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WEATHER
• Meteorology is the study of the
atmosphere and its phenomena
• Aristotle wrote a book on natural
philosophy (340 BC) entitled
“Meteorologica”
– Sum knowledge of weather/climate at
time
– Meteors were all things that fell from
the sky or were seen in the air
– “meteoros” : Greek word meaning “high
in air”
WEATHER VS. CLIMATE
Weather is NOW and is comprised of:
a) air temperature
b) air pressure
c) humidity
d) clouds
e) precipitation
f) visibility
g) wind
Climate represents long-term (e.g. 30 yr) averages
of weather.
• Invention of weather
instruments
– 1500’s Galileo invented
water thermometer
– 1643 Torricelli invented
mercury barometer
– 1667 Hooke invented
anemometer
– 1719 Fahrenheit
developed temp scale
based on boiling/freezing
water
– 1735 Hadley explained
how the earth’s rotation
influences winds in
tropics
– 1742 Celsius developed
the centigrade temp scale
• 1787 Charles discovered
relationship between temp
and volume of air
• 1835 Coriolis used math
to demonstrate the effect
that the earth’s rotation
has on atmosphere
• 1869 first isobars were
placed on map
• 1920 concepts of air
masses and weather
fronts were formulated in
Norway
• 1940’s upper air balloons
3-D view of atmosphere
• 1950’s high speed
computers
• 1960 Tiros 1 first weather
satellite
Air Pressure
= the weight of the air above you from
where you are to the top of the
atmosphere.
Air is compressible • Half of all air
molecules are below
18,000 feet of
altitude; 3/4 are
below 36,000 feet,
etc.
• At sea level air
exerts a force of
about 14 pounds per
square inch
Air pressure is measured with a
barometer
Air Pressure in the News -
Cold air has higher pressure
• Cold air is more dense, so it sinks
down and pushes harder.
– Barometer goes?
• Hot air expands, becomes less
dense, rises, and exerts less
pressure.
– Barometer goes?
Wind
= movement of air
• Wind goes from an area of high pressure
toward an area of lower pressure.
• Wind direction is described by where the air
comes from - a west wind blows from west
toward the east.
Local WindsWhich way would the wind blow?
What time of day is shown in this picture?
More Local WindsWhich way would the wind blow?
What time of day is shown in this picture?
How do meteorologists predict
wind direction?
Coriolis Effect
• Because the Earth rotates on its axis, things
moving from North to South or from South to
North are deflected.
• In the Northern Hemisphere, the deflection is to
the right.
• In the Southern Hemisphere, the deflection is to
the left.
Winds Around Pressure Systems
Let’s take one more look at that map.
A Low Pressure System Over Iceland
Why don’t high pressure
systems show up on satellite
photographs?
The summer Monsoon
• During July, August and half of September,
precipitation comes from thunderstorms
associated with the summer monsoon
• Convection occurs in moist southerly flow
• Because of its high intensity/short
duration, most summer rainfall runs off
quickly and/or evaporates
– Less replenishment of soil moisture and
recharging of aquifers than winter precip
Winds shift to a southerly or southeasterly
direction.
This produces a change in moisture. Together
with daytime heating causes the Arizona
monsoon.
The recipe for monsoon precipitation in Arizona
Middle-level flow
Westerlies
well to the
north
High pressure
belt moves
north of AZ,
bringing midlevel easterly
flow to AZ
Low-level surge of
warm, moist air
H
H
Convection breaks
out over AZ
Mid-level Gulf
moisture enters AZ
from the east
Physical Ingredients for a Monsoon
• Land-Sea differences: land and water have
different heating capacities & water can
store more heat because it is a fluid and
can mix heat down for future release
• Planetary rotation: introduces swirl and
much stronger winds
• Moisture: water vapor, collected over
oceans through evaporation, condenses
over land and marginal seas releasing vast
quantities of heat
• Orography: acts as elevated heat source
which intensifies flow & also conducts flow.
Winter Rains
Air circulation pattern is called the
Pacific/North America or PNA pattern. It is
characterized by:
• Warm, dry weather in the western half of the
country (dominated by a ridge of high pressure).
• Cold, wet weather in the eastern half of the
country (dominated by a trough of low pressure).
Trough:
Cold / Wet
East
Ridge:
Warm/Dry
West