Weather

Basics

Worksheet: Farmers rules

Sheet 1: Sunny tomorrow?

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

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

Check this:

Probabilities

Let’suppose that we are given a forecast for the summer of 35% probability for below “normal rainfall” during June, July, and August.

The probability of normal rainfall is 30% and that of above normal rainfall is 40%. Remember that your home town in a dry basin has experienced “below normal” rainfall about 50% of the time, so a 35% chance in the coming summer means that it is less likely than the average that rainfall will be below normal this season.

Why predictions?

Uncertainty before birthday parties means suspense and surprise when you do not know what the presents might be.

Uncertainty, however, about the weather conditions of your weekend trip can mean that you pack the wrong things and cannot have the barbecue outside.

But if a farmer does not know when to plant his wheat well before the dry season this can mean that the whole harvest is at stake. Farmers found out thousands of years ago the right day to plant.

animation by Julia Heres, Cliparts by Hemera "Big box of art"

If the probability of a weather forecast is 50%, then it is helpful in a limited way. Long time forcasts (6-7 weeks) are most shaky and you never know…

But what would you say if we offer you a long time forecast of 75% probability?

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

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

This is conserved as a rhyme in the German version:
Regnet es am Siebenschläfertag, der Regen sieben Wochen nicht weichen mag.

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 former times? (You´ll find detailed information about this at "Historical background")

About this page:

- Author: Dr. Schrettenbrunner - University of Nürnberg - Germany
- scientific reviewing:
- educational reviewing: Dr. Yvonne Schleicher , Julia Heres
- last update: 08.09.03