Meteorology Chapter 12 Earth Science

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.