# How to read a weather map (with pictures)

Weather maps give us a simplified representation of the current or forecast weather in a given area. The most common is surface analysis, which is what will be discussed in this article. Reading a surface analysis may seem difficult at first, but with a little practice you'll be able to do it in no time.

## Steps

### Part 1 of 4: Learn the Basics of Weather Maps

#### Step 1. Understand the general concepts of precipitation

What most people are concerned about is precipitation, which, in meteorology (the study of weather), is any form of water that falls on the Earth's surface. Forms of precipitation include rain, hail, snow, and sleet.

#### Step 2. One of the most important aspects of interpreting time involves being able to understand the actions that are the product of differences in barometric pressure

High pressure implies dry weather and low pressure is generally associated with humid air (perhaps precipitation).

• A high pressure system is an air mass that contains denser air because its air is cooler or drier than the surrounding air. Therefore, the heavier air falls and moves away from the center of the pressure system (like water pouring onto the ground). With high pressure systems, the weather will tend to be clear.
• A low pressure system is an air mass that has less dense air because it is more humid or hotter. The surrounding air goes inwards, to the center of the system, while the lighter air goes upwards, often causing the formation of clouds and precipitation, as the humid air cools as it rises. You'll see this effect by forcing invisible water vapor to condense into droplets when it makes contact with the outside of a cold glass.) However, no drops will form if the glass is only slightly cold. Therefore, raising air to low pressure will only produce rain if it rises to where the air is cool enough to condense the water vapor into droplets that are too heavy for the rising air to keep them aloft (clouds are simply droplets of water small enough to stay high).
• In very low pressure systems, storms will form (if they haven't already formed). Clouds will begin to form and move across the sky (cumulonimbus forms when humid air rises too high). Sometimes tornadoes form when very high pressure air collides with very hot and humid low pressure air.

#### Step 3. Study a weather map

Try to keep an eye out for one of these maps on the TV news, online, or in your local newspaper (other sources may include magazines and books, but they are probably out of date). Newspapers are a very convenient method of finding a weather map, as they are cheap and reliable publications, and the map can be cut away so that you can take it with you while you learn to interpret the symbols.

#### Step 4. Analyze a small part of your weather map

If possible, find a map that covers a small area (these may be easier to interpret). Focusing on a larger scale can be tricky for a beginner. On this map, you should look at the location, the lines, the arrows, the patterns, the colors, and the numbers. Each symbol counts and they are all different.

### Part 2 of 4: Read Barometric Pressure

#### Step 1. Understand what barometric pressure measures

This is the weight or pressure that the air exerts on the ground and is measured in millibars. It is important to be able to read barometric pressure because pressure systems are associated with certain weather patterns.

• The average barometric pressure system measures 1013 mb (76 centimeters of mercury).
• A typical high pressure system measures around 1030 mb (77.27 centimeters of mercury).
• A typical low pressure system measures around 1000 mb (75.03 centimeters of mercury).

#### Step 2. Learn what the barometric pressure symbols are

To read barometric pressure in a surface analysis, you must check the isobars (iso = equal, bar = pressure), that is, the smooth curved lines that indicate areas with the same barometric pressure. Isobars play an important role in determining wind speed and direction.

• When isobars form closed concentric circles (but not always round), the smallest circle in the center indicates a center of pressure. This can be a high pressure system (represented by an "H" in English and an "A" in Spanish) or a low pressure system (represented by an "L" in English and a "B" in Spanish).
• Air does not flow "down" in pressure gradients, but "around" them, due to the Coriolis effect (Earth's rotation). Therefore, isobars indicate the direction of the wind, counterclockwise around low pressures (cyclonic flow) and clockwise around high pressures (anticyclonic flow) in the Northern Hemisphere, creating, thus, the wind. The closer the isobars are to each other, the stronger the wind will be.

#### Step 3. Learn to interpret a low pressure system (cyclone)

These storms are characterized by an increase in the presence of clouds, winds, temperatures and by a certain probability of precipitation. On a weather map, they are represented by isobars that are close together, with arrows that travel clockwise (southern hemisphere) or counterclockwise (northern hemisphere) and with a "T" on the middle isobar, which forms a circle (however, the letter may vary depending on the language in which the weather report is presented).

• Radar images can show low pressure systems. Tropical cyclones (South Pacific) are also known as hurricanes in America or typhoons on the shores of Asia.

#### Step 4. Learn to interpret a high pressure system

These conditions indicate clear, calm weather with a reduced probability of precipitation. Dry air generally produces a much greater range of high and low temperatures.

### Part 3 of 4: Interpreting the types of fronts

#### Step 1. Observe the types and movements of fronts

These mark the boundary between hot air on one side and cold air on the other. If you are close to a front and you know it is moving towards you, you can expect the weather to vary (for example, clouds, precipitation, storms, and wind) as the front boundary passes over you. Mountains and large bodies of water can distort your path. On a weather map, you will notice some lines with semicircles or triangles on one or both sides (as seen in the image). These indicate the boundaries of the different types of fronts.

#### Step 2. Analyze a cold front

With these weather patterns, rainfall can be torrential and the wind can reach very high speeds. On weather maps, cold fronts are represented by blue lines with triangles on one side. The direction of the tip of the triangles is the direction in which the cold front is moving.

#### Step 3. Analyze a warm front

These involve a gradual increase in precipitation as the front approaches, followed by a rapidly clearing sky and a rise in temperature after the front passes. If the hot air mass is unstable, prolonged storms can occur. Warm fronts are represented by a red line with semicircles on one side. The side of the semicircles represents the direction in which the warm front is moving.

#### Step 4. Study an occluded front

These are formed when a cold front overtakes a warm front. They are associated with various meteorological phenomena (possible storms), depending on whether it is a warm or cold occlusion. Generally, the passage of an occluded front brings drier air (a low dew point). Occluded fronts are represented by a purple line with semicircles and triangles on the same side. The side they are on indicates the direction the occluded front is moving.

#### Step 5. Analyze a stationary front

These indicate a stationary boundary between two different air masses. They have periods of continuous and long rains that persist in a given area for long periods and move in waves. If you see a semicircle bordering one side and triangles along the opposite side, then this is the representation that the front is not moving in either direction.

### Part 4 of 4: Interpreting Other Symbols on a Weather Map

#### Step 1. Read the station models at each observation point

If your weather map has station models, each will indicate temperature, dew point, wind, sea level pressure, barometric trend, and current weather with a series of symbols.

• Typically, temperature is measured in degrees Celsius and precipitation in millimeters. In the United States, temperature is measured in degrees Fahrenheit and precipitation in inches.
• Cloudiness is represented by a circle in the middle; the extent to which it fills indicates how cloudy the sky is.

#### Step 2. Study the lines on the weather map

There are many other lines on weather maps. Two of the most important types of lines represent isotherms and isotachs.

• Isotherms. They are lines that connect points through which the temperature is the same.
• Isotacas. They are lines that connect the points where the wind has the same speed.

#### Step 3. Analyze the pressure gradient

If there is a number in the isobars, such as "1008", it will indicate the pressure (in millibars) along that line. The distance between isobars is known as the pressure gradient. A significant pressure change over a short distance (that is, close isobars) indicates strong winds.

#### Step 4. Analyze the force of the wind

Wind barbs indicate the direction of the wind. These are the lines or triangles that leave the main line at an angle and indicate the intensity of the wind: 50 knots for each triangle, 10 knots for each complete line, 5 knots for each half line.