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Introduction
to
Weather
• Meteorology: the study of the physics,
chemistry, and dynamics (movement) of
the Earth’s atmosphere
– Atmosphere: Envelope of gases bound to
the Earth by gravity
– Weather: State of the atmosphere at a
given time and place; constantly changing
• Weather is described by variables such as:
–
–
–
–
–
Temperature
Pressure
Wind Speed and Direction
Precipitation
Cloud Cover
• Climatology: study of climate types and
patterns and the causes of those
patterns
– Climate: sum of all statistical weather info
that helps describe a place or region
•
•
•
•
Slowly - varying
Averages (monthly, yearly)
Ranges (largest value - smallest value)
Extremes (maximum, minimum)
Climograph: graph showing how average monthly
temperature and precipitation values change
throughout the year at a particular location
90
8
80
7
6
60
5
o
Temperature ( F)
70
50
4
40
3
30
Precipitation (inches)
Climograph for St. Louis, MO
2
20
10
1
0
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Climate data source: www.worldclimate.com
Making climographs and analyzing data: 3.5.3, 4.5.4, 6.2.1, 6.2.2, 6.2.6, 6.5.4
Weather or Climate???
• The average high temperature for the month of
July in Chicago is 84oF.
• Cumulus clouds presently cover the entire sky.
• Snow is falling at a rate of 1 inch per hour.
• The summers here are warm and humid.
• At 3:00 p.m, winds were blowing from the NW at
10 mph.
• Total precipitation at O’Hare Airport for 2003
was 32.02 inches, which is 4.25 inches below
normal.
• Weather vs. Climate: 3.6.5
Observing the Atmosphere
• Temperature: measure of the
motion of the molecules in a
substance
– Hot air: molecules moving
more quickly
– Cold air: molecules moving
less quickly
– Measured using a
thermometer
– Always measured in shady
conditions
– Units: oC, oF
Observing the Atmosphere
• Air Pressure: force per area exerted by
the mass of air above a point
• Measured in:
– Inches of mercury (in. Hg)
– Millibars (mb)
• Average sea-level pressure = 1013.25 mb
or 29.92 in. Hg
• Measured using a barometer
Mercury Barometer
Aneroid Barometer
Observing the Atmosphere
• Wind Speed: How fast is the air moving?
• Measured in:
– Miles per hour (mph)
– Nautical miles per hour (knots)
– 1 knot = 1.15 mph
• Wind direction
– The direction from which the wind is blowing
• A southerly wind is coming from the south.
Observing the Atmosphere
• An anemometer (left) measures
wind speed.
• A wind vane (right) measures wind
direction.
Observing the Atmosphere
• Atmospheric Moisture
– Dewpoint: the temperature (oC, oF) to which
air must be cooled for saturation (and
condensation) to occur
– Air saturation: when the amount of water
vapor in the atmosphere is at its max
possible at the existing temp and pressure.
– Condensation: process by which water
vapor (gas) becomes liquid water
• Process by which clouds form
Observing the Atmosphere
• Precipitation
– Rainfall amount
• Rain gauge
– Snowfall amount
• Yardstick
– Measured in inches,
centimeters, or
millimeters
Observing the Atmosphere
• Radiosonde: balloonborne instrument that
measures upper-air
pressure, temperature,
and moisture
• Data radioed back to
surface station
• Upper-air data
collected 2x per day
Observing the Atmosphere
• Radar: “Radio Detection and Ranging”
• Radar sends out microwave energy pulses
• Energy is reflected by particles in atmosphere
(precipitation)
• Radar dish collects reflected energy and
graphically depicts the location and intensity
of precipitation
• Doppler radar can sense movement of
precipitation, which helps find rotation in
severe thunderstorms
Doppler radar tower
The radar dish is enclosed within
the radome, which protects the
dish from adverse weather
conditions.
How radar works
The larger the reflectivity value (red, orange), the higher
the rainfall and/or hail rate.
Observing the Atmosphere:
Applicable Indiana Standards
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Scientific Inquiry: 3.1.2, 3.1.3, 3.1.4, 4.1.2
Technology and Science: 4.1.5
Manipulation and Observation: 3.2.3, 5.2.4
Communication Skills: 6.2.6
Earth and Processes that Shape it: 3.3.5
Numbers: 3.5.1, 3.5.2, 5.5.1
Shapes and Symbolic Relationships: 3.5.3,
4.5.4, 6.5.4
• Constancy and Change: 3.6.4, 5.6.4
Composition of the Atmosphere
Composition of the Atmosphere
* Troposphere
* 0 - 11 km
* Weather
* Temp decreases
as height increases
* Stratosphere
* Temp increases as
height increases
* Ozone: absorbs
harmful ultraviolet (UV)
radiation from Sun
Composition of the Atmosphere
Force of gravity: 5.3.6,
5.3.13
Composition of the Atmosphere
Earth – Sun Relationships
• The Earth revolves in its orbit around the Sun
every 365 days (1 year).
