Air pressure› Pressure exerted by the weight of air

above› Exerted in all directions› One of the most basic weather elements› Important factor in weather forecasting› Closely tied to cause and effect

relationships

› Low pressure = cloudy conditions and precipitation Cyclones pressure decreases from the outside in.

› High pressure = clear skies and fair weather Anticyclones Pressure increases from outside in

How to measure air pressure?› Barometer

Invented by Galileo

Direct correlation between air pressure and rising mercury

4 global pressure zones› Subtropics and polar highs – sinking

air/prevailing winds Outward air flow

› Equator and subpolar regions – low pressure

Inward and upward Clouds and precipitation

Wind – horizontal movement of air› result of differences in air pressure

Unequal heating of surface

› Flows from high to low› Nature’s way of balancing differences in air

pressure› Solar radiation is ultimate source of most

wind

3 factors control wind› Pressure differences› Coriolis effect› Friction

Pressure differences› Creation of wind› Greater the difference – greater the wind› Isobars

Lines connecting areas of equal air pressure Close – steep pressure and high winds Wide – weak pressure and light winds

› Pressure gradient Pressure changes

Coriolis effect – change in air movement due to Earth’s rotation› All free moving objects, fluids and wind are

deflected right in N. hemisphere› Deflected left in S. hemisphere› Due to 15 degree rotation each hour

Friction› Slows air movement› Can change wind direction› Over ocean – low friction› Over rugged terrain – high friction› Jet stream – prominent air flow feature

Fast moving rivers of air in west to east direction

What about global winds?› Ultimate source is unequal heating› Tropical regions – more radiation is received

than radiated back› Polar regions – less solar energy is received

than lost

The atmosphere balances wind differences out by moving warm air towards high latitudes and cool air toward the equator.(giant heat-transfer system).› Ocean currents also contribute

Local Winds?› Small scale winds› Caused by topographic effects or surface

composition(land or water)

Coastal areas:› Land heated intensely during the day more

so than the adjacent water› Air above land expands, heats, and rises

creating low pressure› Sea breeze develops – cool air over

water(high pressure) moves toward warm land (low pressure)

› At night – land cools more rapidly than sea Land breeze develops Cooler air at higher pressure over land

moves toward the sea where air is warmer and lower pressures

Valley and Mountain Breeze› Function very similarly to coastal breezes

› Valley breeze – Heating during the day generates warm air

that rises from the valley floor

› Mountain Breeze- After sunset, cooling air near slopes results

in cool air moving into valley

Measuring Wind› Direction and Speed› Wind vane - direction

Always point into the wind

› Anemometer – speed

› Prevailing winds Consistently blowing from same direction

El Nino and La Nina

› El Nino – “the child” Warm countercurrents replacing normally

cold offshore waters with warm equatorial waters

Irregular intervals (3-7 years) Marked by amoral weather patterns

 

El Nino Stage

Note the large pool of warm (red) water in the western Pacific where water is piled up by the trade winds

› La Nina Opposite of El Nino Distinctive set of weather conditions Colder than normal air (northwest and plain)

and warms other parts of US –cold event High precipitation in NW Increase hurricane activity

Cost is 20x greater