Chapter 3b: Effects of Weather on Small Unmanned Aircraft Performance
Stability Stability of an air mass determines its typical weather characteristics. When one type of air mass overlies another, conditions change with height. Characteristics typical of an unstable and a stable air mass are as follows: Unstable Air Stable Air Cumuliform clouds Stratiform clouds and fog Showery precipitation Continuous precipitation Rough air (turbulence) Smooth air Good visibility (except in blowing obstructions) Fair to poor visibility in haze and smoke Fronts As air masses move out of their source regions, they come in contact with other air masses of different properties. The zone between two different air masses is a frontal zone or front. Across this zone, temperature, humidity and wind often change rapidly over short distances. Mountain Flying When planning a flight over mountainous terrain, gather as much preflight information as possible on cloud reports, wind direction, wind speed, and stability of air. Satellites often help locate mountain waves. Adequate information may not always be available, so remain alert for signposts in the sky. Wind at mountain top level in excess of 25 knots suggests some turbulence. Wind in excess of 40 knots across a mountain barrier dictates caution. Stratified clouds mean stable air. Standing lenticular and/or rotor clouds suggest a mountain wave; expect turbulence many miles to the lee of mountains and relative smooth flight on the windward side. Convective clouds on the windward side of mountains mean unstable air; expect turbulence in close proximity to and on either side of the mountain. Structural Icing Two conditions are necessary for structural icing in flight: 1. The aircraft must be flying through visible water such as rain or cloud droplets 2. The temperature at the point where the moisture strikes the aircraft must be 0° C or colder. Aerodynamic cooling can lower temperature of an airfoil to 0° C even though the ambient temperature A thunderstorm cell during its life cycle progresses through three stages-( 1) the cumulus, (2) the mature, and (3) the dissipating. It is virtually impossible to visually detect the transition from one stage to another; the transition is subtle and by no means abrupt. Furthermore, a thunderstorm may be a cluster of cells in different stages of the lifecycle. The Cumulus Stage Although most cumulus clouds do not grow into thunderstorms, every thunderstorm begins as a cumulus. The key feature of the cumulus stage is an updraft as illustrated in figure 3-4 . The updraft varies in strength and extends from very near the surface to the cloud top. Growth rate of the cloud is a few degrees warmer. Thunderstorm Life Cycle
Remote Pilot – Small Unmanned Aircraft Systems Study Guide
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