Search the sky for passing jets and look for the condensation trail—contrail—that the jet leaves behind. Because jet exhaust is mainly carbon dioxide and water vapor, when the water vapor mixes with the cold environment of the upper troposphere, it can condense into small water droplets to form a line of white “clouds.” Study the length and thickness of the contrails. You may even take a photo. After about 10 minutes, reexamine the contrails. Are they still there? Have they dissipated, or have they grown thicker? If they have disappeared, or if the jet did not leave a contrail, the humidity of the atmosphere is too low to create clouds and produce rain. So it is safe to say that tomorrow’s weather will probably be very similar to today’s weather. If the contrails are still present and have grown thicker, the humidity of the atmosphere is high enough for cloud formation. And with clouds can come precipitation. So tomorrow’s weather may be very different from today’s.