How to Read a Weather Map

Many of you have probably seen pressure charts used on TV or even on the back of a newspaper. Some of you will understand what the lines, the triangles and different bits and pieces mean.

But if you are not one of those people keep reading and find out what weather forecasters call synoptic charts.

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The word synoptic means the current situation. What this means is – looking at the pressure pattern, the fronts, the wind speed, and direction in the present moment, and in the same time also track how they evolved over the coming few days

It is important to know that pressure, wind, and temperature are in a constant balance trough out the atmosphere, but the atmosphere is always changing and trying to maintain that balance.

Before we talk about the signs on the map lets first learn about atmospheric pressure.

What is atmospheric pressure?

When we talk about atmospheric pressure we talk about amount of pressure that each system produces when it moves over a given location.

  • That is because the atoms and molecules that make up the atmosphere are always moving. Despite their very small size, when they drop on any surface they exert pressure.
  • Air pressure is force exerted by the weight of al column of air above a particular location.
  • At the earth surface this is 14.7 lbs of pressure per square inch (SPI)
  • In meteorology, pressure is measured in millibars (mb) or inches of mercury.

isobars

The pressure pattern will always show you a number of things – we always have areas of high pressure (H) and areas of low pressure (L) and the white lines that circulate around, show where the pressure is equal. They are called isobars.

Sometimes though the pressure gradient – the difference between the high and the low pressure – is much bigger and the air moves faster. That is shown by tighter isobars that you can see on the chart.

When air moves from areas of high pressure to area of low pressure and try and maintain that balance or to try and ease that difference, than there is air moving around and we feel wind moving.

What is high and low pressure?

high and low pressure
Winds move toward low pressure and counter clockwise. Air is heavier and sinks in the middle of high pressure.
  • Adding and removing mass has an effect on atmospheric pressure
  • Ass mass is added to the same volume of air, the molecules become more energetic then the pressure increases and temperatures warms.
  • Ass mass is removed from the same volume of air, the molecules movement slows down – pressure decreases and the temperature cools.

What are the effect of high pressure?

high pressure diagram

  • Air is heavier and sinks in the middle of high pressure
  • In vertical – air moves out from the center of the high toward lower pressure.
  • In horizontal we see air move away from the high center, but ht air doesn’t move straight out but it actually curves turning to the right so the circulation around the high pressure is clockwise.
  • High pressure is usually is associated with clear and cool weather

What are the effect of low pressure?

low pressure

  • In vertical air is drawn in the middle of low pressure area because air is “lighter”.
  • Winds move toward low pressure and counter clockwise.
  • In horizontal air is moving into the low.
  • Usually associated with cloudy and breezy condition. Precipitation accompany many low pressure systems.

Lest take a look at this maps to examine fronts and various types and the weather they can bring.

What is weather front?

weather map with fronts

Fronts are boundaries between two air masses of different temperature and density. When two air masses meet, the warmer air rises above the cooler, dens air. Fronts are associated with an area of low pressure.

They are accompanied by clouds and rain. After the front passes there are changes in temperature, wind direction, moisture and pressure.

Get better understanding of cold fronts. What are they?

cold front

  • Cold fronts occur when a colder air mass replaces a warmer one and there is precipitation along the boundary of the two fronts.
  • The front is marked with blue line with blue triangles (barbs) on the weather map. The barb is pointing at the direction the front is moving.
  • Cold fronts are almost always associated with low pressure.
cold front horizontal
When the denser cold front approaches it causes warm air to rise above it. This rising motion of warm air can lead to cloud development and possibly precipitation.

What is warm front?

warm front

  • Warm front is a boundary where a warm air is replacing a cold air.
  • The warm front is marked on the map with red line with red semicircles (cuss) pointing the direction the front is moving.

When the warm front approaches look for rise in the temperatures and moisture. These conditions all together provide clouds and possibly some precipitation ahead of the warm front while warm, humid and clear conditions will be located behind. clouds and possibly some precipitation even a few thunderstorms could occur.

warm front horizontal
When the warmer air hit the colder it will rises above it. The rising air can lead to clouds and possibly some precipitation even a few thunderstorms could occur.

One thing we tend to see is warm air following a warm front and cold air following a cold front. We also see increased amounts of clouds and rainfall in and around the area that front marks.

Another type of weather you can spot on the weather chart lies between a warm and cold front that’s what we call a warm sector. It’s a fairly cloudy type of air mass and it can bring mist and fog and some outbreaks of light rain from time to time.

Next lest find out about stationary fronts.

stationary front

Stationary fronts characterize  with mild moist air on the warm side and cool and dry air on the other side. The temperature change is not very large on either side of the front.

Stationary front act much like a warm front in the respect that clouds and possibly precipitation are favored on the cool side of the front.

  • Stationary fronts form at boundaries between warm and cold air but neither air mass is moving.
  • Has similar characteristics to a warm front and are often tied to flooding events.
  • Marked by alternating segments of warm and cold fronts with the barbs and cuss pointing in the direction away from the cold and warm air respectively.
  • Wind run parallel to the stationary fronts.

What are occluded fronts?

occluded front

When developing a cold front will tend to catch up with the warm front because it moves faster. When this happens we end up with something called occluded front. In time these occlusions can form their own identities as well.

  • Occluded fronts form when cold, cool and warm air collide.
  • Occluded fronts usually indicate a storm system that is starting to weaken
  • They’re marked by a purple line with purple semicircles and purple triangle next to each other.

So, we hope that now you have an idea about one of the most fundamental tools forecasters use – the weather chart.

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