General Winds
General winds are produced by the broadscale pressure gradients which are shown on synoptic weather maps, but may be modified considerably by friction or other topographic effects. The amount of influence of the surface on general winds is largely dependent on wind speed and the stability of the air. Stable air flowing over even surfaces tends to be smooth, or laminar. Unstable air or strong winds flowing over rough surfaces is turbulent and full of eddies. Forests and other vegetated areas are characteristically rough surfaces and thus contribute to air turbulence, eddies, etc.
Surface winds in the Northern Hemisphere tend to shift clockwise with the passage of fronts. In mountainous topography, however, the effect of the mountains on the windflow usually overshadows this. The windflow is channeled, and, over sharp crests, eddies are produced. At times, waves form over mountains, and, if conditions are favorable, strong surface winds are experienced on the lee side. When the airflow is from higher to lower elevations, the air warms adiabatically and foehn winds are produced.
Wind affects wildfire in many ways. It carries away moisture-laden air and hastens the drying of forest fuels. Light winds aid certain firebrands in igniting a fire. Once a fire is started, wind aids combustion by increasing the oxygen supply. It aids fire spread by carrying heat and burning embers to new fuels, and by bending the flames closer to the unburned fuels ahead of the fire. The direction of fire spread is determined mostly by the wind direction. Thus the fire-control plan, in the case of wildfire, and the burning plan, in the case of prescribed fire, must be based largely on the expected winds.
- Laminar Flow
- Mechanical and Thermal Turbulence
- Frontal Winds
- Effects of Mountain Topography on Surface Winds
- Winds Aloft
- Measuring Surface Winds
Encyclopedia ID: p359
