Atmospheric Circulation and Weather
I.
Composition and Properties of the Atmosphere
A. Lower atmosphere nearly homogenous mixture of nitrogen 78.1% and oxygen
20.9% (argon and co2 = 1%).
B. Density of air is influenced by its temperature and water content.
1. Warm air is less dense than cold air, (because of molecular
movement) it occupies more space.
2. Humid air is less dense than dry air at the same temperature (water
vapor less dense than nitrogen and oxygen molecules it displaces).
C. Atmospheric Pressure
1. Weight of the atmosphere – decreases as you ascend into the
atmosphere. Most pressure is at the surface.
2. Air near the surface is packed densely by its own weight – when this
air is lifted it will expand – expanding air becomes cooler because
going from high to low pressure.
3. Opposite is also true, when air is compressed it becomes warmer –
descending air warms because it is compressed by higher
atmospheric pressure near earth’s surface.
4. Warm air can hold more water vapor than cold air. When air is rising
expanding and cooling water vapor condenses into clouds. The
cooler air can no longer hold as much water vapor.
II.
Atmospheric Circulation
Weather = the state of the atmosphere at a specific place and time.
Climate = a long term average of weather in an area.
Both are influenced by the amount of solar radiation the area receives.
Air does not remain stationary over earth, but flows in large patterns shaped or caused by
unequal solar heating of the earth with latitude and season, and by the rotation of the
earth. The mass movement of air is known as wind.
A.
Uneven solar heating and latitude
1. Poles have less solar heating because
a. sun reaches poles at greater angle energy spread over greater
area. (lower angle of incidence).
2. Equator has more solar heating because
a. equator has high angle of incidence
b. sunlight subjected to less filtration and surface reflection
B.
Uneven solar heating and the seasons:
Mid latitudes receive about 3 times as much solar energy per-day in June than
December – reason is the 23 1/2% tilt of the earth, its orbital inclination. The inclination
of the axis causes the change of seasons.
C.
Uneven solar heating and atmospheric circulation
1. most of suns heat is at the equator.
2. We know that warm air rises and cold air sinks (convection in room
example).
3. The ideal model of atmospheric circulation would be – air heated at tropics
expands, rises, and goes poleward where it loses heat and sinks to the
surface and moves equatorward. This large circuit of air is called an
atmospheric circulation cell. But this is not what happens. Global
circulation is governed by both uneven solar heating and the rotation of the
earth.
III.
D.
The affects of the rotating earth on atmospheric circulation
1. The eastward rotation of earth deflects moving air and water away from its
initial course. The deflection is to the right in the northern hemisphere. The
apparent deflection is called coriolis effect.
2. With a rotating earth, wind and water get deflected. Because of this coriolis
effect a more complex circulation model is needed to explain atmospheric
circulation.
3. In this new model of atmospheric circulation, air still warms and expands at
the equator. This rising air loses moisture and is now drier and more dense.
The air will travel poleward and will become dense enough to fall back
toward the surface at about 30 degrees north and 30 degrees south latitude.
Most of this air near the surface will travel back towards the equator and it is
then deflected to the right coming from the north east (NE Trade Winds or
Easterlies). These winds warm as they approach the equator, evaporating
water and becoming less dense, completing the circuit. This large
atmospheric circulation cell is called the Hadley Cell.
4. A more complex cell – Ferrell Cell operates in the mid latitudes. Air
descending at 30 degrees N/S going poleward is deflected to the right
forming the westerlies.
5. The polar easterlies are formed from air returning from the poles. This cold
air is deflected westward. This circuit is called the Polar Cell. Where the
polar cell meets the Ferrell Cell is called polar front – which creates most
weather in mid – latitudes.
E.
Wind Patterns
1. At the bands between circulation cells air is moving vertically, and the
surface winds are weak and erratic.
2. At the equator where the two Hadley cells converge is called the doldrums.
Also called Intertropical Convergence Zone (ITCZ).
3. Air moving from equator loses precipitation and cools. This cooler dry air
moves poleward but is dense enough to descend around 30 degrees
latitude between Hadley and Ferrell Cells. This area of high pressure and
little surface winds because of vertical movement is called the horse
latitudes.
4. Sailors are most interested in surface winds between cells, surface winds of
Hadley cells are the trade winds or easterlies 15 degrees N and S. Surface
winds of Ferrel Cells called westerlies 45 degrees N and S.
Modifications of circulation due to unequal distribution of land.
A. Local variations occur when land heats up more in summer causing low
pressure and cools more in winter creating high pressure.
1. Land and Sea breezes (small daily monsoons)
a. coastal areas during the day in warm months, land is heated more
than water. This warm low pressure air rises and the cooler air
above the sea replaces it creating a sea – breeze.
b. At night the land cools more rapidly than the water creating high
pressure over land. Air goes from high to low pressure forming
land breeze.
B.
IV.
Monsoons
1. Similar to land and sea breezes but are seasonal and occur on a
large land mass like Asia.
2. Land heated in summer creates low pressure. Cooler air over ocean
(high pressure) moves in to replace the lifting low pressure air over
land (remember air goes from high to low pressure). The moist air
from the ocean warms over land rises and condenses to form
precipitation. The rainy seasons.
3. The opposite occurs in autumn, as the land cools more rapidly that
adjacent ocean. Air cools and sinks over land (high pressure), and
dry surface winds move seaward. The dry season. Monsoon = is a
pattern of wind circulation that changes with the seasons.
Storms
A. Storms form between or within air masses. Air Mass = large body of air with
nearly uniform temperature, humidity, and therefore density throughout. These
are determined by the land which the air passes over.
1. These air masses move within or between circulation cells.
2. Different air masses do not mix well because of different densities.
3. The lighter air mass is wedged upward causing it to expand, cool, and
condense, which contributes to the turbulence between boundaries.
4. The boundary between air masses of different density is called a
front.
B. Two main types of storms – Extratropical Cyclone and tropical cyclone.
Cyclone = huge rotating mass of low – pressure air in which winds converge and
ascend.
1. Extratropical cyclones = form at the boundary between each
hemispheres polar cell and its Ferrel cell – the polar front. (Occur
mainly in winter where temperature and density differences across
front are most pronounced). Principle cause of weather in mid –
latitudes regions.
2. Tropical Cyclones
These form from disturbances within one air mass. These are great
masses of warm, humid rotating air.
a. Large ones called hurricanes (winds of 74 mph)
1. typhoons in Pacific, Tropical cyclone in Indian and willi willis in Australian waters.
b. Winds less than 74 mph called tropical storms and tropical
depressions.
c. Develops from a small tropical depression which forms over a
large warm air mass (Africa). The air mass goes over ocean
waters that must be 79 degrees F or higher and warm humid air
is forced up. The storm begins to develop.
d. The energy is from latent heat of evaporation.