# Challenge: Predict the Weather

Updated Monday 28th January 2008

The science behind weather prediction, part of the BBC/OU's programme website for Rough Science 2

Making a weather station and recording what is happening with the elements isn't very difficult. It gets much harder when it comes to predictions: even the experts with their weather wizardry get it wrong sometimes.

The sun is the driving force behind the weather. As solar energy reaches the earth, equatorial regions heat up more than the poles. In addition to this, the rotation of the Earth and its path around the Sun is always modifying this heating effect.

Due to convection warm air and water at the equator travel towards the poles while cold air and water at the poles travel towards the equator. It is the atmosphere's continual stirring due to convection and rotation that creates and brings to us our changing weather.

What happens at the boundaries between warm and cold masses of air?

When warmer air replaces cooler air, it is called a warm front. Similarly, a cold front describes the situation when a mass of cooler air replaces warm air.

These fronts are shown on weather charts as lines with either red semi-circles (indicating a warm front) or blue triangles (showing a cold front). These symbols are placed on one side of the frontal line to show which way the front is moving.

As cold fronts move more quickly than warm fronts, they can catch up with the warm front ahead. This is called an occlusion.

occlusion

How do fronts change with altitude?

Although the lines on a weather chart show the position of the front at the earth's surface, fronts do not simply rise vertically into the atmosphere - their position changes with altitude. This is due to the fact that hot air rises.

As the warm air mass moves in on the cold air mass, the warmer air glides easily over the top of the cooler air. This results in the warm front extending further forward the higher up you go. So the warm front passes overhead at altitude before it passes at surface level.

This is in contrast to a cold front where the front passes over at ground level first. Because the colder air pushes under the warmer air mass, this cold front lags behind at higher altitude.

What weather is associated with warm and cold fronts?

A warm mass of air will generally hold moisture picked up as it passes over large areas of water though the process of evaporation. As this warm moist air rises it cools and the water vapour condenses into rain - this is known as the dewpoint.

So a warm air mass tends to bring with it plenty of rain and drizzle. As a warm front extends further forward the higher up you go, this rain frequently starts to fall before the front reaches you at ground level. Once the front passes over, the rain often eases off but occasional drizzly patches are likely to follow on behind in the warm sector.

A cold air mass has a much smaller moisture capacity so there isn't the same extensive rain as in a warm air mass. However, as a cold front comes in and drives under the warm front, the warmer and moist air will be forced upwards.

Copyright: Used with permission As the warm air is pushed higher, the moisture it carries condenses and falls as rain. This is why a lot of heavy rain is produced along a cold front but once the cold air mass has come in this often abruptly changes to a clear spell of weather.

As an occlusion is essentially a warm front with a cold front merged into it the weather ahead of it behaves as if a warm front is moving in. Behind the occlusion, however, it acts as a cold front. So you might see drizzly rain before the occlusion, suddenly becoming a heavy downpour as the occlusion passes over and becoming clear weather punctuated by the odd patch of showers.

How many cloud types are there?

There are ten different cloud types which are classified by their shape and height. Meteorologists use the same classification in Latin as for animals and plants - genus, species and variety.

The three basic cloud forms are:

Cumulus - heap clouds
Stratus - layer clouds
Cirrus - hair like or feathery clouds.

These words or variants occur in the compound names for the other seven major types. There is some overlap between the families. Cirrostratus and cirrocumulus are respectively members of the stratiform and cumuliform groups as well as the cirrus family.

The cumulus-like family consists of:
- Cumulus
- Stratocumulus
- Altocumulus
- Cirrocumulus
- Cumulonimbus

The stratus family consists of:
- Stratus
- Nimbostratus
- Altostratus
- Cirrostratus

The cirrus family consists of:
- Cirrus
- Cirrocumulus
- Cirrostratus

Using the meanings of the Latin names as a guide, can you identify what these clouds might be?

As well as observing clouds, how do meteorologists study the weather?

Meteorologists study the weather by recording and analyzing data collected from a range of devices. For our Rough Science weather station, we made a barometer, a hygrometer, a rain gauge, a thermometer, a weather vane and an anemometer.

The barometer and hygrometer are sensitive instruments. We kept them away from direct sunlight, wind and rain. All the other instruments could be placed outside. We kept a weather journal to record all measurements and from this we could detect patterns that enable us to predict the weather.

Rough Science weather station

Why is air pressure measured?

Copyright: Used with permission Air pressure is the weight of all the air molecules above you pushing down. Although air molecules are invisible to the naked eye, they still have weight and because of their thermal energy they keep colliding with each other. Because there is a lot of "empty" space between the air molecules, air can be compressed to fit into a smaller volume.

When air is compressed, the number of air molecules in a given space increases and so it weighs more: the air is said to be under 'high pressure'. The exact pressure is not very informative but the relative changes in pressure from day to day are important indicators of changes in weather.

For example, a large area of warmer air being blown in from another part of the world will cause the air pressure to change many hours, perhaps days, before any indication can be seen or felt. A drop in pressure usually predicts foul weather, while a rise in pressure usually means fair weather ahead.

What are ridges and troughs?

Ridges and troughs are often mentioned on the weather forecast. A ridge is an elongated area of relatively high pressure extending from the centre of a high-pressure region. A trough is an elongated area of relatively low pressure extending from the centre of a region of low pressure.

