Introduction to the Atmosphere
Supplemental Instructor Notes
Composition of Atmosphere
Nitrogen makes up 78% and oxygen 21% of atmosphere.
Nitrogen is added to the air by the decay and burning of organic matter, volcanic
eruptions, and the chemical breakdown of certain rocks, and it is removed by
certain biological processes and by being washed away in rain or snow. Overall,
the addition and removal of nitrogen gas are balanced, and consequently the
quantity present in the air remains constant over time.
Oxygen is produced by vegetation and is removed by a variety of organic and
inorganic processes; its total quantity also apparently remains stable.
The remaining 1% of atmosphere consists mostly or argon gas.
Water vapor and carbon dioxide make up small percent of atmosphere, but have
profound effect on weather, humidity and climate. Water vapor is about 4% of
total volume over tropics and a tiny fraction of 1% in deserts.
Carbon dioxide effects climate because of its ability to absorb infrared radiation,
which is the type of radiation that keeps the lower atmosphere warm. Increased
carbon dioxide warms up the air. Less found in atmosphere in summer when
plants absorb it, more found in winter because of decaying vegetation.
Ozone a minor but vital gas that absorbs ultraviolet solar radiation; it filters out
enough of these rays to protect life forms from potentially deadly effects. It is
made up of 3 oxygen atoms (O3).
Particulates (dust) also found in atmosphere as a result of human and natural
sources. Important ingredients in forming clouds as they act as condensation
nuclei.
Vertical structure of atmosphere is depicted on page 60
Air pressure is highest the closer you are to see level and decreases with increase in
elevation. You can feel it in your ears when going up a mountain or airplane.
Uniform distribution of gases in first 50 miles of atmosphere (homosphere).
Uniformity decreases past this level and gases tend to become more layered
(heteorsphere).
Ozonosphere lies at 9-30 miles up and contains higher than normal level of ozone
Ionosphere is a deep layer of electrically charged molecules and atoms that lie about
40-250 miles out. It aids in long-distance communication by reflecting radio
waves back to Earth. (see page 63, Fig. 3-12)
Difference between weather and climate
Major elements of weather and climate are temperature, pressure, wind and moisture
content.
Principal controls of weather and climate are:
1. latitude
2. distribution of land and water
3. general circulation of the atmosphere (easterlies and westerlies)
4. general circulation of the oceans (currents)
5. elevation
6. topographic barriers
7. storms
Insolation (incoming solar radiation) and Temperature
Spatial and Seasonal Variations in Heating:
Angle of incidence - sun’s rays more intense where the angle is closest to 90 degree.
Chronicle annual march of temperature change. Why is coldest month in January
and hottest month in July? (See Figure 4-18 on page 85).
Length of day. Chronicle daily march of temperature. Why hottest in mid-afternoon
and coldest just before dawn?
Atmospheric Obstruction - clouds, particulate matter, and gas molecules in the
atmosphere absorb, reflect, or scatter insolation.
Latitudinal Radiation Balance - direct rays of sun shift north and south across globe
over the course of the year. (see page 87 of book). Look at langleys=1 calorie per
square centimeter.
Land and Water Contrasts:
Atmosphere is heated mainly by heat radiated from Earth rather than from Sun. Therefore
Earth’s surface is primary control of the heating of the air above it.
The reasons for water not HEATING as fast as earth surface are:
Water has a higher specific heat than land. This is the amount energy required to
raise the temperature of 1 gram of a substance by 1 degree C. The specific heat of
water is about 5 times as great as that of land.
Sun rays penetrate water more deeply than land. Rays are diffused over a greater
volume of matter.
Water is mobile, therefore heat is dispersed because of water mixing and currents.
The cooling effect of evaporation slows down any heat buildup on a water surface.
COOLING
Land cools more slowly than water because heat in water has been stored deeply and is
brought only slowly to the surface for radiation. As the surface water cools, it sinks and is
replaced by warmer upwellings from below. The entire water body must be cooled before
the surface temperature decrease significantly.
This is why the hottest and coldest areas of Earth are found in the interiors of continents.
Mechanisms of Heat Transfer - the persistent shifting of heat from low latitudes to high
and cooling from high latitudes to the low.
