top of page

Wind Damage, Part 1

We know that hail is associated with storms. Usually, storms that drop hail also bring wind, as low-pressure fronts move in and out of the area. You may see damage from either one during an inspection.


In talking about wind damage, we should first cover some basics.

Wind is air moving from areas of high air pressure to areas of low air pressure. The greater the difference in air pressure between two points on Earth, the faster the air will move between them.


These differences in atmospheric pressure are created by uneven heating of the earth’s surface. Because of the earth’s shape and orientation to the sun, more warming takes place along the equator than at the poles. The different rates at which heat is absorbed and released by land, water and the atmosphere itself also contribute to thermal differences.

As air in warmer areas rises, cooler air moves in to replace it. It’s this replacement air that we experience as wind. The force that pulls the replacement air is called “convection.” Convection can cause both updrafts and downdrafts, which create wind with different characteristics, including varying:

  • direction;

  • strength;

  • duration; and

  • speed.


Wind blows faster in open areas where there’s nothing to slow it down. The 2006 IRC (International Residential Code) lists four Wind Exposure Categories, labeled A, B, C and D, according to the degree of obstruction offered by the topography.

An example of Exposure A is an urban environment in which 50% or more of the buildings are taller than 70 feet.

The B Exposure is an urban or suburban environment with closely spaced homes or home-size buildings.

Exposure C is generally open terrain with scattered obstructions less than 30 feet tall.

Exposure D is flat, unobstructed areas, such as shorelines.

Height Above the Ground

Wind slows due to friction with obstructions. Because air is a fluid, obstructions near the ground can affect wind speed at a much greater height. Aerodynamic drag created by buildings taller than 30 feet can actually slow down air that is flowing at an altitude of 1,500 feet. This is because air moves by convection, so flowing air will have an effect on the air surrounding it.

If wind above 1,500 feet is blowing at 100 miles per hour, at Exposure C, it will have slowed to 6