Summer begins in the Northern Hemisphere on June 20, 2012, at 7:09 P.M. (EDT). Each year, the timing of the solstice depends on when the Sun reaches its farthest point north of the equator. This occurs annually on June 20 or June 21 in North America, depending on your time zone.
The word solstice is from the Latin solstitium, from sol (sun) and stitium (to stop), reflecting the fact that the Sun appears to stop at this time (and again at the winter solstice).
In temperate regions, we notice that the Sun is higher in the sky throughout the day, and its rays strike Earth at a more direct angle, causing the efficient warming we call summer. In the winter, just the opposite occurs: The Sun is at its southernmost point and is low in the sky. Its rays hit the Northern Hemisphere at an oblique angle, creating the feeble winter sunlight.
The Sun is directly overhead at its most northern point at "high-noon" on the summer solstice, creating more sunlight in the Northern Hemisphere on this day then any other.
Why isn’t the summer solstice, the longest day of the year, also the hottest day of the year? The reason is that the Earth’s atmosphere, land, and oceans absorb part of the incoming energy from the Sun and store it, releasing it back as heat at various rates. Water is slower to heat (or cool) than air or land. At the summer solstice, the Northern Hemisphere receives the most energy (highest intensity) from the Sun due to the angle of sunlight and day length. However, the land and oceans are still relatively cool, due to spring’s temperatures, so the maximum heating effect on air temperature is not felt just yet. Eventually, the land and, especially, oceans will release stored heat from the summer solstice back into the atmosphere. This usually results in the year’s hottest temperatures appearing in late July, August, or later, depending on latitude and other factors. This effect is called seasonal temperature lag.
Happy summer to all. May it be a good one for you.