1. 5.3WEATHER FORECASTING

2. Air Movement The movement of air caused by the differences in air pressure iscalled wind. The greater the pressure difference, the faster the wind moves. Air moves from areas of HIGH pressure into areas of LOW pressure. 3. Air Movement Differences in air pressure are causedby an unequal heating of the Earth. Equator receives more direct sunlight peryear than other latitudes, so air atequator is warmer = less dense. Warm, less dense air rises and creates areasof LOW pressure. At the poles, air is cooler = more dense Cool, more dense air sinks creating areas ofHIGH pressure. 4. Convection Cells & PressureBelts Air travels in largecircular patterns calledconvection cells. Separated by pressurebelts, bands of highpressure and lowpressure found aboutevery 30 of latitude. The combination ofconvection cells andthe Coriolis effectproduces patterns ofair circulation calledglobal winds. 5. Global Winds Polar Easterlies: wind belts extending from the poles to 60latitude in both hemispheres. Westerlies: wind belts found between 30 and 60 latitude inboth hemispheres. Trade Winds: winds blowing from 30 latitude almost to theequator in both hemispheres. Doldrums: the meeting place of the trade winds from bothhemispheres around the equator; area of low pressure. Also known as the Intertropical Convergence Zone, or the ITCZ. Horse Latitudes: winds located at about 30 north and 30south latitude; area of high pressure. According the legend, this name was given to these areas whensailing ships carried horses to Spain and would occasionally throwthe horses overboard into the sea. 6. Jet Streams A flight from Seattle to Bostoncan be 30 minutes faster than theflight from Boston to Seattle.WHY? Pilots take advantage of thejet stream, a narrow belt of high-speed winds blowing in the uppertroposphere and lowerstratosphere. Do not follow regular paths. Jet streams also affect themovement of storms. 7. Local Winds Move short distances and can blow from any direction. Local features like shorelines and mountains can producetemperature differences causing these local winds. 8. Air Masses Changes in weather are caused by the movement andinteraction of air masses. An air mass is a large body of air where temperature andmoisture content are similar throughout. The temperature and moisture content of an air mass aredetermined by the area over which the air mass forms, or itssource region. Example: Gulf of Mexico warm and wet because the area is warmand has a lot of water evaporating from the region. Different types of air masses are represented by a two-letter symbol. First letter = moisture contento m for maritime (forms over water, wet)o c for continental (forms over land, dry) Second letter = temperatureo P for Polar (forms over polar regions, cold)o T for Tropical (forms over the tropics, warm) 9. COL Fronts D Air masses forming from different areas often do not mix. WARM Due to different densities. WARM air is less dense (lighter) thanCOLD air. The area where two types of airOCCLUDE masses meet is called a front. D Four kinds of fronts includecold, warm, occluded, and stationary.STATIONARY 10. Fronts COLD Forms where cold air moves underwarm air, which is less dense, andpushes the warm air up. Can move quickly and bringthunderstorms, heavy rain, or snow. Cooler weather usually follows a coldfront because the air mass behindthe cold front is cooler and drier thanthe air mass it is replacing. WARM Forms where warm air moves overcold, more dense air and graduallyreplaces it. Generally bring drizzly rain and arefollowed by clear, warm weather. 11. Fronts OCCLUDED Forms when a warm air mass is caughtbetween two colder air masses. The coldest air mass moves under andpushes the warm air mass up. Moves forward until it meets the othercold air mass which is slightlywarmer, therefore slightly less dense. The cold air masses may occasionally mixtogether. Generally bring cool temperatures andlarge amounts of rain and snow. STATIONARY Forms when a cold air mass meets awarm air mass. Move slowly or not at all. Do not have enough force to lift the warmair mass over the cold one so they remainseparated (lack of wind). Generally bring cloudy, wet weather. 12. Air Pressure and Weather Areas having lower pressure than the surrounding areas arecalled cyclones. Air masses come together (converge) and rise. Areas having a higher pressure than surrounding areas arecalled anticyclones. Air masses split apart (diverge) and sink. The sinking, denser air moves out of the centers of these highpressure systems and into areas of low pressure. 