Atmospheric Sciences_001

ATMOSPHERIC SCIENCES

Chapter 7 Part 1

Introduction • The atmosphere is a vital component of the human environment. • It transmits alters the solar energy that controls our climate. • It acts a shield, protecting us from damaging meteoritic impacts and from radiation, such as ultraviolet rays from the sun. • It supports the flight of birds and insects and transports seeds and spores. It’s gases provide the raw materials for life itself: without them, we could not exists.

Weather and Climate: The two aspects of the atmosphere of which we are most aware: 3. Weather-is the name we give to the states of the sky, air, wind and water. Weather elements are rain, snow, heat, wind, thunder, and fog. 5. Climate-our integrated experience is the climate, the characteristic annual cycle of weather.

Atmospheric Sciences (the study of weather): 1.

Meteorology-which is the physics and chemistry of the lower atmosphere.

3.

Aeronomy-the scientific study of the upper atmosphere, which begins 100 km above sea level; electromagnetic forces chemical activity are more important in this field than in meteorology.

5.

Climatology-the study of climate, is concerned with how the earth’s atmosphere behaves over long periods of time

Basic Atmospheric Properties: Composition and Physical State: • The atmosphere is a mixture of gases, with numerous suspended particles, some solid and some liquid. • The lower atmosphere is electrically neutral, containing few free ions; for most part, it is composed of molecules. • The upper atmosphere, by contrast, is extensively ionized; many gases are broken up into single atoms or into free radicals such as hydroxyl (OH-). Because of its special role, water vapor (H2O) is often dealt with separately.

Com pos itio n O f Pu re Dry A ir (W itho ut W at er Va por) In Lo wer A tmos pher e W ith Mol ec ula r W eig ht s and En viro nme nt al R oles

Comp os ition O f Pu re Dry A ir (Wi thou t W ate r Vap or) In L ow er Atmo sphe re Wit h M olec ul ar W ei ght s an d En vir onme nt al R ole s Gases

Formulas

Concentration (% by volume)

Molecular Weight (Kg/mol x 103)

Environmental Roles

Nitrogen

N2

78.09

28.0

Inert as N2; essential to life as N

Oxygen

O2

20.95

32.0

Essential to life; chemically active

Hydrogen

H2

5.0 x 10-5

2.0

Important in atmospheric chemistry

Argon

Ar

0.93

39.9

Inert

Neon

Ne

1.8 x 10-3

20.2

Inert

Helium

He

5.2 x 10-4

4.0

Inert; escapes from earth’s crust

Krypton

Kr

1.0 x 10-4

83.7

Inert

Xenon

Xe

8.0 x 10-6

131.3

Inert

Radon

Rn

6.0 x 10-18

222.0

Radioactive; variable in height and time, because of decay

Active gases

Inert gases

Variable gases Carbon Dioxide

CO2

3.6 x 10-2

44.0

Essential to life; optically active

Ozone

O3

1.0 x 10-6

48.0

Toxic, optically and chemically active

Other traces constituents include sulfur dioxide (SO2), carbon monoxide (CO), oxides of single nitrogen (NOx) and various pollutants

•Water may be present in the atmosphere as gas (vapor), solid, or liquid. Water vapor is always present. •The precipitable water is the liquid water equivalent of the water vapor present n any column of the atmosphere. •In practice, most of the water vapor is in the lowest 5 km.

Thermal and Electrical State: Permanent Layers of the Atmosphere

Troposphere : at the base is called the troposphere; capped by a surface minimum temperature called the tropopause at levels between 10 to17 km above the sea. • Temperature decreases with height in the troposphere, since the main source is solar radiation absorbed at ground level. The rate of decrease, called lapse rate, is about 5.0 K/km. • The troposphere contains most of the water vapor, clouds, and storms of the atmosphere. • Wind tends to be strongest at the tropopause, the level of the jet streams. This is also the level at which jet aircraft cruise.

Stratosphere : above the tropopause, temperature increases with height in stratosphere, reaching a maximum at 50at 55 km, the so called stratopause, where temperature is about as high as at ground level. • It’s warmth is due to the absorption of ultraviolet radiation from the sun by oxygen (O2) and ozone (O3). • Most of the world’s ozone is found in the stratosphere, where it may exceed 5 part per million by volume (ppmv). Hence stratospheric air is lethal to human beings. There is very little water vapor at these heights.

Mesosphere : •

extends from the stratopause at 50 to 55 km to another temperature minimum at 80 km, the mesopause. The mesosphere is a windy and turbulent region, but there is usually too little water vapor for clouds to form.

Thermosphere : above the mesopause, temperature increases indefinitely upward into the thermosphere, the hot upper atmosphere. • The air nearest the earth’s surface is called the boundary layer. The planetary boundary layer (below 1000 m) is the layer in which the wind is affected by friction with the earth’s surface. • The bottom 50 m is often called the surface boundary layer. These layers are very important to the engineer, most of whose work is done at such levels. • The temperatures of the air, sea, and land surface are controlled by unequal heating and cooling by the sun or outgoing radiation.

Thermosphere • This accounts for the familiar changes of heat and cold during a typical day and between seasons. • It also explains why the tropics are warm and polar regions cold. The transport of heat by winds and ocean currents also affects air temperature. • The lower atmosphere is usually electrically neutral, unlike the ionize upper atmosphere. Nevertheless, strong potential gradients do exist, especially in and around thunderstorm.

Energy Outputs And Inputs: Solar Radiation

Solar Radiation: • The sun provides 99.97%of the heat used at the earth’s surface for all natural processes. • The sun is a fairly constant star. We can detect only small variations in the nature and intensity of the radiation it emits. • Hence we speak of the solar constant, which is the intensity of solar radiation reaching the top of the earth’s atmosphere.

Solar Radiation • Measured at right angles to the solar beam, the solar constant is estimated to be 1368 W on each square of the circular outline (disk) of the earth as it faces the sun. The spin of the earth distributes this power over the whole surface of the sphere, whose area is four times as great as that of the disk. Hence the mean solar constant per unit area of the earth’s surface is 342 W/m2. (Surface area of a sphere=4πr²) • Solar radiation resembles that of a blackbody (perfect radiator near 6000 K. The highest intensity occurs near a wavelength of 500 nm, with most of power contained in the range 200 to 5000 nm. Meteorologists call this shortwave radiation, because it is of short wavelength than radiation emitted by the earth itself

Solar Radiation: • The human eye detects light between about 400 and 700 nm which is called visible light waveband • Shorter radiation (200 to 400nm) is called ultra violet, and longer radiation, infrared. • The actual mean intensity (averaged over 24 h) of solar radiation at ground level varies from about 250 W/m2 in subtropical desert to as little as 80 W/m2 in cloudy sub polar areas. • Obviously, it is near zero at night, and day values are considerably higher than average. • At times in clear weather, when the sun is nearly vertically overhead, values approaching the solar constant (1368 W/m2) are observed for short periods.

Solar Radiation