United States Parachute Association > Experienced Skydivers > SIM > Section 5-5

5-5 Weather

A. Determining winds

  1. Surface winds must be determined prior to jumping and should be measured at the actual landing area.
  2. Winds aloft:
    1. Winds aloft reports available from the FAA flight service are only forecasts.
    2. Observations may be made while in flight using navigation systems, for example, global positioning satellite systems (GPS).
    3. Winds can change at any time, so all available information should be checked by the jumper before and during the jump.

B. Hazardous weather

  1. Fronts approach with much warning but can catch the unaware off guard.
    1. Some fronts are preceded by a gust front (a line of sudden and severe weather).
    2. Frontal approach and passage may be associated with rapid and significant changes in the strength and direction of the winds aloft and on the surface.
  2. On calm, hot, humid days, thunderstorms can spontaneously generate and move in unpredictable patterns.
  3. Dust devils are mini-tornados that spontaneously generate on days of high thermal convection activity.
  4. Where to get practical information on approaching weather:
    1. the Weather Channel
    2. www.weather.com
    3. TV weathercasts
    4. pilot assistance (legally responsible to know the weather conditions before flight)
    5. continuous observation

C. Density altitude

  1. Parachute performance is measured at sea level in moderate temperatures and humidity.
  2. Altitude, heat, and humidity influence the density of air
  3. Density altitude is a measure of air density that is calculated according to the temperature and altitude.
  4. As density altitude increases, airspeed increases by:
    1. almost five percent per 3,000 feet up to 12,000 feet MSL
    2. more than five percent per 3,000 feet above 12,000 feet MSL
  5. As density altitude increases, a ram-air canopy pilot can expect the following:
    1. a higher stall speed
    2. a faster forward speed
    3. a faster descent rate
    4. higher opening forces
  6. Additionally, aircraft are affected by higher density altitude in the following ways:
    1. longer distances required for takeoff and landing
    2. reduced propeller effectiveness
    3. poorer turbine and piston engine performance
    4. slower and flatter rate of climb
    5. less useful load
  7. The aircraft pilot is responsible to know the density altitude prior to takeoff, and skydivers are advised to consider the effects of density altitude on canopy performance.