Average 9 am and 3 pm cloud

These maps show the 9 am and 3 pm average annual and monthly cloud distribution across Australia.

Average annual 9am cloud

Product code: IDCJCM0048

What do the maps show?

These maps show the average annual, seasonal and monthly 9 am (3 pm) total cloud amount (TCA) over the period 1976 to 2005. Average annual TCA is calculated by adding 9 am (3 pm) TCA averages over a specified period (1976–2005) and dividing by the number of years in that period (30 years in this case). Similarly, average monthly and seasonal 9 am (3 pm) TCA over the period 1976 to 2005 is calculated by adding monthly or seasonal 9 am (3 pm) TCA averages and dividing by the number of years in the specified period

The annual 9 am and 3 pm cloud amount maps show areas of low cloud values across central Australia, with more cloud in coastal areas, particularly in the north and south. Areas of inland Australia have a lower moisture content in the air (see relative humidity maps) and therefore less cloud cover. Coastal areas have a higher moisture content therefore greater and more frequent cloud cover.

Wet and Dry Season TCA maps (northern wet season: October to April and northern dry season: May to September) show the difference between these two distinct seasons. During the wet season, there is an increase in total cloud amount over northern Australia as this area comes under the influence of the monsoon. At the same time, the sub-tropical ridge moves south and southern Australia experiences drier, less cloudy conditions. Conversely, during the northern dry season there is a decrease in cloud amount in northern Australia as the sub-tropical ridge moves north, while southern Australia is more affected by frontal systems. These bring an increase in rainfall and cloud cover.

How are the values calculated?

The total cloud amount is measured visually by estimating the fraction (in eighths or oktas) of the dome of the sky covered by clouds. A completely clear sky is recorded as zero okta, while a totally overcast sky is recorded as 8 oktas. The presence of any trace of cloud in an otherwise blue sky is recorded as 1 okta, and similarly any trace of blue in an otherwise cloudy sky is recorded as 7 oktas.

Further information and metadata

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