Meteorology: The Basics
Objective
Students will understand the difference between
weather and climate.
Students will comprehend the different types of
air masses and fronts that influence national and
international weather
Students will know how to use clouds to predict
changing weather patterns.
Students will learn how to interpret weather
maps
Students will examine how weather data is
collected, transferred, and used to make
forecasts
Weather Versus Climate
Meteorology: The study of the atmosphere.
Weather: The current state of the atmosphere
in a given geographic area at a given time.
Climate: Long variations in weather for a
particular area.
Weather involves describing / predicting
atmospheric conditions over a short period of
time, whereas, climate involves the conditions
over longer periods of time.
What is an air mass? I
Air Mass: A large body of air that takes on the
characteristics of the area which it is over.
Air masses originate in
a source region
Air masses have unique:
(1) Temperatures
(2) Moisture contents
(3) Air pressures
There are five major
types of air masses
What is an air mass? II
What is an air mass? III
World wide source regions for air masses:
What is an air mass? IV
When air masses move from source
regions into new areas, the following
things happen:
(1) The region where the air mass has
moved undergoes a major change in
temperature and humidity
(2) The Air mass becomes more
moderate
What are Fronts? I
Fronts are boundaries that separate air
masses with differing temperature and
other characteristics
Fronts are observable on surface
weather maps and have symbols that
represent the four different types
What are Fronts? II
Cold front: Occurs when a wedge of cold
air advances toward warmer air
Warm front: Represents the boundary of
warm air moving toward cold air
Stationary front: Occurs when a cold
front remains stationary
Occluded front: Occurs along the
boundary between two polar air masses
with slightly different temperatures.
What are Fronts? III
On surface weather maps, the four types of
fronts are denoted by the following
symbols:
Cross Section of Fronts I
Cold front cross section:
Cross Section of Fronts II
Warm front cross section:
Cross Section of Fronts III
Stationary front cross section:
Cross Section of Fronts IV
Occluded front cross section:
Cross Section of Fronts V
Cold air is generally denser than warm
air
Therefore, when two air masses
interact, the colder air usually wedges
under the warmer air
The warmer air mass generally rises
Fronts and Clouds/Precipitation I
Figure: Weather conditions associated with cross section
A-B on Figure 10. Warm air (mT) lies between the cold front
and warm front. The cold front advances more rapidly than
the warm front, forcing warm air to rise, forming
thunderclouds and heavy rains. Warm air is forced to rise
above the more gently sloping warm front, resulting in the
formation of a series of low to high clouds.
Fronts and Clouds/Precipitation II
Figure: Nimbostratus clouds generate
precipitation along an occluded front.
Fronts and Clouds/Precipitation III
Weather Conditions Associated with a Passing
Frontal System
Conditions
Before Warm Front
Pressure
Decreasing
Winds
Temperature
Clouds
Precipitation
Between Warm/Cold fronts
Small decrease,
then small
increase
South, southeast Southwest
After Cold Front
Increasing
West, northwest
Cool
Cirrus, cirrostratus,
altostratus,
nimbostratus
Warm
Cold
Cumulus,
cumulonimbus
Cumulus,
altostratus
Light-moderate,
increasing
None, then heavy Moderate-light,
rains prior to cold decreasing
front
Using Surface Weather Maps I
Surface data is reported hourly from places
like airports and automated observing
platforms. The types of plots using surface
data include a composite surface map,
weather depiction for aviation, regional plots
for the US, Canada, Mexico and Alaska.
Contoured analyses of various parameters
including temperature and pressure. Finally,
there are meteograms which are time crosssections for individual cities. These data are
updated hourly at around 30 minutes past the
hour.
Using Surface Weather Maps IIA
Simple surface
weather map
from Accuweather.
