Meet the locals: when the weather gives the place character
03 May 2019
Love ‘em or loathe ‘em, Australia’s local weather phenomena can be a defining feature of a city, town or region. From the sweet relief of the Fremantle Doctor taking the edge off Perth’s heat to the blustery southerly buster that can blast cool changes up along the New South Wales coast, our meteorologists around the country introduce you to some of their best-known local weather ‘characters’.
Much weather is variable and complex (which is why forecasting it is a science) but some kinds of weather happen regularly in particular places when certain conditions line up. This is because they are influenced by local geography, such as the position of hills, valleys and proximity to the sea, as well as factors that change regularly with the seasons in that location, such as air temperatures and the level of moisture in the atmosphere.
The Fremantle Doctor
The Fremantle Doctor is a local term given to the afternoon sea breeze that affects the Perth region. In Perth, hot days are usually associated with dry easterly winds during summer and early autumn months, beginning during the morning and continuing into the afternoon. The Doctor is just the tonic as it brings with it much cooler temperatures as it spreads inland from the coastal fringe through to the Darling Scarp. In almost all instances it arrives from a southwesterly direction during the late afternoon, with locals rejoicing that the heat of the day is over.
Stephen McInerney, Meteorologist, Perth
The Bridgewater Jerry
The Bridgewater Jerry is the local name for fog rolling down the Derwent River to Hobart, across the Tasman Bridge and further south. Generally an autumn or winter phenomena, it occurs on cold clear nights when fog forms in the upper Derwent Valley and the Jordan Valley. These two valleys ‘join’ near Bridgewater (perhaps a reason for the name—although its exact origin is shrouded in 'mist-ery') with the fog appearing to flow down the river, trapped by the hills to the west and east. It may cause reduced visibility in places, but in reality it's a thing of beauty and much loved by locals!
Simon McCulloch, Manager Weather Services, Hobart
Image: Bridgewater Jerry, Tasmania, 13 August 2017. Credit: Sohee Kim
Hector ('the Convector')
A thunderstorm forms over the Tiwi Islands, just north of Darwin, almost every afternoon during the wet season (October–April). The formation of this thunderstorm is so regular it has a name: Hector. It was named by World War II pilots who used it for navigation. It's also known as 'Hector the Convector' (convection describes the way heat energy moves between hot and cool parcels of air in thunderstorms).
Hector forms most days due to the perfect collision of sea breezes interacting with the land mass of the Tiwi Islands, and can reach as high as 20 km into the atmosphere. It's not unusual for Hector to dump rainfall of more than 50 mm within a couple of hours during the wet season.
Laura Boekel, Meteorologist, Darwin
Video: Weird Weather: Hector the thunderstorm
Morning glory cloud
Morning glory, also known as a roll cloud, is a jaw-dropping phenomenon that generally occurs early to mid-spring over northwest Queensland, most notably across the Gulf Country and the Gulf of Carpentaria. During the afternoon, sea breezes from the east and west coasts of Cape York collide over the peninsula and collapse back towards the surface, creating a wave in the atmosphere like a ripple moving across a pond. When conditions are just right (there’s an ‘inversion layer’ where temperature increases with height), moist air ascends and condenses into cloud along the leading edge of the westward-moving front, descending at the rear to form turbulent, cylinder-like waves. It reaches the southern Gulf coast in the early morning as a spectacular roll cloud.
While roll clouds can occur anywhere given the right ingredients, the unique orientation of the peninsula and Gulf Coast mean that the morning glory (which can form a series of up to ten roll clouds, one after the other) can be reliably forecast and observed around Burketown and Normanton in the spring months. The clouds can extend up to 1–2 km in height and, amazingly, can span up to 1000 km in length!
Andrew Bufalino and David Crock, Meteorologists, Brisbane
Image: Morning glory cloud above Sweers Island, Queensland, 23 August 2009. Credit: Tex Battle
A 'southerly buster' is a term used to describe an abrupt southerly change that moves up the NSW coast (other countries such as New Zealand and Argentina have southerly busters too). For the change to be labelled a 'southerly buster', southerly winds must exceed 29 knots (54 km/h) and a temperature drop of at least 5 °C must occur within three hours. Southerly busters form when a cold front passes over southeast Australia, with a southerly moving up the coast quickly compared to how its moving inland, where it lags due to mountain ranges. Southerly busters are particularly felt because of the warm days preceding the front caused by hot west to north-westerly winds, which is comparably different to the cool, dense air change associated with the southerly buster.
Ashleigh Lange, Meteorologist, Sydney
Melbourne famously inspired the Crowded House song, 'Four Seasons in One Day', and it's 'change days' when you really experience this. A change day is simply a day when a cold front crosses the city. These days are most noticeable from late spring to early autumn, when the temperature difference between the Australian land mass and the Southern Ocean is at its greatest. Northwesterly winds ahead of the front drag hot air from the interior of the continent over Melbourne, while the southwesterly winds following the front have been moving over the much colder Southern Ocean. This means that when the front moves through, temperatures drop rapidly as the wind changes direction—sometimes 15 °C or more in a matter of minutes. Once these fronts have rounded Cape Otway they often accelerate up the coast towards Melbourne and produce strong, squally winds. Thunderstorms can accompany the cold front, forming as the hot air ahead of the front is lifted by the cold air behind it, meaning the weather can go from hot, to wet and humid, and then to cold in less than an hour!
Tom Delamotte, Meteorologist, Melbourne
The gully winds are a nocturnal strengthening of southeasterly winds down the leeward (western) slopes of the Mount Lofty Ranges and over Adelaide. They mainly occur in the summer and near-summer months, when there is a high pressure system to the south and winds are from the southeast. These winds are reinforced by the downslope acceleration of cooler air, which makes the gully winds gusty and can lead to them being stronger than the prevailing wind. Onset can be sudden and occurs in the evening as the southwesterly sea breeze ceases. In extreme cases, gully winds can cause severe turbulence at Adelaide Airport and damage trees and buildings.
Dr Paul Bierman, Meteorologist, Adelaide
Fog is a common feature of Canberra's morning weather during the cooler months. The peak fog season there runs May–July, with fog typically occurring at the airport on about 25 percent of days.
Most Canberra fogs are 'radiation fogs'. On calm, cloudless nights, heat radiates efficiently from the land out to space, so the temperature of the air near the land decreases. On long nights (such as in June), this radiative cooling often brings the air close to 100 per cent humidity, allowing water vapour in the air to condense and form cloud droplets. The cloud droplets immediately above the ground form fog.
Fog usually clears in the hours after sunrise, when the sun causes temperatures to rise and humidity to fall. As a result, the water droplets turn back into water vapour and the fog 'burns off'. Most fogs at Canberra airport clear by 9 am, but occasionally fogs can last longer than this. I remember a day in June 2009 when a band of thick, higher cloud moved over the fog, preventing the usual burn off. The fog stuck around all day and the temperature failed to reach 4 °C!
Rob Taggart, Meteorologist, Sydney