This is when the difference between the daylight hours and the nighttime hours is most acute. The solstices occur each year on June 20 or 21 and Dec. 21 or 22, and represent the official start of the summer and winter seasons. The vernal equinox and autumnal equinox herald the beginning of spring and fall, respectively. At these times of the year, the sun appears to be directly over Earth’s equator, and the lengths of the day and the night are equal over most of the planet. On March 20 or 21 of each year, the Northern Hemisphere reaches the vernal equinox and enjoys the signs of spring. At the same time, the winds turn colder in the Southern Hemisphere as the autumnal equinox sets in. The year's other equinox occurs on Sept. 22 or 23, when summer fades to fall in the north, and winter’s chill starts giving way to spring in the south. From year to year, there is always some variability in the equinoxes and solstices because of the way Earth's changing tilt matches up with its orbit around the sun. This year, the precise moment of the June solstice came at 7:09 p.m. ET Wednesday. That's the moment when Earth's north pole was tipped precisely toward the sun. Tuesday thus represents the longest day of the year for the Northern Hemisphere's residents, and the longest night of the year south of the equator.
Effect on climate Here’s how the seasonal change affects the weather: Around the time of the June solstice, the North Pole is tilted toward the sun and the Northern Hemisphere is starting to enjoy summer. The density of the solar radiation is higher because it's coming from directly overhead — in other words, the sun's rays are concentrated over a smaller surface area. The days are longer, too, meaning that more radiation is absorbed in northern climes during the 24-hour cycle. Another factor that may come into play is that the radiation takes a somewhat shorter path through the energy-absorbing atmosphere before striking the earth.
These views of Earth from the Spinning Enhanced Visible and Infrared Imager on EUMETSAT's Meteosat-9 satellite shows the terminator line on Dec. 21, 2010 (upper left), March 20, 2011 (upper right), June 21, 2011 (lower left) and Sept. 20, 2010 (lower right). The sequence illustrates how Earth's northern half receives more sunlight in June, while the southern half gets more light in December.
