Pressure Basics  Force per unit area  Barometric/ Air/ Atmospheric pressure is the force exerted by air molecules –Nitrogen (78%) –Oxygen (21%) –Water Vapor (0-4%) –Others (argon, neon…)

The Gas Law: P=DRT  P=pressure; D=density (molecules per unit volume); T=temperature; R=a constant  So, pressure is proportional to the temperature times the density of the air, or we can say that air pressure depends upon air temperature and air density –Increase T, increase molecule speed, increase force –Increase D, increase number molecules, increase force  Easiest to see if we hold one variable constant… –Pump up a bicycle tire (T=same, D=incr, thus P incr) –Tire, hot road (D=same, T increases, thus P increases)

The Gas Law: P=DRT  What if pressure is held constant? –If T increases, D decreases  Molecules speed up and get farther apart  More force exerted by less molecules = constant pressure –If T decreases, D increases  Molecules slow down and get closer together  Less force exerted by more molecules = constant pressure  COLD AIR IS MORE DENSE THAN WARM AIR AT THE SAME PRESSURE

Vertical Pressure Gradient  Very large: 900mb in about 10 miles  Gravity pulls everything, including the atmosphere, toward the earth’s center –Air is compressible –Density increases as altitude decreases –Pressure increases as altitude decreases

Horizontal Pressure Gradient  Much smaller than vertical gradients, but crucial to atmospheric motion  Correction to sea level –Remove influence of elevation differences between stations –Allows us to see subtle horizontal pressure gradients  “Surface” wx map = Constant height map = Sea level pressure map

Pressure, Temperature, Height  Cool column of air –Air more dense, molecules closer together –Pressure decreases more rapidly with height  Relatively low pressure at constant height  Relatively low height at constant pressure  Warm column of air –Air less dense, molecules farther apart –Pressure decreases more slowly with height  Relatively high pressure at constant height  Relatively high height at constant pressure  Pressure decreases upward, relative to the change in density with height

Pressure, Temperature, Height 500 mb examples “Average” global height of 500mb surface Typical global 500mb pattern

Pressure, Temperature, Height Another View

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

Key Figures  6.5, 6.8, 6.9; 7.4, 7.5, 7.7, 7.9, 7.10, 7.16  Remember the following terminology: –The only constant height map we will use is the surface weather map (height = 0; sea level)  We plot (draw) lines of constant pressure (isobars) on this map –All other upper-air maps (any level above sea level) are constant pressure maps (500mb, 300mb, etc.)  We plot lines of constant height (height contours) on these –In either case the general rule is the closer together the lines (stronger or tighter gradient) over a given distance, the stronger the winds