## Worksheet 3.1

ENVIRONMENTAL SCIENCE PUBLISHED FOR EVERYBODY ROUND THE EARTH
ENCYKLOPEDIA ESPERE - main menu (expand)
topic WEATHER: BASICS - menu (expand)

## Farmers rulesSunny tomorrow?

When the weather report says that there is a 70% chance of a sunny day tomorrow in your town, the event is usually defined by a threshold measurement at a nearby weather station.

For example, "a sunny summer day" in Central Europe might mean a measurement of at least 25 oC during the day. The method for estimating the probability of a sunny summer day involves calculations that have been shown to be accurate.

#### Probabilities

Let’suppose that we are given a forecast for the summer which says that there will be a 35% probability of "below normal rainfall" during June, July, and August.

The probability of "normal rainfall" is 30% and the probability of "above normal rainfall" is 40%.

This town is in a dry basin and experiences "below normal rainfall" about 50% of the time.  This means that a 35% chance of "below normal rainfall" in the summer means that it is less likely than average that rainfall will be below normal this season.

If the probability of a weather forecast is 50%, then the forecast is only helpful in a limited way. Long range forcasts (6-7 weeks) are the most unreliable.

But what would you say if we offer you a long range forecast which has a 75% probability of being correct?

This is the “Seven Sleepers’ Rule” of June 27th:

If it rains at Seven Sleepers Day, the rain will stay seven weeks more.

Such predictions are at the one hand regional (i.e. valid for South Germany) and, on the other hand, continental (i.e. they have influence on Central Europe and parts of Northern and Western Europe).

Discuss:

Why were rhymes useful in the past?