“Winter Meadowhawk” dragonflies

The season called “winter” is defined two ways: atmospheric scientists, including meteorologists and climatologists, define winter as the three-month period from December to February; astronomers define winter as the time period that begins on the December Solstice (12/21) and ends on the March Equinox (03/21), although the actual dates for these events may vary slightly.

Several Autumn Meadowhawk dragonflies (Sympetrum vicinum) were spotted on the first day of climatological winter at Occoquan Bay National Wildlife Refuge, Prince William County, Virginia USA. Therefore I think it is appropriate to call them “Winter Meadowhawks.”

The last two photos show the same male, perching on different surfaces. My guess is he was looking for a good source of thermal energy on a cool, windy day.

The Sun is always low in the sky during winter, even at its maximum altitude. Indirect incoming solar radiation (insolation) is less intense than direct insolation. The last photo shows the male dragonfly perched on a south-facing wooden board that is perpendicular to the surface of the Earth, therefore the solar energy received by the board is more intense than the energy received by the ground. This probably explains why the male moved from the ground to the board.

Enrichment

The last photo was taken on 01 December 2017 at 11:33:50 a.m. EST, as shown by the EXIF information for the image. The altitude of the Sun was 28.9° at 11:30 a.m., meaning a ray of sunlight formed an angle of 28.9° with horizontal surfaces such as the ground. At the same time, a ray of sunlight formed an angle of 61.1° with vertical surfaces such as the wooden board shown in the first and last photos. That’s the beauty of mathematics — some simple geometry shows clearly which surface received more intense insolation. Smart dragonflies!

Related Resource: Sun or Moon Altitude/Azimuth Table, U.S. Naval Observatory.

Copyright © 2017 Walter Sanford. All rights reserved.

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