Ch 03.qxd 8/24/04 10:42 AM Page 3-2 Ter ms Used by U.S. Weather Service Calm Light Air Light Breeze Gentle Breeze Moderate Breeze Fresh Breeze Strong Breeze Moderate Gale Velocity m.p.h. Less than 1 1 - 3 4 - 7 8 - 12 13 - 18 19 - 24 25-31 32-38 Estimating Velocities on Land Smoke rises vertically. Smoke drifts; wind vanes unmoved. Wind felt on face; leaves rustle; ordinary vane moves by wind. Leaves and small twigs in constant motion; wind extends light flag. Dust and loose paper raised; small branches are moved. Small trees begin to sway; crested wavelets form in inland water. Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty. Whole trees in motion; inconvenience felt in walking against the wind. Sea like a mirror. Ripples with the appearance of scales are formed but without f oam crests. Small w avelets, still short bu more pronounced; crests hav a glassy appearance and do not break. Large w avelets; crests begin to break.  F oam of glassy appearance. (P erhaps scattered whitecaps.) Small w aves, becoming longer; fair ly frequent whitecaps. Moder ate waves; taking a more pronounced long form; many whitecaps are formed. (Chance of some spray.) Large w aves begin to form; white f oam crests are more e xtensiv e everywhere. (Probab ly some spray.) Sea heaps up and white foam from breaking waves begins to be b lown in streaks along the direction of the wind. Estimating Velocities on Sea Check your glassy water technique before water flying under these conditions. Ideal water flying characteristics in protected water. This is considered rough water for seaplanes and small amphibians, especially in open water. This type of water condition is for emergency only in small aircraft in inland waters and for the expert pilot. to describe the combined motion of all the factors disturbing the surface.) These waves tend to be a chaotic mix of heights, periods, and wavelengths. Because the wind causes the height to increase faster than the wavelength, they often have relatively steep, pointed crests and rounded troughs. With a windspeed of 12 knots, the waves begin to break at their crests and create foam. in which the generating wind blows. When this wind ceases, water friction and spreading reduce the wave height, but the reduction takes place so slowly that a swell persists until the waves encounter an obstruction, such as a shore. Swell systems from many different directions, even from different parts of the world, may cross each other and interact. Often two or more swell systems are visible on the surface, with a sea wave sys- tem developing due to the current wind.  In lakes and sheltered waters, it is often easy to tell wind direction by simply looking at the water’s sur- face. There is usually a strip of calm water along the upwind shore of a lake. Waves are perpendicular to the wind direction. Windspeeds above approximately eight knots leave wind streaks on the water, which are paral- lel to the wind. The height of waves depends on three factors: wind- speed, length of time the wind blows over the water, and the distance over which the wind acts on the water. As waves move away from the area where they were generated (called a  fetch ), they begin to sort them- selves by height and period, becoming regular and evenly spaced. These waves often continue for thou- sands of miles from where they were generated.  Swell is the term describing waves that persist outside the fetch or in the absence of the force that generated them. A swell may be large or small, and does not indicate the direction of the wind. The wake of a boat or ship is also a swell. Unlike wind and current, waves are not deflected much by the rotation of the Earth, but move in the direction Land masses sculpt and channel the air as it moves over them, changing the wind direction and speed. Wind direction may change dramatically from one part of a lake or bay to another, and may even blow in opposite directions within a surprisingly short distance. Always pay attention to the various wind indicators in the area, especially when setting up for takeoff or landing. 3-2 Figure 3-1.The size of waves is determined b the speed of the wind.