Download Chapter 1

Chapter 1
Properties of the Atmosphere
How is the atmosphere characterized?
Preliminaries
• 90% of the atmosphere’s mass is between mean
sea level (MSL) and 12 km (7.5 mi) above MSL
• Atmospheric compostion:
–
–
–
–
N2, 78%;
O2, 21%;
water vapor is variable but very important;
aerosols, clouds, precipitation
• Forms of water in the atmosphere
– Vapor (gas)
– Liquid (cloud droplets and rain drops)
– Solid (ice crystals and ice precipitation)
Temperature
• Defined as a measure of the average
speed (energy of motion) of molecules in a
substance.
• Well, what about liquids and solids?
– Molecules in solids experience vibrations
– Molecules in a liquid have limited path lengths
over which no collisions take place
Temperature scales
Metric system
• Length: meters (m) or kilometers (km)
• Time: seconds (s)
– Speed: m s-1; acceleration: m s-2
• Mass: kilogram (kg)
– Density: kg m-3
• Force: Newton (N, kg m s-2)
• Pressure: Pascal (Pa, N m-2 = kg m-1 s-2)
• Energy: Joule (J, N m = kg m2 s-2)
Distribution of surface
(2 m AGL) temperature
in January and July
T = temperature
What causes
temperature changes?
- Daily variation?
- Annual variation?
T pattern over the U.S. today
Annual variation of T
Seasons (tilt of earth’s axis of
rotation
Vertical variation of T (standard atmosphere)
Tropopause variation vs latitude
Pressure
• Force per unit area (Pa = N m-2)
• Weight of a column of air above a unit
area
– All molecules are summed in that column
Columns with different weights
Vertical profile of pressure
Mercury barometer
p and wind over the U.S. today
Moisture
•
•
•
•
•
Water vapor
Clouds
Precipitation
Water vapor’s benefit
Measurements of water
vapor
Measures of water vapor
• Vapor pressure
– Partial pressure (e) due to water vapor
– A direct measure of the total number of H2O
molecules
• Dew point temperature
– Temperature at which saturation is attained
• Saturation  100% relative humidity (e = es)
• RH = e/es (see p. 11)
– Commonly plotted on surface weather maps
– Depends on (a) amount of water vapor in the air, and
(b) the amount of water vapor that the air can “hold” at
a given temperature
Average vapor pressure (e) and dew point
temperature (D) in Jan. & July
lowest
highest
lowest
highest
Note the relationship between e and D
Saturation vapor pressure and T
RH = Relative humidity
RH = (vapor pressure /
saturation vapor pressure)
x 100%
or
RH = e/es
The curve is exponential,
meaning that the rate of
increase in es with increasing
temperature increases as T
increases.
es = Ae-B/T
Daily variation of T and RH
What is the relation between T and RH?
If the absolute amount of water vapor does not change, then at
low T, RH is high, and at high T, RH is low.
Look at the behavior of RH and T today here in Huntsville.
Td pattern over the U.S. today
Extra!!
Integrated water vapor
TOA
• Precipitable water (PW)
– The depth of liquid water that
would result if all of the water
vapor in the column between
the surface and the “top of the
atmosphere” were condensed.
Water
vapor
• Refer to NSSTC web site
(SuomiNet)
http://vortex.nsstc.uah.edu/mips/data/cu
rrent/surface/
PW
Condensed water
sfc
Phase changes and
latent heating
Latent heating is associated with a
change of phase in water:
water vapor
liquid (cloud drops, rain drops)
ice (ice crystals, ice precip.)
Latent heating is the primary source of
energy of thunderstorms
Latent heating has profound effects on
many atmospheric systems
Advanced concept
Air density (r)
• Number of air molecules per unit volume
– Mass per unit volume: kg m-3
• Cannot directly measure density
• Equation of state is used to calculate r
– p=rRT or r = p/RT
– R is the gas constant
• Density is low on a hot day in Denver, CO
• Density is high on a cold day in
International Falls, MN
Wind
Wind is the movement of air
Wind is measured with
anemometers
Doppler radar/lidar/sodar
Direction – defined as the
direction from which the wind
blows
Speed – mph, knots, or m/s
Symbols
The importance of wind:
Transports temperature and water
vapor horizontally
Strong winds produce damage
Relation
between wind
speed (V) and
the pressure
gradient (PG):
V  1/PG
Strong wind
Weak wind
Winds are
stronger over
the water
surface (lower
friction)
Automated Surface Observing System (NWS)
Fig. 2A
from Ch. 2
A question for thought
• Is there a relation between temperature
and pressure?
Homework
• Test your problem solving skills, p. 17
– Numbers 2, 3