Physical Geography - Week 6 +

Air Parcels

“Bubbles” of air that have different temperature, moisture content than surrounding air

• Warm air parcels rise and expand (Lower density, lower pressure)

• Cool air parcels sink and compress (Higher density, higher pressure)

What causes air to rise?

Air parcels rise by performing “work” on surrounding air and expand due to decreasing pressure. Temps decrease with altitude gained

• As air molecules spread out, KE decreases, and heat decreases

Conversely, air parcels sink by performing “work” on surroundings and sink due to increased pressure

• As air molecules compress, KE increases, and heat increases

Cold Front

Cold air is denser and hugs the ground

Leading edge of the cold air movement is called the front “cold front”

• Advancing cold front forms a steep wall of cold air, and lifts warm air from the ground

• lifting warm air forms clouds, precipitation, and sometimes thunderstorms

• depicted as a protruding series of growing spikes inside concentric circles on a map with isotherms

Warm Front

Enters area of cooler air due to jet stream

Leading edge of the warm air movement is called the front “warm front”

• Rises over area of cold air gradually

• Cool air forms a “wedge” shape as it is being pushed by warm air

Air Masses

• Large bodies of air where the temperature and humidity are horizontally similar

• can come from the polar regions or the tropics

• Cold and warm air meet in the mid-latitude regions, which create fronts/frontal systems, and sometimes even mid-latitude cyclones

• Form by staying in one place long enough to absorb the characteristics of the place

Air Mass Temperatures

Air masses over warm surfaces become warm and vice versa

• Cool air over warm surfaces creates warm air via conduction, resulting air mass is unstable as air rises due to temp difference

• warm air over cold surface = loss of heat in air, air closer to the surface cools down. Increased stability in lower surfaces

Air Mass Humidity

Air gains moisture from water bodies

• Dry air receives more moisture than wet air from evaporation

• warm air over tropical oceans will gain more water vapour than cool air over northern oceans

Characteristics of Air Mass Source Regions

Uniform Surface Properties

• Same over thousands of kilometers

• Relatively flat

• Either land or water but not both

Light Winds

• Required for air to remain stagnant and to create homogeneity

Mid-latitudes are not good source regions

• Too much mixing! Areas with large anti-cyclones are best (polar highs, subtropical highs)

4 Major source areas of Air Masses

High latitude land, ice covered areas

High latitude ice free oceans

Sub-tropical oceans

Sub-tropical deserts

Air Masses over NA in summer and winter

In winter

• Continental Polar and Continental Arctic air masses are larger than maritime tropical air masses

In summer

• vice versa

Mid-Latitude Cyclones

Large, low pressure systems that travel long distances

• Associated with precipitation or even severe weather

They look like hurricanes but do not form in the tropics; rather, they form in mid latitudes

Stages of Mid Latitude Cyclones (Cyclogenesis)

1. Stationary Front - separation of cold easterlies and warm westerlies

2. Frontal Waves - Minor kind formed in boundary of cold + warm fronts causes counterclockwise rotation around a weak low

3. Open Wave - Low pressure system deepens, creates convergence which leads to uplift and cloud formation

4. Mature Cyclone - Low intensifies even more, cumuliform clouds form behind warm front, stratiform clouds ahead

5. Partial Occlusion - Cyclone starts to dissipate as air differences lessen

6. Occluded Low - Low pressure forced north, potential energy no longer driving cyclone; full dissipation

Storms

Related to cloud formation, which are formed by lifting air

Four Types of Air Lifting

Convergent lifting

Convection lifting

Orographic lifting

Frontal lifting

Convergent Lifting

Movement of air into a low pressure system; air moves upward

Intertropical Convergence Zone (ITCZ)

Zone around equator where large cumulonimbus clouds form due to low pressures

Convectional Lifting

As air parcel moves into a warmer area, the air is warmed and lifts as a result

• urban heat islands, plowed fields, cool maritime air moving inland

Orographic Lifting

Cooling of air adiabatically as it moves upwards over a mountain slope

Frontal Lifting

Leading edge of a cold front forces less-dense warm air upwards

• Cold Fronts - push warm air upwards abruptly and cause formation of cumulonimbus clouds. Precipitation, storms

• Warm Fronts - Leading edge forms atop cold front but pushes away cold front, forms cold air into a wedge shape

Thunderstorms

Created by low pressure systems, formed along the boundary of a cold front or as air rises over a mountain

• air floods into the low pressure area and rises and condenses

• As water from air condenses, it releases abundant Latent Heat energy into the surrounding area, causing thunder and lightning

How does precipitation form?

Cloud droplets are 10 micrometers in diameter; rain droplets are 1 mm in diameter

• Droplets clump together, but need to be large and heavy enough in order for precipitation to form

• Velocity of droplet must exceed uplift within the cloud to fall to earth

• Large raindrops fall faster than small raindrops

Development of Hail

Water droplets freeze in clouds, and are circulated within the cloud by updrafts and gravity, causing new layers of water to freeze to the hailstone. Once it gets heavy enough, it can escape the cloud and fall to the earth.

Tornados

Created by a very low pressure parcel of air during a thunderstorm

• Warm air rises into thunderclouds while cool air falls from it along with precipitation.

• Warm air rising creates low pressure area which intensifies up/downdrafts

• Warm, moist air flows into tornado. Moisture condenses and releases latent energy which fuels tornado

• Results in a spinning column of air

Hurricanes

Low Pressure Areas in the upper atmosphere 

Collision of cool and warm air masses over the tropics fuels the hurricane

Massive energy emission due to change of states of water vapour