precipitation

12. Cirrus (Ci) Cirrostratus (Cs) Cirrocumulus (Cc)

13. Middle Clouds Altitude: 2000-7000m Temperature: Between 0 o C and -25 o C Composition: ice crystals, water droplets, or a combination of both Appearance: –Thicker and larger than cirrus clouds –Sun is dimly visible –Completely or partially cover the sky –Rarely produce precipitation that reaches the ground

14. Altocumulus (Ac) Altostratus (As)

15. Low Clouds Altitude: ground to 2000m Temperature: temperatures above -5 o C Composition: mostly water droplets Appearance: –Low lying thick gray clouds –Sun is obscured –Completely cover the sky –Generally light, but steady precipitation

16. Stratocumulus (Sc) Nimbostratus (Ns) Stratus (St)

17. Clouds With Vertical Development Altitude: height of Convective Condensation Level (CCL) –generally about 1000-2000m –Clouds tops can be as high as 20,000m (stratosphere) Composition: water drops, supercooled water drops, and ice crystals Appearance: –White puffy clouds –Cotton, Cauliflower –No precipitation with “fair weather” cumulus –Significant storms with cumulonimbus

18. Cumulus (Cu) Cumulus Congestus (CuCon) Cumulonimbus (Cb)

19. Mountain Wave Cloud

20. Cap Cloud

21. Lee-Wave Clouds

22. Lenticular

23. Mammatus

24. Nacreous Clouds

25. Noctilucent Cloud

26. Fog –a cloud (stratus) in contact with the ground –Restricts visibility to 1000m or less –If this visibility restriction is not met, then it is called mist The air needs to be saturated for fog to develop

27. Radiation Fog Air becomes saturated due to radiational cooling Conditions for development: –Clear night sky –Light winds (calm winds would favor dew) –Humid air at the ground with dry air aloft Generally occurs over land where rain or snowmelt has occurred the day before Is often burned off by the sun a few hours after sunrise

28. Advection Fog Air becomes saturated due to advective cooling Conditions for development: –Warm humid air advecting over a cold surface –The cold surface chills the air to its saturation point at the lowest layers Warm air flowing over snow covered ground or a cold water surface (Great Lakes)

30. Steam Fog Air becomes saturated due to the addition of water vapor Conditions for development: –In the winter when cold dry air flows over an unfrozen lake –The lower layer warms and becomes more humid due to evaporation  this mixes with the cold dry air aloft to form fog Fog resembles smoke coming out of a smokestack

32. Upslope Fog Air becomes saturated due to expansional cooling Conditions for development –As humid air ascends up a mountain it expands and cools, thus reaching saturation Sometimes the fog reaches the top of the hill and spreads as a stratus cloud over a valley – this is called high fog

33. Precipitation Processes Most clouds do not bring any rain or snow For clouds to precipitate the cloud particles must be large enough for their terminal velocity to be greater than the updraft in the cloud For this to happen drops need to be about 2mm in diameter, but cloud drops are only 10-20μm in diameter – so how does it rain (or snow)?

34. Warm Air Clouds Collision-Coalescence Process –Droplets that grow by colliding and then coalescing (merging) with one another Droplets with larger diameters have a larger terminal velocity, so as they move through the cloud, they “pick up” smaller droplets As droplets become large enough they fall out of the cloud as precipitation

35. Cold Air Clouds Bergeron-Findeisn Process –The growth of ice crystals in a cloud at the expense of supercooled water droplets Same idea as warm air clouds  as the frozen particles grow they overtake more droplets and fall out of the cloud

37. Virga Once a large droplet leaves the base of the cloud there is no guarantee that it will reach the surface Often the drop will evaporate Virga –Water or ice particles that vaporize before they reach the earth’s surface

38. Types of Precipitation Rain –Diameters between 0.5 and 6mm Drops break apart if diameter gets too large Drizzle –Diameters between 0.2 and 0.5mm –Generally occurs in stratus clouds –Occur with fog and contribute to low visibility

39. Types of Precipitation Snow –An agglomeration of ice crystals in the form of flakes –Crystals come in 4 types Needles Dendrites Plates Columns –Snow flakes vary in size, but can be as big as 5-10 centimeters in diameter –Snow Pellets: supercooled droplets that collide and freeze on an ice crystal –Snow Grains: like drizzle, except they freeze before reaching the ground

41. Types of Precipitation Ice Pellets –Also called sleet –Snowflakes that partially or completely melt and then refreeze before hitting the ground Freezing Rain (or drizzle) –Liquid drops that supercool and partially freeze on contact on cold surfaces at the ground –This forms a coat of ice on road, trees, and stuff Hail –Chunks of ice –Forms in thunderstorms with strong updrafts that cause ice rock to grow –Mostly melt before hitting the surface

44. Weather Radar A remote sensing tool for determining the location, movement, and intensity of areas of precipitation National Weather Service uses a WSR-88D –WSR – weather surveillance radar Reflectivity Mode –Location, movement, and intensity of areas of precipitation –Maximum range of 285 miles Velocity (Doppler) Mode –Air motions directly toward or away from the radar associated with the circulation of the weather system –Maximum range of 143 miles

45. Doppler Effect A shift in the frequency of sound waves emanating from a moving source –How the sound of a train or ambulance changes as it moves towards and then away from you Doppler radar monitors the motion of precipitation toward or away from the radar –Meteorologists can detect circulations and rotations (tornados) and thus give advanced warnings

46. Reflectivity Mode Doppler Effect

47. Radar Stuff Clear Air Mode –Very sensitive radar setting –Radar can detect dust particles or swarms of bugs that collect along boundaries of air masses –These boundaries are potential sites for thunderstorms development Ground Clutter –Nearby objects (buildings, trees) that reflect back to the radar