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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

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Precipitation I

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Precipitation I


  • 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 mm

    • Cloud droplet: 20 mm

    • Raindrop: 2000 mm

  • 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

  • Curvatureeffect: 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 mm).

  • 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 m2 60 m/s 134 mi/hr

Baseball (3.66 cm)145 gm 42 cm2 33 m/s 74 mi/hr

Golf ball (2 cm) 46 gm 14 cm2 32 m/s 72 mi/hr

Hail stone (0.5 cm radius) .48 gm .79 cm2 14 m/s 31 mi/hr

Raindrop (0.2 cm radius) .034 gm .13 cm2 9 m/s 20 mi/hr

m – mass, g – gravity acceleration, C-shape coefficient of proportionality, r-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

Ice Crystal Particles

  • 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?

  • 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

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




Skip the sections “Cloud seeding and precipitation” and “Precipitation in clouds”

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