Precipitation Growth in Warm Clouds • Cloud droplets are typically 10 microns in size. small raindrops are typically 1000 microns (almost one million droplets) • How does one go from one to the other? • Simplest explanation is that the droplet grows by condensation of water vapor on its surface. But this would take several days. • One way for raindrops to form is by a processes known as collision-coalescence. • This process requires lots of droplets - high absolute humidity - seen only in the tropics.
Collision-Coalescence • Warm cloud because process because it only occurs in the tropics. • As the cloud droplet is formed it is pulled down by gravity but then is moved upward by the rising air within the cloud. Hence within a cloud one has droplets moving in all directions and they can collide and form larger droplets by coalescing. • Once a drop grows to a size when the force of gravity exceeds the uplift from the rising air, the drop moves downward through the cloud picking up other droplets as it falls.
Collision-coalescence process Fig. 4-31, p. 114
Precipitation Growth in Cold Clouds • Outside of the tropics there are just not enough cloud droplets to form rain drops by the collision-coalescence process. • A Swedish atmospheric physicist, Dr. Bergeron, would vacation in the fall in the mountains. He often took an early morning walk along a path that led through a pine forest and encountered uplift fogs. • He noted that on the days when the temperature was above 0 C, the fog went all the way to the ground, whereas if the temperature fell below -10 C, the fog lay above the tops of the trees.
Why? • He further noted when the temperature fell below -10 C, that the pine needles were covered with ice. • The reason is that the vapor pressure of water above ice is less than the vapor above water.
Attraction of water vapor to ice versus water Fig. 4-33, p. 116
Saturation vapor pressure over ice and over water Fig. 4-35, p. 117
Accretion and Aggregation • It should be noted that the ice crystal growth gives a ‘snow flake’ which eventually will also begin to fall and then rise in the updraft. • These ice crystals then begin to aggregate into larger snowflakes. • The ice crystal can also collide with a supercooled water droplet which instantly freezes - accretion. • As the ice crystals descend at some point the temperature can rise above 0 C, and the crystal melts to form a raindrop.
Process of aggregation Fig. 4-32, p. 115
These picture shows fall-streaks. They consist of ice particles that have fallen out of a cloud and evaporate before they reach the ground. Fig. 4.33
Warm Front Fig. 9.13
FORMS OF PRECIPITATION • RAIN - DROPLETS OF WATER GREATER THAN 0.5 MM IN DIAMETER. WHEN DROPLETS SMALLER THAN 0.5 MM CALLED DRIZZLE. • MUCH RAIN STARTS OUT ALOFT AS ICE CRYSTALS • SNOW - ICE CRYSTALS. IF AIR IS COLD (LOW HUMIDITY), WE GET LIGHT AND FLUFFY SNOW (POWDER). IF AIR IS WARM THAN ABOUT -5 CELSIUS, THEN WE GET WET SNOW (GOOD FOR SNOWBALLS). • SLEET - SMALL PARTICLES OF ICE. RAINDROPS ENCOUNTER FREEZING AIR ON DESCENT. IF FREEZING NOT COMPLETE - FREEZING RAIN. • HAIL - LAYERS OF ICE FORM AS THE HAILSTORM TRAVELS YUP NAND DOWN IN A STRONG CONVECTIVE CLOUD • RIME - FORMED BY FREEZING OF SUPERCOOLED FOG ON OBJECTS.
The effects of airflow over a mountain Fig. 4-42, p. 122
Clouds and Precipitation near Mountains • As air ascends mountain it cools adiabatically, clouds form, and precipitation occurs. • Above this altitude the relative humidity stays at 100% • At the peak of the mountain the absolute humidity is determined by the saturation vapor pressure at -12C. • As the air descends its absolute humidity remains the same as at the peak
Clouds and Precipitation near Mountains • As the air descends it is compressed, so it warms • Hence the saturation vapor pressure will increase, and the relative humidity will decrease • The net effect of the air ascending and descending the mountain is that the air becomes drier and warmer. • On the island of Hawaii, the west side of the coast (westerly winds) has rain forests, the eastern side has deserts.