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Weather and Climate

  
 
What is the difference between weather and climate?
 
 
Weather is the day to day changes in the atmosphere.
 
 
Climate is the overall pattern of weather, usually based on an average over 30 years.
 
   
 
Climate graphs
 
 

Climate graphs are a way of showing how temperature and precipitation vary throughout the year for a particular place.

   
 
How to construct a climate graph…
 
 
Draw and label the axes (remember you need two names for the y axis – ‘temperature’ and ‘rainfall’ ).
Work out the right scale for each axis (look at your highest temperature and rainfall figures).
Draw the graph using bars for rainfall and a line for temperature.  Remember to plot the temperature points in the middle of the months on the x axis.
 
   
 
Why does it rain?
 
 
There are three main types of rainfall – relief, convectional and frontal.
In all three situations, rainfall occurs because air rises. Air contains water vapour and as the air rises it cools and condensation can occur. This means that the water vapour returns back into minute droplets of water and falls as rain!
 
  Convectional Rainfall  
   
  Relief Rainfall  
   
  Frontal Rainfall  
   
 
Rainfall
 
 
Rain is a type of precipitation.
Precipitation is where water vapour in the air is cooled and condenses into water droplets.
Other types of precipitation are snow, sleet, hail, dew or fog.
Rainfall is measured with a rain gauge.
 
   
 
Temperature
 
 
Temperature is how hot or cold it is.
Temperature can be measured using a thermometer such as the liquid-in-glass type, where the liquid (mercury or spirit) in the bulb expands and rises up a capillary tube. 
Temperature is measured in either oCelsius or oFahrenheit.
Temperatures are measured in the shade, so that the heat of the sun does not heat up the instrument being used.
  
   
 
Wind
 
 
Wind is simply moving air. 
 
Wind speed is how fast the air is moving.
Wind speed is measured by an anemometer.  This has metal cups which rotate in the wind.
The stronger the wind, the faster the anemometer turns.
 
   
 
Beaufort scale
 
 
The Beaufort Scale is a measure for the intensity of the wind based on conditions.  It can be used to estimate wind speed.
The Beaufort scale is divided into a series of values, from 0 for calm winds to 12 and above for hurricanes.
 
   
 

Beaufort Scale

Effects

Speed (kph)

0

Smoke rises vertically

0

1

Smoke drifts

1-5

2

Wind felt on face, leaves rustle

6-11

3

Leaves and small twigs move

12-20

4

Small branches move

21-30

5

Small trees sway

31-40

6

Large branches sway, umbrella used with difficulty

41-50

7

Whole trees sway

51-60

8

Twigs break off trees, hard to walk into wind

61-74

9

Chimney pots and slates blow off

75-87

10

Trees uprooted

85-100

11

Rarely occurs inland

101-115

12

Disastrous, widespread damage

115+
 
 
Wind direction
 
 
Wind direction is measured using a wind vane.
 
   
 
Sunlight
 
 
Sunlight is the amount of daylight.
The duration and the intensity of sunlight is measured using a Campbell-Stokes sunshine recorder.
These focus light from the sun onto a piece of card where it leaves a burnt trace. The more sunshine there is, the longer the line.
 
   
 
Air pressure
 
 

Air pressure is the force exerted on the Earth by the weight of the air above.

It is measured with a barometer.

 
   
 
Visibility
 
 
Is how far we can see, it’s measured by a visibility meter, in metres or kilometres.
 
   
 
Humidity
 
 
Is the percentage of water vapour in the air. It’s measured with a hygrometer.
 
   
 
Cloud types
 
 
There are four main cloud types:
 
*cirrus - the Latin for 'hair' for clouds that are wispy
*cumulus - the Latin for 'pile' for heaped looking clouds
*stratus - from stratum, the Latin word for 'layer‘, for sheets of cloud
*nimbus - the Latin word for 'cloud' for low, grey rain cloud
 
   
 
Cloud cover
 
 

Cloud cover is the fraction of the sky covered in clouds. 

Cloud cover can be measured by observation. 

It is measured in oktas.

 
   
 
Synoptic charts
 
 
Synoptic charts are charts of surface pressure.  They help meteorologists predict the weather.
Isobars are lines joining areas of equal pressure.
Pressure is decreasing towards the middle. This is a low pressure system.
Pressure is shown in millibars.
 
   
 

Satellite images

  
 
Satellite cameras detect heat and light from the Earth.  
 
Visible satellite images record light from the sun, reflected back to the satellite by cloud tops and land and sea surfaces. They are equivalent to a black and white photograph from space.  
 
Visible satellite images can only be taken during the day.
The infrared satellite images record the heat that is given off by an object. The coldest objects such as fronts and high clouds show up the whitest.
 
   
 

Depression

  
 
Satellite images are used to predict and track the weather.
The centre of the depression can be recognised by a swirl of cloud.

The line of continuous cloud from this centre marks the line of the fronts. 

The white dots at the rear of the depression mark the shower clouds.
 
   
 
Low pressure systems (Depression)
 
 
When air rises, it creates a low pressure system. 
Air rising means clouds form and there is a possibility of rain.
  
   
 
High pressure systems (Anticyclone)
 
 
When air sinks and warms this causes a high pressure system. There are clear skies and no rain.
 
In Summer, clear skies mean that there are no clouds to stop the sun shining through and so days can be warm.
At night, however, there are no clouds to stop the heat escaping so nights can be cool.
In Winter, high pressure systems give rise to clear, bright days.  However, clear skies also mean that there are no clouds to stop the heat escaping and so ice and frost can form.
 
   
 

Winter Anticyclone

  
 
•In winter, the cloudless skies we get with anticyclones allow heat to escape.
•The ground cools quickly at night, cooling air above it.
•Water vapour condenses and freezes on cold surfaces, giving frost.
•It also condenses on dust giving fog.
•Days are often clear, cold and bright.
•Water can freeze into ice.
 
   
 

Summer Anticyclone

  
 
•In summer the cloudless skies mean the sun is strong and days are hot.
•Evenings are cool.
•Ground cools at night, so water vapour condenses and forms dew.
•No rain which can lead to drought.
•On hot days hot air rises quickly, cools and forms big black clouds leading to thunderstorms.
 
   
 
Why do the British Isles have unpredictable weather?
 
 
The answer has a lot to do with air masses…
Air masses are huge blocks of air.  They can be damp or dry, warm or cold, depending on where they came from and over what type of surface they have travelled. 
 
For example, an air mass that has travelled over the sea will increase its moisture content and be more likely to produce rainy weather.
 
   
 
What happens to the weather when the air masses meet?
 
 
Depressions (low pressure system) form when a cold air mass meets a warm air mass.
The junction between these two different air masses is called a front.  A front is associated with a change in the weather.
 
   
 
What happens at fronts?
 
 
A warm front means that warm air is coming. 
At a warm front, warm air is rising over cold air. 
This usually produces clouds and rain. 
 
   
 
A cold front means that cold air is coming. 
At a cold front, cold air pushes under the warm air. 
This produces strong winds and heavy rain.