Air Pressure and Wind
Isobars
-A type of isoline -Differences in air pressure are mapped with isobars -Connect points of equal atmospheric pressure -Note pattern of isobars around highs and lows
Forces of Wind Direction
-Pressure Gradient Force -Coriolis Effect -Friction
Pressure Gradient Force
The force created by a change in air pressure over a given distance -Change in air pressure = wind The stronger the PGF, the stronger the wind Strong PG = isobars drawn close together = strong winds Weak PG = isobars drawn further apart = less wind -Wind moves "down" the gradient Wind always moves from high to low pressure
Coriolis Effect
-Impacts wind above 2300 feet -Deflects wind to the right in the northern hemisphere, to the left in the southern -So strong that upper level winds are adjusted by 90º- becoming parallel to isobars
Measuring Air Pressure
-Inches of mercury: the height of a column of mercury in a liquid barometer (Avg sea level pressure = 29.92) -Millibar: a measure of force per unit area (Avg sea level pressure = 1013.25 mb; range at the surface 960-1050mb)
Depicting Lower vs. Upper Atmosphere Wind
-Lower: wind naturally moves across (perpendicular to) isobars due to friction -Upper: wind moves parallel to isobars according to Coriolis effect
Friction
-Only at the surface -Slows wind at surface so Coriolis force is reduced -Winds move perpendicular to isobars
What to remember about wind
-Wind moves down a PGF from high to low pressure -Coriolis Effect at upper atmosphere causes wind to deflect right in Northern Hemisphere (wind blows parallel to isobars) -Friction at lower atmosphere reduces Coriolis Effect and slows wind (wind moves perpendicular to isobars) -Anticyclonic wind diverges clockwise out of a high -Cyclonic wind converges counterclockwise
Anticyclones vs. Cyclones
Pressure Gradient Force, Coriolis Effect, and Friction dictate circulation Winds move differently around highs and lows -Northern hemisphere: 1. diverge (move away) clockwise out of high pressure systems- called anticyclones 2. converge (come together) counterclockwise into low pressure systems- called cyclones
Station Models
Represent a place (city, town) on a map (a circle) -A standardized system to present weather data -Various information is dawn around and within the circle Temperature is given is degrees Fahrenheit Air pressure notation -To save space, the notation is shortened -Usually, pressure ranges from 960-1050 mb -The first "9" and "10" is omitted, a swell as the decimal point -To interpret: -If the first number is between 0-5, place 10 in front -If the first number is between 6-9, place 9 in front -Then, add a decimal point before the last number
Air Pressure
The force exerted by the atmosphere on a surface Gravity pulls air towards the surface -Air is compacted downward -Pressure is higher at the surface -Decreases with altitude Pressure, density, and temperature closely Rising or warm air = low pressure Sinking or cool air = high pressure
What direction is the wind blowing
Wind arrow indicates direction of the wind -Remember: wind direction is described as the direction from which the wind is blowing Feathers indicate wind speed -Full feather = 10 knots -Half feather = 5 knots
