The NW7US Beacon

Happy Autumnal Equinox!

From the U.S. National Weather Service:

Although meteorological, or climatological, fall began in the Northern Hemisphere on September 1, astronomical fall in the Northern Hemisphere (and astronomical spring in the Southern Hemisphere) began this morning at 5:05 a.m. EDT, the time when the equinox occurred. At that point, Earth’s axis, while still tilted 23.5 degrees, was pointing neither towards nor away from the sun, making night and day the same length. Also at that time, the angle of the sun’s rays was perfectly perpendicular to the equator.

The difference in the sun’s relative angle to the equator is apparent in the images below, which use data for incoming shortwave energy at Earth’s surface from NOAA’s Global Forecast System weather model. Darker yellow and orange areas indicate higher levels of incoming shortwave solar energy. On the left is the predicted shortwave energy for this morning’s equinox. The energy from the Sun can be seen to straddle the equator almost perfectly.

On the right is the same data for Northern Hemisphere’s summer solstice, which occurred on June 21, 2011, when the Earth’s axis was most directly pointed towards the sun. Because of this, the energy from the sun is noticeably shifted northward, causing the Northern Hemisphere to experience summer. The effect of clouds on blocking and absorbing solar energy is also visible in these images. Areas with high cloud cover have much lower solar energy at the surface than the surrounding areas.

A high-resolution version of this image is available from NOAA’s Environmental Visualization Laboratory…

http://www.nnvl.noaa.gov/MediaDetail.php?MediaID=846&MediaTypeID=1