The behaviour of atmospheric pressure systems – highs and lows – is what determines the wind blowing over the ocean, which, in turn, is what generates the waves we ultimately ride. But why do those highs and lows behave the way they do?
To begin, let’s just assume that there are already some areas of the Earth’s surface where the air pressure is higher than average, and other areas where it is lower than average. Never mind how they got there.
Want to check the charts? Go HERE.
Now, in simple terms, high pressure is where the air is pushing down hard onto the surface, and low pressure is where the air is pushing down less hard, or even rising upwards. It is as if there were an excess of air in a high pressure and a deficit of air in a low pressure. As a result, the air flows away from the high, where there is an excess, and towards the low, where there is a deficit. In more technical terms, there is a pressure gradient from the high to the low, and the air flows ‘down the pressure gradient’ to try to equalise it. The greater the pressure gradient, the faster that air will try to flow.
But the air doesn’t go in a straight line from areas of high to low pressure. It turns as it goes, and ends up circulating around those systems. The air circulates around low pressures in a cyclonic direction (hence the word cyclone). Cyclonic means anticlockwise in the northern hemisphere and clockwise in the southern hemisphere. Correspondingly, air circulates around high pressures in an anticyclonic direction (hence the word anticyclone). You guessed it, anticyclonic means clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.
But why does the air circulate that way? Indeed, why does it circulate at all? To explain, we need to bring in the Coriolis force (see previous article HERE). Instead of going in a straight line from the high to the low, the Coriolis force deflects the air to the right in the northern hemisphere and to the left in the southern hemisphere.
Now, think of a cell of low pressure surrounded by high pressure in the northern hemisphere. The air is entering the low from all sides, flowing down the pressure gradient. The air flowing in towards the centre of the low is constantly turning right due to the Coriolis force, resulting in an anticlockwise circulation around the low. Likewise, think of a cell of high pressure surrounded by low pressure. The air flowing out from the centre of the high to the periphery will turn to the right due to the Coriolis force, resulting in a clockwise circulation around the high. In the southern hemisphere, the whole thing is reversed.
If you are confused, think of a low pressure like a roundabout. In Spain, where I surf in the northern winter, people drive on the right-hand side of the road. As a result, cars veer to the right as they enter the roundabout, but they end up circulating around the roundabout in an anticlockwise direction. In South Africa, where I surf in the southern winter, people drive on the left-hand side of the road. Therefore, cars veer to the left as they enter the roundabout, but they end up circulating in a clockwise direction.
If hot air rises why does it form 'highs' while cold air forms 'lows'? How do high and low weather systems work?
If hot air rises, why does it form 'highs' while cold air forms 'lows'? I would have thought that high pressure at sea level would have been caused by denser atmosphere air.— Ray
At the beginning of 2013, Australia experienced a record-breaking heatwave that extended over a greater area for a longer period of time than ever experienced before.
This exceptional weather was caused by a combination of factors.
The monsoon which normally brings cooler, wetter conditions from the north failed to show up.
Meanwhile, a stationary high pressure cell in the Tasman Sea forced hot air from the north across southern Australia and prevented cooler air from further south penetrating inland.
The result: a column of hot air circulating around the continent drying and frying southern Australia.
The monsoon finally arrived three weeks late in mid January bringing rain to the Top End, Tropical Cyclone Peta lashed the Pilbara, and Oswald drowned the east coast of Australia.
So why do lows produce cooler weather across Australia if cooler air is denser, and highs can produce hot, dry weather given that hot air rises?
To understand how high and low pressure systems form and interact you have to look beyond the Earth's surface temperature to the troposphere, says Associate Professor Steve Siems from the Weather and Climate Program at Monash University.
"The surface pressure measures the mass or density of air above it, so even if it's warm at the surface, if there's a lot of air above it, you can get a high pressure, or if it's cold at the surface but there's not much air above it you could get a low pressure system."
