JET CONTRAILS & WEATHER
THICK &/or LINGERING CONTRAILS - indicate changing weather and/or precipitation
DISSIPATING or NO CONTRAILS - indicate continuing and/or clearing weather.
The plume of condensed water
vapor in the exhaust
trailing behind high flying jet aircraft is called a “contrail” - short for condensation
trail. Contrails are long white streaks of ice crystals left behind in the exhaust
of flying jet aircraft. At 8 to 12 km (5-7.5 mi.) above the ground, the jet engine
pulls in very cold, dry air and spews out hot, water-filled exhaust. The hot water
vapour mixes with the colder surrounding air. In doing so, it expands and freezes in (1 or 2 seconds) forming a trail of ice-crystal clouds. Contrails will only form into long-lasting visible cirrus-like cloud formations when the air temperature is at -40C or lower (-40F) and the humidity level is at 70% or greater.
Contrails can provide a clue to upcoming weather. If a jet leaves no trail or a short trail; or if the trail fades quickly - it is an indication that the air is relatively dry and sinking, which suggests that fair weather is likely to continue. On the other hand, if the exhaust trails linger for an hour or more; or they spread across the entire sky - the surrounding air is probably moist and rising, foretelling the arrival of a storm in a day or two. Reading contrails gives you a glimpse as to the weather over the next 12 to 24 hours.
Sky light blue to dark blue. Bright moon. Jet contrail disappears immediately or doesn't form. all indicate: Continuing Good Weather, Clear.
Large halo circling moon or sun. Jet contrail lingers thickly before falling apart indicate: Possible Change.