This map shows
fronts, relative
pressure, and
weather conditions
Using Surface Weather Maps IIB
The black lines are called isobars, which are lines of
equal pressure (using the pressure unit called
millibars)
The winds are highest
where the isobars are
closest together
The winds are lowest
where isobars are
furthest apart
The contours show
what is known as a
“pressure gradient”
Using Surface Weather Maps III
Surface Observation Weather Symbols
Surface weather observation maps display
current conditions such as wind speed, wind
direction, cloud cover, dew point temperature,
and pressure for a number of sites
Using Surface Weather Maps IV
Collecting Weather Data I
NWS sites, airports, and other locations have
weather stations
Airplanes and ships collect weather data and
report it to central locations
There are weather stations located on buoys in
ocean basins
The National Weather Service (NWS) and other
government / international organizations collect
weather data in the atmosphere via weather
balloons that are sent into the atmosphere
Volunteer weather observers also report conditions
Collecting Weather Data II
NOAA’s NWS and other organizations launch
weather balloons which make an ascent into
the Earth’s upper atmosphere (35 km up) to
collect data about temperature, pressure, wind
speed/direction, and other conditions
Weather balloons carry a device called a
radiosonde, which collects weather information
and transmits it back to Earth
There are over 900 balloon launch sites, most
of which are located in the Northern
Hemisphere
Collecting Weather Data III
Weather balloons rise until they burst
The weather balloon falls to the surface of the
Earth and a parachute helps to slow it down
About 20% of the radiosondes used in weather
balloons are recovered by the NWS to be
refurbished for future launches
Watch weather balloon videos
Developing a Forecast I
There are several methods used to develop a
forecast:
(1) Climatologic Forecast: what have the
conditions been like on average over a long
period of time?
(2) Analog Forecast: A weather forecast that
compares current weather conditions to those
that have occurred in the past.
(3) Persistence Forecast: A weather forecast
that assumes the conditions at the time of the
forecast will not change (if it was 90 degrees
yesterday, it will be about 90 degrees tomorrow)
Developing a Forecast II
(4) Numerical Weather Prediction (NWP)
Forecast: uses the power of computers to make
a forecast. Complex computer programs, also
known as forecast models, run on
supercomputers and provide predictions on
many atmospheric variables such as
temperature, pressure, wind, and rainfall. A
forecaster examines how the features predicted
by the computer will interact to produce the
day's weather.
Despite these flaws, the NWP method is
probably the best of the five discussed here at
forecasting the day-to-day weather changes
Weather Satellites
Meteorologists find it useful to look at weather on a very large scale.
Because geostationary weather satellites can only make images of
relatively small portions of the earth's surface, several images
were combined to form this picture of North and South America.
Satellite Imagery Links
Other Important Terms I
Jet Streams: Narrow bands of fast, high-
altitude, westerly winds. Moves at around 115
m.p.h.
Doppler Effect: A change in wave
frequency occurs as an object moves closer
or farther from the source point.
Station Model: A record of weather for a
certain place at a certain time.
Other Important Terms II
Trade Winds: Air sinks from high altitudes and moves
towards the equator in a westerly manner. When the
air is warm enough it will rise and move northward.
Prevailing Westerlies: Wind progress towards the
poles in an easterly manner. Once cooled the air mass
returns south again. This is the major driving force of
weather in the U.S.
Air Mass Modification: The exchange of heat or
moisture with the surface over which the air mass
travels.
Coriolis Effect: Particles in the northern hemisphere
move to the right or clockwise. Particles in the
southern hemisphere rotate to the left or counter
clockwise. This is a result of the rotation of the Earth
on its axis.
The intertropical convergence zone (ITCZ) is a dynamic
weather area located near the equator. Sunlight at the
equator warms the air, making it rise and thereby creating
low pressure at the equator, along with rain-producing
cumulonimbus clouds. The rising air eventually cools and
sinks back to the surface at latitudes roughly 30° north
and south of the equator. The sinking cool air creates highpressure systems that keep those latitudes relatively dry.
The sinking air is drawn back to the low pressure near the
equator, creating a flow of air known as the trade winds.
Eventually, the trade winds converge near the equator and
create the ITCZ.
The prevailing winds include the doldrums near the equator, the
northeast (or southeast) trade winds somewhat above (or below) the
equator, the prevailing westerlies near the middle latitudes, and the polar
easterlies near the poles. The winds are named with respect to the
direction from which they blow. For example, the northeast trade wind
blows in the northern hemisphere from the east toward the west.
