#V RISING OUT OF FLAME FREE#

Overall, mesas tend to decelerate general winds because energy must be expended to create local reversals of wind flow called “separation eddies” that form upwind and downwind of steep sided barriers near separation eddies and on top of the mesa, expect 20 feet winds to be decelerated below what might be expected for the general area. On the lee side of the peak, a turbulent reversal or wind eddies of general wind flow is possible.

Some acceleration of general winds is likely around the flanks and over the top of isolated mountains peaks with gently inclined slopes.

Isolated peaks tend to divert general wind flow horizontally and vertically.

See Section for Effective wind speed.Įstimating Surface (20 feet) Wind Speed in Mountain Terrain Effective Wind Speed is used in place of midflame wind speed when winds are blowing upslope and to determine size and shape (length-to-width ratio) for those fires. It is generally derived from the Surface (20 feet) Wind based on sheltering from an upper canopy or flame height based on fuel bed depth.Įye-Level Winds are frequently used to represent midflame wind speeds, though that may represent an overestimate for shallow and sparse fuelbeds that have lower flame heights or an underestimate for shrub and crown fuels with deep fuelbeds.Įffective wind speed is the combined effect of Midflame Wind Speed and the slope equivalent wind speed in the direction of maximum spread (head fire). This is the wind input required for estimating fire spread using the Rothermel surface fire model. Midflame Wind Speed is the estimated wind speed at a height above the surface fuel equivalent to the height at midflame. The duration of a gust is usually less than 20 seconds. weather observing practice, gusts are reported when the peak wind speed reaches at least 16 knots and the variation in wind speed between the peaks and lulls is at least 9 knots. Wind Gust is a sudden, brief increase in speed of the wind. Measured near the earth’s surface, at an observing station, customarily at some height (usually 20 feet or 10 meters) above the average vegetative surface and a distance equal to at least 10 times the height of any obstruction to minimize the distorting effects of local obstacles and terrain. The corresponding land breeze is lighter, perhaps 5-10mph.

Land and Sea Breeze Circulations during the day, sea/lake breeze can reach 10-15mph at the peak of solar heating in the afternoon.

Peak speeds can be as much as double those of slope winds, reaching 10-15mph at their peak. Their development each day generally lags 1-3 hours behind that of slope winds.

Valley Winds are similar to and linked with slope winds.

Downslope winds are very shallow and of a slower speed than upslope winds, generally 2-5mph. The cooled denser air is stable and the downslope flow, therefore, tends to be laminar. In individual draws and on slopes going into shadow, the transition period consists of (1) dying of the upslope wind, (2) a period of relative calm, and (3) gentle laminar flow downslope. The transition from upslope to downslope wind begins soon after the first slopes go into afternoon shadow and cooling of the surface begins. Upslope winds generally range from 3-8mph. The strength of upslope winds is also influenced by the length and steepness of the slope as well as the exposure. They can react quickly to insolation on the slope, with upslope breezes starting within a few minutes.

Slope Winds are driven by heat exchange at the slope surface.

They differ from those which would be appropriate to the general pressure pattern in that they are limited to near surface and are controlled by the strength of the daily solar cycle. Thermal, convective, drainage, and convective winds are all caused by local temperature differences generated over a comparatively small area by local terrain and weather. They may be influenced and modified considerably in the lower atmosphere by terrain and vegetative structure.

#V RISING OUT OF FLAME FREE#

Synoptic scale, gradient, free air, ridgetop are large-scale winds produced by broad scale pressure gradients between high- and low-pressure systems. Examples include frontal winds, Foehn winds, thunderstorm winds, whirlwinds, surfacing or low-level jets (reverse wind profiles), and glacier winds. This graphic of different winds shows generally the source elevation and duration of different general (synoptic), local (mesoscale), and gust (microscale) winds.Ĭritical winds dominate the fire environment and easily override local wind influences. Adjusting Surface (20 feet) Wind to Midflame Wind Speed.Worksheet for Estimating 20 feet Surface Winds.

Estimating Surface (20 feet) Wind Speed in Mountain Terrain.

V rising out of flame