Weather Data (Cambridge International IGCSE Geography)

Calculations using Weather Data

Spec point — 2.4.2 · Make calculations using information from weather instruments

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

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 are 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

Exam Hint

• Always make sure you state wind direction clearly, for example:

  • The wind is coming from the east
  • It is a westerly wind

• Saying the wind is in a northerly direction isn't clear enough and can be misinterpreted

Pressure

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

• The mean (average) sea level pressure is 1013 mb

• 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 clouds and leads to light winds and settled weather conditions

• In depressions (low pressure), the air rises and blows in an anticlockwise direction around the low (in the northern hemisphere)

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

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

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

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

Describing Pressure Systems

Relative humidity

• This is measured using the relative humidity table. Find it by looking for the intersection of the wet bulb thermometer line's depression and the dry bulb temperature line

• Humidity is shown as a percentage of temperature

• Temperature determines relative humidity; the more water vapour the air can hold, the higher the temperature.

  • If air is 20 °C and holds 4 grams of water, it 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

Worked Example Explain how a wind vane is used to show the direction from which the wind is blowing

[2 Marks]

• The arrow / pointer turns / spins round / pushed by wind [1] and points to N/E/S/W / compass direction (from which wind is blowing) [1]