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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.