Froude Number

Froude Number

The Froude Number represents the flow of air when it comes in contact with a barrier, in Vermont's case, the Green Mountains. It is an estimation of whether the flow can make it over the mountains or not and is basically a ratio of the wind perpendicular to the mountain chain versus the atmospheric stability.

Under northwesterly flow, if the Froude Number is low (< 1), or subcritical and blocked, the air will not make it over the mountain and the precipitation will back up and fall primarily along the spine of the Green Mountains and the western slopes. If it is very low (< 0.5), the precipitation will back further west into parts of the eastern Champlain Valley and the western slopes of the Green Mountains.

If the Froude Number is high (> 1), or supercritical and unblocked, the air will flow freely over the mountains and deposit the heaviest precipitation on the east side. A Froude Number near 1 is critical and the heaviest precipitation will likely fall along the barrier. For more information on the Froude Number, please click here.