Atmospheric circulation accounts for 75-80% of horizontal heat transfer.
Oceanic circulation also transfers heat. Currents are mainly driven by air blowing
over the surface.
o Enormous elliptical loops elongated east-west and centered approximately
at 30 degrees of latitude. These loops called gyres flow clockwise in the
Northern Hemisphere and counterclockwise in the Southern Hemisphere. (
o
Measuring temperature
Degrees Fahrenheit = (degrees Celsius * 1.8) + 32
Degrees Celsius = (degrees Fahrenheit - 32) divided by 1.8
Atmospheric Pressure and Wind
Atmospheric pressure is the determinate of wind which can influence landscapes.
Atmospheric pressure is the force exerted by the gas molecules on some area of Earth’s
surface or on any other body, including ours. You can feel pressure when you go up in an
airplane. Pressure and wind are major elements of weather and climate. At sea level, the
pressure exerted by the atmosphere is about 14.7 pds per sq. inch. This value drops with
increasing altitude because the farther away you get from Earth and it gravitational pull,
the fewer gas molecules are present in the atmosphere.
Density is the amount of matter in a unit volume. The density of a gas is proportional to
the pressure on it. The reverse of this statement is also true: The pressure a gas exerts is
proportional to its density. The denser the gas, the greater pressure it exerts. Therefore, at
low altitudes the gas molecules of the atmosphere are packed closely and are more dense
because of gravitational pull of Earth. More density = more pressure. At high altitudes the
air is less dense and therefore less pressure.
Temperature and Pressure: when air is heated molecule become more agitated and more
collisions. This results in higher pressure. Decrease in temp produces decrease in
pressure.
Generalizations:
Very warm surface conditions often produce low pressure at the surface (thermal
low).
Strongly rising air often produces low pressure at the surface (a dynamic low)
Very cold surface conditions often produce high pressure at surface (thermal high)
Strongly descending air often produces high pressure at the surface (dynamic low)
Atmospheric pressure is measured with a barometer in units called millibars. 1 bar =
1000 millibars = 14.7 pds per square inch. Average sea-level pressure is 1013.25
millibars. (bring barometer to class and look at weather station reading.)
Isobars are lines connecting points of equal atmospheric pressure on a map. As with other
types of isolines, the relative closeness of isobars indicates the horizontal rate of pressure
change, or pressure gradient.
Wind is horizontal air movement. Updrafts and downdrafts are vertical movements, and
ascents and subsidences are large-scale vertical movements. Winds represent nature’s
attempt to even out the uneven distribution of air pressure over Earth.
The direction of wind movement is determined principally by the interaction of three
factors: pressure gradient, the Coriolis effect, and friction.
Pressure gradient - winds move air from high pressure to low.
Coriolis effect due to rotation of earth. Winds that move parallel to isobars is called
geostrophic wind - this is when pressure gradient and Coriolis effect are in
balance. Deflection to the right in the Northern Hemisphere and deflection is
always to the left in the Southern.
Friction - effect of friction extends to only about 5,000 ft. above ground. Higher than
that most winds follow a geostrophic course. Friction slows winds down and
limits effect of Coriolis.
Cyclones and Anticyclones.
Anticyclone is a high pressure center and low pressure is cyclone.
1.
Anticyclone:
In Northern Hemisphere, upper air winds move parallel to the isobars and clockwise
In NA the friction area creates a divergent clockwise flow, with air spiraling out away
from the center of the anticyclone.
In Southern Hemisphere, upper air winds move counterclockwise and parallel to isobars.
In friction area of SA air diverges in counterclockwise pattern.
2.
Cyclone:
In NA upper air parallels isobars in counterclockwise pattern
In NA friction area a converging counterclockwise flow exists
In SA upper air move clockwise and parallel to isobars
In SA friction area wind spiral inward in clockwise spiral.
In a cyclone and anticyclone also effect vertical air movement. In a cyclone air converges
and rises. In an anticyclone air descends and diverges. This accounts for cloudy days
when low pressure exists and clear days with high pressure.
Wind Speed:
Determined primarily by the pressure gradient.
