HONORS GEOGRAPHY: METEOROLOGHY-THE BEHAVIOR OF AIR

- in the atmosphere is unending circulation of water - energy of sun evaporates huge quantities of water from oceans - winds transport the moist air to other regions - where condenses and makes clouds - some of which makes rain and snow - if it falls in ocean, the cycle starts again - if it falls on land, it returns to sea in complex journey - this cycle of moving water from liquid, vapor and back - the Hydrologic cycle - from ocean to atmosphere to land and back to ocean - vapor comes from many sources - if from the oceans, rivers or lakes - evaporation - if from living things like animals, trees or people - transpiration - vapor condenses into clouds - wind pushes clouds to land - precipitation falls to the ground as snow, sleet, hail or rain - may take form of glacier - may travel underground in rivers - called groundwater - run off flows to the sea - and the cycle starts over again - think of air molecules as countless billions of bees - swarming around, bumping into each other - they also collide with anything around them - each molecule of air travels about a thousand times its diameter - we’ve already learned that temperature is determined by the average speed of its molecules - kinetic energy - so at room temp. avg speed is @ 1,000 mph - if temp raised, avg speed would increase - if temp lowered, avg speed would decrease - the cooler it gets the slower the speed - until it reached -273'C or -459'F - this is supposedly the coldest temp possible - absolute zero - molelcules have min. amt of energy - so no thermal motion - temperature scales - most scientists use temp. scale called absolute or Kelvin Scale - after British scientist Lord Kelvin who first introduced it - the Kelvin scale has no negative numbers - unlike fahrenheit or Celsius (formerly centigrade) - this makes it convenience for scientific calculations - the Fahrenheit scale was dev. early 1700's by G. Daniel Fahrenheit - he assigned number 32 as temp when water freezes - and 212 at point water boils - b/t freezing and boiling points are 180 equal division - called degrees - this kind of thermometer is called a fahrenheit thermometer - b/c it measures hotness or coldness of things in degrees F - The Celsius scale introduced later in 18th century - 0' is temp at which water freezes - 100' is temp at which water boils - space b/t freeze and boil is divided into 100 equal degrees - so each degree Celsius is 1.8 times higher than Farh - increase in 1'C = increase of 1.5'F - on Kelvin Scale degrees are called Kelvins - abbreviated K - each degree is exactly same as Celsius - and temp of 0'K = -273'C - converting from ‘C to K - add 273' to Celsius temp - ex: K =’C + 273 - there are three steps to converting Celsius to Fahrenheit - take the temperature in Celsius and multiply by 1.8 - add 32 degrees - the result is Celsius converted to Fahrenheit - i.e. 35' celsius X 1.8 = 63 + 32 = 95' Fahrenheit - in most of the world, temp readings are taken in Celsius - in US, temps above the surface are taken in C - while temps at the surface are taken in F - in this class temps will be given in C, and we’ll figure out the F - as long as air temp is above absolute zero, molecules will move about - on avg spring day might collide 10 billion times each second - with each other, houses, trees, flowers, the ground, people - each time it bounces off a thing it gives a tiny push - push divided by the area is called pressure force Pressure = area - at sea level air molecules exert force of @ 14.7 lbs psi - even though we can’t actually feel it, we can detect changes in it - as we ride in a plane, our ears will pop - air collisions b/t outside and inside ear equalize - means higher you go, the less the air pressure - but why are there fewer molecules at high levels? - air molecules, like everything else, held to earth by gravity - squeezing air molecules close together at earth’s surface - this is called air density - we know that air molecules have weight - which exerts a force over the earth - amt of force over an area is called atmospheric pressure/ air pressure - the higher we go, the less air pressure over us - a column of air 1sq in from sea level to top of atmosphere - weight is approx. 14.7 lbs. - so normal pressure at sea level is 14.7 lbs psi - if more molecules are put into the column, pressure goes up - if less molecules are in the column, the pressure lowers - change in air density = change in air pressure - pounds per square inch (PSI) is one way of expressing pressure - but on surface weather maps, they use millibar - but need to know what a Newton (N) is - amt of force required to move an object w/a mass of one kilogram (kg) so that it increases its speed at a rate of 1 m/sec each sec. - a bar is a unit of pressure - force of 100,000N on surface area of 1 sq meter - because bar is particularly large unit we use millibar - 1 bar = 1000 mb - pressure readings on all surface weather maps are expressed in millibars - at sea level standard value for atmospheric pressure is 1013.25 mb - we measure atmospheric pressure using a barometer - so atmospheric pressure is also referred to as barometric pressure - mercury barometer - glass tube is put open side down into dish of mercury - closed end has air pumped out to make a vacuum - in high pressure, air pressure pushes down on dish - causing mercury to rise in tube - in low pressure, air pressure eases up on dish - lowering the mercury in the tube - each millibar is expressed in number of inches of mercury pushed - ex: 850 mb = 25.10" of mercury is pushed up - ex: 1110 mb = 32.78" of mercury - ex: sea level mb? (1013.25 mb) = 29.92" - most common type of home barometer is called an aneroid barometer - contains no fluid - inside is small flexible metal box called aneroid cell - before cell is sealed, air is partially removed - better to measure pressure changes in the air - small changes in pressure causes cell to expand or contract - size of cell is