Weather maps show a simplified representation of the current or predicted climate in a given area. The most common map that you may happen to read is the one relating to the surface analysis, which is also the subject of this article. At first, reading surface analysis on a weather map may seem complicated, but with a little practice you'll be able to do it in no time.
Steps
Part 1 of 4: Basic Concepts
Step 1. Understand the basic concepts of precipitation
What interests people most are precipitation which, in meteorology (the study of climate), represents the fall of water in any form on the earth's surface. Precipitation includes rain, snow, hail and sleet.
Step 2. Understand what a high pressure system is
One of the main aspects of the interpretation of the weather involves the ability to understand the events caused by the different air pressures. High pressure implies a dry climate. A high pressure system is a denser mass of air because it is colder or drier than the surrounding air. As a result, the heavier air falls down and away from the center of the system, like water being poured onto the ground.
With high pressure systems, the weather tends to be calmer or clearing up
Step 3. Understand what a low pressure system is
Low pressure is typically associated with humid air which, at times, turns into precipitation. A low pressure system is a less dense mass of air than the surrounding one, because it is more humid and / or warmer. The surrounding air is drawn towards the center, as the lighter one tends to rise upwards, causing clouds and precipitation because the humid air cools as it rises.
- You can see this phenomenon when invisible water vapor forcibly condenses into droplets on the outer surface of a cold glass. However, drops do not form if the glass is only slightly cold; in the same way, the low pressure air that rises produces rain only if it reaches an altitude where it is cold enough to trigger the phenomenon of condensation and therefore the formation of drops too heavy to be supported by the rising air. Clouds are simple drops of air that are small enough to "float" in the air.
- When there are very low pressure systems, thunderstorms are on the way (if they haven't already broken out). Clouds begin to form and move across the sky; cumulonimbus clouds develop when humid air reaches very high altitudes. In some cases, tornadoes arise when very high pressure air collides with the very hot and humid air of a low pressure system.
Step 4. Study a weather map
Observe one that is presented during the news on television, online or in the newspaper. Other possible sources are magazines and books, but the maps may be out of date. Newspapers are a good place to find weather maps, as they are cheap, reliable, and can be cropped to take the page you are interested in with you as you learn to interpret the symbols.
Step 5. Analyze a small section of the map
If possible, look for one that takes into account a small area, as it is easier to read; it is not easy for beginners to concentrate on a large scale. Observe the locations, lines, arrows, systems, colors and numbers on the map; they are all different and important symbols.
Part 2 of 4: Reading the Air Pressure
Step 1. Understand what air pressure measures
This is the force or pressure that air exerts on the ground and is measured in millibars (mbar). It is important to be able to read the air pressure, because systems are associated with certain weather conditions.
- The average value of the air pressure corresponds to 1013 mbar (759.8 mmHg).
- A typical high pressure system measures approximately 1030 mbar (772.6 mmHg).
- A typical low pressure system measures approximately 1000 mbar (750 mmHg).
Step 2. Learn the air pressure symbols
In order to read a surface analysis on the weather map, look at the isobars (from the Greek "isos" which means "equal" and "baros" which can be translated as "weight"); these are simple curved lines that indicate areas that have the same pressure and play a fundamental role in determining wind direction and speed.
- When isobars form closed, concentric (but not always circular) rings, the smaller ring in the middle indicates the center of the pressure system. It can be a high pressure system (usually indicated with "A" in Italian or with "H", if English abbreviations are preferred) or a low pressure system (indicated with a "B" or an "L").
- The air does not flow "down" following the pressure gradient, but "surrounds" it due to the Coriolis effect (the rotational movement of the Earth). Consequently, in the Northern Hemisphere the direction of the wind is indicated by the isobar lines in a counterclockwise direction around the low pressure cores (cyclonic flow) and in a clockwise direction around the high pressure cores (anticyclonic flow). The closer the isobars are to each other, the stronger the winds.
Step 3. Learn to interpret a low pressure (cyclone) system
These storms are characterized by an increase in cloud cover, winds, temperatures and a greater probability of precipitation. They are represented on the map with very close isobars, crossed by arrows that rotate clockwise (in the southern hemisphere) or counterclockwise (in the northern hemisphere). Typically, in the medial isobar, which forms a closed circumference, you can read a "T" or another letter, depending on the language chosen by the person who compiled the map.
