Meteorology
meteorology = study of atmosphere
1. weather = present state of atmosphere and describes
current conditions
a. caused by interaction of air, water, and solar energy
b. variation can take place over minutes, hours, days,
weeks, or months
2. climate = average of all weather
conditions of area over
long period of time
energy balance on Earth:
1. Earth radiates back to space about as much energy as it
receives over a year’s time
2. sun feels hotter in afternoon due to rays striking Earth more
directly
3. morning and evening rays strike Earth at lower angle and
energy is spread over larger area
4. solar radiation reaching poles is less intense
5. tropics and other places maintain fairly constant average
temperatures due to heat being redistributed around the
world
6. continual motion of air and water reallocate heat energy
among surface, oceans, and atmosphere
air mass = large body of air that has same properties as
region over which it develops
1. source region = area over which an air mass forms
2. originate in parts of world where winds are light
3. can be several thousand kilometers in diameter and
several kilometers deep
4. temperature determined by whether it comes from tropics
or polar regions
5. humidity determined by whether it comes from land or sea
6. classified according to source region
a. “c” = continental (dry)
b. “m” = maritime (moist)
c. “P” = polar (cool) high latitudes
d. “T” = tropical (warm) low latitudes
e. “A” = arctic
7. weather maps use two symbols together to describe
temperature and humidity
Ex: mT = warm moist
8. five major types affect weather in United States
a. mT = maritime tropical
1) forms over warm waters of Gulf of Mexico and
tropical North Atlantic Ocean
2) holds warm, moist air
3) comes into U.S. from Pacific and Atlantic oceans and
Gulf of Mexico
4) summer = hot, humid weather, thunderstorms,
hurricanes
5) winter = mild, often cloudy weather
b. mP = maritime polar
1) come from cold ocean water (North Pacific Ocean)
2) holds cold, moist air
3) forms over Pacific Ocean in winter and summer as well as
North Atlantic in summer
4) winter = rain and snow to Pacific Coast; summer = cool,
foggy weather
5) maritime polar Atlantic = form over North Atlantic
Ocean
a) winter = cold, cloudy weather and precipitation
b) summer = cool weather
with low clouds and fog
6) can produce heavy snow
and cold temperatures
c. cT = continental tropical
1) form over the deserts of south-western U.S. and Mexico
2) flow over North America only
in the summer
3) brings in clear, dry and very hot weather
4) produces a warm, dry climate
d. cP = continental polar (Canadian)
1) forms over interior of Canada and Alaska covered by ice
and snow
2) move southeastward across
Canada and into northern U.S.A.
3) brings in cold, dry air (summer)
4) during winter can produce
extremely cold temperatures
e. A = arctic air
1) basically same as continental polar air mass, but much
colder
2) brings most frigid outbreaks of winter
3) usually associated with very high pressure systems
4) develops over latitudes above 60oN in ice and snow
covered regions of Siberia and Arctic Basin
5) during winter this region receives almost no solar
radiation but continues to radiate heat out to space
causing it to be very cold area
9. air mass modification = occurs when air mass starts to gain
some of characteristics of new surface beneath it
a. usually an exchange of heat or moisture with surface
over which air mass travels
b. modified some as move away from source region
c. eventually modified to have same characteristics as new
surface it moves over
WEATHER SYSTEMS
Factors responsible for air movement:
1. Earth’s surface is curved rather than flat
a. causes uneven heating in areas around world
b. equator heated more than any other place causing air to
have low density and rise
c. poles are cooler so air is more dense and sinks and
moves along the surface
2. Coriolis effect = apparent deflection of objects (air) moving
across earth’s surface
a. caused by an apparent force that results from earth’s
eastward rotation (to the right)
b. objects in Northern Hemisphere moving south would be
turned west (deflected right)
air movement = northeast to southwest
c. objects in Southern Hemisphere are deflected to the left
air movement = southeast to
northwest
d. combines with global wind systems
that transport colder air to warmer
areas and warmer air to colder
areas
3. convection cells = looping pattern of flowing air
a. North and Southern hemisphere each have three
b. form by air rising and sinking at various latitudes
across surface
WIND SYSTEMS:
Victor Starr (American meteorologist) = designed model of major wind
systems which suggested that major wind patterns were controlled
by combination of unequal heating and Earth’s rotation
winds = formed by the movement of air from one place to another
1. caused by differences in temperature and pressure
2. named for direction from which they blow (flow)
Global winds:
1. doldrums = windless zone at equator (0o latitude)
a. receives much of sun’s energy
b. warm air rises producing low pressure area
c. cooler high pressure air warmed so
rapidly that winds formed don’t reach area
d. any winds that form are very weak
2. trade winds = winds that move from north or south toward equator
a. occur usually at 30oN and 30oS latitudes
1) air sink, warms, and moves toward equator in westerly direction
2) when air reaches equator, it will rise again and move back toward
30o latitude where it sinks and process starts over
3) Northeast trade winds = in Northern Hemisphere, blow from NE to
SW
4) Southeast trade winds = in Southern Hemisphere, blow from SE to
