Transcript Radiation

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