As NSW and Queensland prepare to face the wrath of Cyclone Uesi the Bureau of Meteorology (BoM) has answered one of the most puzzling questions relating to Australia’s recent extreme weather conditions – why can one area be pouring with heavy rain but there are blue skies just a short distance away?
The bureau explains the type of rain all depends on how high clouds can grow, indicating the vertical stability of the atmosphere.
There is generally two types of rainfall which will determine how isolated or widespread a downpour will be, convective and stratiform.
Convective precipitation is how BoM describes isolated bursts of rain and can often occur during showers and thunderstorms.
It is most commonly a short but intense burst of rainfall and is usually the cause of localised flash flooding.
This is due to an unstable atmosphere where tall clouds with gaps form, producing the variable rainfall.
“This is usually the reason why one town can receive significant rainfall and another town just 20 km away can get nothing,” the experts at BoM explain on their website.
Stratiform precipitation is what BoM considers light to moderate rainfall and is usually due to a stratiform “layer” or flat cloud, which produces uniform widespread rainfall.
This type of rain is usually lighter but can last for longer periods of time.
“You'll often see this type of rain with cold fronts and northwest cloud bands,” BoM meteorologists said.
Bring an umbrella or batten down the hatches?
To determine if your local area is going to receive a drenching like the recent storms in Sydney or just a sprinkle of wet weather, BoM advise to pay attention to two factors when checking the weather forecast – what is the chance of rain and how much possible rainfall is expected?
The ‘chance of any rain’ predicts the likelihood of rainfall in your area and is available whenever the minimum measurable amount of rainfall (over 0.2mm) can be forecast over a 24-hour period, but this alone will not tell you how intense the expected downpour could be.
How much rain to expect is answered by 'possible rainfall' and is determined by two values.
“The first value means the location has a 50 per cent chance of receiving at least that amount of rain. The second represents a 25 per cent chance of receiving at least that amount,” the BoM website explains.
Although it’s possible there can still be a chance of rain with 0 per cent of rain expected to fall, this doesn’t mean you will stay completely dry, but just that the amount of possible rainfall is less than 0.2mm.
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