How to use Isobar map

How to use Isobar map

Windy.app's got isobars on the map. They'll help you better forecast wind speed and direction. The forecast shows isobars for 10 days.

To activate the layer, open the map and press the isobar sign in the right upper corner.

Isobars map in Windy.app

Available for all Android users. iOS soon!

What is an isobar?

Atmospheric pressure is the pressure of air on the Earth's surface.

It's lower in areas with warmer air and vice versa. Air masses always tend to move to lower pressure areas.

Points with the same atmospheric pressure on the map are connected with lines – isobars. The air tends to move to a lower pressure area using the shortest way - at a right angle to the isobar. The perpendicular is called a normal line.

Wind and isobars

The pressure difference is the only thing speeding wind up. The quicker the pressure changes from spot to spot, the higher the wind speed is. All the other forces, such as friction, only reduce wind speed and/or change its direction.

The wind would be perpendicular to the isobar if the Earth wasn't spherical and rotating around its axis. But instead, all moving objects, including wind, are subject to the influence of the so-called Coriolis force.

It deflects wind from the normal line to a certain angle. In the Northern Hemisphere air is rotated clockwise (to the right) and in the Southern - counterclockwise (to the left) with reference to the movement direction.

The friction between air and sea is lower, that's why it's deflected more there. The angle is usually 40-50° above land and can reach 70-80° above sea - then the wind blows practically along the isobar.

If isobars are close to each other

The higher the pressure difference between two areas is, the higher the wind speed is. That's why isobars that are close to one another mean high wind speed.

Close Isobars high pressure

Isobars on the Windy.app's map

Cyclones and anticyclones (H and L)

L on the isobar map is a cyclone with a low-pressure area in the circle. A cyclone means cloudy and rainy weather.

H is an anticyclone with a high-pressure area in the circle. Anticyclone means calm and sunny weather.

In the Northern Hemisphere:

• Wind moving counterclockwise around a low-pressure area (L)

• Wind moving clockwise around a high-pressure area (H)

Cyclones and anticyclones on the Windy.app's map

 

Read more:

How atmospheric pressure works. Simple explanation

Basic Meteorology for Yachtsmen and Yachtswomen