Unit 4 Lesson 6 Precipitation

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Grade 9 Geography Unit 4 Lesson 6

Types of Precipitation

There are different kinds of precipitation: (1) convectional, (2) orographic and (3)

cyclonic.

As well, three types of heat or energy are involved in the arrangement ofmolecules in the atmosphere. Most of the energy on earth's surface comes fromthe sun. The other forms of energy are sensible heat and latent heat.

Heat is the movement of molecules. The fastest the molecules are moving the

warmer it is. This heat is sensible heat and can be felt. Sensible heat transfer canalso occur by convection. In this process, a fluid is heated by a warm surface,expands and rises creating an upward flow.

Orographic Precipitation

Orographic precipitationresults when warm moist

air moving across theocean is forced to rise by

large mountains. As the airrises, it cools. Why? Ahigher elevation results in

cooler temperatures.

Cold air cannot hold as

much moisture as warmair. As air cools, the water

vapour in the aircondenses and water

droplets form. Clouds forms and precipitation (rain or snow) occurs on thewindward side of the mountain (see diagram). The air is now dry and rises overtop the mountain. As the air moves back down the mountain, it collects moisture

from the ground via evaporation. This side of the mountain is called the leewardside. It receives very little precipitation.


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Types of Precipitation2

Convectional Precipitation

Convectional precipitation results

from the heating of the earth'ssurface. The warm ground heats

the air over it. As the air warms,the air molecules begin to move

further apart. With increaseddistance between molecules, themolecules are less densely

packed. Thus, the air becomeslighter and rises rapidly into the

atmosphere. As the air rises, itcools. Water vapour in the air condenses into clouds and precipitation. This type

of precipitation is common in the Prairies and Ontario.

Cyclonic Precipitation

Cyclonic or Frontal precipitationresults when the leading edge

of a warm, moist air mass(warm front) meets a cool and

dry air mass (cold front). Themolecules in the cold air aremore tightly packed together

(i.e., more dense), and thus, thecold air is heavier than the

warm air. The warmer air massis forced up over the cool air.

As it rises, the warm air cools,

the water vapour in the aircondenses, and clouds and

precipitation result. This precipitation is common in Atlantic Canada.

This type of system is called Frontal Precipitation because the moisture tends to

occur along the front of the air mass.

A cyclonic storm is a large, low pressure system that forms when a warm airmass and a cold air mass collide. This collision often occurs under the polar-front

jet stream which spreads cold, dry arctic air near warm, moist tropical air. The

rotation of the earth causes the air to circulate in a counterclockwise directionaround an area of low pressure.


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The figure illustrates the patterns of wind flow, surface pressure, fronts, andzones of precipitation associated with cyclonic precipitation in the Northern

Hemisphere. Around thelow pressure (L), winds blow

counterclockwise and

inwards.

NOTE: the direction of airflow would be in the

opposite direction in theSouthern Hemisphere.(clockwise but still inward).

Can you explain why?HINT: Earth rotational spin

and prevailing winddirection.

West of L, cold air traveling from the north and northwest creates a cold frontextending from the cyclone's centre to the southwest. Southeast of L, northward

moving warm air from the subtropics produces a warm front. Precipitation occursat the centre of the low pressure AND along the fronts where air is being uplifted.

Why does the precipitation occur at the low pressure system?

? As the warm air mass moves inward toward centre of the low pressure,

the air rises and cools creating precipitation.

Why does precipitation occur along the fronts?

?The warm, less dense air is forced to rise above the colder dense air attwo locations: (1) the leading edge of the warm front and (2) the leading

edge of the cold front.

A variety of weather occurs as a cyclonic storm passes over: showers,downpours, hail and snow. The Ice storm of 1998 was a large cyclonic stormthat stalled over eastern North America.

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Unit 4 Lesson 6 Precipitation