Download Week 4

4-1
Stability of Air
However, even if the air is stable it may be forced
upwards by a variety of mechanisms.
What causes air to rise?
It may have to rise over a mountain—orographic
lifting.
If temperature gradient is large enough, air will
become unstable and vertical movement will occur.
Warm air may be forced over cold air in a weather
system—frontal wedging.
Three possible conditions can occur:
Convergence—air flowing together forcing air to
rise.
Localized Convective Lifting—unequal heating of the
surface leading to localized regions of warm and
bouyant air.
Winds
Coriolis Effect
In chapter 12 we studied the vertical motion of air
and the formation of clouds and precipitation.
On a nonrotating planet air would flow straight from
high pressure towards lower pressure regions.
What about horizontal motions of air—winds?
However, as soon as air starts to move the Earth’s
rotation modifies this motion.
Horizontal motion of air is caused by variations in
air pressure:
How will air flow, from high to low or low to high
pressure?
Are points on the Earth all moving at the same rate of
speed?
4-2
What happens to air which is moving poleward?
Cyclones and Anticyclones
You have probably all seen weather maps with high
and low pressure zones shown.
How will air flow around these?
Towards the equator?
Low pressure zones are called Cyclones.
High pressure zones are called Anticyclones.
How do high and low pressure zones develop?
How are they maintained?
How and where does this occur?
Global Circulation and Climate
Later we will discuss the global circulation pattern
which develops but first let us look at a more
localized example:
Is the Earth heated uniformly?
Which areas receive large amounts of energy?
Suppose we have a land region near water.
Which areas receive little energy?
Which is easier to heat up, land or water?
Thus, during the day:
As we have seen, winds are driven by such uneven
heating of the surface.
4-3
Influence of Continents
In our discussion so far we have ignored the fact that
the surface of the planet is not uniform.
In many areas of the southern hemisphere there are
relatively few land masses.
As we have seen, water has a much greater ability to
hold heat than land.
Thus land regions heat up (or cool down) quickly
relative to water.
Large land masses (e.g. Siberia) will cool down
substantially during winter:
Circulation pattern we have discussed comes closest
to explaining the motion here.
However, large land masses, as seen in the northern
hemisphere, modify the circulation pattern.
During the summer months the land warms up:
Circulation Patterns and Climate
During the summer the tropical region moves
somewhat up and over the summer hemisphere.
As we have seen, the circulation pattern results in:
The subtropical high and polar front also move
poleward.
Cloudy and very wet regions near the equator
(typically within 10° of the equator: the tropics).
During the winter the reverse occurs:
Desert regions around the subtropical high (20–30°
latitude).
Tropics move away from the winter hemisphere.
Stormy regions near the polar front (~60° latitude).
Subtropical high and polar front move towards the
equator.
As the sun moves north and south of the equator the
locations of these zones changes.
4-4
Mediterranian Climate (Dry-Summer Subtropics)
This type of climate occurs along the west coasts of
continents in regions at latitudes ~30–45° (just
poleward of the subtropical high deserts).
Marine West Coast
Situated poleward of the mediterranian climate.
e.g. us, much of northern Europe.
e.g. California, and as the name implies, regions
around the Mediterranian sea.
Again mild winters and summers due to winds off
the ocean.
Mild winters and summers. Why?
Too far poleward to come completely under the
influence of the subtropical high in summer.
During the summer the subtropical high moves over
these regions—
However, its influence is felt yielding a reduction in
rainfall during the summer.
During the winter these regions feel the
cyclone/anticyclone sytems of the middle latitudes.
Ample precipitation year round.
Middle Latitude Deserts and Steppes
Humid Continental Climate
As air is forced over mountains it expands, cools and
often drops much of its moisture. Examples?
This type of climate is seen in the central to eastern
U.S.
As air flows down leeward side of the mountain the
now dry air is compressed and heats up.
Summers and winters more severe than west coast.
Why?
==> region of low precipitation on leeward side.
Often referred to as rainshadow deserts.
Precipitation generally greater in summer than
winter:
Examples?
Less influenced by dry polar air from Canada in
summer, more by humid air off the Gulf of Mexico.