What can a synoptic map tell us about the weather

A synoptic map is a map of the weather, so it is also known as a weather map or weather chart.

Meteorological services of every country create such maps. But you might wonder why you would need them since the weather forecast is easily accessible. Any site can tell you: “Tomorrow it's going to be 20 °C, with heavy rain at 14:00, and wind up to 7 m/s”. So how can a synoptic map help?

In this lesson of the Windy.app Meteorological Textbook (WMT) and newsletter for better weather forecasting you you will learn what a synoptic map is and how to read it.

What is a synoptic map?

The word “synoptic” originates from the Greek “synopsis”, which means "a general view," and literally "seeing something all at once”. A synoptic map (also known as weather map or weather chart) is a map of the weather, meaning a view of the weather “from above”. These maps can be made for any territory, from an individual region of a selected country to an entire hemisphere.

The weather is a combination of a number of factors, but it is always shaped by processes on a large scale, covering thousands of kilometers; cyclones and anticyclones, trade winds, easterlies and westerlies, and monsoons. All of these winds are caused by differences in atmospheric pressure. That is why lines of equal pressure called isobars are the main element of any synoptic map. Since the centers of cyclones and anticyclones are the areas with the lowest and the highest pressure, respectively, equal pressure lines are arranged around them.

This is what a cyclone (L — low atmospheric pressure area) looks like on a synoptic map in the Windy.app for iOS

Another important element of a synoptic map is atmospheric fronts, narrow borderlines between air masses of different temperatures. If cold air is moving toward warmer air, it's a cold front. If warm air is moving towards colder air, it's a warm front. After a cold front has passed, it normally gets colder, after a warm one, warmer. Dangerous weather events, such as wind gusts, thunderstorms, tornados, and icy spots, are often associated with atmospheric fronts. Atmospheric fronts are very active in cyclones, and almost non-existent in anticyclones.

The simplest synoptic map: an anticyclone, a cyclone, and atmospheric fronts. Cold fronts are marked with blue, and warm fronts with red / Windy.app

All the other elements of a synoptic map are optional. But on demand, for more readability, we can add a lot of useful information. For example, we can show gusts of wind or precipitation zones.

So why look at a synoptic map? Let us give you an example. Imagine a short rain is forecast for tomorrow, but the weather is expected to be mostly clear. You decide to go for a bike ride, and you are caught with your bike in the middle of nowhere in a thunder and wind storm. After that, the sky is quickly covered with thick clouds, and the rain gets much weaker, but goes on and on. Nothing pleasant about it, is there? The opposite can also happen: you decide that you'd better stay home, but the rain never actually comes. Why does this happen?

The reason is that the meteorological forecast model predicts weather in specific “spots” (sites), just like our app. This forecast is, however, not very precise: for instance, a cumulus cloud can appear 100-200 km away from the place where the model predicted it would form (and you stayed home to avoid getting wet). Or it can end up being much bigger than expected, and cause dangerous phenomena that the model didn´t predict. For a weather forecast, these errors are not very significant, but for an actual person at an actual location, this forecast feels absolutely inaccurate and completely useless.

A synoptic map helps you understand how much you can trust any given weather prediction.

How to read a synoptic map?

There are two types of synoptic maps: analysis maps and forecast maps:

An analysis map shows the state of the atmosphere at the present moment, based on the data from weather stations.
Forecast maps are created based on meteorological models, and show the state of the atmosphere in the future.

The Windy.app has both types of weather maps.

Aside from the cold and the warm fronts, the synoptic map shows occlusion fronts, which are places where cold fronts overtakes warm fronts, and are marked in purple / Windy.app

By the way, analysis maps also work for predicting the weather. The thing is, synoptic processes are not very quick. If you look at the freshest analysis map, and then at a couple of the previous ones, you can certainly imagine where the cyclones, the anticyclones, and the fronts are going to move the next day.

