It’s a Sun Reveler’s favorite day of the year (in the Northern Hemisphere), today the Summer Solstice will take place at approximately 23:09 UTC (for those not familiar with Universal Time, that’s 7:09 PM EDT). At that time the Earth’s Northern Hemisphere will be at its most inclined position toward the Sun during its entire orbit (see image 1 below). During the Summer Solstice, the Northern Hemisphere receives more sunlight than on any other day of the year as it is tilted towards the Sun; from this point on, the days will begin to get shorter.
Earth's Orbit Around Sun. Northern Hemisphere is tilted towards the Sun at the Summer Solstice
Even though today will be the longest day of the year in the Northern Hemisphere, temperatures usually don’t peak until about July or August. Summer is essentially just getting started, but if we receive the most sunlight today why don’t the temperatures peak until several weeks later?
The Earth’s atmosphere and oceans absorb the radiation from the Sun and then reradiate this energy over time. Water has a very high heat capacity and considering that Earth is covered in about 70% of water, this takes a long time to heat-up and reradiate. Think of a pot of water on a stove, you wouldn’t expect the water to instantaneously boil as soon as the burner is turned to high-heat; the Earth doesn’t heat this quickly either.
Number of +90F degree days by U.S. region in 2011 versus 2012
Although temperatures typically don’t spike until July or August, today will feel like the “boiling point” has been reached across the Northeast, Great Lakes, and Southwest states. Heat advisories and excessive heat warnings have been posted across several states, including the cities of Phoenix, Philadelphia, New York, Boston, and even as far north as Portland, ME. For many cities in the Northeast this will be the start of the first heat wave (3 or more consecutive days of +90F temperatures) of the year, but recall that by this time last year, many cities in the Northeast had already had 1 or 2 heat waves. In fact, taking a look at the total number of +90F degree days last year compared to this year through the same time period (January 1st-June 19th) the Northeast region as a whole has not seen a single day this year with temperatures of 90F or greater, while last year there had been 2. Conversely, the Southwest and Southern Rocky Mountain states have seen several more days of +90F weather this year.
Temperatures vs. Normal for today (June 20th, 2012)
As we approach our most inclined position towards the Sun later today, hopefully you can find some air conditioning, or at the very least, some shade. Folks in the Northeast can take comfort in the fact that the weather hasn’t been as bad as last year, but in the Southwest, there may be more cause for complaint and grief this year. Happy Summer Solstice and stay cool!