force of the wind

2008-08-21 15:01:37.000 – Jeff Wehrwein,  Summit Intern

Looking north from Mount Chocorua

On Tuesday afternoon, Stacey and I took advantage of the cool weather in the valley and hiked Mount Chocorua, which is just outside North Conway and south of Mount Washington by about 25 miles. We took the Champney Falls trail to the Piper trail, which together form a relatively tame 3.8-mile route to the summit. Along the trail we found Champney Falls and a healthy population of mushrooms, which have enjoyed the unusually plentiful rain this month.

I was surprised by how windy it was as we hiked, though glad for the cool breeze. Mount Chocorua is only 3475 feet tall, but it has a bare rocky summit with rare alpine vegetation. Standing on the summit, the gusts of wind were eager to knock me off if I wasn’t paying attention. This got me thinking about the wind, which has been relatively calm on the summit for my past few shifts. Stacey pointed out that at a lower altitude, the force of the wind is actually stronger because the air is more dense. So a 30 mph wind at sea level exerts more force than a 30 mph wind at the summit of Mt. Chocorua, which in turn exerts more force than a 30 mph wind at the summit of Mount Washington.

This morning, I did a bit of research into just how much the force of the wind varies at different altitudes. The formula for the force of the wind is based on differential equations called the Navier-Stokes equations. I’m not much into differential equations, but the main idea is that the force of the wind depends on the density of the air. This makes sense – denser air has more particles colliding with you and exerting a force. The density of the air, in turn, depends on the temperature and air pressure, according to the ideal gas law. Putting the two together, the force of the wind is proportional to air pressure divided by temperature. Therefore, higher pressure creates a stronger force, but higher temperature creates a weaker force.

A past intern at the Observatory created a wind force calculator that takes a given wind speed, pressure, and temperature, and approximates the force of the wind. I crunched out a few examples, which I’ll share here. On Mount Chocorua, I estimate that the wind was around 40 miles per hour, the pressure was around 900 millibars, and the temperature was around 55 F. According to the calculator, I was feeling a force of about 40 pounds. When I arrived on the summit yesterday, it was colder and windier than I had seen since May. The wind was gusting up to about 70 mph, the temperature was 35 F, and the pressure was 807 millibars. These conditions exert a force of 112 pounds, which explains why I had trouble walking from one end of the deck to the other.

A typical winter day with 100 mph winds, 10 F temperatures, and pressure of 800 millibars exerts about 240 pounds of force, which is like an NFL linebacker trying to push you across the deck. A hypothetical category 5 hurricane with winds at 160 mph, temperatures around 80 F, and pressure around 920 millibars would exert a force of 583 pounds. Finally, the world record wind of 231 mph exerted a force of roughly 1280 pounds.

Jeff Wehrwein,  Summit Intern