Warm Up 3/18/08 The wet adiabatic rate of cooling is less than the dry rate because ____. a. of the dew point b. of the release of latent heat c. wet air is unsaturated d. dry air is less dense Cool air acts as a barrier over which warmer, less dense air rises, in a process known as ____. a. divergence c. orographic lifting b. frontal wedging d. subduction Orographic lifting is associated with ____. a. mountains c. fronts b. flat plains d. rivers Answers: 1) b. 2) b. 3) a.

Cloud Types and Precipitation Chapter 18, Section 3

Types of Clouds Clouds are classified on the basis of their form and height Cirrus clouds are high, white, and thin Cumulus clouds consist of rounded individual cloud masses, they normally have a flat base and the appearance of rising domes or towers Stratus clouds are sheets or layers that cover much or all of the sky There are three levels of cloud heights: high, middle, and low

Three cloud types make up the family of high clouds (above 6000 meters): cirrus, cirrostratus, and cirrocumulus All high clouds are thin and white and are often made up of ice crystals Clouds that appear in the middle range (~2000-6000 m) have the prefix alto- Middle clouds may cause infrequent light snow and drizzle There are three members in the family of low clouds (below 2000 m): stratus, stratocumulus, and nimbostratus Nimbostratus clouds are the main precipitation makers Vertical development clouds have their bases in the low height range, but extend through the middle or high altitudes Cumulonimbus may produce rain showers or thunderstorms

Classification of Clouds

Concept Check What does the Latin word stratus mean? Stratus means “to cover with a layer”.

Fog Fog is defined as a cloud with its base at or very near the ground A blanket of fog is produced in some West Coast locations when warm, moist air from the Pacific Ocean moves over the cold California Current and then is carried onshore by prevailing winds Fogs also can form on cool, clear, calm nights when Earth’s surface cools rapidly by radiation When cool air moves over warm water, enough moisture may evaporate from the water surface to produce saturation; as the rising water vapor meets the cold air, it immediately condenses and rises with the air that is being warmed from below

Distribution of Fog

Concept Check Compare and contrast clouds and fogs. Clouds and fogs are physically the same. Fogs are clouds with their bases at or very near the ground.

How Precipitation Forms For precipitation to form, cloud droplets must grow in volume by roughly one million times Bergeron Process – a theory that relates to the formation of precipitation to supercooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water Supercooled – water in the liquid state below 0ºC (in the atmosphere pure water can reach -40ºC without freezing) Supersaturated – the condition of air that is more highly concentrated than is normally possible under given temperature and pressure conditions Because the level of supersaturation with respect to ice can be quite high, the growth of ice crystals is rapid enough to produce crystals that are large enough to fall Collision-Coalescence Process – a theory of raindrop formation in warm clouds (above 0ºC) in which large cloud droplets collide and join together with smaller droplets to form a raindrop

The Bergeron Process

Collision-Coalescence Process

Concept Check What must happen in order for precipitation to form? Cloud droplets must increase in volume by about one million times.

Forms of Precipitation The type of precipitation that reaches Earth’s surface depends on the temperature profile in the lowest few kilometers of the atmosphere Temperature profile is the way the air changes with altitude Rain means drops of water that fall from a cloud and have a diameter of at least 0.5 mm At very low temperatures light, fluffy snow made up of six-sided ice crystals forms Sleet, glaze, and hail are all formed from water becoming supercooled on its trip down to the surface

Assignment Read Chapter 18 (pg. 504 – 522) Do Chapter 18 Assessment #1-30 (pg. 527-528) Study for Chapter 18 Quiz!