In this article we calculate the pressures that can be generated by the various hurricane category winds. The pressure can be through direct pressures of winds encountering buildings, or through the Bernoulli reduction in pressures due to rapid airflow.
Due to the confusion of US-UK or Imperial units, we calculate in the metric system, and then convert to pounds force per unit area. We start with the Bernoulli equation, which is actually energy conservation in fluid or gaseous flows. The energy density is (1/2 x density x velocity squared), or 1/2 ρ v^2. The pressure from blocked air is the momentum change per unit time, or the momentum ρv being stopped, times the rate of the flow of the air or v, giving (ρ v^2).
The density of air at sea level is ρ = 1.275 kg/m^3 (kilograms per cubic meter). We start with the wind speeds of hurricane categories, in mph, and convert them to m/sec with v (m/sec) = v (mph) x 0.447. We then square that and multiply by 1/2 ρ. The 1/2 ρ v^2 is in units of kg/(m sec^2) which is the same as the force per square meter, or force in Newtons N/ m^2, giving the pressure. Finally, we convert that to pounds force per square foot with:
1 Pascal = 1 N/m^2 =0.02089 pounds force/ft^2.
The last row of 185 mph is where the maximum steady wind speeds of hurricanes end. Based on the increase of velocity squared by about √ 2 for each category, it is also where a category 6 would start.
Pressures for Hurricane Categories
| Category | v-mph | v-m/sec | N/m^2 | lbf/ft^2 |
| 1 | 74 | 33.1 | 699 | 14.6 |
| 2 | 96 | 42.9 | 1,174 | 24.5 |
| 3 | 111 | 49.6 | 1,571 | 32.8 |
| 4 | 130 | 58.1 | 2,155 | 45.0 |
| 5 | 157 | 70.2 | 3,142 | 65.6 |
| 6* | 185 | 82.7 | 4,363 | 91.1 |
As an application, we look at double-sided mobile homes, which range in area from about 500 square feet to about 3,000 square feet. We take one at 1,000 square feet, which can be doubled or tripled for larger sizes. At the start of a category 1, it could have its roof lifted by 1,460 pounds of force by the reduction in force as the Bernoulli decreased pressure from the overhead wind. Tie down installation costs about $2,000. At a maximum wind speed hurricane of 185 mph, the lift force would be 9,110 pounds of force.
I had thought as had others that one should just open some window to balance the pressure. That doesn’t work since the wind speed inside would still be rather small. Only if you had the same wind speed inside, with all of the water blown in, would the Bernoulli force be fully cancelled.
I started this work with respect to the still standing but open tower of the Champlain South building in Surfside, Florida. The open side would be a wind catcher, slowing it down, while the wind would rapidly skim around the undamaged side. That would put a lot of pressure on the side of the building with apparent collapsed columns. It turns out that for the tower that was still in tact, one of the columns had shifted, and the rebar in the columns appeared to be of substandard size. We were much relieved when they demolished both of the remaining towers.
