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The hot and cold of drought in NSW

Ben Domensino, Monday July 30, 2018 - 14:36 AEST

Some parts of drought-stricken NSW are having both their coldest and warmest July in more than 20 years, when considering maximum and minimum temperatures separately.

It's been a dry month in NSW, with most of the state receiving less than 20 per cent of the usual July rainfall and some places on track to register their driest July on record.

As of 9am on Monday, Forbes had only received 0.8mm of rain since the start of July and no rain is forecast to reach the gauge before the end of the month. This is likely to be the driest July in 143 years of records at Forbes.

Further north, Nyngan and Brewarrina are both likely to register their first rainless July since 1970.

Broken Hill's paltry 0.4mm of rain this month follows the site's driest first half of a year on record, with data available back to the late 1800s.

The lack of rain in recent months has taken a noticeable toll on the landscape in NSW, turning the ground a barren shade of brown. The combinations of absent rainfall and enhanced evaporation has starved the soil of moisture.

While a cropless countryside is an unmistakable sign of drought in Australia, a lesser know meteorological signal that times are dry is the coincidence of hot days and cold nights.

During drought, reduced moisture on the ground and in the topsoil causes air immediately above the ground to become drier than usual. This has a significant effect on overnight temperatures because drier air can cool down further than more moisture-laden air.

Put simply, drier air allows the temperature to drop further before condensation starts to occur. When condensation occurs - which causes fog, frost and dew - latent heat is released into the atmosphere, which slows the cooling process down. So, the drier the air, the lower the dew point, and the more the air can cool down before condensation kicks in.

This cold night effect is enhanced even further during drought by a lack of cloud in the absence of rain-bearing systems. Clear skies allow the previous day's heat to radiate into space at night, while clouds trap some of this warmth in.

In contrast to cool nights, clear daytime skies during a drought typically cause maximum temperatures to be above average, thanks to abundant solar heating.

The unmistakable cold night-warm day signal of drought has been evident in NSW this month.

With just one day left in the month, Parkes is on track to register both its lowest average minimum and highest average maximum for July in 20 years of records. Nearby, Dubbo is also having its second coldest July in 16 years based in minimums and its equal warmest July in more than a century based on maximums.

- Weatherzone

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