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ES 5.3 PPT ES 5.3 PPT Presentation Transcript

  • 5.3WEATHER FORECASTING
  • Air Movement The movement of air caused by the differences in air pressure is called wind.  The greater the pressure difference, the faster the wind moves.  Air moves from areas of HIGH pressure into areas of LOW pressure.
  • Air Movement Differences in air pressure are caused by an unequal heating of the Earth.  Equator receives more direct sunlight per year than other latitudes, so air at equator is warmer = less dense.  Warm, less dense air rises and creates areas of LOW pressure.  At the poles, air is cooler = more dense  Cool, more dense air sinks creating areas of HIGH pressure.
  • Convection Cells & PressureBelts Air travels in large circular patterns called convection cells.  Separated by pressure belts, bands of high pressure and low pressure found about every 30 of latitude.  The combination of convection cells and the Coriolis effect produces patterns of air circulation called global winds.
  • Global Winds Polar Easterlies: wind belts extending from the poles to 60 latitude in both hemispheres. Westerlies: wind belts found between 30 and 60 latitude in both hemispheres. Trade Winds: winds blowing from 30 latitude almost to the equator in both hemispheres. Doldrums: the meeting place of the trade winds from both hemispheres 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 30 south latitude; area of high pressure.  According the legend, this name was given to these areas when sailing ships carried horses to Spain and would occasionally throw the horses overboard into the sea.
  • Jet Streams A flight from Seattle to Boston can be 30 minutes faster than the flight from Boston to Seattle. WHY? Pilots take advantage of the jet stream, a narrow belt of high- speed winds blowing in the upper troposphere and lower stratosphere.  Do not follow regular paths.  Jet streams also affect the movement of storms.
  • Local Winds Move short distances and can blow from any direction.  Local features like shorelines and mountains can produce temperature differences causing these local winds.
  • Air Masses Changes in weather are caused by the movement and interaction of air masses.  An air mass is a large body of air where temperature and moisture content are similar throughout.  The temperature and moisture content of an air mass are determined by the area over which the air mass forms, or its source region.  Example: Gulf of Mexico  warm and wet because the area is warm and has a lot of water evaporating from the region.  Different types of air masses are represented by a two-letter symbol.  First letter = moisture content o m for maritime (forms over water, wet) o c for continental (forms over land, dry)  Second letter = temperature o P for Polar (forms over polar regions, cold) o T for Tropical (forms over the tropics, warm)
  • COL Fronts D Air masses forming from different areas often do not mix. WARM  Due to different densities.  WARM air is less dense (lighter) than COLD air.  The area where two types of air OCCLUDE masses meet is called a front. D  Four kinds of fronts include cold, warm, occluded, and stationary. STATIONAR Y
  • Fronts COLD  Forms where cold air moves under warm air, which is less dense, and pushes the warm air up.  Can move quickly and bring thunderstorms, heavy rain, or snow.  Cooler weather usually follows a cold front because the air mass behind the cold front is cooler and drier than the air mass it is replacing. WARM  Forms where warm air moves over cold, more dense air and gradually replaces it.  Generally bring drizzly rain and are followed by clear, warm weather.
  • Fronts OCCLUDED  Forms when a warm air mass is caught between two colder air masses.  The coldest air mass moves under and pushes the warm air mass up.  Moves forward until it meets the other cold air mass which is slightly warmer, therefore slightly less dense.  The cold air masses may occasionally mix together.  Generally bring cool temperatures and large amounts of rain and snow. STATIONARY  Forms when a cold air mass meets a warm air mass. Move slowly or not at all.  Do not have enough force to lift the warm air mass over the cold one so they remain separated (lack of wind).  Generally bring cloudy, wet weather.
  • Air Pressure and Weather Areas having lower pressure than the surrounding areas are called cyclones.  Air masses come together (converge) and rise. Areas having a higher pressure than surrounding areas are called anticyclones.  Air masses split apart (diverge) and sink.  The sinking, denser air moves out of the centers of these high pressure systems and into areas of low pressure.
  • Convergence Divergence aloftaloftDivergence at Convergence atsurface surface
  • Air Pressure and Weather How do cyclones and anticyclones affect the weather?  As the air in the center of a cyclone rises, it cools and condenses to form clouds, which produce precipitation. Jeff Verszyla  In an anticyclone, the air KDKA Chief Meteorologist sinks, gets warmer, and absorbs moisture. This brings dry, clear weather. By keeping track of cyclones and anticyclones, meteorologists are able to accurately predict the weather for a particular area.
  • Weather Forecasting To accurately forecast the weather, meteorologists need to measure various atmospheric conditions including: To do this, they use special instruments to collect this data both near and far above Earth’s surface.
  • High in the Sky Weather balloons carry electronic equipment used for measuring weather conditions as high as 30 km. above Earth’s surface.
  • 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.
  • Measuring Wind Direction Wind direction can be measured by using one of the following devices:  Windsock: cone-shaped cloth bag open at both ends.  Wind enters the bag through the wide end and leaves through the narrow end.  Therefore, the wide end points into the wind.  Wind Vane: a device shaped like an arrow with a large tail; attached to a pole.  As the wind pushes the tail of the wind vane, it spins on the pole until the arrow points into the wind.
  • 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.
  • Radar and Satellites Radar is a measuring system used to find the location, movement and amount of precipitation. It can also detect what form of precipitation a weather system 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 weather systems in order to track storms and measure certain conditions.
  • Weather Maps In the United States, the National Weather Service (NWS) and the National Oceanic and Atmospheric Administration (NOAA) collect and analyze weather data.  The NWS produces weather maps based on information gathered from about 1000 weather stations across the US.  On these maps, each station is represented by a station model, which is a small circle showing the location of the weather station.  Various symbols surround these station models, all representing specific weather data.
  • Weather Maps Weather maps you see on TV include several different lines connecting 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 low pressure.  Isotherms: lines connecting areas of equal temperatures.  Isohypse: lines connecting areas of equal height.  Isohyet: lines connecting areas of equal precipitation.
  • Barometric Pressure Coding Most barometric pressure readings fall somewhere between 950 mb. and 1060 mb. Meteorologists use codes to identify pressure levels and the general rules are as follows: When DECODING:  Add a decimal point between the last 2 digits of the coded 3 digit number.  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.
  • 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: 432 778 021