Atmospheric Circulation

1. Global Convection Currents

• Equator = warm rising air

• Poles = cold sinking air

• Air travels from H (high) to L (low) pressure• H-L Pressure Zones: Horse latitudes and Doldrums are

places where air flow slows and wind speed dies down • Prevailing winds: Trade winds, Westerlies, and Polar

Easterlies are characterized by winds that travel in one main direction (from HL)

2. Global Winds

• Hadley Cells - cells of atmospheric convection– Driven by

solar heating – Air rises near

equator– Air sinks near

the poles

3. Air Movement• Convergence – L pressure

systems have a CCW rotation causing it to rise and cool inside– Condenses– Forms clouds– Cause of “bad weather”

• Divergence – H pressure systems have a CW rotation causing air to sink – Diverges – Form clear air– Cause of “good weather”

• Jet Streams – form at boundaries between convection cells– Fast moving

ribbons of air in the upper atmosphere

• Air Masses – large uniform volume of air that has the same– Density– Temperature– Humidity

4 types - • Maritime Polar - mP• Maritime Tropical - mT• Continental Polar - CP• Continental Tropical - cT

4. Air Masses

Weather Formation

5. Wind Formation

• Variations of solar energy input lead to

• temperature differences which lead to

• pressure variations in the atmosphere

• The wind blows to try to correct these imbalances in pressure.

• Regions of L and H pressure moving through an area cause changes in weather as different air masses pass by.

6. Atmospheric Stability

• Unstable air –Temp of air mass > Temp of surrounding air = rising air

• Stable air – Temp of air mass < Temp of surrounding air = sinking air

7. Weather Fronts

• Air Fronts – boundaries where 2 different air masses meet and do not mix– Caused by density

differences– Causes changes in

elements of weather• Temperature• Dew Point• Wind Direction• Cloud Cover• Type of Weather

• 4 main types –– Cold– Warm– Stationary– Occluded

• Cold Front – leading edge of colder air that is replacing warmer air. – Warm air rises– Usually form with

areas of L pressure– Weather:

• Heavy rain• Hail• Strong winds

– Summer months: thunderstorms and severe storms

• Warm Front – Leading edge of warmer air that is replacing cooler air.– Warm air rises slowly

over colder air – Weather:

• Clouds• Gradual to

increasing rain• Fog• Clears and warm

after– Winter months:

hazardous weather conditions• Sleet• Freezing rain

• Stationary Front – boundary between 2 different air mass with different densities but neither is advancing– Air masses do

not move together

– Weather:• clear to partly

cloudy• If a lot moisture

= rain/storms– Ice storms– Flooding

• Occluded Front – when a cold front catches up to a warm front– Cold fronts move

much faster than warm fronts

– Cold takes over– Weather:

• Warm or cold• Clear to partly

cloudy• If a lot of moisture =

rain/storms– Ice storms– Flooding – Thunderstorms

Types of Weather

8. Types of weather• Sunny

– H pressure• Clouds

– Air is lifted and cooled • Rain• Snow• Freezing Rain/Ice

– Freezing air near Earth’s surface• Hail

– Supercooled water drops grow like snowballs• Sleet

– Frozen rain drops (transparent) • Dust Storms

– Strong, dry cold front• Fog

– Stratus cloud on the ground– Moist, warm front

• Hurricane• Lightning

– Discharge of electricity in the atmosphere• Thunderstorm• Tornado• Wind

– Imbalance of heating in the atmosphere

9. Elements of Weather and Weather Instruments

• Temperature Thermometer • Wind Speed Anemometer• Air Pressure Barometer • Moisture Hygrometer

• Precipitation Rain Gauge or Doppler Radar

• Cloud Cover Satellite Imaging

10. Severe Weather

• Severe weather hazards – dangerous weather phenomena that threaten life and property

• Thunderstorms – caused by unstable air masses– Form

cumulonimbus clouds

– Warm, moist air is forced up rapidly by fronts, mountains, or daytime heating

• Tornadoes - violently rotating column of air– Forms in a severe

thunderstorm called a supercell

– Wind coming in starts to swirl and forms a funnel

– The spinning air creates a L pressure area

– Cold, dry polar air meets warm, moist tropical air

– Unstable conditions exist

• Hurricanes – giant, spiraling tropical storms with high wind speeds of 120 mph +– Spiral towards L

pressure center– Form over oceans– Warm, moist air

rises rapidly – The “eye of the

storm” is calm

• El Nino - a climate pattern representing an unusual warming of the eastern tropical Pacific Ocean sea surface temperatures– Trade winds weaken– Warmer water in the

East– Occur every 3-7 years– Reduced upwelling of

deep nutrient rich waters

• La Nina - a climate pattern representing a cooling of the eastern tropical Pacific Ocean sea surface temperatures– Trade winds

strengthen – Warmer water in

the West (normal)– Usually occur after

an El Nino – Enhanced upwelling

of deep nutrient rich waters

Weather Maps

11. Weather Maps

• Weather maps – allow meteorologists to understand current weather and predict future weather

• Station Model – symbols used to show weather conditions– Temperature– Wind

• The wind barb (parts ff and dd combined) is like an arrow and points in the direction the wind is blowing.

– Weather– Dew point– Sky cover– Pressure

Pressure/Front Map

Temperature/Front Map