There are a few important rules you need to remember to read a synoptic map successfully. Let's discuss them in a bit more detail:

The Coriolis force makes wind and cyclones and anticyclones blow in a circle, along the isobars. In the northern hemisphere, it blows anti-clockwise in cyclones and clockwise in anticyclones. In the southern hemisphere, it's the other way around. This will help you understand where the air is coming from to your area. By the way, cyclones and anticyclones themselves normally travel from west to east.
The the distance between isobars is another thing to pay attention to on a weather map: the closer they are together, the stronger the wind, and the more active the atmospheric fronts.
An anticyclone doesn't always mean clear sky and sunny weather, as is the common belief. In its eastern part, where the wind blows from the north, light rainfalls are not uncommon, and it can suddenly get cold and damp. If a cold front crosses the eastern part of an anticyclone, even thunderstorms can be expected.
It is often very windy on the north and on the south of an anticyclone, which can bring snow storms in winter. In the center and on the west of an anticyclone, it is normally calm, clear and sunny. In summer, when the sun heats up the land surface a lot, it can get very hot in the center of an anticyclone. In winter, however, when the sun is barely warming the earth, the center of an anticyclone can get very cold, and night cooling through radiation becomes even more prominent.
In the case of cyclones, it's less important to know in which part of it we are, exactly. The weather there is determined by the atmospheric fronts that move around the center of the cyclone. In a new cyclone, the cold front is almost always on the west, and the warm front is on the east, but in dissipating cyclones, fronts can be located anywhere. So let's talk a bit more about fronts.
A cold front is very active. Most dangerous weather phenomena happen in cold fronts. A cold front is heterogeneous. In some places, it can cause big destruction, while a neighboring zone won't even get a drop of rain. Near a cold front is always a potential danger. Rain in cold fronts rarely lasts long, it is normally strong and short. A cold front is most active during the afternoon. In the evening and in the morning, the wind and other dangerous phenomena weaken, and for the night, the front can effectively disappear. By the way, the northern wind (which blows on the west side of the cyclone) often brings windy and rainy weather, even if the cold front is rather far from you.
A warm front is calmer. Precipitation is not as intense, but can be long, and is relatively equal around a vast area. If a warm front is moving towards you, it is very unlikely that you would avoid rain. A warm front rarely brings dangerous phenomena, but as the temperature goes down to around 0°C, ice rain and icy spots are possible. At night, warm fronts can become more active, and dangerous phenomena more characteristic of a cold front can happen. In winter, snowfalls on warm fronts are more abundant than on cold fronts. A warm front is normally located in the eastern part of a cyclone.
Occlusion fronts are known for strong and long-lasting precipitation, sometimes for many days in a row. Occlusion fronts are normally located in the central and northern part of a cyclone. There are also troughs, in our app, they are marked by yellow lines. A trough is a part of a cyclone where some features of a front are observed, but there is no front itself. You need to pay close attention to troughs, as they can quickly develop into real fronts. In this case, the weather turns bad quickly and unpredictably.
If there is no defined cyclone or anticyclone above you, the area would have something called a "weak pressure gradient". In this case, the weather depends on a wide range of factors and can be so different that it becomes a task for a professional synoptic expert.

Here, areas with a long (top) and short (bottom) distance between isobars are clearly visible. The most active weather processes will take place at the bottom of the map / Windy.app

A very long trough on the weather map in the Windy.app for iOS

You can see that Eastern Coast of North America is covered with cyclones, with many warm and occlusion fronts / Windy.app

So, at the end, how exactly do you use a weather map? Let's say, you saw a nice sunny day is forecast. Look at the map: if you are in the middle of an anticyclone, then it will most certainly be the case. If there's a front moving towards you, and the model is still predicting good weather, weigh the risks, and decide if you can afford them. This is most important for meteorosensitive activities (such as mountain hiking), where bad weather can be life-threatening.

There is no doubt that you can´t go without prognostic models to make a weather forecast. However, a synoptic weather map is an important source of valuable information that is always nice to have.

Where to get a synoptic map?

In the Windy.app you can access a synoptic map from three places: 1). Home screen of the app, 2). Spot screen, and 3). Weather Station screen.

To get weather fronts and isobars on the map, activate the namesake Atmospheric Fronts and Isobars feature in Weather Map Settings.

Weather Fronts and Isobars in the Windy.app for iOS

There is also a wind map on the Windy.app site similar to one you see in the app. For now it has just one wind layer but we are working on implementation of others.

Learn more about reading weather maps in the Windy.app blog.

Text: Eugenio Monti, a meteorologist and a climatologist

Cover photo: Simon Hurry / Unsplash