NW
c. 30oN and S latitudes are also called horse latitudes
1) at times winds become calm stranding ships for days or weeks
2) those carrying horses began throwing them
overboard when food supply ran out
d. convergence:
1) wind from both hemispheres move together
from two different directions
2) air converges, is forced upward, and creates
area of low pressure
3) ITCZ (intertropical convergenence zone) = large area near
equator where convergence occurs
3. prevailing westerlies
a. located between 30o to 60o latitudes (north and south)
1) circulation pattern opposite of trade winds
2) surface winds move toward poles in generally easterly
direction
b. responsible for movement of weather across United
States and Canada
c. Northern Hemisphere = blow SW to NE
Southern Hemisphere = blow NW to SE
d. are often particularly strong winds
4. polar easterlies = cold, dense, horizontal air currents
a. occurs at the 90o latitudes forcing air up at 60o latitudes
b. have weak winds but provide large amounts of energy to
westerlies
1) strongest where flow off Antarctica
2) polar front = form where easterlies meet westerlies producing
stormy area
3) characterized by cold air
c. causes changes in weather across United States
d. subpolar lows = belt of low air pressure at about 60oN and 60oS
latitude
e. Northern Hemisphere = blow
NE to SW
Southern Hemisphere= blow
SE to NW
jet streams = narrow belts of fast, high altitude, westerly winds
1. located in upper troposphere and lower stratosphere
2. strong, high speed, high pressure air
3. flow from west to east
4. two in each hemisphere
a. polar jet stream
1) separates prevailing westerlies from polar easterlies
(60o latitude)
2) altitude of 10-15 km, ~100km wide, 2-3 km thick
3) maximum wind speed = ~ 500 km/hr
4) winds change speed and direction
5) control path of storms and
effect airline routes
b. subtropical jet stream
1) separates trade winds from prevailing westerlies
(30o latitude)
2) warm equatorial air meets cooler air of middle latitudes
3) do not change much in speed or position
5. average wind speed range = 60 km/h up to 350 km/h
6. tend to wander up and down as they flow around Earth
7. wind speed, depth, positions in latitude and altitude change
from season to season or even day to day
8. large scale weather systems generally follow path of these
winds
LOCAL WINDS:
1. breezes = gentle winds that extend over distances of less
than 100 km
2. sea breezes = flow of air from the sea to the land
a. cool, dense air from over the water flows inland and
forces warm air over land up
b. forms 3-4 hours after sunrise and peaks in early
afternoon
c. caused by the water being cooler than land during the
day
d. surrounding area temperature
may drop by as much as
5oC within an hour
3. land breezes = flow of air from the land to the sea
a. cool air from land flows over the warmer water and pushes
warm air over sea up
b. occurs at night because land loses its heat to the
atmosphere while water retains its heat
c. starts to form in late evening and peaks near sunrise
4. valley breeze = flow of air up the
slopes of mountains
a. occurs during the day
b. caused by warm air from valleys
moving up slope
5. mountain breeze = flow of air from
mountain peaks to valley
a. occurs during the night
b. mountains cool more quickly
6. monsoon = seasonal winds
a. part of the year it blows from
the land to the ocean → winter
usually
b. part of the year it blows from the
ocean to the land → summer usually
c. summer monsoon causes rainy
season with warm temperatures and
huge amounts of rain
front = boundary formed between two air masses that have
collided
1. area where temperatures and humidities change
2. greater the difference between air masses, greater the
change in weather
3. can stretch over thousands of km across Earth’s surface
4. interaction between colliding air masses can cause dramatic
changes in weather
Four Types of Fronts
1. warm front = warm air is pushing ahead and
displacing colder air
a. lighter warm air slides over heavier cold air
b. presence of high cirrus clouds
c. stratus clouds form as front continues
to move into an area
d. may produce precipitation over a
large area (rain, snow, sleet, or freezing rain)
e. weather map symbol =
2. cold front = cold air is advancing and displacing warmer air
a. heavier, cold air is shoving under warm air, pushing it
upward rapidly
b. cumulus and cumulonimbus clouds form
c. produces rain and thunderstorms that can be violent
d. squall line = long line of heavy thunderstorms just ahead
of front
e. fair, cool weather usually follows
f. weather map symbol =
3. stationary front = two air masses collide and neither can
advance the other
a. front does not move; boundary between air masses stalls
b. pressure differences cause warm front or cold front to
stop moving forward
c. usually remains for several days
d. widespread clouds can form on both sides of frontal
boundary
e. light winds and precipitation may occur across entire
frontal region
f. weather map symbol =
4. occluded fronts = created when cold, warm, and cool air
come in conflict
a. two cooler air masses merge, forcing warm air between
them to rise
b. often called occlusions
c. two types:
1) cold occlusion = cold air is shoving under cool air at
Earth’s surface
2) warm occlusion = cool air rises over cold air at the
surface
d. usually see strong winds and heavy precipitation
e. weather map symbol =
5. polar front = boundary where cold polar air meets
warmer air of middle latitudes
a. circle between 40o and 60o latitudes N and S
b. winter - average position is across middle of U.S.
