Contrail Meteorology

I would just like to explain my observations that the contrails (or vapor (vapour) trails) of jet airplanes that we see high in the sky above us reveal quite a bit about the atmospheric conditions at the altitude of the contrail. Typical altitudes for long-distance flights are around ten kilometers or 6-7 miles.

A jet engine will leave no contrail at low altitudes simply because the air is more dense there. Air at low altitudes can hold more water vapor (vapour) because of it's higher density. (Let's alternate the two spellings). The long white contrails only become visible when the water from the burned fuel condenses upon being exposed to the cold outside air. By the way, it is my observation that jet contrails most resemble altocumulus clouds and could be categorized as such.

When an aircraft leaves only a short contrail, the air at that altitude is of a low relative humidity. That means that the air is holding only a fraction of the evaporated water that it potentially could due to it's temperature and density. Air can hold more water when it is warmer and when it is denser. The vapour from the jet at first condenses, forming the contrail, but then shortly afterward reevaporates because the air has plenty of capacity to hold more water vapor.

When the contrail lasts for a significant period of time, this indicates that there is a higher relative humidity in the air at that altitude. Meaning that the air already has much of the vapour that it could potentially hold due to it's density and temperature. The water vapor condenses upon meeting the cold air outside the jet engine and remains condensed for a while due to the scarcity of room in the air for more vapour.

The air with the highest relative humidity is that in which the contrail spreads. A jet contrail is usually a narrow white line, as seen from the ground, but if there is already a lot of vapor in the air, some of that will condense into the new water that has been introduced into the air by the jet exhaust. This will cause the contrail to spread in lateral directions so that it becomes a broad line across the sky.

Contrails also reveal the direction of the wind at their altitudes. Winds high in the sky may not be the same at all as those nearer the ground. On one recent day I noticed that the low cumulus clouds, with an altitude of about a kilometer, were moving eastward with the wind while a contrail much higher in the sky was moving southward. The two air movements were actually perpendicular to one another.