Wind
Wind can tell us many things about weather conditions. A person
knowledgeable
about local weather can take the temperature and dew point,
wind data and
altimeter setting and make a pretty good estimate of what the
present weather
is. He can throw away all but the wind and still give you an
approximate weather
outlook. There is a simple key to understanding wind:
Variations in pressure
from place to place create wind, because air tends to
flow from high pressure to
low, as water flows from high ground to low. This
phenomenon has led to the
creation of a model to explain worldwide air
circulation. Because cooler air is
denser than warmer air, its pressure is
higher. As air at the equator warms and
rises, the pressure in the area
lowers. Heavier, cooler air tends to flow toward
the low, and as it does, it
causes the warmer air to flow upward and poleward,
where it cools and
develops higher pressure. This air, in turn, will then tend
to flow back
toward the equator. The same principle of circulation--from areas
of high
pressure to areas of low--also governs the circulation around the high-
and
low-pressure areas that move across our country. Isobars are lines drawn on
a
map to link points of equal atmospheric pressure. The textbook rule says
that
wind flows parallel to the isobars, but this appears to conflict with
the basic
principle of flow. If air flows from high pressure to low,
shouldn't its
movement be perpendicular to the isobars? The answer is that
the rotation of the
earth deflects the wind flow about 90 degrees, causing
the air to circulate
around the pressure systems instead of directly from one
system to another. In
the Northern Hemisphere, the flow is clockwise around
high-pressure areas and
counterclockwise around lows. This fact gives us a
start on wind-based weather
information. If you stand with your back to the
wind, low pressure will be to
your left. If there's a strong crosswind
component aloft, worse weather is ahead
if the wind is from your left and
better weather if the wind is from your right.
Knowing these basics of wind
flow will never take the place of a good weather
map and the information we
get from the National Weather Service, but it can be
helpful when the
forecasts are all going sour. The strength of wind flow is
governed by how
much the barometric pressure changes within a given distance. As
they tell
you in basic meteorology, the closer the isobars are to each other,
the
stronger the wind. Strong winds mean the highs are very high and the lows
are
very low. Air assumes the properties of the area over which it flows:
Warm
air comes form the south, cold air from the north. Moist air comes from
over
water, dry air from over large land masses. The vigor with which the
wind blows
is an indication of how much moisture, or how much dry air, is
overspreading the
area. Winds in the middle and high levels tell the
meteorologist a lot, too.
Wind speeds around highs differ from those around
lows. A high is usually large
in area and the pressure change near its center
tends to be gradual. Also,
because of friction effects, the surface winds
tend to flow clockwise and
outward (about 30 degrees) around a high. The wind
is thus light and the weather
usually good near its center. The flow around a
low is counterclockwise and
inward, so the wind tends to accelerate nearer
the center, which is usually
small compared to the center of a high. The
strength and direction of wind flow
also provide excellent clues to
approaching weather. If a cold front passes your
location, a good
northwesterly flow of cold, dry air settling in behind it is an
indication
that the high behind the front is of respectable size and strength.
If the
flow behind the front is weak, the high probably doesn't amount to much,
and
the odds of having a prolonged period of good weather following the front
are
slim. If the weatherman mentions an approaching low-pressure system but
the
winds outside are light, the low is probably far away or weak. If the
wind is
freshening rapidly, the low is probably strong. The stronger the
wind
circulating around the low, the more severe will be any storm associated
with
it. Furthermore, as lows deepen the circulation around them increases
while the
movement of the low across the country slows. Severe blows occur
when the upper
winds are cold and the lower winds are warm. This creates
instability and ideal
conditions for vertical development. The reverse
situation causes long periods
of relatively stable bad weather in the
wintertime: Warm air running over cold
air is the cause of snow and ice
storms as well as of foggy days. Wind often
tips us off about what is going
to happen to winter weather. Again, if the
northwesterly flow following a
cold frontal passage is strong, the usual pattern
of weather behind such a
front is likely to hold. If the northwest wind is weak,
it means warm, moist
air will probably soon start overspreading the cold air
near the surface. If,
after the front passes, the wind shifts to the northeast,
watch out. The flow
ahead of a cold front is usually from the southwest, so a
surface-wind shift
to the northeast as the front passes your area means that
nothing much is
working to push out that southwesterly flow. The situation is
likely to
become one of cold, northeasterly breezes at the surface, warm,
southwesterly
breezes aloft, and poor weather until the basic pattern changes.