13. ConvergenceDivergence aloftaloftDivergence at Convergence atsurface surface 14. Air Pressure and Weather How do cyclones andanticyclones affect the weather? As the air in the center of acyclone rises, it cools andcondenses to form clouds, whichproduce precipitation.Jeff Verszyla In an anticyclone, the air KDKA Chief Meteorologistsinks, gets warmer, and absorbsmoisture. This brings dry, clearweather. By keeping track of cyclones andanticyclones, meteorologists areable to accurately predict theweather for a particular area. 15. Weather Forecasting To accurately forecast the weather, meteorologists needto measure various atmospheric conditions including: To do this, they use special instruments to collect thisdata both near and far above Earths surface. 16. High in the Sky Weather balloons carry electronic equipment used for measuring weather conditions as high as 30 km. above Earths surface. 17. Measuring Air Temperature andPressure A tool used to measure air temperature is called a thermometer. Most use a liquid (alcohol) sealed in a narrow glass tube. When air temperature increases, the liquid expands and moves up the glass tube. When it decreases, the liquid shrinks and moves down the tube. A barometer is an instrument used to measure air pressure. A mercurial barometer consists of a glass tube sealed at one end, placed in a container full of mercury (Hg) which will either rise or fall with changes in air pressure. An aneroid barometer contains a closed vessel called an aneroid cell, which contracts or expands with changes in air pressure. 18. Measuring Wind Direction Wind direction can be measured byusing one of the following devices: Windsock: cone-shaped cloth bag open atboth ends. Wind enters the bag through the wide endand leaves through the narrow end. Therefore, the wide end points into the wind. Wind Vane: a device shaped like an arrowwith a large tail; attached to a pole. As the wind pushes the tail of the windvane, it spins on the pole until the arrowpoints into the wind. 19. Measuring Wind Speed An instrument used to measure wind speed is called an anemometer, which consists of three or four cups connected by spokes to a pole. The wind pushes on the hollow sides of the cups and causes the cups to rotate on the pole. The motion sends a weak electric current which is measured and displayed on a dial. 20. Radar and Satellites Radar is a measuring system used to find the location, movement and amountof precipitation. It can also detect what form of precipitation a weathersystem is carrying. Example: Doppler radar used in a local TV weather report (based on the Doppler Effect: change in frequency as objects move toward a source). Weather satellites orbiting the Earth can also provide images of weathersystems in order to track storms and measure certain conditions. 21. Weather Maps In the United States, the National Weather Service (NWS)and the National Oceanic and AtmosphericAdministration (NOAA) collect and analyze weather data. The NWS produces weather maps based on informationgathered from about 1000 weather stations across the US. On these maps, each station is represented by a station model, whichis a small circle showing the location of the weather station. Various symbols surround these station models, all representingspecific weather data. 22. Weather Maps Weather maps you see on TV include several different linesconnecting points of equality. For example Isobars: lines connecting areas of equal air pressure. Normally drawn at 4 mb. intervals. Isobars forming closed circles represent areas of high or lowpressure. Isotherms: lines connecting areas of equal temperatures. Isohypse: lines connecting areas of equal height. Isohyet: lines connecting areas of equal precipitation. 23. Barometric Pressure Coding Most barometric pressure readings fall somewhere between950 mb. and 1060 mb. Meteorologists use codes to identifypressure levels and the general rules are as follows: When DECODING: Add a decimal point between the last 2 digits of the coded 3 digitnumber. Add a 9 or a 10 in front of the coded 3 digit number. If the coded 3 digit number is between 000 and 500 ADD A 10 If the coded 3 digit number is between 501 and 999 ADD A 9 When CODING: Drop the 9 or 10 at the beginning of the full figure. Drop the decimal point between the last 2 digits. 24. Barometric Pressure Coding Decode the following coded barometric pressure readings:978.2 mb. 950.1 mb. 1014.5 mb. Code the following full figured barometric pressure readings:432778021