General Weather Topics

Air Pressure Changes

Areas of high and low pressure are caused by ascending and descending air. As air warms it ascends, leading to low pressure at the surface. As air cools it descends, leading to high pressure at the surface.

In general, low pressure leads to unsettled weather conditions and high pressure leads to settled weather conditions.

In an anticyclone (high pressure) the 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.

In a depression (low pressure), air is rising and blows in an anticlockwise direction around the low (in the northern hemisphere). As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. This is why the weather in a depression is often unsettled, there are usually weather fronts associated with depressions.

  • Rising air creates a low pressure, while sinking air creates high pressure.
    • With high pressure, sinking air suppresses weather development.
    • High air pressure produces clear sky, dry and stable weather.
  • In a low pressure zone, wind is circulated inwards and upwards rapidly.
    • As a result, air rises and cools; clouds and precipitate are formed.
    • Low air pressure produces unstable weather conditions like rain or storms.
    • Storms are the deep centre areas of low pressure zones. It brings strong wind and rain.

Air pressure changes all the time and changes in air pressure give a indication of future atmospheric developments. However, not all air pressure changes influence weather conditions, for example it has been observed that rapid movement towards a low pressure in an Offshore area may lead to a sharp drop in air pressure without any information-content for future weather developments being evident.

  • static air pressure:  This is the typic situation during high pressure periods (~1025hPa), when the weather is usually stable and fair (In wintertime perhaps foggy). You may observe regular daily variations of the pressure.
  • increasing air pressure: A slow and consistent increase is a sign of improving weather conditions. Rapid increases is a sign for shorter periods of improved weather and can also be a sign of high winds lasting perhaps for several days.  
  • falling air pressure: A slow constant decrease of air pressure is a sign for a slow worsening of weather conditions, usually occurring at the end of a high-pressure period. Initially the pressure decrease may be difficult to observe and some cyclic variations may be seen. A rapid fall of air pressure is a sign of an approaching low and of bad weather. depending upon the rate of decrease you should expect strong winds up to gale force.

ET – Evaporative Transpiration

You can use ET to determine when and how much irrigation water is needed. A common use is with grass irrigation. As an example, if your irrigation system applies 12.7mm of water during one irrigation event, with no rainfall and an ET loss value for 2 days in a row of 6.35mm, you would need to irrigate after 2 days. (12.7 / 6.35 = 2)