Download Meteorology

Meteorology
Chapter 12
Earth Science
2013-2014
The Causes of Weather
• Meteorology – the study of atmospheric
phenomena.
– Meteor – ancient Greek meaning “high in the air”
• Hydrometeors
– Clouds, raindrops, snowflakes, fog, dust, rainbows.
– Cloud droplets and forms of precipitation
containing water in any phase (solid, liquid, gas).
The Causes of Weather
• Lithometeors
– Smoke, haze, dust, other condensation nuclei
• Electrometeors
– Visible or audible manifestations of atmospheric
electricity
Weather and Climate
• Weather – the current state of the
atmosphere.
– Variations that take place over minutes, hours,
days, weeks, or months.
• Climate – long-term variations in weather for
a particular area.
– Usually averaged over 30+ years
Weather and Climate
• Summary:
– Meteorology
• The study of the atmosphere.
– Weather
• The current state of the atmosphere, including shortterm variations that affect our lives.
– Climate
• The average weather over a long period of time.
A Question of Balance
• How is the radiation distributed around the
planet?
– Sun’s rays are more direct during the afternoon
than in the morning or evening.
– Helps to explain why the tropics are hotter than
the poles.
– But the heat must be redistributed around the
world in order to maintain fairly constant average
temperatures.
• Continual motion of air and water.
Air Masses
• Air mass – large body of air that takes on the
characteristics of the area over which it forms.
• Source region – the region over which an air mass
forms.
• Air masses can form over land or water.
• Covers thousands of square kilometers.
Air Masses
• Classifying air masses
– Classified according to their source regions.
– Main types (see pg 303):
•
•
•
•
•
Warm and dry continental tropical (cT)
Warm and humid maritime tropical (mT)
Cold and dry continental polar (cP)
Cold and humid maritime polar (mP)
Arctic (A)
– Same as a continental polar mass, but much colder.
Air Masses
• Source regions
– All five main types can be found in North America.
– Origins will change, however.
– See page 303 for a diagram/map.
Air Masses
• Air mass modification
– Air masses do not stay in one place indefinitely.
– They move and transfer heat from one area to
another.
– When it moves, it will take on some of the
characteristics of the new surface beneath it.
Weather Systems
• Earth rotates from west to east.
• Causes the Coriolis effect – moving particles
are deflected to the right in the northern
hemisphere and to the left in the southern
hemisphere.
– Transports colder air to warmer areas and warmer
air to colder areas.
Global Wind Systems
• Trade winds
– Occurs at 30° north and south latitude.
– Air sinks, warms, and moves toward the equator
in a westerly direction.
– Called horse latitudes.
Global Wind Systems
• Prevailing westerlies
– Flows between 30° and 60° north an south latitude
opposite to the direction of the trade winds.
– Surface winds move toward the poles in a generally
easterly direction.
• Polar easterlies
– Lies between 60° latitude and the poles.
– From northeast to southwest in the northern
hemisphere.
– Reverses in the southern hemisphere.
Jet Streams
• Narrow bands of fast, high-altitude, westerly
winds.
• Can flow at speeds up to 185 km/hr at elevations
of 10.7 to 12.2 km.
• Resemble jets of water.
• Boundaries between wind zones.
Large-Scale Weather Systems
• Disturbances form along jet streams and give rise
to large-scale weather systems.
• Transport surface cold air toward the tropics.
• Transport surface warm air toward the poles.
• Jet stream moves air of different temperatures
from one region to another, affecting the
intensity of weather systems.
Fronts
• Front – narrow region separating two air
masses of different densities.
• Four main types:
– Cold fronts
– Warm fronts
– Stationary fronts
– Occluded fronts
Fronts
• Cold Front
– Cold, dense air displaces warm air and forces the
warm air up along a steep front.
– Rising warm air cools and condenses.
– Sometimes results in clouds, showers, and
thunderstorms.
– Represented as a solid blue line with blue
triangles pointing in the direction of the front’s
motion.
Fronts
• Warm Fronts
– Advancing warm air displaces cold air.
– Develops a gradual frontal slope instead of a steep
boundary.
– Characterized by extensive cloudiness and
precipitation.
– Represented by a solid red line with solid red
semicircles pointing in the direction of the front’s
motion.
Fronts
• Stationary Fronts
– Two air masses meet and neither advances into
the other’s territory.
– Frequently occurs when two air masses have
become so modified in their travels that the
temperature and pressure gradients between
them are small.
– Represented by a combination of cold- and warmfront symbols.
