waves_726.jpgDid you ever take a photograph only to later realize that the picture contained unseen details? Normally nighttime clouds over the city are pale yellow as they are lit by reflections from sodium vapor street lights. Thus Gothamist was surprised to see a bright white cloud out our window. We're not sure why it was so white (our ignorant guess is that sodium vapor lamps aren't used in Central Park), but the whiteness was unusual enough that we took a few pictures. When we looked at the blown up photos on our computer we saw something more interesting than a white cloud –waves. Kelvin-Helmholtz to be precise.

Kelvin-Helmholtz waves form when fluids of different densities rub past each other. Wind-blown ripples on a water body are probably the most commonly seen Kelvin-Helmholtz wave, but they also occur in the atmosphere. On Sunday night cooler, drier air near the ground was displacing warmer moister air above. The air along the boundary of the two air masses the air is rolling from right to left like a series of logs (check the two animations). On the upward portion of the roll the cold air cools the warm air enough to form a cloud. As the air sinks, the cloud dissipates at first, but as the air gets better mixed more of it is saturated and the cloud grows in size. The rolls are tiny, the whole cloud, in which Gothamist counts eleven waves that get less and less distinct from right to left, only stretches from 7th to 5th Avenues.

No cool, dry air is in sight for the rest of the week. The National Weather Service, AccuWeather and Weather.com are in nearly complete agreement. Warm and humid with a chance of afternoon or evening showers through Saturday.