We welcome the summer solstice with a mini heat wave. The solstice occurs at 7:09PM DST Wednesday, June 20, 2012 and is the time the sun is perpendicular to the Tropic of Cancer. It is the longest day of daylight and marks the celestial start to summer. Sunrise Wednesday was at 5:43 AM and sunset was 8:37 PM. That's a total of 14 hours 54 minutes. Plenty of time to enjoy any and all things outdoors... that is, if you can stand the heat and humidity on some summer days.
To help visualize what happens on the solstice, take a look at the diagram below. The earth revolves around the sun along an imaginary axis. Notice the earth is tilted (23.5°). The solstice occurs at the time the sun is directly over the Tropic of Cancer, which is at 23.5°. For every place North of the Tropic of Cancer, the sun is at its highest elevation - directly overhead at high noon.
- Solstices & Equinoxes - Courtesty NOAA
For the Northern Hemisphere, the summer solstice means the start of summer, whereas in the Southern Hemisphere the solstice marks the start of the winter season (the Earth is titled away from the sun, so less daylight, which leads to cooler temperatures). The reason we have seasons is because of the tilt of the Earth's axis. It also has to do with the amount of daylight and the atmospheric path length.
You may wonder, if the sun is at it's highest elevation at noon on the solstice and we have the longest hours of daylight, why it isn't the hottest day of the year, too. The main reason is because it takes much longer for the sun to warm the ocean, which absorbs and radiates the sun's energy over time. Typically, the hottest days occur in July and August. The deep, ocean water takes much longer to heat compared to the land. You also have to remember the Earth is made up of about 70% water. With all that water, it's going to take longer to warm up and, in turn, give us our hottest temperatures of the year based on the moderating effects of the ocean waters.
The amount of daylight will now slowly decrease until the winter solstice. The sunrise/sunset times surrounding the solstice, though, only fluctuate by a few seconds, so enjoy the added daylight and time outdoors!
Happy summer solstice!