Precipitation I

RECAP Moisture in the air (different types of humidity). Condensation and evaporation in the air (dew point). Stability of the atmosphere: determines the type of clouds Cloud formation: moist air rises up in the atmosphere where it expands adiabatically, cools down, saturates and forms clouds. Chapter 7: precipitation. Any form of water (liquid or solid) that falls from a cloud and reaches the ground.

From droplets to raindrops Typical sizes (diameter) ♦ Condensation nuclei: 0.2  m ♦ Cloud droplet: 20  m ♦ Raindrop: 2000  m The cloud droplets need to grow in order to become raindrops! Growth is determined by the balance of condensation (C) and evaporation (E) ♦ C>E the droplet grows ♦ C<E the droplet gets smaller ♦ C=E the droplet stays the same (in equilibrium), hence: Saturation (equilibrium) vapor pressure

The growth of cloud droplets Curvature effect: The saturation water vapor pressure depends on the curvature of the water surface. The larger the curvature the easier it is for the water molecules to leave the surface of the liquid water. The saturation vapor pressure for small droplets is higher therefore they require more vapor to keep their size Bottom line: the smaller the droplet, the more difficult it is to grow. Small droplets don’t make it as raindrops.

The growth of cloud droplets If small droplets were to survive and grow, they require supersaturation of the air and the help of condensation nuclei Condensation nuclei: jump start the formation of the cloud droplets. ♦ Hygroscopic nuclei: condensation begins at RH<100%. ♦ Decrease the starting curvature of the initial droplet. ♦ Solute effect: salt particles partially dissolve in the water and decrease the saturation vapor pressure.

From droplets to raindrops Condensation by itself is a very slow process, cannot produce raindrops, only cloud droplets (~20  m). The cloud droplets may stay suspended in the air by air currents for a long time Those which happen to descend below the cloud, evaporate and don’t make it to the ground There must be other ways to grow raindrops: ♦ Collision-coalescence process: first must discuss how droplets fall. ♦ Ice-crystal (Bergeron) process

Terminal Velocity Gravity makes things fall, constant force -> acceleration Air-drag force increases with the speed of the falling object Eventually gravity and air-drag become equal -> no net force -> the body falls at constant speed. This velocity is called terminal velocity. The terminal velocity depends on ♦ Shape of the body; ♦ Size of the body; ♦ Mass of the body; ♦ Air properties (density).

Terminal Velocity-Examples Falling objectMass Area Terminal velocity Skydiver75 kg 0.7 m 2 60 m/s 134 mi/hr Baseball (3.66 cm)145 gm 42 cm 2 33 m/s 74 mi/hr Golf ball (2 cm) 46 gm 14 cm 2 32 m/s 72 mi/hr Hail stone (0.5 cm radius).48 gm.79 cm 2 14 m/s 31 mi/hr Raindrop (0.2 cm radius).034 gm.13 cm 2 9 m/s 20 mi/hr m – mass, g – gravity acceleration, C-shape coefficient of proportionality,  -air density, A - area of the cross section of the body, v- velocity

Collision and Coalescence Processes Larger drops fall faster, overtake and absorb smaller drops: ♦ coalescence Larger drops grow faster than smaller drops. Larger drops are the first to hit the ground, the first raindrops are very large and heavy. Smaller drops evaporate before they reach the ground. Smaller drops are absorbed on the way down

Droplet growth in warm clouds Warm clouds: above freezing temperature Thick clouds are associated with strong updraft winds. This maximizes the time the droplet spends in the cloud -> more time for coalescence to take place ♦ Thick clouds produce large rain drops. ♦ Thin clouds produce at most a drizzle.

“Cold” clouds The temperature of a “cold” cloud drops below the water freezing point. Below 0 deg C the cloud water droplets are supercooled. The smaller the droplet, the lower the temperature at which it will freeze. Below -40 deg C almost all droplets freeze and form ice crystals. Small particles in the air serve as ice nuclei: ♦ deposition nuclei ♦ freezing nuclei ♦ contact nuclei

Saturation Vapor Pressure The saturation vapor pressure P s, depends on the temperature. It increases with temperature. P s over water is larger than it is over an ice surface at the same temperature. unsaturated Super- saturated saturated

Ice Crystal Particles The saturation vapor pressure above a water surface is larger than the saturation vapor pressure above an ice surface. Water molecules evaporate more easily than ice molecules There are many more condensation nuclei than ice nuclei -> there are many more water droplets than ice crystals in the cloud How does snow form?

Ice-crystal (Bergeron) process Water vapor molecules migrate towards the ice crystals. Cloud ice crystals grow at the expense of the water droplets.

Ice crystals in the clouds Accretion Fracture Aggregation Skip the sections “Cloud seeding and precipitation” and “Precipitation in clouds”