Pressure BasicsPressure Basics

Force per unit areaForce per unit area Barometric/ Air/ Barometric/ Air/

Atmospheric pressure Atmospheric pressure is the force exerted by is the force exerted by air moleculesair molecules– Nitrogen (78%)Nitrogen (78%)– Oxygen (21%)Oxygen (21%)– Water Vapor (0-4%)Water Vapor (0-4%)– Others (argon, neon…)Others (argon, neon…)

The Gas Law: P=DRTThe Gas Law: P=DRT P=pressure; D=density (molecules per unit volume); P=pressure; D=density (molecules per unit volume);

T=temperature; R=a constantT=temperature; R=a constant So, pressure is proportional to the temperature times So, pressure is proportional to the temperature times

the density of the air, or we can say that air pressure the density of the air, or we can say that air pressure depends upon air temperature and air densitydepends upon air temperature and air density– Increase T, increase molecule speed, increase forceIncrease T, increase molecule speed, increase force– Increase D, increase number molecules, increase forceIncrease D, increase number molecules, increase force

Easiest to see if we hold one variable constant…Easiest to see if we hold one variable constant…– Pump up a bicycle tire (T=same, D=incr, thus P incr)Pump up a bicycle tire (T=same, D=incr, thus P incr)– Tire, hot road (D=same, T increases, thus P increases)Tire, hot road (D=same, T increases, thus P increases)

The Gas Law: P=DRTThe Gas Law: P=DRT What if pressure is held constant?What if pressure is held constant?

– If T increases, D decreasesIf T increases, D decreases Molecules speed up and get farther apartMolecules speed up and get farther apart More force exerted by less molecules = constant More force exerted by less molecules = constant

pressurepressure– If T decreases, D increasesIf T decreases, D increases

Molecules slow down and get closer togetherMolecules slow down and get closer together Less force exerted by more molecules = constant Less force exerted by more molecules = constant

pressurepressure

COLD AIR IS MORE DENSE THAN WARM COLD AIR IS MORE DENSE THAN WARM AIR AIR AT THE SAME PRESSUREAT THE SAME PRESSURE

Vertical Pressure GradientVertical Pressure Gradient Very large: 900mb in about 10 milesVery large: 900mb in about 10 miles Gravity pulls everything, including the atmosphere, Gravity pulls everything, including the atmosphere,

toward the earth’s centertoward the earth’s center– Air is compressibleAir is compressible– Density increases as altitude decreasesDensity increases as altitude decreases– Pressure increases as altitude decreasesPressure increases as altitude decreases

Horizontal Pressure GradientHorizontal Pressure Gradient Much smaller than vertical gradients, but Much smaller than vertical gradients, but

crucial to atmospheric motioncrucial to atmospheric motion Correction to sea levelCorrection to sea level

– Remove influence of elevation differences Remove influence of elevation differences between stationsbetween stations

– Allows us to see subtle horizontal pressure Allows us to see subtle horizontal pressure gradientsgradients

““Surface” wx map = Constant height map = Surface” wx map = Constant height map = Sea level pressure mapSea level pressure map

Pressure, Temperature, HeightPressure, Temperature, Height

Cool column of airCool column of air– Air more dense, molecules Air more dense, molecules

closer togethercloser together– Pressure decreases more Pressure decreases more

rapidly with heightrapidly with height Relatively low pressure at Relatively low pressure at

constant heightconstant height Relatively low height at Relatively low height at

constant pressureconstant pressure

Warm column of airWarm column of air– Air less dense, molecules Air less dense, molecules

farther apartfarther apart– Pressure decreases more Pressure decreases more

slowly with heightslowly with height Relatively high pressure at Relatively high pressure at

constant heightconstant height Relatively high height at Relatively high height at

constant pressureconstant pressure

Pressure decreases upward, relative to the Pressure decreases upward, relative to the change in density with heightchange in density with height

Pressure, Temperature, HeightPressure, Temperature, Height500 mb examples500 mb examples

“Average” global height of 500mb surface

Typical global 500mb pattern

Pressure, Temperature, HeightPressure, Temperature, HeightAnother ViewAnother View

Pressure, Temperature, HeightPressure, Temperature, Height Thus, a constant pressure map showing contours Thus, a constant pressure map showing contours

of height has the same use as a constant height of height has the same use as a constant height map showing contours of pressuremap showing contours of pressure– Large pressure gradient = stronger winds (i.e.,sea level)Large pressure gradient = stronger winds (i.e.,sea level)– Large height gradient = stronger winds (i.e., 500 mb)Large height gradient = stronger winds (i.e., 500 mb)

Key FiguresKey Figures 6.5, 6.8, 6.9; 7.4, 7.5, 7.7, 7.9, 7.10, 7.166.5, 6.8, 6.9; 7.4, 7.5, 7.7, 7.9, 7.10, 7.16 Remember the following terminology:Remember the following terminology:

– The only The only constant height map constant height map we will use is the we will use is the surface surface weather mapweather map (height = 0; sea level) (height = 0; sea level) We We plotplot (draw) lines of constant pressure (isobars) on this map (draw) lines of constant pressure (isobars) on this map

– All other All other upper-airupper-air maps (any level above sea level) are maps (any level above sea level) are constant pressure mapsconstant pressure maps (500mb, 300mb, etc.) (500mb, 300mb, etc.) We plot lines of constant height (height We plot lines of constant height (height contours) contours) on theseon these

– In either case the general rule is the closer together the In either case the general rule is the closer together the lines (stronger or tighter lines (stronger or tighter gradientgradient) over a given distance, ) over a given distance, the stronger the windsthe stronger the winds