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# Weather map

A weather map is a tool used to display information quickly showing the analysis of various meteorological quantities at various levels of the atmosphere. Maps using isotherms show temperature gradients, which can help locate weather fronts. Isotach maps, analyzing lines of equal wind speed, on a constant pressure surface of 300 mb or 250 mb show where the jet stream is located. Two-dimensional streamlines based on wind speeds at various levels show areas of convergence and divergence in the wind field, which are helpful in determining the location of features within the wind pattern. A popular type of surface weather map is the surface weather analysis, which plots isobars to depict areas of high pressure and low pressure. Special weather maps in aviation show areas of icing and turbulence.

## Uses

A weather map is used to display an overview of one or more atmospheric variables at a specific time in the free atmosphere. They are used for the analysis and display of observations and computer analyses, including forecast fields derived by computer models.

## Plotted winds

Winds have a standard notation when plotted on weather maps. More than a century ago, winds were plotted as arrows, with feathers on just one side depicting five knots of wind, while feathers on both sides depicted of wind. The notation changed to that of half of an arrow, with half of a wind barb indicating five knots, a full barb ten knots, and a pennant flag fifty knots.

## Types of analyses performed

### Isobaric analysis

An isobaric analysis involves the construction of lines of equal mean sea level pressure on a geographic map. The innermost closed lines indicate the positions of relative maxima and minima in the pressure field. The minima are called low pressure areas while the maxima are called high pressure areas. Highs are often shown as H's whereas lows are often shown as L's. Elongated areas of low pressure, or troughs, are sometimes plotted as thick, brown dashed lines down the trough axis.

### Isotach analysis

Isotachs are lines of equal wind speed drawn on weather maps. They are helpful in finding maxima and minima in the wind pattern. Minima in the wind pattern aloft are favorable for tropical cyclogenesis. Maximum in the wind pattern at various levels of the atmosphere show locations of jet streams.

### Isotherm analysis

Isotherms are lines of equal temperature drawn on weather maps. Isotherms are drawn normally as solid lines at a preferred temperature interval. They show temperature gradients, which can be useful in finding fronts, which are on the warm side of large temperature gradients. By plotting the freezing line, isotherms can be useful in determination of precipitation type.

### Streamline analysis

A streamline analysis is a series of arrows oriented parallel to wind, showing wind motion within a certain geographic area. C's depict cyclonic flow or likely areas of low pressure, while A's depict anticyclonic flow or likely positions of high pressure areas.

## Types of weather maps

### Surface weather analysis

A surface weather analysis is a type of weather map that depicts positions for high and low pressure areas, as well as various types of synoptic scale systems such as frontal zones. Mesoscale boundaries such as tropical cyclones, outflow boundaries and squall lines also are analyzed on surface weather analyses. Isobars are commonly used to place surface boundaries from the horse latitudes poleward, while streamline analyses are used in the tropics.

### Aviation maps

Aviation interests have their own set of weather maps. One type of map shows where VFR (visual flight rules) are in effect and where IFR (instrument flight rules) are in effect. Weather depiction plots show ceiling height (level where at least half the sky is covered with clouds) in hundreds of feet, present weather, and cloud cover. Icing maps depict areas where icing can be a hazard for flying. Aviation-related maps also show areas of turbulence.