Air in a high pressure area compresses and warms as it descends. The warming inhibits the formation of clouds, meaning the sky is normally sunny in high-pressure areas. But haze and fog still might form. The opposite occurs within an area of low atmospheric pressure.

Weather forecasters measure air pressure with a barometer. Barometers measure the current air pressure at a particular location in "inches of mercury" or in "millibars" (mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars, a more modern unit or measure of pressure.

Copyright: Used with permission We made a barometer from a plastic tube partly filled with water (coloured with log wood to make it easier to read). The top of the tube was bent over and sealed-off by binding with wire. The bottom of the tube went into a jam jar of water filled to a constant level.

As the air pressure changed it applied a change in force on the water in the jam jar which changed the height of the liquid in the tube. Changes in height of the liquid in the tube therefore showed changed in air pressure.

What is the best way to measure changes in temperature?

A thermometer will enable you to keep track of temperature to see if it is rising or falling. Our thermometer was rather similar in design to the barometer but smaller. Here temperature changes caused the gas in the jam-jar to expand pushing the level of water in the tube up or down. Up meant warmer weather, down meant colder weather.

Why do meteorologists measure humidity?

Relative humidity is the ratio of water vapour in the air at a given temperature to the maximum amount that could exist at that temperature. On hot summer days, the higher the relative humidity, the greater the discomfort. It is harder to sweat and stay cool as perspiration evaporates less readily and the body feels more hot and sticky.

If the humidity is high there is more moisture in the air and so more chance of clouds forming and rain falling if the temperature drops.

The forecaster bases the possibility of rain on current weather patterns, including wind and humidity, as well as the effect of terrain and long-term weather statistics.

How can we make a humidity meter?

We used human hair to make our hygrometer. Hair is composed of many millions of cells. The cell walls are sensitive to moisture and will change shape and size as the moisture content in the air changes. Hair can therefore indicate the level of moisture in the air by expanding and contracting.

Our meter was made by simply attaching one end of a long strand of hair (15cm) to the base (bottom left hand corner but invisible in the picture) and the other end to the end of a stick pointer which was pivoted near to this point.

To make the meter perform correctly a weight was attached just the other side of this pivot (string and nut). When the air is moist, the hair will expand and lengthen, making the pointer pivot down. When the air is dry, the hair will contract and shorten, making the pointer pivot up. When we made our regular hydrometer observations, we made a mark to indicate where the pointer rested. Over time, you'll be able to see the humidity patterns that will help you forecast the weather.

What do we use to measure rainfall?

This is a device that measures the amount of water collected, over a given time, when it rains. Measurements over long periods of time will give you an idea of how wet the area is in which you live.

Basically, any measuring glass left outside can serve as a rain gauge. If you want the measurements to be compared from day to day you must use the same container and length of collection time every time you collect the rain.

What causes the winds and controls their direction?

Horizontal variations of pressure across the earth cause wind.

In the northern hemisphere the air rotates clockwise around a high pressure area and counter-clockwise around a low. The air would like to move directly from high to low to equalize the pressure but cannot do so because the earth is rotating and it ends up rotating too.

The fact that the air does not go directly but moves around means that the weather associated with the air 'hangs' around for much longer than you would expect. It can also mean that the weather condition covers a much wider area.

Winds are described in terms of the direction from which they come. This makes sense as the winds tend to bring the weather and temperatures with them, so knowing where they have come from can often give information about the weather they might bring.

For example, in the UK a north wind might well have come from the cold polar regions and so will bring cooler weather or air with it - "When a north wind blows it doth bring snow".

Obviously storms are associated with high winds and an anemometer may give some indication of the increase in wind or for that matter the passing of a storm as the wind drops.

How can we make a wind speed measuring device?

Our anemometer was made from four cups fixed to a wooden cross that could move freely round. The cups were made from the nuts of a calabash tree. The rotating device was connected to a dial that moved with it. The dial rotation was counted using a simple sand clock to get an estimate of relative wind speeds.

How do we find out wind direction?

A simple wind vane was constructed to show the direction of the winds. To find north, south, east and west, we used a magnetic compass. Once these directions were known, we attached an arrow to the top of the device and the movement of the pointer gave an indication of wind direction.

How should the measurements be recorded?

You should make measurements from each of your weather instruments in your weather station at least once a day and record them in a logbook. Keep an orderly chart and if you plot the results on a graph as you go along it will be easier to notice patterns in your weather data. You should also make a note of observations such as the sky's colour, cloud cover etc.

As you start to build up months and years of observations and measurements you will be able to start to make predictions about the weather that are much better than guesses.

How accurate were the Rough Scientists' weather predictions?

We successfully made a weather station with a variety of measuring devices and did try to predict the weather, however, it took us a couple of days to make the instruments, which didn't leave us much time to spot any patterns!

Our weather station would be useful over a much longer observation time.

The BBC and the Open University are not responsible for the content of external websites.

Build Your Own Weather Station - from the Discovery website

The Atmosphere in Motion: Air Pressure, Forces and Winds on the on the Lyndon State College site

Books

The Basic Essentials of Weather Forecasting by Michael Hodgson, pub ICS Books Inc

Weather Forecasting by Michael Hodgson, pub ICS Books Inc

Mountain Weather - a guide for skiers and Hillwalkers by William Burroughs , pub The Crowood Press

Weather To Travel by Maria Harding, pub Powergen

For further information, take a look at our frequently asked questions which may give you the support you need.