c. summer - north of Great Lakes
d. wave = bend formed in a cold or stationary front
Pressure Systems:
high pressure = form when air is cooled
1. air molecules more densely packed
2. result of cold, dense air that is sinking and spreads outward
from centers when reach Earth
3. warming of air decreases its relative humidity and water
vapor is evaporated
4. Northern Hemisphere → rotates clockwise; Southern
Hemisphere counterclockwise
5. usually means clear weather
6. weather map symbol = H
low pressure = form when air is warmed and rises
1. molecules farther apart → less dense → rises
2. rising air must be replaced by air from outside system, so net
flow is toward center and upward
3. forms along fronts where warm air meets cold air
4. Northern hemisphere counterclockwise; Southern
hemisphere clockwise
5. cause most of weather changes across United States
6. associated with clouds and precipitation
7. weather map symbol = L
8. wave cyclone = specific type of low pressure system
a. main producers of inclement weather in middle latitudes
b. usually begins along stationary front
c. part of front moves south as cold front and another part
moves north as warm front
d. sets up counterclockwise circulation
* Air moves from an area of high pressure to one of low
pressure.
GATHERING WEATHER DATA:
thermometer = used to measure temperature
1. bimetal thermometer = bar made of two strips of different
types of metals
a. curve when heat
b. straighten when cooled
2. thermograph = measures temperature change by recording
movement of bar
3. electrical thermometer
a. as temperature rises, electric current flow increases
b. current flow translated into temperature readings
barometer = instrument measures atmospheric pressure
1. mercurial barometer = uses mercury to determine
atmospheric pressure
a. pressure presses on liquid Hg in well at base
b. squeezes Hg up to certain height in tube
c. height varies with pressure
d. expressed by how high Hg rises in tube
e. standard atmospheric pressure = 760 mm
1) indicates average atmospheric pressure at sea level
2) 760 mm or 29.92 inches of Hg =
one atmosphere
f. millibars(mb)) - used on weather maps
1 mb = 0.001 standard
atmospheric pressure
2. aneroid barometer = works without liquid
a. most common type used today
b. seal metal container from which air has been removed
1) pressure decreases → sides bulge out
2) pressure increases → sides bend inward
c. shown by pointer on a scale
d. usually marked in mm (inches of Hg) or mb
e. altimeter = barometer used to measure altitude above sea
level
1) lowered pressure reading interpreted
as increased altitude
2) must be corrected for local weather
conditions to be accurate
anemometer = used to measure wind speed
1. expressed as meters/second, miles/hour, or knots
2. one knot = 1850 m/s
wind vane = used to determine the direction of the wind on
Earth’s surface
* points into the wind
hygrometers = used to measure relative humidity
1. psychrometer = works on principle that evaporation causes
cooling
a. wet bulb thermometer has a water soaked wick wrapped
around its bulb
b. other is a dry bulb thermometer
c. wet bulb usually shows a lower temperature
d. readings give signs of how dry the air is
e. difference between wet bulb and dry bulb is used to
determine RH by using a chart
f. when both thermometers read the same the air is
saturated
2. hair hygrometer = based on principle that human
hair stretches when it is humid
a. one end of a bundle of hair is fixed while
the other is attached to pointer
b. when air is humid → stretches; when air is dry →
shrinks back
Measuring Precipitation:
1. rain gauge = measure amount of rainfall
a. measure in inches
b. measure only precipitation that falls in one spot
c. represents depth of water that rain would leave
if it did not soak into ground, flow away, or evaporate
d. types vary
1) wide mouth funnel over cylindrical container
2) funnel and small divided bucket
3) water caught in large bucket and weighed continuously
2. measuring stick = measure amount of snowfall
a. snow is measure by both depth of accumulation and
water content
b. measured in inches and tenths of an inch
c. on average, 10 inches of snow equals 1 inch of rain
d. dry snows are deeper than equal weights of wet snow
Automated Surface Observing System (ASOS) = collects
multiple data at regular intervals
1. ceilometer = measures height of cloud layers and estimates
amount of sky covered by clouds
2. continuously record air pressure, temperature, wind speed
and direction, runway visibility, cloud ceiling, and precipitation
intensity
3. many units being used at major airports
