The variable weather in Melbourne means aviation hazards are ever-present.
Fog/low cloud
Fog is a significant hazard for Melbourne and can affect many aerodromes around the Melbourne Basin (Melbourne, Essendon, Moorabbin, Avalon and Point Cook) at the same time. Fog and low cloud events are often associated with a phenomenon known as the Melbourne (Spillane) Eddy. In this situation, moist south-westerly winds develop and move low cloud over the Melbourne Basin.
Radiation fog is not common around Melbourne, but is in the Yarra Valley. Low cloud is also often associated with a southerly air flow. Moisture from Port Phillip Bay can produce low cloud and possible drizzle, for hours over the basin. In moist flows, sea fog and low cloud can also affect the basin via the bay.
Thunderstorms
Thunderstorms can occur in the Melbourne Basin at any time of the year. The main thunderstorm activity is during spring and summer, with these storms more likely to bring large hail and damaging winds. The triggers include frontal systems, troughs and sea breeze convergence.
Thunderstorms can be tracked when they form on the ranges to the north and are then steered by a north to north-westerly flow over the Melbourne area. Large thunderstorm bands can occur with fronts and can move quickly through Melbourne. In these airstreams, and for the Melbourne Basin, it is best to monitor thunderstorms near Ballarat, as the storm cells typically track from this direction. Meanwhile, when thunderstorms develop on a sea breeze convergence line, they can have weak steering and hence be slow moving and last for several hours.
Showers
In a southerly flow, Melbourne is exposed to showers, with reduced visibility and sometimes long periods of low cloud. Moorabbin is most affected by showers when the air stream is from the south-west. The Melbourne area is normally protected from precipitation from the north due to a rain shadow from the ranges. However, occasionally bands of showers can move over the area from the north.
Winds
The strongest winds are typically prefrontal northerly winds. These winds can strengthen due to funnelling through Kilmore Gap to the north of Melbourne. Consequently, Melbourne Airport can experience stronger winds than Essendon, despite the proximity of the aerodromes.
An afternoon southerly sea breeze occurs mainly in spring and summer and can affect all aerodromes, resulting in runway changes. Moorabbin and Avalon can often experience an early afternoon bay breeze followed by a stronger late afternoon southerly sea breeze.
Turbulence
The west-to-east orientation of the Great Dividing Range means northerly winds will produce the most turbulent flows in the basin. Severe turbulence can result ahead of strong cold fronts. Windshear can also occur when strong northerly winds aloft do not completely mix to the surface. This results in a strong change in wind speed with height, which can make take-off and landing difficult.