Pressure systems are regions where atmospheric pressure is either higher or lower than the surrounding environment. Atmospheric pressure is the weight of the air above us pressing down on the Earth's surface and is measured in hectopascals (hPa). There is no specific pressure value that automatically makes a system 'high' or 'low'. Instead, it is the pressure relative to the surrounding area that matters.
Reading the weather map
Weather charts use symbols to identify pressure systems. High pressure systems are marked with 'H', while low pressure systems are marked with 'L'. The number displayed alongside them indicates the mean sea level pressure at the centre of the high or low pressure system.
Generally, the lower the pressure at the centre of a low, the more intense the system may be. A shallow low usually has a central pressure above 1000 hPa, a moderate low ranges between 980 and 1000 hPa, and a deep or intense low falls below 980 hPa. Tropical cyclones, which are a specific type of low pressure system which forms over tropical waters, can have pressure centres even lower. A typical high pressure system will often have a central pressure of around 1020 hPa or higher.
Video: Understanding synoptic charts
The weather map or synoptic chart, as us meteorologists call them - you see them on the news, in the paper, and in our videos, but do you know what all the lines and symbols mean? You can tell a lot about the current weather just from looking at the features on a map. So, let me take you through the basics.
Isobars
Now the white lines on a chart, these are called isobars, and they're lines of constant or equal atmospheric pressure. And you'll see these on a weather map no matter where you are in the world. They give us an indication of the airflow around weather features, and you can also interpret the wind strength from isobars. The general rule being that where the isobars are the tightest, most squeezed together, that's where the wind will be the strongest, and where they're the furthest apart or the slackest, that's where the wind will be the lightest.
Highs
Now it's time to look at the H's and the L's. These are also to do with pressure. They mark areas of local pressure extremes. So if we look at the H for example, that's indicating an area of high pressure. Much like the peak of a mountain, no matter which way you moved away from the H, the pressure is going to drop. In the southern hemisphere the wind flows anticlockwise around a high-pressure system. An interesting point is that the air in the centre of a high-pressure system is sinking, and that tends to lead to more settled weather conditions.
Lows
Lows on the other hand, are like the bottom of a valley. They're the local minimums of atmospheric pressure. In the southern hemisphere the wind flows clockwise around a low-pressure system. And in the centre of the low, the air is rising. So lows on the map are normally associated with clouds, wet, and stormy weather. You can also see that the isobars are quite close together around and low, so this is also indicating an area of windy conditions.
Fronts
Now it's time to talk about fronts. Fronts are the lines and shapes that you see on the weather map, and they indicate a boundary between two different air masses. So we'll start by looking at the cold front: the blue line with the triangles. Now these indicate an area of where cold air is moving in and pushing warm air upwards. So we tend to expect to see a blanket of cloud along the leading edge of a front, which can produce fairly consistent rain. As a front passes over you, you'll notice a change in wind direction and also temperature. In summer there can be quite a sharp drop in temperature behind a cold front.
Warm front
Now the red lines and semi-circles over here are indicating a warm front. A warm front is where warmer air will replace cooler air. Just like a cold front there can be quite a marked temperature change when a warm front moves through, but it's typically less dramatic than what we would see with a cold front. And we do see these less frequently around Australia than we do cold fronts. Now there's other types of fronts too: stationary and occluded fronts, and if you'd like to know more about these, check out the blog linked in our video description.
Trough
We also see dashed lines like this one over here: now this is an area called a trough. It's an area of relatively low atmospheric pressure compared to its immediate surroundings. Like cold fronts, troughs are separating two different air masses. Usually moist air on one side, and drier air on the other. As the trough moves towards the moist air, it causes the air to lift, and so we see cloud and showers and thunderstorms often developing on a trough.
Monsoon
Now another type of trough that we see in the Australian region is the monsoon trough, shown by this dot-dashed line. Now a monsoon is a seasonal wind pattern and it typically brings heavy rain. It moves with the seasons. So over our winter months we see the monsoon trough move to the north of the equator, far away from Australia, but in the summer it will shift back into the southern hemisphere and bring a return of the wet season to Northern Australia.
So there you have it, the basics of a synoptic chart or weather map. A meteorologist's road map to understanding and predicting what weather we might experience. You can find synoptic charts and other maps and forecasts on the bureau's website.
High pressure systems
A high pressure system is an area where the atmospheric pressure is higher than its surroundings.
In the southern hemisphere, winds circulate anticlockwise and outward around a high. This movement is linked to air sinking from higher levels of the atmosphere towards the surface. As the air descends, it warms and becomes increasingly stable.
This stability is what gives high pressure systems their reputation for delivering settled weather. The sinking air discourages cloud formation, meaning the skies are often clear, rainfall is limited, and winds are generally light.
Low pressure systems
A low pressure system is an area where atmospheric pressure is lower than the surrounding environment.
In the southern hemisphere, winds circulate clockwise and inward around a low. Air moving away from surrounding high pressure systems converges towards the centre of the low. Once it reaches the centre, it has nowhere else to go but upward.
As this air rises, it cools. Cooling causes moisture in the air to condense, forming clouds. If the rising air remains warmer than the air around it, it will continue rising and create instability in the atmosphere. This process can produce towering cloud formations that extend high into the sky.
The result of the instability is often cloudy, wet and windy weather. Low pressure systems are commonly associated with rain, thunderstorms, gusty winds and rapidly changing conditions.
Compared with highs, they are generally smaller systems, but they frequently produce more significant weather.
Understanding the forecast
The next time you look at a weather map, pay attention to the highs and lows. These pressure systems are the key drivers of day-to-day weather, influencing everything from cloud cover and rainfall to wind strength and temperature.
While a high pressure system often signals calm and sunny conditions, a low pressure system usually indicates more active weather ahead. By understanding how these systems work, you can gain a clearer picture of the forces shaping the atmosphere above us.