• The Earth rotates on its axis every 24 hours
(1 day).
• The Earth’s axis is tilted 23.5o from the vertical.
• The seasons are due to the tilt of the Earth’s
axis and the spherical shape of the Earth’s
surface.
• Revolution, Rotation, Tilt, and Seasons: 4.3.8, 6.3.3, 6.3.5
During the summer, the Northern Hemisphere is tilted
towards the Sun. Here, locations receive the Sun’s most
direct rays, and have longer periods of daylight hours.
During the winter, the Northern Hemisphere is tilted away
from the Sun. Periods of daylight are shorter, and the
Sun’s rays are less direct.
During the vernal (spring) and autumnal (fall) equinoxes,
neither hemisphere is tilted towards or away from the Sun.
On these dates, every location on Earth receives 12 hours
of daylight and 12 hours of darkness.
Earth’s spherical shape: 5.3.7
What happens to sunlight when it enters the atmosphere?
*Note that the atmosphere itself does not absorb a large
percentage of incoming solar radiation (sunlight).
How does the Earth’s atmosphere warm?
About 50% of the incoming solar radiation is absorbed
by the Earth’s surface. Most of the molecules in the
Earth’s atmosphere do not absorb visible or shortwave
radiation.
The Earth’s surface warms, and emits energy in the
form of infrared or longwave radiation, which is invisible
to the human eye.
Certain molecules in the atmosphere, called
greenhouse gases, do absorb the Earth’s infrared
radiation. This absorption warms the atmosphere.
Also, the greenhouse gas molecules emit their own
infrared radiation, which is absorbed by other
molecules, and the atmosphere warms even more!
Greenhouse Gases
-Carbon Dioxide
-Water Vapor
-Methane
-Nitrous Oxide
All of these gases are occur naturally in the
atmosphere, but human activities, such as the burning
of fossil fuels (coal, oil, and natural gas), have caused
atmospheric concentrations of these gases to
increase dramatically since around 1850.
Human effect on atmosphere: 3.3.7, 6.3.13
Annual cycle of the amount of sunlight received at the surface.
Global Heat Transfer
• The tropics receive more solar energy
than the poles every year.
• How come the tropics aren’t getting
progressively warmer and the poles
aren’t getting progressively colder?
• Answer: Redistribution of heat by
atmospheric circulations and ocean
currents!
Annual Cycle of Air Temperature
What determines temperature?
• Latitude: locations at lower latitudes typically
experience higher temps year-round than
higher latitude locations, because the lower
latitudes receive more solar energy
• Proximity to water: locations near water,
especially a cool ocean current, have smaller
annual temp ranges than landlocked
locations
• Elevation: locations at higher elevations
usually have cooler conditions than locations
at lower elevations
Latitude
Water vs.
Land
Effects of oceans on climate: 4.3.4, 6.3.11
Elevation
The Global Water Cycle
• Evaporation: liquid water becomes gas (vapor)
and enters the atmosphere
• Transpiration: water vapor is released into the
atmosphere by trees and plants
• Condensation: water vapor in atmosphere
becomes tiny liquid droplets
• Precipitation: liquid or frozen particles fall to the
ground and return moisture to the surface
• Runoff: excess precipitation flows downhill over
surface towards stream, river, or lake
• Percolation or Infiltration: movement of water
through underlying rock and soil.
Clouds
• On Earth, water naturally occurs in all 3
phases or states of matter (gas, liquid, solid)
• Clouds are composed of tiny liquid water
droplets or tiny ice crystals.
– Clouds are not made of water vapor (Otherwise,
we wouldn’t be able to see them!)
• In nature, clouds form when the temperature
of air is lowered to its dewpoint temperature.
• Water phases and water cycle: 5.3.4, 5.3.8, 6.3.9
• Clouds: 5.3.5
Cloud Types
• Three main cloud types:
– Cirrus: High, thin, wispy clouds made of ice crystals.
Usually occur above 20,000 ft (6000 m)
– Stratus: Low, uniform, horizontal grayish clouds with
bases below 6,500 ft (2000 m)
• Fog: Stratus cloud in contact with the ground.
• Nimbostratus: Dark gray stratus clouds from which
precipitation is falling
– Cumulus: Relatively small, puffy clouds that resemble
cotton balls and occur on fair weather days
– Note: All cloud photos on following pages can be found online at
www.cloudman.com
Cumulus
Cirrus
Stratus
Fog
Cloud Types
• Category subdivisions:
– Cirrocumulus: high, small, white, rounded patches
arranged in a wavelike or mackerel pattern; made of ice
crystals
– Cirrostratus: widespread, high cloud sheet made of ice
crystals
– Altocumulus: roll-like patches or puffs that often form in
waves or parallel bands; found between 6,500 ft and
23,000 ft; made of ice crystals and liquid water droplets
– Altostratus: uniformly gray layers that completely or
partially cover the sky; found between 6,500 ft and
23,000 ft; made of ice crystals and liquid water droplets
Cirrocumulus
Cirrostratus
Altocumulus
Altostratus
Cumulonimbus (Thunderstorm) Cloud
-Cumulus – type
cloud extending from
near the surface to
the top of the
troposphere.
