Weather

Basics

3. Folklore Weather Forecasting

Worksheet 1: Farmers rules
Sunny 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.

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 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?
You will find detailed information about this in Worksheet 2 of this unit.

About this page:

author: Dr. Schrettenbrunner - University of Nürnberg - Germany
educational reviewing: Dr. Yvonne Schleicher, Julia Heres - University of Nürnberg - Germany
last update: 2003-09-08