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
Annual rainfall in mm | Description of the amount |
0 - 249 |
Very low |
250 - 499 |
Low |
500 - 999 |
Moderate |
1000 - 1999 |
High |
Over 2000 |
Very high |
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
Temperature (°C) | Description |
Below -10 |
Very cold |
-10 to -1 |
Cold |
0 - 9 |
Cool |
10 - 19 |
Warm |
20 29 |
Hot |
30 and above |
Very hot |
Describing Temperature Ranges
Temperature Range in °C | Description |
0 - 3 |
Very small |
4 - 8 |
Small |
9 - 19 |
Moderate |
20 and above |
Large |
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
Wind Speeds (km/h) | Description |
Below 50 |
Calm, light, moderate or strong winds |
50 - 100 |
Gale |
101 - 118 |
Storm |
119 and above |
Hurricane |
Exam Tip
- 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
- 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
Pressure System in mb | Description |
1013 |
Sea level pressure |
1013 and above |
High pressure or anticyclone |
1013 and below |
Low pressure or depression |
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
Worked example
Explain how a wind vane is used to show the direction from which the wind is blowing
[2]
- 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]