-Produces heavy rain,
strong surface winds,
hail, lightning, and
occasionally
tornadoes.
-Composed of ice
crystals (near top)
and liquid water
droplets
Air Pressure and Winds
Air pressure is the weight of the air above you.
Air pressure decreases with altitude as you go
up, because less air remains above you.
Air Pressure and Winds
• Horizontal (east/west, north/south)
differences in air pressure cause the air
to move  Wind!
• The force that causes the air to move is
called the pressure gradient force.
• The pressure gradient force always
moves air from higher pressure towards
lower pressure, in an attempt to
equalize pressure everywhere.
• Air pressure and wind: 4.3.2
Forces and motion: 5.3.11
The greater
the pressure
difference
between two
locations, the
faster the
wind will blow
from high
pressure to
low pressure.
Basic Weather Map Features
• Isobars: lines that connect values of equal air
pressure, usually drawn in millibars (mb)
• High Pressure Centers (H)
• Low Pressure Centers (L)
• Front: boundary between two different air
masses
– Cold front: cold air mass is advancing
– Warm front: warm air mass is advancing
– Stationary front: neither air mass is moving
In N. Hemisphere:
Air flows
counterclockwise
and into a low
pressure center.
Air flows clockwise
and out of a high
pressure center.
Mid-latitude
Cyclone
Locations under the influence of a mid-latitude cyclone (low
pressure system) experience more clouds and precipitation
than locations under the influence of a high pressure center.
At the intersection of air masses (fronts) warm air is forced
to rise, which allows it to cool and form clouds.
Cold front
cross – section
What to look for when a cold front passes by:
- Surface pressure reaches a minimum, then begins to rise
- Wind direction shifts from southerly to westerly or
northwesterly
- Surface air temperatures decrease
- Narrow band of precipitation along and just ahead of cold
front, sometimes consisting of heavy rain and thunderstorms
What to look for when a warm front passes by:
- Surface pressure decreases
- Wind direction shifts from easterly to southerly
- Surface air temperatures rise
- Wide band of light-to-moderate precipitation ahead of front
Warm front
cross – section
Predicting the Weather Based on Sky,
Wind, and Pressure Observations
• Weather will generally remain fair when:
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–
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Wind blows gently from W or NW
Barometer remains steady or rises
Cumulus clouds are in the summer afternoon sky
Morning fog breaks by noon (evidence of clear sky above)
• Expect precipitation when:
– Cirrus clouds thicken and are followed by lower clouds (altostratus,
then stratus or nimbostratus)  signals approach of warm front
– Puffy cumulus clouds begin to develop vertically
– Sky is dark and threatening to the west
– Southerly wind increases in speed with clouds moving from the
west  signals approach of cold front)
– Barometer falls steadily  signals approach of a low pressure
center or front
Predicting the Weather Based on Sky,
Wind, and Pressure Observations
• Weather will generally clear when:
– Barometer rises rapidly
– Wind shifts to the west
• Temperature will usually fall when:
– Wind blows from, or shifts to, N or NW
– Night sky is clear and wind is light
– Barometer rises steadily in winter
• Temperature will usually rise when:
– Wind is from the south, particularly with cloud
cover at night or a clear sky during the day
Wind Direction
Barometric Pressure
General Forecast
SW to NW
30.10 to 30.20
steady
Fair, little temp change for
1 – 2 days
SW to NW
30.10 to 30.20
rising rapidly
Fair, warmer weather and
rain within 2 days
SW to NW
30.20+
steady
Fair, little temp change
SW to NW
30.20+
falling slowly
Fair, slowly rising temps
for about 2 days
S to SE
30.10 to 30.20
falling slowly
Rain within 24 hours
S to SE
30.10 to 30.20
falling rapidly
Rain within 12 - 24 hrs,
wind speed will increase
SE to NE
30.10 to 30.20
falling slowly
Rain within 12 - 18 hrs,
wind speed will increase
SE to NE
30.10 to 30.20
falling rapidly
Rain within 12 hrs, wind
speed will increase
SE to NE
30.00 or below
falling slowly
Rain will continue 1 or
more days
Wind Direction
Barometric Pressure
General Forecast
SE to NE
30.00 or below
falling rapidly
Rain with high winds in few
hours; clearing within 36
hours - colder in winter
E to NE
30.10+
falling slowly
Summer, with light winds: rain
in 2 – 4 days; Winter: rain or
snow within 24 hrs
E to NE
30.10+
falling rapidly
Summer: probably rain in 12
– 24 hours
S to SW
30.00 or below
rising slowly
Clearing within a few hours.
Then fair for several days
S to E
29.80 or below
falling rapidly
Severe storm within few hrs;
then clearing within 24 hrs –
colder in winter
Becoming W
29.80 or below
rising rapidly
End of storm – clearing and
colder