The "Climatology" Technique

  • a forecast that is based on "climatology" or average weather
  • example:
    • Lets say the climatological records show that it rains only 1 day out of 100 during the summer months in LA.
    • Then, if you forecast "no rain" for any day in July and August, the probability you will be wrong is 1/100.
    • Therefore, the probability you will be right is about 99%
  • NOTE: precip forecasts are usually given as probabilities
    • ex: "there is a 60% chance of rain today"
  • this means that there is a 60% chance of measurable precip at any random place in the forecast area

Accuracy and skill in forecasting

  • What is an accurate forecast?
    • your forecast for tonights min temp is 0F
    • if the actual min was 1F, is in inaccurate?
    • if the actual min was 10F, is in inaccurate?
    • Accuracy (in forecasting) is arbitrary and relative - it's not clearly, objectively defined
  • How does a forecast show skill?
    • must be more accurate than utilizing persistence or climatology
    • persistence is "hard to beat" on a time frame of minutes to hours
    • skill is often shown on time frame of hours to a few days
    • Beyond 10 days, forecasts are generally no better than those generated by climatology


Climatology shows that the skies over Vancouver, Canada are cloudy 327 days a year. If you were to forecast cloudy skies for every day of the year, how accurate will your forecasts be? Do your forecasts show skill? Explain.