Meteorology

Chapter 12Earth 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 short-

term 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 warm-front 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.