Understanding Air Pressure

Air Pressure Is exerted in all directions: down, up, sideways Sea level pressure = 1 kilogram per square inch.

Measuring Air Pressure Barometer – used to tell the air pressure around you (in millibars) Increase in AP = Mercury rises Decrease in AP = Mercury lowers

Wind A result of horizontal differences in air pressure. Flows from high to low pressure Solar radiation = ultimate source for wind.

Control Wind Three factors: 1.) Pressure Differences 2.) Coriolis Effect 3.) Friction

Pressure Differences Pressure Gradient – read from isobars Close isobars = high winds Far apart isobars = low winds

Coriolis Effect 1.) A right angle to the direction of air flow 2.) Affects wind direction and wind speed. 3.) Strong wind = strong deflection 4.) Strong at poles and weak at equator

Friction Slows down air movement which changes direction. Above layer = parallel to isobars (low friction) Jet streams = fast moving “rivers” of air

Pressure Centers and Wind Fancy for: Cyclones

High Pressure vs. Low Pressure Anticyclones Pressure increases from outside to center Winds blow outward and clockwise Net flow = outward Cyclones Pressure decreases from outer to center Winds blow inward and counterclockwise Net Flow = inward

Weather and Air Pressure Rising air = cloud formation and precipitation. Sinking air = clear skies Have to have convergence and divergence!

Weather Forecasting Low pressure system = bad weather!

Global Winds Atmosphere = Air conditioner Warm air goes toward high latitudes Cool air towards equator.

Non – Rotating Earth Model Where hot air goes to the poles when it reaches the troposphere.

Rotating Earth Model 4 pockets: 1.) Trade Winds 2.) Westerlies 3.) Polar Easterlies 4.) Polar Front

Influence of Continents Southern Hemisphere = continuous pressure system. Land + Water = different pressure systems

Monsoons Seasonal changes in the wind direction. LOTS OF RAIN!!!

Regional Wind Systems Section 19.3

General Info Circulation in the middle latitudes is complex and does not fit the convection system described for the tropics.

Local Winds Caused by either topographic effect or by variations in surface composition. Land and Water

Valley and Mountain Breezes Valley Breeze – heat during the day = warm air that rises Mountain Breeze – cooling at night = air moving into the valley

Measuring Wind Two basic measurements Direction and Speed Labeled by the direction they flow Ex. North wind – north towards the south Measured by a wind vane

Wind Direction Wind Speed Prevailing wind – blows fro the same direction consistently Ex. Westerlies (US) Wind Speed Measured by an anemometer

El Nino Warm countercurrents that become unusually strong and replace cold offshore water with warm water. 3 – 7 years

La Nina When surface temperatures in the eastern Pacific are colder than average Distinctive set of weather patterns.