Social > Explainer: what influences air temperature?

Explainer: what influences air temperature?

Air temperature matters to us all, every day. We dress to be comfortable in it. We carry out our business activities, from agriculture to construction, with it in mind. Extremes of temperature, such as heatwaves and very cold weather, can put lives in danger. While temperature is largely caused by the sun and weather systems there are other factors that influence it. So, what are the main influences and how might they affect where you are?

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We measure temperature at weather stations using specialised equipment, following strict international standards. The weather stations are carefully located to offer representative information for the area. But there's not one on every corner and temperature is affected by both broad-scale and very local influences. It can be interesting to think about what factors are playing into the temperature where you are.

Weather

Where the wind comes from

Perhaps one of the greatest influences on temperature is where the wind is blowing from.

Winds coming off the water will carry cooler maritime air over the warm land. If a weather station is near the coast it could read cooler than the adjacent inland.

An example of this is a sea breeze like the Fremantle Doctor along the west coast of Western Australia. Sometimes the sea breeze lingers along the coast, meaning that more inland locations don’t feel any relief.

Winds coming over the land will generally be warmer, particularly if they're coming from lower latitudes or the desert.

Southern Victoria and Melbourne are famous for rapid temperature swings, especially in the summer. Ahead of a cold front, hot northerly winds blow from the deserts of central Australia. These push Melbourne's temperature into the mid-40s. But once the cold front moves through, the winds suddenly swing around to the southwest, coming off the cold Southern Ocean. This is why the temperature can drop by up to 20 ºC within the space of just an hour or so.

Image: Winds blowing from hot desert areas can bring heat to regions thousands of kilometres away

Temperatures in the upper atmosphere

Temperatures above the surface have a large impact on those on the Earth’s surface. The warmer the air above the surface, the warmer the surface will become.

Meteorologists often look at the temperature 5000 ft (approximately 1500 m) above the surface to forecast what we'll experience on the ground. Temperatures high enough above the surface aren't influenced by daytime changes caused by the sun, or low-level winds such as coastal sea breezes.

Air mixing throughout the day brings this upper-level air down. It becomes warmer as it approaches the surface. Air is more compressed at lower levels, because air pressure increases as height decreases. This compression increases its temperature at a rate of around 10 °C per kilometre – a process known as adiabatic warming.

The air mixing happens most effectively on a sunny day with some wind around. Cloudy days don’t allow for much mixing. A good rule of thumb is to add 15–17 °C to the temperature 5000 ft above the surface to get the surface temperature on a sunny day.

Sunlight and shade

Air temperature is officially measured in the shade at 2 m off the ground, away from influences that can cause fluctuations in the reading. This is because the thermometer can become hotter or colder than the air around it, which would make readings inaccurate. Think of how the metal surface of a car feels much hotter in the sun than the air does.

It stands to reason you'll feel warmer in the sun than in the shade. It's useful to know that the official temperature reading doesn't account for this.

Humidity and wind chill

Humidity and wind chill can both influence how cold or warm you feel. They're not taken into account where temperature is measured. But we do factor them into our apparent ('feels like') temperature.

Shape of the land

Mountains and high places

Topography has a huge influence on temperatures. In general, temperatures decrease as height increases, at an average rate of 6.5 °C for every 1000 m. The decrease in temperature with height may seem counterintuitive – after all you're moving closer to the sun. The reason for this is that the atmosphere becomes thinner with height, due to gravity. A thinner atmosphere makes it harder for air to retain energy from the sun. This is why you’re more likely to find snow on higher mountains like Mount Kosciusko (2228 m) than Mount Dandenong (633 m).

Valleys

The opposite is true as well. Valleys are often warmer during the day than surrounding areas. Valleys that dip below sea level record some of the highest temperatures on the planet. Think of Death Valley in California, which often reaches above 50 °C in summer.

The reason for this is that valleys are excellent at collecting and holding hot air that sinks. But doesn't hot air rise? Well, yes but while the warm air at the valley floor does rise, it can become trapped within the walls of the valley before it gets a chance to escape. Instead, as it rises it cools, and then sinks back down, heating as it goes, through adiabatic warming. This whole cycle can create a circulation known as a valley breeze.

Air moving down into the valley from above can also add to the heat. This is called katabatic wind, and once again adiabatic warming is at play.

Sloping surfaces around the valley also mean a greater surface area is available to capture solar heat, compared to a flat surface.

At night, valleys can become colder than surrounding areas. Colder air is more dense, so it will sink towards the bottom of a valley. This is called a mountain breeze. Also, as valleys are relatively sheltered, there is less mixing of warmer air from above.

Bodies of water

Lakes, seas and oceans help to moderate the temperature of surrounding land areas. This is because they retain large amounts of heat. They also take a long time to heat up and cool down compared to the land.

This is why coastal cities like Sydney or Cairns don't experience the night-time or seasonal temperature extremes that inland locations like Dubbo or Mount Isa do.

The origin of ocean currents also influences temperatures on land. Warmer currents translate to warmer land temperatures and vice versa. This is why the west coast of Western Australia is generally warmer than coastal New South Wales. The warm Leeuwin current brings warmer water from the north right down the coast.

The best global example of this is the Atlantic Gulf Stream. It explains why southern Europe remains much warmer than the northeast of the USA in the winter despite being at similar latitudes.

Urban areas and heat

Built-up landscapes full of concrete, asphalt and glass absorb more heat during the day than grassy or forested areas. The heat isn't just from the sun. It's also from vehicle exhaust and electronic sources, such as air conditioners. This means higher temperatures during the day. It also means warmer nights as a lot of this heat becomes trapped and can't easily escape. This is called the urban heat island effect.

Large urban areas can often be several degrees warmer than the surrounding countryside. This is one reason why the official Melbourne weather station was moved from the central business district to Olympic Park in Richmond. Over the decades, an increase in roads, buildings and vehicles around the old site on Lonsdale St meant that it was no longer measuring the most representative conditions for Melbournians.

Image: Melbourne's weather station at Olympic Park

How to check the temperature

You can see the current temperature from your nearest weather station on our website or the BOM Weather app.

To find your nearest weather station and know where the temperature is being measured:

• From the website home page, open the page for your state/territory and under 'Observations', select 'All observations'. Under 'Latest weather' you can select clickable maps of weather stations for your state or the capital city area.
• On the app, scroll to the bottom of the forecast screen for your location. Select the 'i' after the 'Last updated' time. You can then view the weather station and how far away it is.

You can also check the apparent ('feels like') temperature on the website and app:

• From the website home page, open the page for your state/territory and under 'Observations', select 'Latest observations'. 'App Temp °C' is the apparent temperature for each location.
• On the app, the 'feels like' temperature appears below the current temperature.