Australia Weather News

Humidity is the amount of water vapour in the air. Water vapour is a gas and therefore invisible but can be more responsible for human discomfort than high temperatures. High humidity reduces the effectiveness of human and animal cooling processes such as sweating.

The most common measurement of humidity that we see in the press and on weather web sites is relative humidity but it is one of the least useful indicators of atmospheric water vapour and its effects. It is expressed as a percentage of the partial pressure of the water vapour in the air compared to the maximum possible that the air can hold at that temperature. On a cold winter morning the humidity can often approach 100% and when that occurs, fog forms but in reality, the amount of moisture in the air is not nearly as high as what might occur on a summer afternoon when the temperature is over 30 and the relative humidity is a lower 70%.

A better measure of humidity is dew point. This is the temperature at which dew or fog will form. A dew point of 20 degrees means the moisture content of the air is very high (imagine a fog at 20 degrees).

February is the most humid month along the east coast of Australia. Dew points of 20 degrees occur in Sydney on about 25% of February afternoons but less than 20% in Melbourne and less than 10% in Adelaide. In Brisbane dew points frequently exceed 20 degrees and in tropical north Queensland and around Darwin, a 20 degree dew point would occur almost every day. High dew points can make the apparent temperature feel a lot higher as well. For example, in the past week, it has been very humid in Queensland. In recent days in Cairns a temperature of 33 degrees and a dew point of 28 degrees meaning the effective temperature felt over 40 degrees.

The most humid places on Earth naturally occur in the tropics. Cities such as Jakarta, Kuala Lumper and Singapore are always humid due to their proximity to the Equator while cities such as Chennai and Kolkata in India, Hong Kong, inland cities in the SE of USA and Australia’s Darwin have extremely high humidity for many months of each year.

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