The Weather

Observing and recording

Powerpoint Presentation © Pitsford Hall weather stationNorthamptonshire Grammar SchoolAbridged version without video clips

What is weather?

Weather describes the state of the atmosphere at any particular time.

Weather can be described in terms of temperature, precipitation (snow, rain & hail), wind speed and direction, visibility and cloud amounts.

What is Climate?

Climate describes the average weather of a particular part of the world at different times of the year

In Britain we would expect cool summers and mild winters with moderate rainfall throughout the year

The Weather Station

A weather station makes continuous measurements of different aspects of the weather.

Weather stations use standard instruments so that their readings can be compared.

HOWEVER…

You can make weather measurements with equipment a lot cheaper and simpler than the kit shown on the previous slide which is an official Met Office station. Please see www.metlink.org for more information.

Temperature

Temperature is recorded using thermometers housed inside a Stevenson screen

Weather stations record both air temperature and the temperature of the ground

Temperature: The Stevenson Screen

Why is the screen painted white?

Why is it raised on legs above the ground?

Why has it got louvred sides?

Precipitation

Rainfall, snow, hail and fog.

Rainfall is measured in a raingauge.

Some raingauges record rainfall automatically whilst others are emptied everyday by an observer

Wind

The direction and strength of the wind are both measured

A wind vane measures direction

An anemometer records strength

Wind strength can also be measured using the Beaufort Scale

The Beaufort Scale

Pressure

Pressure is the weight of the atmosphere

When air rises pressure falls

When air sinks pressure increases

Pressure controls the type of weather

Barometers and barographs record pressure

Recording pressure

Sunshine

A note is made of the number of hours of bright sunshine each day

Sunshine is traditionally measured using a Campbell-Stokes sunshine recorder

Cloud

The amount of the sky obscured by cloud

Different types of clouds

Sometimes even the speed and direction in which the clouds are moving are recorded using a nephoscope

Cloud Types - Cumulonimbus

Cloud Types - Cumulus

Cloud Types - Stratus

Cloud Types - Cirrus

Britain’s Weather

The Seasons

Temperature

In summer it gets warmer the further south you go

In winter it gets warmer the further west you go

In Britain temperature is affected by latitude and the sea

Temperature: Latitude

Temperature: Summer & Winter

In winter the northern hemisphere is tilted away from the sun so the sun’s rays are weaker

In summer the northern hemisphere is tilted towards the sun so the sun’s rays are stronger

Temperature: Summer & Winter

During the course of the year the Earth tilts towards and away from the sun

The mid-points of this cycle are called the equinoxes (spring and autumn)

Temperature: The Sea

The North Atlantic Drift is a warm ocean current originating in the Caribbean

It moves across the Atlantic keeping the west of Britain warmer than the east in winter

Winter in Canada & Britain

Rainfall

In Britain it rains the most in the west and in mountainous areas

Rain is brought across Britain from weather systems called depressions

In summer showers result from convection on warm days

How it rains

The surface is heated by the sun

Air rises Air expands and cools Air condenses Water droplets grow to

form raindrops Air can rise in different

ways

Relief rainfall

Cyclonic Rainfall

Depressions are areas of low pressure formed when cold and warm air meet

The warm air rises above the cold air to form a front

There are two types of fronts depending on the way in which the air masses are moving.

Cyclonic Rainfall: Fronts

Warm fronts form where warm air moves towards cold air

Cold fronts form where cold air moves towards warm air

As air rises at fronts both are responsible for rain

Convectional rainfall

Common on hot summer days inland

Hot air rises quickly and condenses to form cumulonimbus

Water freezes at the top of the cloud forming hail

Associated with lightning

Fair Weather

Fair weather is produced by high pressure

High pressure forms anticyclones

Air sinks and prevents the formation of rain clouds

Anticyclones cause heatwaves in summer and frosty/foggy mornings in winter

Forecasting the Weather

Modern forecasts use computers to simulate the likely weather based on careful observations from 100s of weather stations round the world

Satellite images and radar give a better picture of the weather over a wide area

A Satellite Image

A Radar Image

Cyclonic rainfall moving in from the west

The brighter the colour, the heavier the rainfall

Mountains intensify the rain such as over the Scottish Highlands

The Weather Map

Weather maps summarise the weather over a wide area using symbols

What symbols are used for: pressure, wind direction & strength, cloud cover and rainfall?

Weather

This Powerpoint presentation was produced specially for the Department of Geography & Geology at Northamptonshire Grammar School

May 2005

© M J Lewis