Transcript Weather - Precipitation (font fix).

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…
1 – It tries to cross an area of high elevation
(orographic or relief).
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).
Orographic (Relief) Precipitation
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 (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 (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.