Contrails?????

I have a few Q's about contrails.
I know how they are made, sort of like when I breath into cold air a cloud forms. (something like that;)

My question is how come a jet traviling at the same alt. has a contrail one day but not another? Isnt always cold up there. :?

Another Q is, what is the lowest altitude a jet can produce a contrail? (FL350?)

I believe it depends on the OAT.

I think the temperature is a big factor. There probably isn't a set altitude where contrails form. It may be -15 degrees Celsius at FL200, then another temperature another day. The contrail's length probably depends on the winds and if they are strong enough that day to scatter the contrail.

Quote:

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Originally Posted by NASA

Contrails only form at very high altitudes (usually above 8 km) where the air is extremely cold (less than -40 degrees C). Other clouds can form at a range of altitudes, from very close to the ground, such as fog, to very high off the ground, such as cirrus clouds.

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Contrails are very similar to you seeing your breath on a cold day. A lot of factors determine contrail properties; wind, temperature, humidity, etc.

Check out http://en.wikipedia.org/wiki/Contrail and http://cimss.ssec.wisc.edu/wxwise/class/contrail.html

Hey thanks! :D

I have seen contrails as low as 15000 from an Airliner(Namely when NWA had flight 18 in from Tokyo on the Lendy arrival over my house near Manhattan)

Yes it is all based on OAT moisture dew points etc... Think about how when it is 20 degrees outside and you blow warm air out. That air contains moisture like the exhaust of a jet. The tempature diffrences causes that moisture at altitudes like 350 and such to instantly turn into ice crystals hence creating the contrail you will see. The reason it lasts longer than your breath is because the air on average is about negative 50 or so at those altitudes or less.