A rainbow forms when raindrops separate white sunlight into a spectrum. As
sunlight passes through a drop of water, it is first bent and then reflected from
the back surface of the drop toward the viewer’s eye. The amount of bending,
known as refraction, differs for light of different colors. Red light bends the
least and violet light bends the most. Here, a primary rainbow arcs through the
sky after a rainstorm. Primary rainbows have red on the outside arc; the colors
of secondary bows are reversed .
References I
Understanding Weather and Climate, Aguado and
Burt, Prentice Hall
Scuba Diving NJ and NY Web Site,
http://njscuba.net/index.html
West Virginia State University Weather Page,
http://fozzy.wvstateu.edu/~weatherlink/http://fozzy.
wvstateu.edu/~weatherlink/
Weather Systems,
http://www.mhhe.com/earthsci/geology/mcconnell/
wea/front.htm
References II
Unisys Weather, http://weather.unisys.com/
University of Illinois WW2010 Web Site,
http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/sf
cobs/home.rxml
NOAA Radisonde Web Site,
http://www.erh.noaa.gov/gyx/weather_balloons.ht
m/
Nature of Severe Weather
At any given time there are more
than 2000 storms occurring around
the Earth.
Important Terms
Thunderstorm: A storm caused by the collision
of two different air masses.
Air-Mass Thunderstorm: Most common type
of thunderstorm. Occurs because of air rising that
has been heated unequally within a single air mass.
Usually small and not severe.
Sea-Breeze Thunderstorms: Storms caused
by an extreme temperature difference between sea
and land.
Frontal Thunderstorms: Usually produced by
advancing cold fronts that collide with warm fronts.
Important Terms
Supercells: A severe storm that can be
self-sustaining. It is characterized by
intense rotating updrafts. Can last for
hours and have wind speeds of over
150 m.p.h.
A meteorologist tracks a tornado as part of ongoing weather
observations to better understand the earth’s atmosphere. Since the
19th century, scientific forecasting has greatly improved. Weather
radar can detect and track tornados, hurricanes, and other severe
storms.
Sidney Tornado, 2001
Doppler radar measures the speed and direction of the movement of
clouds, in addition to cloud density. In this image of a thunderstorm over
Oklahoma, Doppler radar shows a mesocyclone, a rotating mass of air
that may signal that the formation of a tornado is imminent.
A snow blizzard that struck the East Coast of the United States in
January 1996, left the Boston area with about 46 cm (about 18 in)
of snow. A snowstorm is called a blizzard when visibility is less
than 0.40 km (0.25 mi) and the snowfall is accompanied by strong
winds.
Hurricanes cause most of their damage with strong winds, but they can also spawn
tornadoes and cause sudden storm surges, or rises in water level. Hurricanes are
defined as tropical storms that have winds of 119 km/h (74 mph) or more and are
given ratings from 1 (minimal) to 5 (catastrophic) based on wind speed, height of
storm surge, and barometric pressure.
Meteorologists use satellite images taken in infrared light to determine
the heat of areas of the atmosphere. The data is translated into a visible
image, which is often enhanced, or colored with shades representing
temperatures. In enhanced images, red usually represents the hottest
temperatures and violet usually represents the coolest temperatures.
Tropical storm Sam hit Christmas Island and the Cocos Islands off the western coast of
Australia in January 1990. Astronauts aboard the space shuttle Columbia took this picture
of the storm after it had started to lose strength, but the circular motion of the clouds is still
apparent. Tropical storms have wind speeds from 63 to 119 km/h (39 to 74 mph), faster than
those of a tropical depression and slower than those of a typhoon.
Most weather services use satellite information in their forecasts. Pictures such as this one
of Hurricane Gloria reveal patterns and movement that provide clues about what the storm
might do next. The weather is continuously monitored and photographed by satellites
worldwide, but it is still difficult to predict weather because of the many variables involved.
Infrared sensors help meteorologists to interpret the pictures by determining the height and
temperature of the clouds.
Questions?