Weather Data (Cambridge (CIE) O Level Geography)

Calculations using Weather Data

Rainfall

• Daily, weekly, monthly and annual rainfall totals 

• Always measured in mm

• Always plotted on a bar graph

• Mean (average) monthly and annual rainfall over a minimum of 30 years

Describing Annual Rainfall Amounts

Temperature

• Measured in °C or °F

• Always plotted as a line graph

• Calculations are:

  • Diurnal (daily)

  • Mean daily temperature

  • Mean monthly temperature

  • Annual range

  • Mean annual range

Describing Temperatures

Describing Temperature Ranges

Wind

• Wind is measured in speed:

  • Knots (Kts)

  • Miles per hour (mph)

  • Kilometres per hour (km/h)

• Direction is quoted from where the wind is blowing and not where it is going to, using compass points

• This is important as it informs of what temperature and moisture is being brought with it

• The most frequently occurring wind is called the prevailing wind (UK's prevailing wind is from the southwest)

• The direction giving the strongest wind is called the dominant wind

• Wind is plotted on a map with wind barbs

Describing Wind Speeds

Pressure

• Pressure is measured in millibars (mb) and based against average sea level pressure

• Mean (average) sea level pressure is 1013mb

• It is not totalled over time but on a time-basis depending on circumstances:

  • Following a storm or potential storm, pressure readings would be needed hourly or more

  • Forecast for a weekend would be a daily measurement

  • Forecast for sailing may need an early morning reading and then later etc. 

• On a weather chart, lines joining places with equal sea-level pressures are called isobars

• Isobars identify features such as anticyclones (areas of high pressure) and depressions (areas of low pressure)

• Anticyclone (high pressure) winds tend to be light and blow in a clockwise direction (in the northern hemisphere)

• Also, the air is descending, which reduces the formation of cloud and leads to light winds and settled weather conditions

• Depressions (low pressure), air is rising and blows in an anticlockwise direction around the low (in the northern hemisphere)

• The rising air cools, causing water vapour to condense which form clouds and perhaps precipitation

• This is why the weather in a depression is often unsettled, there are usually weather fronts associated with depressions.

• High pressure area surrounded by lower pressures are described as a high-pressure system even if 1013mb are not reached

• Low pressure areas surrounded by higher pressure can have a central pressure of more than 1013mb

Describing Pressure Systems

Relative humidity

• Measured via the relative humidity table and is found by looking at where the depression of the wet bulb thermometer line intersects with the dry bulb temperature line

• Humidity is shown as a percentage of temperature

• Relative humidity is controlled by temperature, the higher the temperature the more water vapour the air can hold:

  • If air is 20°C and holds 4 grams of water is has a relative humidity of 27%

  • 4g more water is added, so air is now 8g @ 20°C = 52% and so on until it reaches saturation and moisture will start to condense (20°C still, but air holds 15g of water = 100% humidity)

  • However, if the temperature rises, then the air can again hold more water, so the relative humidity drops (15g @ 23°C = 83% humidity or 15g @ 32°C = 50% humidity etc.)

  • But, if the temperature drops, then the air holds too much water, so the excess water is released until equilibrium is reached