Summer officially kicks off today, with the summer solstice marking the longest day of the year on June 20, at least in the Northern Hemisphere. This year’s summer solstice takes place a day earlier than it’s been for the past three years, due to the fact that 2012 is a leap year—this February got an extra day, to keep our calendar year of 365 days in sync with the astronomical year, which is about 365.24 days.
The solstices are the results of Earth’s north-south axis being tilted 23.4 degrees relative to the ecliptic, the plane of our solar system. This tilt causes different amounts of sunlight to reach different regions of the planet during Earth’s year-long orbit around the sun. Today the North Pole is tipped more toward the sun than on any other day of 2012. (The opposite holds true for the Southern Hemisphere, where today is the winter solstice, the shortest day of the year.)
At high noon on the summer solstice, the sun appears at its highest point in the sky—its most directly overhead position—in the Northern Hemisphere. On the summer solstice, the Northern Hemisphere receives more sunlight than on any other day of the year—but that doesn’t mean the first day of summer is also the hottest.
Earth’s oceans and atmosphere act like heat sinks, absorbing and re-radiating the sun’s rays over time. Even though the planet is absorbing lots of sunlight on the summer solstice, it takes several weeks to release it. As a result, the hottest days of summer usually occur in July or August.
So, now the countdown starts for the hottest days in Baghdad. We have already seen 48 degree Celsius last Friday.