Chapter 25 Weather Section 4 Forecasting Weather Notes 25-6

History of Forecasting People at one time thought there were weather gods that caused wind, rain and thunder People at one time thought there were weather gods that caused wind, rain and thunder Then they used the moon and stars to try and forecast Then they used the moon and stars to try and forecast True forecasting began with the invention of weather instruments (thermometer and barometer) True forecasting began with the invention of weather instruments (thermometer and barometer)

History of Forecasting Telegraph (1844) allowed for information to be shared quickly Telegraph (1844) allowed for information to be shared quickly Led to the first national weather service Led to the first national weather service Army Signal Corps (1870) was the first true government agency Army Signal Corps (1870) was the first true government agency In the next 20 years it was called Weather Bureau In the next 20 years it was called Weather Bureau Then it was changed to the National Weather Service (1970) Then it was changed to the National Weather Service (1970)

Weather Observations Weather conditions at every weather station all over the world are taken every 6 hours Weather conditions at every weather station all over the world are taken every 6 hours Barometric pressure (rising or falling) Barometric pressure (rising or falling) Wind speed and direction Wind speed and direction Precipitation Precipitation Temperature Temperature Humidity Humidity Cloud type, amount and height Cloud type, amount and height General weather conditions and visibility General weather conditions and visibility

Weather Observations Automated systems also collect data Automated systems also collect data Larger station send up radiosondes Larger station send up radiosondes Each station puts the information into an international code and sends it to a central location Each station puts the information into an international code and sends it to a central location Weather centers around the world share information Weather centers around the world share information

Weather Observations World Meteorological Organization (WMO) World Meteorological Organization (WMO) Sponsors a program called World Weather Watch (1873) Sponsors a program called World Weather Watch (1873) Promotes the quick exchange of weather information Promotes the quick exchange of weather information Helps establish or improve services in developing countries Helps establish or improve services in developing countries Part of the United Nations Part of the United Nations

Making a Weather Map Each time new data is taken, a weather map is updated Each time new data is taken, a weather map is updated Observations are translated into internationally recognized symbols Observations are translated into internationally recognized symbols They are plotted in a cluster around the station from which the data came They are plotted in a cluster around the station from which the data came Cluster of symbols is called a station model Cluster of symbols is called a station model See page 513 for station model symbols See page 513 for station model symbols

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Making a Weather Map Common weather symbols Common weather symbols Cloud cover Cloud cover Wind speed and direction Wind speed and direction Weather conditions (precipitation/storm) Weather conditions (precipitation/storm) Air temperature Air temperature Dew point Dew point If dew point and air temp are close to the same the relative humidity is high If dew point and air temp are close to the same the relative humidity is high Atmospheric pressure (millibars) Atmospheric pressure (millibars) Line below the number that indicates whether the pressure is rising or falling or steady Line below the number that indicates whether the pressure is rising or falling or steady

Isobars Lines are drawn to connect points of equal atmospheric pressure Lines are drawn to connect points of equal atmospheric pressure Called isobars Called isobars Closely spaced: rapid change in pressure and high winds Closely spaced: rapid change in pressure and high winds Widely spaced: slow change in pressure and low wind speeds Widely spaced: slow change in pressure and low wind speeds Circles: centers of high or low pressure; H for high and L for low Circles: centers of high or low pressure; H for high and L for low

Fronts Fronts and areas of precipitation are marked Fronts and areas of precipitation are marked Fronts are shown by sharp changes in wind, temperature or humidity Fronts are shown by sharp changes in wind, temperature or humidity Areas between stations reporting precipitation are colored green or marked with symbols for precipitation Areas between stations reporting precipitation are colored green or marked with symbols for precipitation See page 514 See page 514

Types of Forecasts Daily forecast: Daily forecast: Predict for 48 hours Predict for 48 hours Extended forecasts are usually 5 days to a week Extended forecasts are usually 5 days to a week Long range forecast: Long range forecast: Cover monthly and seasonal outlooks Cover monthly and seasonal outlooks

Types of Forecasts They study recent weather maps and compare it with maps for the previous 24 hours They study recent weather maps and compare it with maps for the previous 24 hours They can see paths of storms and trend of temperatures They can see paths of storms and trend of temperatures Use computers to make predictions Use computers to make predictions Must interpret the maps because computers make generalizations Must interpret the maps because computers make generalizations Use data and personal experience Use data and personal experience

Types of Forecasts Accurate forecasts are about 3-5 days Accurate forecasts are about 3-5 days Accuracy drops with each added day Accuracy drops with each added day 6-10 day forecast show large scale movements of air masses 6-10 day forecast show large scale movements of air masses Can’t show precise information for individual areas Can’t show precise information for individual areas Controlling the weather (precipitation) has been done by cloud seeding Controlling the weather (precipitation) has been done by cloud seeding Have also tried to control lightning without a lot of success Have also tried to control lightning without a lot of success