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A solstice is an astronomical event that happens twice each year as the sun reaches its highest or lowest excursion relative to the celestial equator on the celestial sphere.
As a result, on the solstice the sun appears to have reached its highest or lowest annual altitude in the sky above the horizon at local solar noon. The word solstice is derived from the Latin sol (sun) and sistere (to stand still), because at the solstices, the sun stands still in declination; that is, the seasonal movement of the sun's path (as seen from Earth) comes to a stop before reversing direction. The solstices, together with the equinoxes, are connected with the seasons. In many cultures the solstices mark either the beginning or the midpoint of winter and summer.
The term solstice can also be used in a broader sense, as the date (day) when this occurs. The day of the solstice is either the longest day of the year (in summer) or the shortest day of the year (in winter) for any place outside of the tropics.Solstice is one of our favorite times of year—in the heart of the Sonoran Desert it's blazingly hot (106 today), incredibly dry (6% humidity), and often windy. But the black-throated sparrows are singing madly, the summer Anna's hummingbirds are arriving back from California, the nighthawks and elf owls are trilling and peeping all night long—and it means the rains are coming, the desert is ready, the anticipation is palpable.
The Arizona Monsoon is a well-defined meteorological event (technically called a meteorological 'singularity') that occurs during the summer throughout the southwest portion of North America. During the winter time, the primary wind flow in Arizona is from the west or northwest—from California and Nevada. As we move into the summer, the winds shift to a southerly or southeasterly direction. Moisture streams northward from the Pacific Ocean and the Gulf of Mexico. This shift produces a radical change in moisture conditions statewide.
Such a change, together with daytime heating, is the key to the Arizona monsoon. This wind shift is the result of two meteorological changes:
- The movement northward from winter to summer of the huge upper air subtropical high pressure cells, specifically the so-called Bermuda High (H).
These two features combine to create strong southerly flow over Arizona. The southerly winds push moisture north-ward from Mexico. The exact source region for the moisture of the Arizona monsoon is unknown. Researchers have proposed the Gulf of Mexico and/or the Gulf of California as the source regions but conclusive evidence has so far been elusive.
- In addition, the intense heating of the desert creates rising air and surface low pressure (called a thermal low) in the Mohave (L).
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| MyRadar iPhone app showing the monsoon action on June 16, 2012 |
After a rainy night, our local vultures were sunning themselves in the morning sun in trees along the big Raven Wash. I love when they hold their wings out like this - perhaps the only time you could call vultures beautiful!
