Weather - Precipitation

Precipitation RECALL… As air goes up, it cools down! As air cools down, water (vapour) condenses more than it evaporates!

Precipitation Air can rise for a number of reasons… 2 – It rises because it has absorbed heat from the earth’s surface (convectional). 3 – It rises because a bulldozer mass of cold air is pushing it up (cyclonic). 1 – It tries to cross an area of high elevation (orographic or relief).

Orographic (Relief) Precipitation

Mountains create a barrier, and this barrier creates orographic precipitation. As air rises, it cools. The air mass itself expands, while the particles squish together and contract (not able to hold as much moisture). Condensation occurs, and water droplets are released into the air. These are still light enough to be carried by the air!

Orographic (Relief) Precipitation As more vapour condenses, water droplets become larger and heavier! When they are too heavy to remain aloft, they fall as…

Orographic (Relief) Precipitation The moisture content of air is measured in Relative Humidity. This is just a quick and easy way of saying… (Amount of moisture in the air / moisture capacity) X 100%.

Orographic (Relief) Precipitation (Textbook p.135)

Convectional Precipitation (Textbook p.136)

Convectional Precipitation Convectional precipitation occurs when the ground is subject to very intense heating. The air above the ground heats too, and rises. As the air rises, it cools and the water vapour held within now condenses. This condensed vapour forms clouds in a very high column (sometimes leads to hail). Eventually, the moisture in these clouds gets heavy enough to fall…

Convectional Precipitation Air actually grabs the water drops and drags them downwards! This results in cooler air descending (those nasty drafts and gusts of wind you feel just before a storm). The rain eventually cools the ground and stops the cycle. Bonus stuff from convectional precipitation… Hail (formed when moisture in the cloud reaches high enough altitude); Thunderstorms; Tornadoes.

Convectional Precipitation

Cyclonic (Frontal) Precipitation

Cyclonic precipitation occurs when warm and cold fronts collide. This is called a “cyclonic storm”. It often occurs under the Polar Front jet stream. The rotation of the earth causes the air to spin in a counter-clockwise direction (giving us our common image of a “spinning” cyclone).

Cyclonic (Frontal) Precipitation

Cyclonic precipitation has two fronts: Cold front (the leading edge of the cold air pushing into the warm air mass); Warm front (the leading edge of the warm air mass on the other side). Both of these fronts create precipitation. Fun fact: most of the precipitation we receive in Ontario is cyclonic.