Pressure Gradient - What Makes the Wind Blow

Map drawn by Digital Atmosphere. Weather Graphics.com

The solid thin lines on this map are isobars - lines of constant pressure. If you were to walk or drive along one of these lines your barometer reading (pressure) would not change (assuming you could do it instantly.). The map is a snapshot in time. As the weather systems move so will the isobars. The closer isobars are together the stronger the winds. Full disclosure: This is generally a true statement but there are factors that make it not so. That will be the subject of another post.

So on this map the isobars are closer together around the low center just north of Lake Superior. The spacing is wider to the southwest and the separation is greatest in the area of high pressure extending from off the Pacific Northwest coast to the northern Gulf of Mexico. If you look closely at the station model plots within the high pressure area, the wind speeds are light - less than 10 knots (9 mph). Around the low pressure center near Lake Superior wind speeds are 10 to 20 knots (9 to 18 mph). If you are unfamiliar with the station model plot format go to the station model plotting section on this web site to learn more.

The pressure gradient is the change in pressure over a specified distance. If the isobars (pressure lines) are close together the gradient is greater than if the lines are far apart.