What is a wind rose and how is it interpreted?

A wind rose is a graphical representation of the wind speed and wind direction for a specified period for a particular location. The wind rose can be representative of the wind for a day, month, year or a long-term average by month or year. There are two graphics for each wind rose. One graphic depicts the average wind speed by wind direction in meters per second. The other graphic represents the percentage of frequency by wind direction.


How do I interpret a synoptic chart?

When looking at a synoptic chart the first thing to take note of are the isobars. These are lines joining places of equal pressure. Areas with high numbers are known as areas of high pressure and low-pressure areas are indicated by lower numbers. Wind blows from high-pressure areas to low-pressure areas. However, due to Coriolis force wind doesnt blow in a straight line from high- to low-pressure areas but blow in a clockwise direction around a low-pressure area and in an anti-clockwise direction around a high-pressure area (in the Southern Hemisphere). Winds thus tend to blow sub-parallel to the isobars and from the isobar patterns on a synoptic chart it is possible to estimate from which direction the wind will be blowing at any location. The closer the isobars are together, the steeper the gradient between areas of high and low pressure and the stronger the wind. By studying the patterns shown by isobars, forecasters can make predictions about how the weather conditions will develop. 

Air does not just move horizontally but also vertically. Descending (or sinking) air is associated with high-pressure areas and this produces clear skies and generally fair weather. Ascending (or rising) air on the other hand is associated with low-pressure areas causing the formation of clouds with possibly precipitation. Troughs of low pressure and ridges of high pressure can also be identified. Ridges are areas of high pressure that generally result in dry conditions in their immediate vicinity. A ridge of high pressure may be associated with coastal showers when it brings onshore winds along the east coast in advance of the ridge itself. These onshore winds can produce widespread coastal showers. The zone of interaction of the ridge with nearby areas of low pressure or troughs can be unstable and produce storms or rain in any area. Troughs are regions of relatively low pressure which often precede a cold front. These areas of relatively low pressure are unstable and tend to have high moisture associated with them. Consequently, they are good sources of thunderstorms.

Temperatures from a large number of weather stations are also plotted on the synoptic chart. This information is used to determine the location of fronts. By seeing where temperature changes significantly across a small area, it is possible to locate the position of these fronts. Cold fronts have triangles along the line indicating the position of the front and warm fronts have half-circles. Fronts occur at the boundaries of converging air masses which come together from different parts of the world. Since air masses usually have different temperature, they cannot mix together immediately owing to their different densities. Instead, the lighter, warmer air mass begins to rise above the cooler, denser one.

Fronts are usually associated with regions of low pressure, also known as depressions. As the sector of warm air is forced to rise, the cold air begins to engulf it. The leading edge of the warm air is marked by the warm front. The cold front marks the rear edge of the warm air and the leading edge of the ensuing cold air. When the warm air is completely lifted off the ground and is no longer in contact with the surface of the earth, this may be marked on a synoptic chart by an occluded front. Fronts are usually accompanied by clouds of all types, and very often by precipitation. Precipitation is usually heavier although less prolonged at cold fronts than at warm fronts, since the uplifting of warm air is more vigorous due to the undercutting of cold air, resulting in increased atmospheric instability.