Hadley cells are two prominent tropical circulations. Around the world in equatorial
latitudes, warm air rises, producing a region of relative low pressure at the surface. This
air ascends to great heights, mostly in thunderstorm updrafts. By the time this air reaches
the upper troposphere at elevations of about 50,000 ft., it has cooled. The air moves
poleward, eventually descending at latitudes of about 30 degrees where if forms bands of
high pressure at the surface. There are 7 basic surface components to this exchange of air:
1.
2.
3.
4.
5.
6.
7.
Sub-tropical high - pressure cell in ocean basin centered at about 30 degrees of
latitude. These are associated with a general subsidence of air from higher
altitudes in the form of a broadscale, gentle downdraft. Areas associated with
warm, tropical sunshine and an absence of wind. Associated with deserts and
frequently called horse latitudes. Sailing ships of 16th and 17th century stuck
there and had to throw over their horses to conserve drinking water.
The trade winds issue from the equatorward sides of the sub-tropical highs. They
dominate more of the globe than any other wind system. They are very consistent
in direction, speed and frequency. They blow from northeast above Equator and
from southeast below Equator. Trade winds dominate more of the globe than any
other wind system. They lie about latitude 25 degree North and South.
Intetropical convergence zone (ITZ) - air from 2 hemispheres meet. Air also
ascends here and creates thunderstorms and beginning of Hadley cell process.
Narrow band of clouds can often be seen over this region - especially over oceans.
Westerlies - winds flow basically from west to east around the world in the
latitudinal zone between 30 - 60 degrees both north and south of the equator.
They are a result of the winds from the poleward sides of the STHs.
Polar Highs - persistent features above polar ice caps.
Polar Easterlies - winds that move generally from east to west, are cold and dry,
but quite variable and lie between polar highs and about 60 degree of latitude.
Subpolar lows - low pressure at about 50 to 60 degrees latitude. Often contains
the polar front. It is a meeting ground and zone of conflict between the cold winds
of the polar easterlies and the relatively warmer westerlies. Characterized by
rising air, widespread cloudiness, precipitation, and generally unsettled or stormy
weather conditions.
Atmospheric Moisture
Moisture is the 4th element of weather and climate. (temperature, pressure, wind,
moisture)
Water Vapor is a colorless, odorless, tasteless, invisible gas that mixes freely with
the other gases of the atmosphere. Water vapor may condense to form haze, fog,
cloud, rain, sleet, hail, or snow, producing a skyscape that is both visible and tangible.
More than half of all water vapor is found within 1 mile of Earth’s surface, and only a
tiny fraction exists above 4 miles. The hydrologic cycle is the unending circulation of
our planet’s water supply (see page 140).
Conversion of moisture from liquid to gas is called evaporation. It can take place at
any temperature, but higher temperatures cause molecules to move faster and collide
more forcefully. The impact of such collisions near the water surface may provide
sufficient energy to allow molecules to break free from the water and enter the air.
Evaporation effect can be seen in boiling water.
Amount and rate of evaporation on water contingent on:
Temperature of both air and water, most prevalent along subtropical high regions
Amount of water vapor already in the air
Whether the air is still or moving.
Humidity is the amount of water vapor in the air. Most familiar humidity measure is
that of relative humidity. This describes how close the air is to saturation with water
vapor. It is a ratio (expressed as a percentage) that compares the actual amount of
water vapor in the air to the water vapor “Capacity” of the air.
Relative humidity= actual water vapor in air/capacity * 100
Air can be brought to saturation (100% relative humidity) simply through a decrease
in temperature.
The temperature at which saturation is reached is the dew point. The dew
point varies with the moisture content in the air. There is an inverse
relationship between temperature and relative humidity.
Sensible temperature - this is the temperature your skin feels based upon
temperature and relative humidity. On a warm humid day perspiration
does not evaporate very fast off our skin, so temperature feels warmer than
it really is. On a warm dry day moisture evaporates quickly off our skin so
it seems cooler than it really is. Cold, damp weather acts as conduit of
body heat, while cold dry weather we can conserve our body heat. See
heat index for comparisons. (p.145)
Condensation is the opposite of evaporation. It is the process whereby water vapor is
converted to liquid water. It usually is the result of air being cooled to a temperature
below the dew point. It is necessary to have a surface on which condensation can take
place. If no such surface is available, no condensation occurs. Particulates in atmosphere
serve as starting vehicles for condensation nuclei. Air is supercooled if air is below
freezing, but fine droplets have not yet condensed or frozen. Supercooled air promote the
growth of ice particles in cold clouds by freezing around them.
