Forecasts, mountains and rain

Forecasters draw maps of temperature, pressure and humidity. The maps show air masses: large bodies of air that are warm and dry, or cool and moist. Where two air masses meet, the weather often changes. Here we read the same facts in a table instead of on a map. Computers run models, programs that use today’s data and the rules of physics to predict the next hours and days. A forecast for tomorrow is usually quite reliable, but a forecast for two weeks ahead is much less certain, because small changes in the data grow bigger with time. Good forecasts warn farmers, fishers, pilots and families about rain, wind and heat.