Upper Level Data:
1. radiosonde = balloon borne package of sensors
for measuring at high altitudes
a. measure temperature, air pressure, and humidity
b. readings sent back by radio signal to ground station that
tracks movement of balloon
c. can determine wind speed and direction by tracking how
fast and in what direction balloon is moving
2. weather radar = used to locate areas of rain
a. waves are reflected from precipitation particles
b. screen shows location and shape of precipitation
areas
c. also gives a rough idea of how heavy precipitation is
3. Doppler radar = used to determine movement of weather
system
a. provides good estimates of wind speeds in
precipitation areas
b. show echoes with more detail
c. can also show wind direction
d. echoes and winds show up on color screen
1) different echo strengths or wind speeds show up as
different colors
2) makes it easier to spot strong thunderstorms and even
tornadoes
4. weather satellites = observes weather that is missing by
computer models and local weather
a. observations track clouds but not necessarily
precipitation
b. provides continuous pictures of clouds
c. make it possible to track lows, hurricanes, and
thunderstorms
d. can see nearly half of Earth’s surface
e. every 30 minutes its television cameras take infrared
as well as visible pictures
WEAHTER ANALYSIS/ FORECASTING
forecast = prediction of future weather conditions
- most are made for periods of a few hours up to 5 days
meteorologist = person who studies weather
1. make measurements of temperature, air pressure, winds,
humidity, and precipitation
2. gather data using radar, computers, and instruments
attached to balloons
3. interpret information from local weather observers, balloons,
satellites, and weather stations around world
weather map = collection of weather data from many
collecting stations
surface weather map = essential in following rapidly
changing weather or in compensating for poor computer
model forecasts
1. data is taken hourly at stations all over the world
2. United States has about 700 National Weather Service
stations
3. official U.S. weather maps are made every 3 hours
(made at the National Meteorological Center in Camp
Springs, Maryland)
station model = symbol that describes local weather of
collecting station
1. where possible direct readings are given
2. in other cases, codes are used
3. models and codes are determined
by World Meteorological Organization
4. show various measurements
a. wind direction and speed
1) shown by a line attached to circle
2) direction line points is from
which wind is blowing
3) small lines at end of the line are
used to indicated wind speed
b. cloud cover shown by circle
c. other data includes atmospheric pressure, temperature,
dew point, types of precipitation, types of clouds, and
change in barometric pressure
isopleths = lines that connect points of equal or constant values
1. can represent different weather variables
2. isotherm = lines that connect locations with same
temperature on weather map
- number on end of line indicates temperature at all points
along line
3. isobars = lines that connect locations with the same air
pressure on a weather map
a. number at end of line represents air pressure at each
location on line
b. can be given in millibars or inches of mercury
c. indicates the locations of highs or lows
d. if close together → great pressure difference over
small area → strong winds
e. if spread apart → less difference → gentle winds
TYPES OF FORECASTS
SHORT-TERM:
1. daily = predict weather conditions for 48 hour period
2. digital forecasting = relies on numerical data from computer
calculations
a. main method used by modern meteorologists
b. highly dependent on density of data available (more data,
more accurate forecast)
3. most accurate and detailed forecasts; can be made for 3 to 5
days
4. accuracy decreases with each additional day
LONG-TERM:
1. long-range = cover monthly and seasonal outlooks
2. extended = up to 6 to 10 days
3. analog forecasts = compares current weather patterns to
patterns that took place in past
a. uses assumption that weather systems behave in similar
fashion
b. also look for patterns that are analogous to current
patterns
c. to be accurate past event must be similar through all
levels of atmosphere and over large area
d. used for monthly or seasonal forecasts
4. computer model = copy of atmosphere in computer
a. contains data on wind, temperature, pressure, humidity,
clouds, and precipitation
b. can predict future weather patterns
5. now better than human forecasters at predicting movements
and strengths of highs and lows