Cloud Development and Precipitation

Cloud Development and Precipitation Prof. John Toohey-Morales, CCM St. Thomas University Miami Gardens, Florida

Adiabatic Cooling and Warming • Rising air expands & cools, while sinking air is compressed and warms • Adiabatic process is one in which there is no heat exchange with the surrounding air

Stability: Key to Cloud Development • The stability of a layer in the atmosphere is determined by comparing the temperature of a rising parcel of air (which cools at the adiabatic lapse rate) to that of its surroundings (which cool at the environmental lapse rate) • Dry adiabatic rate of cooling is 10°C per 1 km • Moist adiabatic rate is 6°C per 1,000 meters • Environmental rate averages 6.5°C per 1 km

Stability: Key to Cloud Development (question) • If a parcel of unsaturated air with a temperature of 30° C rises from the surface to an altitude of 1000 m, the unsaturated parcel temperature at this altitude would be about a. 10° C warmer than at the surface. b. 10° C colder than at the surface. c. 6° C colder than at the surface. d. impossible to tell from the data given

Stable Air • An absolutely stable atmosphere exists when a rising air parcel is colder and heavier (more dense) than the surrounding air • If released, the air parcel returns to its original position

Factors Affecting Stability • A stable layer results when there is a relatively small difference in temperature between the surface air and the air aloft • If air aloft warms or surface air cools, the atmosphere stabilizes • Some examples of processes leading to surface cooling are • Nighttime radiational cooling • Influx of cold air • Air cooled by a colder surface

Stable Air (question) • If an air parcel is given a small push upward and it falls back to its original position, the atmosphere is said to be a. stable. b. unstable. c. isothermal. d. neutral. e. adiabatic.

Unstable Air • An absolutely unstable atmosphere exists when a rising parcel of air is warmer and lighter (less dense) than the air surrounding it • If released, the air parcel would continue to rise

Factors Affecting Instability • An unstable layer results when the environmental lapse rate steepens or the temperature of the air drops quickly with height • If air aloft cools or surface air warms it becomes unstable • Some examples of processes leading to surface warming are • Daytime solar heating • Influx of warm air • Air warmed by a hotter surface

Unstable Air (question) • Which of the following environmental lapse rates would represent the most unstable conditions in a layer of unsaturated air? a. 1° C per 1000 m b. 3° C per 1000 m c. 6° C per 1000 m d. 9° C per 1000 m e. 11° C per 1000 m

Ways Clouds Form

Way Clouds Form (question) • An example of orographic clouds would be a. clouds forming over a warm ocean current. b. clouds forming on the windward slope of a mountain. c. clouds forming behind a jet airplane. d. clouds formed by surface heating.

Rain Shadow • Air dries on the leeward side of mountains because of water vapor condensing and remaining as cloud droplets and precipitation on the windward side • Rain shadow is the region on the lee side of the mountain with low precipitation and dry air

Producing Rain: Collision and Coalescence • Cloud droplets forming on condensation nuclei are extremely small with a diameter of 0.02 mm so they stay suspended by slight upwards air currents • Some larger droplets may form through random collision or by forming on larger condensation nuclei

Producing Rain: Collision and Coalescence (continued) • Larger drops fall faster than smaller drops, overtaking, colliding and coalescing with smaller droplets, eventually growing too heavy and falling to the ground due to gravity

Producing Rain: Collision and Coalescence (continued) • Warm clouds with strong updrafts (like Cumulus Congestus) can sustain the weight of more larger drops, leading to bigger raindrops and heavier rain

Producing Rain: Collision and Coalescence (question) • The merging of liquid cloud droplets by collision is called a. coalescence. b. riming. c. accretion. d. deposition. e. condensation.

Ice-Crystal Process • Most common type of rain formation at our latitudes (mid to high latitudes) • Small supercooled water droplets can exist at below freezing temperatures • Ice-Crystals are also present, but in smaller quantities (because ice nuclei are scarce) • Bergeron process: Ice-Crystals grow at the expense of the surrounding water droplets through accretion, with the liquid water freezing on contact with the ice

Ice-Crystal Process (question) • Most rain at middle latitudes is produced by the ice crystal process. This is because a. ice crystal nuclei are more plentiful than condensation nuclei. b. most clouds form in cold regions of the atmosphere. c. ice crystals evaporate more slowly than water droplets. d. most rain occurs during the winter.

Snow to Rain • Ice-Crystals grow, fall, collide, and stick to one another forming a snowflake • If snowflake melts before hitting the ground, it falls as a raindrop • Most rain begins as snow

Snow to Rain (question) • Which of the following statements is not correct? a. Generally, the smaller the pure water droplet, the lower the temperature at which it will freeze b. Ice nuclei are more plentiful in the atmosphere than condensation nuclei c. Much of the rain falling in middle northern latitudes begins as snow d. Ice crystals may grow in a cold cloud even though supercooled droplets do not.

Go to Visualization Sleet and Freezing Rain

Sleet & Freezing Rain (continued) • A raindrop which freezes before reaching the ground is called a. snow. b. graupel. c. sleet. d. glaze.

Go to Visualization Hail • Produced by accretion • Up to 7” in diameter • Causes hundreds of millions of dollars in damage annually • For a hailstone to grow to golf ball size it must remain in the cloud for 5 to 10 minutes • The hailstones may cycle within a Cumulonimbus, or be too heavy and drop

Measuring and Detecting Precipitation • A rain gauge is used to collect and measure precipitation starting at 0.01” (less is called a “trace” of rain) • 10” of snow will melt to 1” of water (total precipitation) • Radar (radio detection and ranging) shows where precipitation is and its intensity • Doppler Radar allows us to observe rotating winds inside T-Storms to detect tornadoes

Cloud Development and Precipitation