Radar images can show low pressure systems. Tropical cyclones (which develop in the South Pacific Ocean) are also referred to Hurricanes close to America or typhoons close to the coastal regions of Asia.
Step 4. Learn to interpret a high pressure system
This weather condition indicates a serene, clear climate with a reduced chance of precipitation. Drier air typically results in more extreme temperatures, both high and low.
These systems are represented with isobars, an "A" in the midline and arrows showing the direction in which the winds blow (clockwise in the northern hemisphere and counterclockwise in the southern hemisphere). Just like cyclones, high pressure systems are visible through radar images
Part 3 of 4: Interpreting the Types of Fronts
Step 1. Observe the types and movement of the fronts
The fronts are the contact or "border" surfaces between the masses of hot air and those of cold air. If you are close to a front and you know it is moving towards you, you can expect changes in weather conditions (such as cloud formation, precipitation, thunderstorms and winds) as the front passes over your location; mountains and large bodies of water can alter the path of the fronts.
You can recognize them on the weather map because they are indicated with semicircular lines, "lying" triangles or with both symbols. These drawings indicate the boundary lines between the various types of fronts
Step 2. Analyze a cold source
When this weather system is present, the rains can be torrential and the winds can reach high speeds. On the map it is indicated with blue lines and triangles. The direction in which the apex of the triangle points represents that in which the cold front is moving.
Step 3. Study a warm front
As it approaches, this kind of system brings with it a gradual increase in rainfall, followed by a sudden improvement and rise in temperatures after its passage. If the warm air mass is unstable, the weather can be characterized by prolonged thunderstorms.
You can locate the warm front on the map thanks to a red line with semicircles. The convex side of the semicircle indicates the direction the front is moving
Step 4. Study an occluded front
It is formed when a cold front reaches a warm front. It is associated with several meteorological phenomena (including thunderstorms), depending on whether it is a hot or cold front. Its passage generally leads to drier air (lower dew point).
Occluded fronts are represented by purple lines with semicircles and triangles facing the same direction, which is identical to where the front itself is heading
Step 5. Study a stationary front
It is a non-moving front which is located between two masses of air; it is characterized by long periods of constant rain over an area and tends to move in waves.
On the maps it is represented by a line with semicircles on one side and triangles on the opposite, to indicate that the front is not in motion
Part 4 of 4: Interpret the other Symbols on the Weather Map
Step 1. Interpret the symbols that represent the observation points at each weather station
If the map has symbols, each of them represents temperature, dew point, sea level pressure, pressure trend and current conditions through a series of symbols.
- The temperature is usually noted in degrees Celsius and rainfall in millimeters. In the United States, the same quantities are reported in degrees Fahrenheit and inches.
- Cloud cover is indicated with a circle in the center; the greater the colored area of the circle, the greater the cloudiness.
Step 2. Study the lines on a weather map
There are many other lines and the two most important are isotherms and isotachias.
- Isotherms: are the lines on the map that connect the points with the same temperature.
- Isotachia: are the lines on the map that join the points where the wind maintains the same speed.
Step 3. Analyze the pressure gradient
The presence of a number on an isobar, for example "1008", indicates the pressure expressed in millibars along that line; the distance between the isobars is called the pressure gradient. A large change at a short distance (close isobars) indicates strong winds.
Step 4. Analyze the strength of the wind
The wind vectors indicate the direction in which the wind is blowing. The triangles or segments extending diagonally from the main line represent intensity; each triangle corresponds to 50 nodes, each complete segment corresponds to 10 nodes and half segment is equal to 5 nodes.
Advice
- Isobars can be modified or deformed by terrain reliefs such as mountains.
- Don't be put off by the apparent complexity of a weather map; the ability to read it is a skill that should not be neglected.
- If you are interested in climate systems and peculiarities, you can consider joining a club or a meteorological society.
- This type of map is based on radar and satellite images, instrumental records from weather stations and computer analyzes.
- THE meteorological fronts often derive from the nucleus of zones of low pressure.
