Saturday, 15 August 2015

Pre-Eclipse 2017 Road Trip: Nebraska Clouds and Contrails

On eclipse day we don't want to see any clouds in the way of the Sun and Moon. To make things really good, we don't want to see any clouds at all.
Long train on left, Blue Sky overhead, hazy horizon - Western Nebraska
Change in sky condition as we proceed East on I-80
But perfectly clear sky is rarely the case in the middle of the day in the middle of America late in the summer. A typical late August day in the Midwest is mostly clear in the morning with increasing clouds through out the day. With the eclipse occurring at noon, the full heating effect of the Sun will be diminished and clear sky should hold if the day starts that way.

The clouds are formed as the humidity and temperature rise. With the Sun in eclipse starting just before 11am the full heat of the midday Sun will not be felt. The net effect will be a slowing of evaporation and net temperature increases.

New Contrail being formed

Another type of cloud often observed in the American Midwest is the contrail. Left behind by jets crossing the country at high altitudes these contrails can sometimes be a real bother in an otherwise clear sky.
Persistent Contrails coming out of Lincoln Nebraska

Despite rumors to the contrary, contrails largely consist of water with trace amounts of exhaust from the jet engines. The formation of contrails depends on the relative humidity. The higher the humidity, the better chance of seeing contrails.

For this reason, the western part of Nebraska and eastern Wyoming have better conditions. The lower humidity means that jets do not leave behind contrails.

For more about contrail formation see:

...More random thoughts about TSE2017 to follow...

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