Chapter 1Properties 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

Note the relationship between e and D

highest

lowest

highest

lowest

Saturation vapor pressure and T

RH = Relative humidity

RH = (vapor pressure / saturation vapor pressure) x 100% orRH = 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

Integrated water vapor

• 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.

• Refer to NSSTC web site (SuomiNet)

http://vortex.nsstc.uah.edu/mips/data/current/surface/

Watervapor

TOA

sfc

Condensed water

PW

Extra!!

Phase changes and latent heating

Latent heating is associated with a change of phase in water:

water vaporliquid (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

Air density ()

• 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

– pRT 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

Advanced concept

WindWind is the movement of air

Wind is measured withanemometersDoppler 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 horizontallyStrong 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