What produces Precipitation?
Result of ice-crystal formation - at high altitude ice crystal and water droplets start to
coalesce.
Collision/coalescence - as large droplet fall they hit other small droplets and join
together. Occurrence more common in the tropics and middle latitudes.
Forms of Precipitation:
Rain - result of condensation and precipitation in ascending air. There can be a
distinction between rain, showers and drizzle.
Snow - water vapor is directly converted to ice without an intermediate liquid stage.
Sleet - small raindrops that freeze during descent and reach the ground as small
pellets of ice.
Glaze - rain that turns to ice the instant it collides with a solid object.
Hail - supercooled water droplets that fall and rise with updrafts and downdrafts and
pick up ice crystals along the way.
Atmospheric lifting and precipitation:
Convective lifting results in convective precipitation that is typically showery with
large raindrops falling fast and furiously but only for a short duration.
Orographic lifting results in orographic precipitation and rain shadows.
Frontal lifting results in frontal precipitation
Convergent lifting and precipitation
Global Distribution of Rainfall:
Dry at subtropical latitudes
Dry in midlatitudes where distant from moist air masses.
Dry in high latitudes because water surfaces are scarce and cold, so little opportunity
exists for moisture to evaporate into the air.
Atmospheric Flows and Disturbances
AIR MASSES - the troposphere is composed of many large, variable parcels of air that
are distinct from one another. Such parcels are called air masses.
Characteristics of air mass:
Large, more than 1000 miles across and several miles deep.
Uniform properties in the horizontal dimension. Temperature, humidity and stability
are relatively homogenous at horizontal level.
Recognizable entity and travel as one. Distinct from the surrounding air.
Origin:
 Air mass develops its characteristics by remaining over a uniform land or
sea surface long enough to acquire the temp/humidity/stability
characteristics of the surface - it becomes stagnate.
 To become stagnate, it needs to be stable air (high pressure). Most air
masses are associated with anticyclonic conditions.
 Source region must be extensive, physically uniform, and associated with
air that is stationary or anticyclonic. Ideal source regions are ocean
surfaces and extensive flat land areas that have a uniform covering of
snow, forest, or desert.
 Broad view is that air masses can originate almost anywhere in the low or
high altitudes, but rarely in the mid-latitudes. Source regions are large land
and water bodies.
 Air masses are classified according to their source region: Key words are:
1. Arctic/Antarctic - A
2. Continental polar - cP
3. Maritime polar - mP
4. Continental tropical - cT
5. Maritime tropical - mT
6. Equatorial - E
An air mass modifies the weather of the regions into which it moves. It takes
source-region characteristics into other regions.
North American continent is a prominent area of air-mass interaction. Lack of mts.
Trending east to west permits polar air to sweep southward and tropical air to flow
northward unhindered by terrain, particularly over the eastern two-thirds of continent.
Rockies can impede Pacific air masses and modify their characteristics.
Middle Latitudes are the principal battleground of tropospheric phenomena where polar
and tropical air masses meet and come into conflict. It is where most fronts occur, and
where weather is most dynamic and changeable from season to season and from day to
day.
Midlatitude Cyclones (low pressure system)
Four varieties of motion occur in a typical midlatitude cyclone
entire system moves east to west in general flow of westerlies.
cyclonic counterclockwise airflow
cold front advances
warm front advances.
In each hemisphere, these migratory disturbances are more numerous, better
developed, and faster moving in winter than in summer. Also, they follow a much
more equatorward tracks in winter.
Midlatitude anticyclone (high pressure)
Has air converging into it from above, subsiding, and diverging at the surface,
clockwise in the Northern Hemisphere and counterclockwise in the Southern.
Anticyclones contain no fronts, the weather is clear and dry with little or no
opportunity for cloud formation.