Moisture in the Atmosphere Precipitation Clouds Fog

Precipitation Necessary Conditions 1) A source of moisture - water bodies, plants, etc. 2) Heat for evaporation to get water vapour into the air. 3) Condensation - occurs when there is 100% relative humidity  (i.e. when the dew point is reached)

Precipitation for condensation to begin, the air has to cool. This is because the colder the air is, the less water vapour it can hold. Most of the time, water vapour needs to rise in order to cool.

Precipitation adiabatic cooling: the cooling of the air by expansion. When air is forced to rise it expands resulting in lower air pressure. When a gas expands it does work and therefore uses up energy. The loss in energy results in a loss of heat. 

Precipitation dry adiabatic lapse rate: when air is forced to rise, it cools at a rate of 1 deg. C / 100 m as long as the air temp. is above the dew point. wet adiabatic lapse rate: occurs when the dew point is reached and condensation of the air takes place. As a result of condensation, the air does not cool as fast when it rises (i.e. 0.6 deg. C / 100 m).

Precipitation Note: There is also something known as the environmental lapse rate - the rate at which air temperature decreases with altitude under stable conditions at a rate of 1 deg. C per 156 metres. Stable conditions indicate that the air is not being forced to rise.

Precipitation 4) Nuclei: water vapour needs particles to condense onto.  Nuclei sources: dust, pollution, sea salt. 5) Rain droplets need to coalesce into large enough droplets to fall to earth. - ice particles aid the process as well as wind and electrical charge from lightning.

Processes of Precipitation With each process of precipitation, there is a mechanism to get moist air to rise. Once it begins to rise, the water vapour in it cools, condenses, forms clouds and precipitation.

Processes of Precipitation

Processes of Precipitation With orographic precipitation, the mechanism to get the air to rise is the mountains.

Processes of Precipitation

Processes of Precipitation With convectional precipitation, the mechanism is the heat of the ground warming the air above it.

Processes of Precipitation

Processes of Precipitation With cyclonic or frontal precipitation, the mechanism is two different air masses meeting. Air masses are large bodies of air that form over a period of days or weeks above extensive, fairly uniform surface areas of the world. They take on thetemperature and moisture characteristics of their source region (i.e. the land or water they are over).

Clouds

Mr Burns

Mr Byrne

Clouds Condensed water vapour Consists of water droplets or ice crystals

Cloud Varieties There are three main types of clouds: -Cirrus: high clouds -Stratus (layers): light rain clouds -Cumulus: vertical, with flat base, thick layer clouds

BUTT

Clouds change continuously and many combinations of the three types can exist.

cloud classification Based on Height (Altitude)

Cirrus Variety High Clouds Ice crystals

Cirrus Clouds -between 7,000 to 13,000 metres -indicate arrival of a warm front and rain -highest clouds…wispy -do not produce precipitation

Cirrostratus Clouds have layered appearance between 7,000 to 13,000 metres very thin appearance do not produce precipitation Associated with the ring around the sun

Cirrocumulus Clouds -layered also, but thicker -may cover the entire sky -between 7,000 and 13,000 metres -do not produce precipitation

Alto Clouds ‘Middle’ Clouds

Altocumulus -white and gray in colour -puffy appearance -from 2,000 to 7,000 metres

Altostratus -bluish-gray, never white -high amount of condensed water vapour -from 2,000 to 7,000 metres

Stratus Clouds Low Clouds

Stratocumulus -appear white, but have many dark patches - Layered and fluffy -not thought of as rain clouds -clouds at lower altitudes

Stratus -gray in colour -uniform base covering the entire sky -produce a steady, light drizzle of very minute droplets -found at lower altitudes

Nimbostratus -layered and dark in appearance -high moisture content -produce steady, prolonged precipitation (Nimbus = rain) -found at lower altitudes

Vertical Clouds: Cumulus Clouds

Cumulus -generated by rising air currents -can reach heights of 13,000 metres -often white in colour -with a dark flat base -looks like cauliflower at the top

Cumulonimbus -very thick, up to 13,000 metres in height -violent, circulating air currents -can produce baseball size hail -thunder and lightening are associated with these -look anvil shaped -rainfall is sudden, heavy and short lived

Other Clouds

Lenticular -usually thin -formed over a mountain peak -technically called altocumulus standing lenticularis

Fog

Three Conditions for Condensation There must be sufficient water vapour in the air There must be a condensation nuclei- smoke, dust etc- for water particles to attach to and form around Dew point has been met- air is saturated with water and can’t hold anymore

Dew Usually no wind and no clouds Air has high water vapor content Land cools off at night (by radiation) Air temp. low to the ground falls below the dew point ( cold temperature holds less vapor) Small droplets form as air condenses.

Frost If dew point is below freezing, condensed water vapour will form frost. Sometimes it may change from vapour to solid….this is sublimation

Fog Air close to a cool surface is cooled enough to reach dew point Water vapour condenses to form low lying cloud Two types: radiation and advection

Radiation Fog Cloudless and windless nights As ground cools in the evening, air above it cools and condenses (dew point reached) Seen in low lying valleys as cool air sinks If there is any wind, fog will not form- why?

Advection Fog Horizontal movement of air When warm, moist air passes over a cool area (ocean, snow) and air temp drops suddenly Often forms on the ocean- fog banks can travel