LEC 09: WEATHER & AIR MASSES

Lake Effect Snow

Air Masses

Air mass picks up moisture & energy from lake, releases snow downwind

Adiabatic Processes

Processes that occur with no net heat exchange between a parcel of air and environment

Orographic

Atmospheric Lifting Mechanism

Air masses move laterally, pushed up and over mountain

Windward & Leeward

In the direction of wind flow (mid-latitude western side) heavy rain

away from wind flow, aka. downwind (mid-latitude eastern side) dry

Chinook Winds

Warm and dry winds that flow downslope—adiabatic compression

Convectional

Atmospheric Lifting Mechanism

Surface warms, heats air masses, which rise

Convective Storms

Storms that form when warm, moist air rises → cools, and condenses into clouds, often producing heavy rain, thunder, and lightning.

Convergent

Atmospheric Lifting Mechanism

Air masses converging on low-pressure zone push air upwards.

Important in tropics where temperature differences are small.

Frontal Lifting

Atmospheric Lifting Mechanism

Caused by collisions among air masses

Mainly a mid-latitude phenomenon

Cold Fronts

Advancing cold air quickly lifts warm air

Steep Front

Cold Fronts

Strong precipitation, storms, severe weather

Move relatively fast

Warm Fronts

Advancing warm air slides over cold air

Gradual Lifting

Warm Fronts

Drizzle and overcast skies

Move relatively slow

Rossby Waves

Giant meanders in the jet stream

Caused by upper-atmosphere variations in pressure:

highs (ridges) and lows (troughs)

Mid-latitude Cyclones

Weather systems that are centered on low pressure zone causing surface convergence

Draws in warm air from south and cold air from north (N. Hem.)

Cold and warm fronts rotate around central low

Formation of Tropical Cyclones

Start as a line of mild low pressure (easterly wave)

Air converges on low pressure and rises

Rising air strengthens the low pressure systems and air starts to rotate → cyclone

Formation of Hurricanes

1. Warm ocean surface generates convection and vertical movement

2. Moist air condenses and releases latent heat – rain starts

3. Creates warmer buoyant air that continues to rise

4.Rising air causes low-pressure zone at the surface. Winds converge & spiral inwards

5. Continued convection from the warm ocean surface intensifies storm