calibrated to represent different pressures - any change in size is shown by an indicating arm - barometer readings tell us a lot about what the weather will be - 870 mb lowest recorded sea level pressure during typhoon Tip 1979 - 899 mb Hurricane Allen Aug. 1980 - 980 mb deep low pressure system - 1013.25 mb average sea level pressure - 1050 mb strong high pressure system - 1064 mb highest recorded sea level pressure in US Montana 12/83 - 1084 mb highest recorded sea level pressure, Siberia 12/68 - 28 to 29 inches and rising meas that rain will continue for 12 to 24 hours followed by clearing and cooler - 29.01 to 29.40 inches and falling means that rain will fall within 2 hours - 29.41 to 29.70 inches and rising means clouds will diminish throughout the day - 29.71 to 30.00 inches and falling means that the skies will become partly cloudy and humidity will rise - 30.01 to 30.50 inches and rising means that it will be mostly sunny and warm - 30.51 to 31.00 inches and falling means that clouds will increase throughout the day with a 10% chance of rain late in the day. - air pressure decreases rapidly with height - new sea level, pressure usually close to 1000 mb - normally decreasing 10 mb for every 100 meters in elevation - at higher levels, pressure decreases much more slowly - at 5.5 km (3.5 mi) pressure is about 500 mb - half of sea level pressure - means that half of all molecules are below us - Mt. Everest at 9 km or 29,000', pressure about 300 mb - at about 50 km or 30 mi. pressure is about 1 mb - meaning 99.9% of all atmostphere is below us - yet atmosphere extends up for many kilometers - helps define “thin air” - decrease of pressure w/height makes sure rising air always cools - think of a balloon and let’s call it a parcel - this parcel can expand or contract freely - but external air/heat c/n mix w/air inside - as parcel moves, d/n break apart but remains single unit - air molecules inside parcel defines air density - avg speed of molecules directly related to air temp - and molecules colliding inside rep. air pressure - at surface parcel has same temp as surrounding air - if carried up, enters region where air pressure is lower - lower pressure outside lets molecules inside expand the parcel - because no new molecules inside parcel, less kinetic energy from collisions because of less density - less kinetic energy = lower temperature - therefore air that rises always expands and cools - if parcel lowers to earth, returns to where pressure outside higher - squeezing parcel back to original size - increasing average speed of molecules inside - more kinetic heat - so air that sinks (subsides) gets warm - subsiding air always warms by compression - as air subsides and warms, can hold more water vapor - means must need more water to reach saturation - so sinking air prevents formation of clouds - as air rises and cools, c/n hold more water - so clouds can form by condensation - so cloudy skies are often due to rising/cooling air - while clear skies may be result of sinking (warm) air - in other words when air rises or descends, air pressure temp and density all change - this applies to air at the surface, but aloft it’s different - we already said air higher up tends to cool or be cold - because of less kinetic energy - if the air were confined to a city block to the top of atmosphere - air density would be the same - think of an ice cube - when warmed it is less dense - when cold it is more dense - the same goes with air that is aloft - let’s look at our column of air over the city again - column 1 and 2 are equal size and same amt of molecules - if column 1 air was cold, the molecules w/b denser - but same amt of molecules as 2 - if column 2 was warmed, the molecules w/b less dense - still the same amt of molecules as 1 - column one would be shorter than 2 - because same amt of molecules are compressed - and warm air expands the column - weather rule: it takes a shorter column or cold, dense air to exert the same amount of pressure as a taller column of warm, less dense air. - now let’s move up the cold column until half way and mark the spot - then from that line cross over to the warm column - notice there are more molecules above that line than below - more pressure above means high pressure above - weather rule: warm air aloft is normally associated with high pressure while cold air aloft usually means low atmospheric pressure - now what would happen if the surface temperature changed - on cold surface, molecules w/b densest at the bottom - mercury barometer would read high pressure - if warm surface, molecules less dense - mercury barometer would read low pressure - heat from the sun can warm the upper atmosphere - and we all know that warm air molecules have more kinetic energy - because they are moving faster - this heating of the atmosphere moves the molecules - we know it as wind - when the sun goes down, these winds also cool - this is called diurnal fluctuation of pressure - diurnal means daily - so upper level heats at its max. @ 10a.m. - when sun is heating atmosphere - and 10p.m. - when upper atmosphere cools - upper level has minimum pressure new 4am and pm - these changes in winds are known as thermal tides - because the fluctuation of winds is like the tides of the ocean - movement of wind aloft can also change surface air pressure - in some places winds aloft will cause air to funnel into an area - causing air to crowd together like cars entering a freeway - this piling up is called convergence - upper level convergence increases air pressure on surface - in other regions winds aloft will cause air to go in many directions - this is called divergence - upper level divergences decreases air pressure on surface SUMMARY - Changes in surface air pressure can be brought on by changes in air density above the surface. They can be caused by: - Changes in air temperature - sunlight warming upper portion of atmosphere, causing winds - convergence and divergence of air brought by wind patterns