Fronts
• Occluded Fronts
– Cold air mass moves so rapidly that it overtakes a
warm front.
– Cold air wedges the warm air upward.
– Precipitation is common on both sides of the
front.
– Represented by a line with alternating purple
triangles and semicircles that point toward the
direction of motion.
Pressure Systems
• High-Pressure Systems
– Air sinks and spreads away from the center.
– Moves in a clockwise direction in the northern
hemisphere due to the Coriolis effect.
Pressure Systems
• Low-Pressure Systems
– Air rises and must be replaced by air from outside
the system.
– The net flow is inward toward the center and then
upward.
– Flows in a counterclockwise direction in the
northern hemisphere.
Gathering Weather Data
• Meteorologists measure the atmospheric
variables of temperature, air pressure, wind,
and relative humidity to make accurate
weather forecasts.
• The quality of the data is important.
• Several types of technology are used.
Surface Data
• Thermometer
– Device used to measure temperature.
– Usually contains mercury or alcohol that expand
when heated.
• Barometer
– Device used to measure air pressure.
– Two types, but both use expansion and
contraction of materials to measure pressure.
Surface Data
• Anemometer
– Device used to measure wind speed.
– Simplest has cupped arms that rotate as the wind
blows.
• Hygrometer
– Device used to measure relative humidity.
– Sometimes uses wet- and dry-bulb thermometers.
– Measures temperature difference as water from the
wet-bulb cools and compares to a chart.
Surface Data
• Automated Surface Observing System
– Network of stations that capture “snapshots” of
all data.
– Uses all of the instruments discussed and:
• Rain gauge – to measure rainfall.
• Celiometer – measures the height of cloud layers and
estimates the amount of sky covered by clouds.
Upper-Level Data
• To make accurate forecasts, meteorologists
must gather atmospheric data at heights of up
to 30,000 m.
• Requires more sophisticated technology.
Upper-Level Data
• Radiosonde
– Balloon borne package of sensors.
– Measure temperature, air pressure, and humidity.
– Data is sent back by radio signal.
– Can also determine wind speed by tracking the
balloon.
– Still quite expensive.
Weather Radar
• Radar – radio detection and ranging.
• Made of several parts:
– Transmitter
• Generates electromagnetic waves that leave the transmitter
through antennae.
• Programmed to ignore small cloud droplets and bounce off of rain
droplets.
– Amplifier
• Increases the wave signals of the scattered waves.
– Computer
• Analyzes signal and displays on a screen to show where rain is
falling.
Weather Radar
• Doppler Radar
– Many advanced systems take advantage of the
Doppler effect.
• Change in wave frequency that occurs in energy (sound
or light) as the energy moves towards or away from an
observer.
– Provides a good estimation of the wind speeds
associated with precipitation areas, including
those that are experiencing severe weather such
as thunderstorms and tornados.
Weather Satellites
• Cameras are mounted on weather satellites to
take pictures and send them back to Earth at
regular intervals.
• Help to predict weather.
Infrared Imagery
• Infrared imagery detects differences in
temperatures.
• Does not require light in order to work as
other cameras do.
• Can also determine the temperature of clouds
to better assess what type they are, etc.
Weather Analysis
• Station model
– Record of weather data for a particular site at a
particular time.
– Uses meteorological symbols to represent data.
Surface Analysis
• Isopleths
– Lines that connect points of equal or constant
values.
– Isobars – lines of equal pressure.
– Isotherms – lines of equal temperature.
– Similar to contour lines on a topographic map.
Short-Term Forecasts
• In the early days of forecasting,
meteorologists observed current weather
conditions, compared them, then
extrapolated.
• The results served as the basis of forecasts.
• Weather forecasting is too complicated to rely
on this.
Short-Term Forecasts
• Digital Forecasts
– A forecast that relies on numerical data.
– Many of the principles in meteorology can be
expressed by equations.
– Computers are necessary to prevent forecasting
from taking an excessive amount of time.
– The more data that exists, the more accurate the
forecast is able to be.
Short-Term Forecasts
• Analog Forecasts
– Compares current weather patterns to patterns
that took place in the past.
– Assumes that weather systems behave in a similar
pattern.
– Useful mainly for monthly or seasonal forecasts.
Long-Term Forecasts
• Regardless of the forecasting method used, all
forecasts become less reliable when they
attempt to predict long-term changes in the
weather.
• Heat absorbed in particular locations can
effect the weather.
• Short-term are the most accurate.