An American Airlines 757 and JetBlue A320 contrailing over Cleveland in 2008. Photo: Dave Reed - OPShots.net
Sometimes planes leave contrails, and sometimes they don’t. It depends on the weather, and specifically it depends on the weather at altitude. It’s also very localized. A plane might leave a trail in one region, and another plane a mile away might not leave a trail.
NASA have put together a contrail forecast page that you can use to roughly predict when contrails are likely for a given region, and a given altitude. The following image should be the current forecast
The presence of a circle at a particular point simply tells you if the conditions are favorable for contrail formation at the altitude indicated by the color of the circle. The size of the circle only varies so different altitudes can be shown at the same point, but the smaller the circle, the higher the altitude.
The Mb scale on the left is the measure of atmospheric pressure in millibars. This can roughly be translated to altitude, as pressure decreases fairly uniformly with altitude. Planes actually use the air pressure to measure their altitude using an altimeter, but you have to set it to the local sea level pressure in order to get an accurate result for landing and take-off. To avoid confusion, planes flying above 18,000 feet all set their altimeters to the same reference, 29.92 inchs of mercury, or 1013.25mb (for sea level).
via Contrail Science.
