Australian Weather

Today, 3:24AM UTC

As we head into the cooler months fog will become a more common sight, but what exactly is it?  In its simplest definition, fog is simply a cloud at ground level. More specifically it is microscopic water droplets suspended in the air near to the surface of the earth, reducing visibility to less than 1000m.  There are several different types of fog, with different mechanisms of formation, with radiation fog being one of the most common. Radiation fog is caused by radiational cooling overnight at and near the surface of the earth and requires a few key elements for its formation. Before we take a look at these key elements, we should first take a closer look at radiational cooling. Simply put, it is the cooling of the earth's surface, and near surface air, due to the loss of longwave radiation. At night more longwave radiation escapes to space than incoming shortwave radiation to the surface, resulting in a net cooling effect. The first of the key elements mentioned above is high relative humidity, providing the moisture needed to form fog droplets. The second and third elements are calm-to-light winds and rapid cooling. Look out for surface high pressure systems as these generally bring light winds, in addition to enhancing radiational cooling at the surface – ticking two of the key elements.   Clear skies are also a key factor in the formation of radiation fog. Cloud acts like a blanket, preventing long wave radiation from escaping to space and leading to a warmer surface. Warmer air can hold more moisture than cooler air and as such condensation is more unlikely to take place.  So how exactly does radiation fog form?  As daytime heating ceases, the earth's surface and the layer of air just above it begin to cool via a process called conduction. If there is enough water vapour in the air, and sufficient cooling at the surface, this low-level air will reach saturation.   It should be noted here that air is an extremely poor conductor of heat and as such only a very shallow layer near the ground is cooled. This is where light winds come into play. A slight breeze will help mix the air a little, with cooler air then being distributed through a deeper layer.  As cooling continues, near-surface air will condense onto objects as water droplets (known as dew). If the winds are too light, this is where the process ends, with only a few centimetres of the near-surface air cooled sufficiently. Conversely, if winds are too strong, warmer and drier air is mixed down, effectively stopping any condensation taking place.  If cooling continues however, with just the right amount of wind, excess water vapour in the saturated layer just above the surface condenses, forming fog droplets. As radiational cooling continues, the fog layer deepens and can reach a couple of metres deep.  Radiation fog is usually relatively short lived once the sun rises. As the sun slowly warms the ground throughout the morning, it warms the surface just above it, causing the water vapour (fog) to evaporate from the ground up. Whilst it is common to hear the phrase ‘burning off’ when referring to fog disappearing, in actual fact, it is more dissipating.  

20 May 2022, 3:17AM UTC

New model data confirms that a La Niña-like pattern could persist in the coming months, increasing the likelihood of above-average rain in parts of Australia in winter and spring. The Pacific Ocean has been in a distinctive La Niña pattern since late last year and during the last fortnight, this pattern has no signs of weakening. Image: Current sea surface temperature anomalies in the Pacific Ocean, showing a distinctive La Niña pattern with cooler-than-average water in the central and eastern equatorial Pacific Ocean and warmer-than-average water in the western equatorial Pacific Ocean. Source: NOAA The graph below shows the latest model-based prediction for La Niña (and its counterpart El Niño) from the Columbia University’s International Research Institute for Climate and Society (IRI) and the U.S. Climate Prediction Centre (CPC). This data was published on May 19, 2022. Image: El Niño-Southern Oscillation (ENSO) forecast for the next 10 months, according to the U.S. CPC/IRI. The blue bars show the probability of La Niña occurring during each three-month period, with grey and red bars representing the probability of neutral and El Niño conditions, respectively. Source: CPC/IRI The graph above shows that, according to the latest IRI/CPC model forecast, La Niña has a 62 percent chance of persisting through the southern hemisphere winter and a 58 percent chance of occurring in spring. The continuation of a La Niña-like pattern in the coming months is also being predicted by most of the international forecast models that are analysed by the Bureau of Meteorology. The first graph below shows that all models surveyed by the Bureau expect sea surface temperatures in the equatorial Pacific Ocean to be near or exceeding La Niña thresholds in late winter. The second graph shows that by mid-spring, four out of seven models are in a La Niña state. Image: Comparison of international seasonal forecast model’s predictions for sea surface temperature anomalies in the central equatorial Pacific Ocean in August 2022 (top) and October 2022 (bottom). Source: Bureau of Meteorology The Bureau of Meteorology’s outlooks currently only go out until October. However, the IRI/CPC graph near the top of this article shows that their modelling gives roughly equal weighting to La Niña and neutral (neither La Niña nor El Niño) conditions in the Pacific Ocean during the southern Hemisphere’s summer. The main takeaway from these datasets are: La Niña or a La Niña-like pattern are predicted to linger through the middle of 2022, possibly encompassing both winter and spring in the Southern Hemisphere. This would increase the likelihood of above-average rain and below-average daytime temperatures over large areas of Australia during winter and spring, as shown in the map below. There is still some uncertainty between forecast models, especially beyond the Southern Hemisphere’s spring. Image: Average winter-spring rainfall (top) and rainfall (bottom) deciles from 13 La Niña years combined, indicating average rainfall and temperature footprints of La Niña. Source: Bureau of Meteorology. The impacts of this ongoing La Niña are also likely to be enhanced by a developing negative Indian Ocean Dipole (IOD) in the coming months. A negative IOD also increases the likelihood above-average rain and cooler-than-average daytime temperatures in Australia. In fact, a number of forecast models suggest that we could see a strong negative IOD this winter and spring.