When the
high behind the front lacks the strength to push any farther, the
front
becomes stationary and the weather goes bad. Wind carries many
messages. These
vary by area; a northeast wind can mean one thing in one part
of the country and
something else in another. The messages can change with
the seasons, but they
always mean something. Many things other than pressure
systems affect the wind.
The relationship between wind and surface terrain is
important and logical. Wind
moving up a mountain slope is an updraft; wind
moving down a slope is a
downdraft. Mountains make and modify the general
winds, too. As air moves
through a mountain pass, its velocity can increase
to much more than the average
wind speed in the area because of a venturi
effect. The friction effect of rough
terrain can also alter the general wind
flow over a mountainous area. Wind is
formed in and by mountains even when
there is not much general air-mass flow.
During the day, heated air flows up
the mountainsides, creating updrafts at the
ridges and lifting moisture to
build the cumulus clouds that decorate mountain
ranges on summer days.
Thunderstorms frequently develop along the ridges on warm
and muggy days. In
the evening, when the heating ceases, the reverse happens and
cool air flows
down the mountain slopes. The proximity of water to land can
cause wind. Sea
breezes develop and flow from water to land during the day as
the land warms.
Cumulus clouds are likely to form over land, and the air beneath
them will be
bumpy even though the air over the adjacent water is smooth. In the
evening,
the flow will be likely to reverse itself and move from land to water.
This
is an example of just basic circulation. Plain dry ground also has an
effect
on wind. Surface friction can cause the wind to change direction, usually
by
about 30 degrees when measured at 2,000 feet and at the surface. In
the
Northern Hemisphere, the change is counterclockwise as you descend, so
a
southwesterly wind at 2,000 feet would change to a more southerly wind at
the
ground. Surface friction also causes a decrease in wind speed; this
explains why
the wind tends to blow hardest in the plains. There's less
friction as the wind
moves across the flat, almost treeless surface, so the
velocity is nearly as
strong on the ground as it is a few thousand feet high.
Even the time of day
does things to wind, and this is at least in part
related to friction effects.
As previously mentioned, heating on a summer day
often causes cumulus clouds and
an increase in surface wind. After sunset,
however, the cooling of the surface
results in increased stability at the
lower levels; the clouds dissipate and the
wind dies. Radiational cooling
maximizes the effect of friction, so that even if
pressure systems are the
cause of a daytime wind, the surface wind can become
calm at night. ? “
Surface winds accelerate and undergo horizontal divergence
when blowing from
a rough to a smooth surface. Surface winds slow and undergo
horizontal
convergence when blowing from a smooth to a rough surface.
Horizontal
divergence causes air to descend and horizontal convergence causes
air to
rise” (online weather studies, 162). ? “ A mountain range that
intercepts a
flow of humid air may induce a cloudy, rainy climate on its
windward slope and a
dry climate on its leeward slopes. The dry region may
extend some distance
downwind of the mountain range” (Online Weather Studies,
p.115). ? “
Mountain-wave clouds form when a mountain range deflects the
horizontal wind
into a wave-like pattern that stretches downwind. Clouds
develop on the wave
crests where air ascends and is cooled by expansion.
Clouds are absent in the
wave troughs where air descends and is
compressionally warmed”(online weather
studies,
p.129).
Bibliography
“Forecast U.S. Winds” The Weather
Channel. May 2000. http://www.weather.com/
Null, Jan. “Winds of the Wind.”
Weatherwise. May/June 2000. http://www.weatherwise.org/00mj.null.html
Ruzic,
David N. “Lecture-Discussion #8 on Wind Energy.” Board of Trustees
University
of Illinois. 1998. http://starfire.ne.uiuc.edu/ne201/course/topics/wind/
(14
May. 2000). Tran, Diem, and Bernard A. Blair. Online Weather Studies.
Boston:
American Meteorology Society,
1999.