Adelaide experiences a diverse range of weather conditions, with the best flying days occurring in autumn but with aviation hazards in all seasons.
Winds
An afternoon south-westerly sea breeze is a regular feature during summer. Typically, a west-south-westerly sea breeze will arrive in the late morning, slowly turning south-southwesterly during the late afternoon, then southeasterly by the evening.
Thunderstorms
These can occur in any month but are slightly more common in summer and spring. While thunderstorms can form on the Adelaide Plain, they are more likely to form to the north-west before being steered towards Adelaide. Severe thunderstorms with large hail, heavy rain and damaging wind gusts are likely to form in this way, before rapidly intensifying over the warm Spencer and St Vincent Gulf waters and bearing down on Adelaide.
In cooler months, thunderstorms are usually associated with the passage of cold fronts. This type is often smaller than summer storms, but wind gusts can be as strong, with possible short-lived tornadoes. colloquially called ‘coldies’.
Typically, thunderstorms will reach the eastern districts of South Australia (east of the Mount Lofty Ranges) the day before, and this can act as a ‘heads ‘up’ for thunderstorms at Adelaide.
Another type of thunderstorm that can occur year-round is the mid-level thunderstorm. These form due to the passage of mid- to upper-level troughs and the presence of midlevel wind convergence and are usually less hazardous, with wind gusts and rainfall often not reaching the ground (virga).
Turbulence and wind shear
Gully winds, most often observed on summer nights, typically occur when a high-pressure system centered south of Adelaide takes hold quickly after a cool change. These can produce significant turbulence at Adelaide Airport. Strong easterly winds coming over the Mount Lofty Ranges, exacerbated by the cooler air accelerating down the western slopes of the ranges, can produce damaging winds and turbulence about the foothills, particularly in the eastern suburbs; this can also extend to the Adelaide CBD or even Adelaide Airport, sometimes resulting in a low-level rotor – a localised violent overturning of the atmosphere – resulting in severe turbulence.
On the eastern side of the ranges, particularly in winter, strong westerly winds and associated turbulence and mountain waves are very common, impacting flight paths approaching from the east. Strong westerly winds can often produce low-level wind shear, particularly on cool winter mornings when a north-easterly katabatic (drainage) flow has formed at the surface.
Low-level wind shear at Adelaide Airport happens most often during early mornings when vigorous low-level north-westerly winds are struggling to reach the surface because of an inversion forming. In these cases, the surface winds are usually observed at about 15 knots from the north-west and produce significant vertical wind shear.
Low cloud
The greatest risk of broken low cloud occurs overnight and in the early morning through a process known as topographic blocking. The prevailing wind is blocked by the topography, in this case the Mount Lofty Ranges. This typically occurs in a light north-westerly flow that does not have enough momentum to lift over the ranges.
Conditions can go from CAVOK to BKN005 in a matter of minutes, potentially lasting into the early afternoon. Broken low cloud at Edinburgh and Parafield Airports can sometimes act as precursors to low cloud at Adelaide Airport.
Fog
Adelaide Airport averages about 4 or 5 fogs a year, predominantly in winter, although rare cases have been recorded in December. When there is low-level moisture (for example, after showers the day before) with calm wind conditions and clear skies overnight (for example, the quick establishment of a high-pressure system), radiation fog is a high risk.
Another scenario includes moisture banking up against the Mount Lofty Ranges, such that low stratus forms and then lowers to the surface, resulting in fog.
Precipitation
In a north-westerly synoptic flow, showers and rain are usually heavier, and usually associated with more significant visibility reductions and lower cloud bases.
However, in a south-westerly flow, the showers tend to be less problematic for aviation because of the colder and drier air. Showers just after a significant cold front can still ‘pack a punch’; the showers in the cold pools of air trailing these fronts (usually lasting less than a day) often produce small hail and strong wind gusts.