Air pressure is measured in hectapascals. There isn't a particular number of hectopascals that distinguishes a high pressure system from a low pressure one; it's the relative differences between the two that count, and the interplay between them that causes weather.
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Highs and lows
A high pressure system occurs where the air mass above the Earth is denser than in surrounding areas, and therefore exerts a higher force or pressure.
"In a high pressure system you have dense air subsiding downwards and pushing down at the surface, Because the surface air is being pushed down from the air above it spreads outwards, which is why we generally have fair weather [in mid-latitude regions] when we have high pressure systems".
In summer, high pressure systems are associated with warm day time temperatures in mid-latitude regions. However, during winter high pressure systems over the centre of Australia bring "dry, cooler conditions with rain along the southern portion," says Siems.
Air in high pressure systems moves in an anticlockwise direction (in the southern hemisphere), while air in low pressure systems moves in a clockwise direction due to the rotation of the Earth.
At the surface of the Earth air flows from high pressure systems into low pressure systems.
"When you have a low pressure system you're dragging air inwards and its being pulled upwards", explains Siems. As the warm humid air spirals upwards, it cools and clouds form.
In winter, these may be thick enough to give rain or snow, and explain why people living in the southern states associate low pressure systems with cold and sometimes stormy weather.
In summer, lows travelling over warm water in the tropics can produce cyclones.
"You can get lots of convection or upward air movement going on in the tropics" explains Siems.
During January and February it is also common to see 'heat lows' — a hot dome-shaped pocket of low pressure air — form in Central Australia caused by intense rapid solar heating of the land surface.
"A heat low is when you have so much heating over a broad area. The air is hot and it expands and this heating is deep enough that the surface pressure does register a low," explains Siems.
While not responsible for the latest heatwave, heat lows combined with highs in the Great Australian Bight usually bring drier conditions to the southern part of Australia.
Anatomy of the January heatwave According to the Bureau of Meteorology January's heatwave started well before January.Several extreme heat conditions were recorded between September to December. At the same time central South Australia received its lowest July to December rainfall and below average rain fell across almost all south eastern Australia. Then, from the end of December, a series of slow-moving high pressure systems in the Great Australian Bight blocked the arrival of the monsoon in the north and cooler air from lows in the south."Combined with the delayed monsoon, [the blocking high systems] allowed tremendous heat to build up over the continent," says Dr Karl Braganza from the Bureau of Meteorology.
To see how the heatwaves progressed check out BOM's special report on the January heatwave.
Associate Professor Steven Siems from the Weather and Climate Program, School of Mathematical Sciences, Monash University spoke to Annie Hastwell
Published 31 January 2013
The Earth's atmosphere exerts pressure on the surface. Pressure is measured in hectoPascals (hPa), also called millibars. Standard pressure at sea level is defined as 1013hPa, but we can see large areas of either high or low pressure. These areas are all relative to each other, so what defines a high will change depending on the area around it.
Weather chart
On a weather chart, lines joining places with equal sea-level pressures are called isobars. Charts showing isobars are useful because they identify features such as anticyclones (areas of high pressure) and depressions (areas of low pressure).
Ascending and descending air
Areas of high and low pressure are caused by ascending and descending air. As air warms it ascends, leading to low pressure at the surface. As air cools it descends, leading to high pressure at the surface.
In general, low pressure leads to unsettled weather conditions and high pressure leads to settled weather conditions.
Anticyclone (high pressure)
In an anticyclone (high pressure) the winds tend to be light and blow in a clockwise direction (in the northern hemisphere). Also, the air is descending, which reduces the formation of cloud and leads to light winds and settled weather conditions.
Depression (low pressure)
In a depression (low pressure), air is rising and blows in an anticlockwise direction around the low (in the northern hemisphere). As it rises and cools, water vapour condenses to form clouds and perhaps precipitation. This is why the weather in a depression is often unsettled, there are usually weather fronts associated with depressions.