Here’s Your Answer

Photo by MaryHelen Swanson

Q: Why do airplanes leave trails in the sky some days and not others?

A: For the answer this week we asked Post Review weatherman Jeremy Grams, who is a professional meteorologist. (He is also an alumnus of North Branch High School, by the way)

He said the sight of this phenomena in our sky is related to specific weather conditions. Vapor trails or contrails form from water vapor being exhausted by an aircraft engine as a byproduct of combustion. They need very cold temperatures (below -40°F), which is typically found at the cruising altitude of jet flights above 25000 feet.

If the air at that level is dry (i.e, the relative humidity is low), a contrail may not form or may dissipate in seconds.

If the air at that level is moist, the contrail can persist for minutes and potentially hours if turbulence is weak.

In the picture, above, the contrails are mixed with the naturally-forming cirrus clouds. This is typically a very good meteorological situation for contrails to form and remain persistently thick.

There was relatively high moisture in the upper-levels (above 25000 feet) Monday morning in advance of the next approaching upper-level low pressure system centered along the British Columbia coast.

Turbulence was light owing to the presence of upper-level high pressure overhead. Thus, noticing contrails with increasing cirrus clouds can often spell a change in the weather pattern.

The forecast calls for very warm temperatures Tuesday and then a cold front passage and much cooler temps with a chance for rain on Wednesday.

If you have a question you would like answered, send your question to editor.postreview@ecm-inc.com and watch for your answer in the Post Review.

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