About weather fronts
The Earth receives more energy from the sun at the equator than at the poles, keeping the tropics warm. The atmosphere and oceans constantly try to even out temperature differences around the globe. Winds in the atmosphere and currents in the oceans take heat from the tropics towards the poles.
A front is what we call the boundary where different air masses meet as winds push them along. Fronts usually bring a change in weather.
Each air mass has its own temperature and humidity characteristics. When air masses meet, these interact in different ways. This means fronts can bring a wide range of weather, including:
- changes in temperature
- rainfall
- significant storms.
Types of fronts
There are several types of frontal systems, including:
- cold fronts
- warm fronts
- stationary fronts
- occluded fronts.
Cold fronts are more common in Australia and have a greater impact. They often bring rainfall to southern Australia, and are most common during winter.
Frontal systems can bring rainfall to southern Australia at any time of year. They have most impact during winter months.
Cold fronts
How cold fronts form
A cold front forms when comparatively cold, dense air moves toward the equator and encounters warmer air. The air is deflected by the Coriolis force – an effect created by Earth's rotation. It pushes cold fronts from west to east in the mid-latitudes.
This means south-west Western Australia is normally the first place to get strong cold fronts. Typically, that cold front moves through to the continent's south-east within a few days.
Video: Ask the Bureau: What is a cold front?
It brings a change in the weather, particularly to cloud, rainfall, a sudden change in wind direction accompanied with gusty winds, and usually a sharp drop in temperature. It’s a change in air mass, from warmer conditions to cooler conditions. As a cold front moves over an area, colder air behind the change is actually quite dense and cold, it wedges in underneath the warmer air ahead of it.
This warm air ahead will actually rise, and as it rises in the atmosphere, it starts to cool and condense into cloud droplets. These cloud droplets can then form cloud and hence rainfall. Also, as the cold air moves through, we also get quite a significant change in wind direction, typically in Australia this from a north-westerly direction in the warm area ahead of the change, around to a south-westerly direction behind the change. Also, with this colder air moving over a fixed point, we get this large drop in temperature as we go from the warm air to the colder air.
The main focus of the rainfall would generally occur on and ahead of the frontal boundary.
The type of rainfall we experience can be fairly consistent in nature as it’s falling from a blanket type of cloud, which can often have embedded thunderstorm activity within it.
Once you move into the colder air behind the change, we start to get into that typical more showery weather, where we see that cumuliform type of cloud, where we get shower followed by a bit of sunshine, bit of cloud and more showers, and on and off. Cold fronts occur all year round, but they have different impacts depending on the time of year. In summer, hot and dry conditions ahead of the front, combined with gusty winds and a strong wind change, can lead to increased fire danger. While in winter, damaging winds and heavy rain are possible with the passage of a strong cold front, while snow can develop in the coldest air behind the change.
So, cold fronts are a frequent occurrence through southern parts of Australia, they’re also the major contributor to rainfall through the cooler months.
Why cold fronts bring bad weather
Cold fronts have a sloping surface, shallowest at the leading edge. As they wedge under warmer air, they lift it up. Air cools as it rises, so if that warmer air is moist, the water vapour condenses as it is raised and cloud forms. Cloud forms and there may be:
- strong or gusty winds
- cooler temperatures
- rain, hail, thunder and lightning.
Some of our most hazardous weather is associated with cold fronts. We often issue severe weather warnings, generally due to expected wind gusts over 90 km/h or heavy rain. Learn more about our severe weather and coastal hazard warning services.
Many cold fronts bring short, sharp bursts of heavy rain. As cold fronts move quickly, they don't often bring prolonged rain that leads to floods. A series of fronts within a few days can raise the risk of flood, as rainfall builds up and the ground is saturated.
Cold fronts can also produce cool season tornadoes. South-west Western Australia is one of the more tornado-prone regions in the world. Learn more about tornadoes.
When are cold fronts likely?
Cold fronts occur at any time of year but they have different impacts throughout the year.
Spring
Cold fronts are generally stronger in southern Australia during spring. This is when the Australian continent is starting to warm up but the sea to the south is still very cold. The greater temperature difference can mean stronger fronts.
Winter
In winter, inland air is cooler but damaging winds and heavy rainfall can happen as a strong front passes. Snow can develop behind the front in the coolest air.
Cold fronts in winter bring:
- snow to the alpine regions
- rain across southern Australia
- strong winds.
Summer
In summer, strong north-westerly winds ahead of a cold front can bring very hot, dry air from inland over south-east Australia. This can bring extreme fire danger.
The wind change with the front can rapidly change the direction of any fires. This increases the fire danger. For example, Victoria's devastating bushfires on Black Saturday in February 2009 involved a sharp westerly change. The change followed strong, gusty north-westerly winds ahead of the cold front.
Autumn
In autumn, cold fronts can bring an early taste of winter. Autumn weather is typically settled with clear skies for much of southern Australia. This is due to the reduced temperature difference between land, sea and upper atmosphere. Cold fronts break this pattern of stable weather.
Warm fronts
How a warm front forms
A warm front forms when a comparatively warm air mass moves into a region with cooler air. The warm, less dense air slides over the sloping surface of the cooler air mass. A warm front is the opposite to a cold front in that warm air pushes out the cold.
As the warm air mass is pushed up into the atmosphere, it cools. This can create cirrus and other high clouds. As the warm front passes, lower clouds may form. This can lead to rain and sometimes storms.
Warm fronts usually move slower than cold fronts.
Warm fronts in Australia
We don't often see warm fronts in Australia as they tend to occur south of us. However, they are common across Europe, North America and parts of northern Asia.
The changes in weather from a warm front may be more gradual than when a cold front passes through.
Stationary fronts and occluded fronts
Much less common in Australia are stationary and occluded fronts.
Stationary fronts
Stationary fronts are very slow-moving or nearly stationary. They can form when a warm air mass and cold air mass meet, but neither is strong enough to push the other out of the way.
A stationary front can last for days, depending on winds and whether one of the fronts strengthens or weakens. The area below the stationary front may have days of cloudy and rainy weather.
Occluded fronts
Occluded fronts form when one front meets and overtakes another.
Cold fronts usually move faster than warm fronts. This means occluded fronts most often form from a cold front overtaking a warm front, heading in the same direction.
The cold front coming from behind meets the cold air that was ahead of the warm front. It has closed in – occluded – the warm front.
This pushes warm air from the warm front upward, which can lead to rain along the occluded front.
How frontal systems affect Australia
Frontal systems can bring rain to southern Australia at any time of year. They typically have the greatest impact during winter.
Fronts generally move across southern Australia from west to east. They can be in the Australian region from a couple of days to a week.
Frontal systems vary in their intensity and speed:
- More intense (stronger) systems generally bring heavier rainfall.
- If frontal systems move slowly, rain may fall for extended periods and be heavy at times.
Climate drivers related to frontal systems
Cut-off lows
Cut-off lows are low pressure systems that break away from the main belt of low pressure south of Australia. They enhance rainfall in southern Australia. Learn about cut-off lows.
Subtropical ridge
The subtropical ridge is a belt of high pressure around the globe in the middle latitudes. It brings dry and stable conditions to large parts of Australia. Learn about the subtropical ridge.