DISCOVER

# What Can Isobars Tell You About Wind Speed?

Updated July 20, 2017

The National Weather Service states that atmospheric pressure "is the force per unit area exerted by the atmosphere at a given point." Being a primary weather parameter that can act as a proxy for other variables, such as temperature and humidity, makes atmospheric pressure extremely important for understanding and predicting atmospheric phenomena. Pressure is depicted on surface maps by isobars, which are lines of equal pressure. They can be used to estimate and predict wind speed.

## Identification

Commonly seen as circles around areas of high and low pressure, isobar contours are the most prominent features on standard surface maps. The contours use millibars (mb) as their unit and have an interval of 4 mb. Air flows from high-pressure areas to low-pressure areas, so approximate wind direction can be determined using isobars. Surface maps are often generated every hour by the National Weather Service and other meteorological organisations.

## Sea Level Pressure

Adjusting data from all weather stations to account for differing pressures at varying heights is done by converting all data to sea level pressure. This allows meteorologists to get a truer picture of pressure differences, and thus more accurate wind speed estimates. Standard sea level pressure is considered to be 1013.25 mb, or 6.66 Kilogram per square inch.

The pressure gradient is the geographical space between lines of equal barometric pressure. Tightly packed isobars correlate with higher wind speeds, while more distance between contours indicates environments with less pressure change, resulting in calmer winds. Mathematically, the magnitude of the force can be displayed as ∆P/∆s, where P represents pressure, s is distance, and ∆ means "change in." Simply put, the change in pressure between two points divided by the change in distance between these same points is the pressure gradient. This gradient determines the magnitude of the force, which according to the University of Illinois WW2010 Project "is responsible for triggering the initial movement of air."

## Deriving Wind Speed

Performing exact wind speed calculations means determining acceleration, for which you must have the density of the air, and then converting it to speed. This involves some rather complicated equations and generally the use of an assumed average air density that is not always correct in individual cases. Thankfully, using contour intervals, estimates can be derived. "How to Read and Interpret All the Basic Weather Charts" states: "A pressure difference of 4 mb every 60 miles would give winds of about 50 mph." Considering that change in pressure gradient exhibits a positive and linear relationship with speed, a 4-mb difference every 30 miles would bring approximately 120-mph winds.

## Expert Insight

Prediction of winds from isobars differs based on geographic area, as there are forces and effects other than pressure gradient acting on the air. These include friction and the Coriolis effect. To become proficient at these estimations, you need to become familiar with the region of focus. This involves consistent studying of surface maps and how contour intervals relate to velocity in the area.