Weather Forecasting

• Weather forecasting


  Involves observers, meteorologists, and machines
• Computers


  • Make forecasts more accurate
  • Weather satellites orbiting the Earth take photographs of clouds from space
• Forecasting process


  1. Observers and meteorologists use observations from ground and space

  2. Apply formulas and rules based on past experience

  3. Make forecasts
• What we want to know in weather forecasting

  • Temperature
  • Wind speed
  • Wind direction
  • Clouds (type, extent, altitude)
  • Precipitation (type, amount, location)
  • Visibility (fog, haze)
  • Humidity
  • Trends in all of these
  • Timing of significant changes
  • Occurrence of extreme events
• Physical processes in weather

  • Thermal - atmospheric dynamics driven by temperature gradients from uneven solar heating
  • Pressure gradient forces - immediate cause of horizontal motions
  • Moisture - effect of water vapour content on air density, and release of latent heat impacts convection
• Numerical Weather Prediction


  Physical processes reduced to simplified set of equations describing changes in physical quantities over time and space, initialized with latest observations and stepped forward in time to produce a forecast
• Requirements for Numerical Weather Prediction

  • Extensive set of simultaneous measurements over wide area (synoptic observations) to initialize it
  • Fast, powerful computer
  • Adequate representation of physical processes
• Forecast range


  • 3-5 days is limit of reasonable quantitative forecasts
  • Medium-range forecasts (5-10 days) limited to large-scale pressure field and winds, not detailed conditions
• Low pressure


  • Cloudy skies
  • Steady precipitation
  • Counterclockwise rotation of air
• High pressure


  • Clear/Sunny skies
  • Calm weather
  • Clockwise rotation of air
• Weather fronts move from west to east because the prevailing winds are from the west (Prevailing Westerlies)
• Weather tools


  • Thermometer
  • Barometer
  • Anemometer
  • Rain gauge
• Thermometer


  • Measures the amount of heat in the air in degrees
• Barometer


  • Measures the weight of the air
• Anemometer


  • Measures the velocity or speed of air movement in miles or kilometers
• Rain gauge


  • Measures the levels or amount of precipitation in inches or cm
• Doppler radar


  • Tool used by meteorologists to forecast the weather
• Weather satellites


  • Polar orbiting - see same swath of Earth every 12 hours
  • Geostationary - hover over same spot on Earth by orbiting over equator at Earth's rotation speed
• Weather satellites are primarily used to monitor the weather and climate of the Earth
• Weather balloons


  • Fitted with radiosondes, used by meteorologists to forecast the weather
• ASOS


  • Automated Surface Observing System, used by meteorologists to forecast the weather
• NOAA supercomputers


  • Collect, process and analyze billions of observations from weather satellites, weather balloons, buoys and surface stations from around the world
• AWIPS


  • Advanced Weather Interactive Processing System, workstation used by meteorologists to forecast the weather