HONORS GEOGRAPHY: METEOROLOGY

WINDS - the intense heat at the equator causes powerful convection - when air mass is warm, air molecules move faster - causing them to push outward and the air mass expands - just like our balloon example of parcels - that expansion causes drop in density - air mass is now lighter than surroundings - and the air parcel rises - that is what convection is - when air mass cools, the process is reversed - and the parcel sinks - this process is constantly taking place in our atmosphere - the sun provides the heat - which is not uniform or regular - there are many factors that effect how the sun heats the earth - the season - the latitude - the cloud cover - re-radiation from land or sea - winds - the air rises, cools and spreads - then starts to sink and return to the surface - where the air is rising, you have a low - where air is sinking, you have a high - since atmosphere is always trying to balance itself (equilibrium) - air moves from low to high pressure - always ins this direction, never reversed - this is better known as wind - think of an inflated balloon - but stop the air from escaping - inside is a pocket of high pressure - when air released, air moves from high to low - equalizing or balancing the air pressure difference - weather rule: the difference in air pressure over a horizontal distance - pressure gradient force - greater the diff in pressure b/t 2 air masses - the greater pressure gradient force and stronger the winds - isobars are sometimes used to indicate increments of air pressure - lines are drawn b/t points of equal pressure - these are isobars - closed isobar curves indicate high or low pressure - usually labeled with an H or L - air that rises at the equator eventually rises to the troposphere - approx. 10 km or 6 miles - it can rise no further - from there is spreads toward the poles - gradually cooling and sinking to middle latitudes - @ 30 degrees north and south - this sinking air causes a high pressure condition - bringing usually fair, dry conditions - most of the world’s deserts are located at these latitudes - some of this sinking air is forced out by the weight above - and returns to the low pressure at the equator - these are known as trade winds - at the equator the winds die out - these are called the doldrums - from old English word meaning dull - early sailors feared being stranded here - no wind to steer the ships - area b/t equator and 30 degrees N & S is called the Hadley Cell - after the guy who discovered these wind patterns - now, while some of the winds return from middle latitudes to equator - some actually continue to their respective poles - at approx. 60 degrees N & S this air meets cold polar air - areas where these two masses meet called polar fronts - diff. in temp b/t these 2 masses causes warmer air to rise - most of this air moves back toward the equator - sinking to the ground at the middle latitudes - contribute to high pressure in these regions - the circulation that occurs b/t 30 & 60 degrees is Ferrel Cells - after Wm Ferrel who identified them 1856 - the rest of the air that rises at polar front continues toward the poles - as it nears the poles, cools and sinks - and returns to 60 degrees N & S - these are Polar Hadley Cells - are weaker than tropical ones - b/c less solar energy reaches polar regions - the air d/n flow in a straight path north and south - this is b/c earth’s rotation causes any fluid object to appear to turn to the right of the direction of motion in the North - and to the left in the South - this effect is called the Coriolis Effect - after Gustave-Gaspard de Coriolis identified it in 1835 - this effect explains the flow of weather systems - causes N. Hem. winds to travel counter clockwise in high press. - & clockwise in the S. Hem. in low pressure - but lower pressure winds blow in the opposite direction - this is why winds in Ferrel Cells blow North from West to East - these are called Westerlies - in the South, Ferrel Cell winds blow east - called Easterlies - global air/wind patterns create principal winds systems - like the trades, westerlies, easterlies, - they also influence smaller scale winds - like monsoons - many areas experience these wet, warm rains - S.W United States, Chile - but strongest occur in S. Asia, N. Australia, Africa - monsoons bring tremendous amts of rain and causes massive flooding - floods have killed thousands of people in Bangladesh, India, SE Asia - the most dramatic, though, are in India - in winter, when sun is low in sky, - air over Siberia cools dramatically - cold air aloft is denser than warm air aloft - so it sinks, causing strong high pressure - this high pressure causes winds that blow over S over India - and out toward the sea - dissipating clouds and rain - in summer this high weakens significantly - developing a low pressure over N. India - draws warm, moist air in from Indian Ocean - producing heavy rain - small scale winds are result of localized diff. in pressure or temperature - ex: coastal areas, local winds may develop on clear sunny days - as sun heats the land, land heats up faster than water - air rises and is replaced by cooler air from sea - the sea breeze - generally occurs in sp ring and summer - when temp diffs. b/t land and sea are highest - the reverse occurs at night - land cools at a faster rate than water - air over the sea is still warm - which rises - air on land is pushed out to sea - the land breeze - as wind blows over mountains and sinks on other side - creates high pressure and clear skies - the compression of air also raises the temperature - resulting in a warm wind - there are several world examples of warm downslope winds - the Chinook on the east side of the Rockies - the Foehn in Switzerland - these winds rapidly melt snow and cause a rain shadow - winds forced through valleys strengthen - just like narrowing of a hose will create more jet stream power - in south of France wind formed by Rhone Valley - the Mistral - brings cold, dry and sqaully conditions from North - squall: short, intense thunderstorm - other winds result when intense heating of inland creates low pressure - the Sirocco - hot, dry winds to Mediterranean from Sahara Desert - pick up moisture from the sea - by time it reaches Europe, they are warm and humid - the Khamsin - also originates in the Sahara - brings hot, dry air to Southern Egypt - often devastates the crops there - The Jet Stream - high speed superhighway going from 80 to 190 mph - form because temperature differences below in lower atmosphere - creates higher pressure gradient aloft - greater the difference the faster, more powerful wind - that’s reason why jet stream strongest in winter - when temp contrast are greatest - high air pressure seeks low pressure - this movement is what causes wind - to find where the low is - place your back to the wind - on your left will be the low pressure system - on your right will be the high