Figure 16.42: One of the geostationary satellites that monitors conditions over the United
States. ( 2 )
are lower, they get a more detailed view of the planet.
Forecasting Methods
There are many ways to create a forecast, some simple and some complex. Some use only
current, local observations, while others deal with enormous amounts of data from many
locations at different times. Some forecasting methods are discussed below.
Perhaps the easiest way to forecast weather is with the ’persistence’ method. In this method,
weassumethattheweathertomorrowwillbeliketheweathertoday. Thepersistencemethod
works well if a region is under a stationary air mass or if the weather is consistent from day
to day. For example, Southern California is nearly always warm and sunny on summer days,
and so that is a fairly safe prediction to make. The persistence method can also be used for
long-term forecasts in locations where a warm, dry month is likely to lead to another warm
dry month, as in a Southern California summer.
The ’climatology ’method assumes that the weather will be the same on a given date as
it was on that date in past years. This is often not very accurate. It may be snowing in
Yosemite one New Year’s Day and sunny and relatively warm on the next. Using the ’trend’
method, forecasters look at the weather upwind of their location. If a cold front is moving in
their direction at a regular speed, they calculate when the cold front will arrive. Of course,
the front could slow down, speed up, or shift directions, so that it arrives late, early, in a
strengthened or weakened state, or never arrives at all. Forecasters use the ’analog’ method
when they identify a pattern. Just like an analogy compares two similar things, if last week
a certain pattern of atmospheric circulation led to a certain type of weather, the forecaster