Sometimes it is—weather being a classic example of chaos theory, where anything can happen on any given day, based on the laws of probability—but generally the hottest time of year comes much later.
To understand why it generally is not, it’s important to first understand why you might think the Summer Solstice SHOULD be the hottest day.
The Earth is heated by the Sun – trivial, but critical.
The Sun imparts the most heat on that part of the Earth which is closest to it—not because it is literally closest to the Sun, but because the Earth is round, and the part which is closest to the Sun also happens to be oriented such that the sun is directly overheat (orthogonal to the surface, in math-speak). In this orientation, the Sun’s rays are most concentrated there.
Because the Earth is tilted, with respect to its orbit of the Sun, the Sun appears to be directly over different parts of the Earth at different times of the year. On the Summer Solstice (on or around June 21) the Sun appears directly over the Tropic of Cancer, at ~23 degrees North Latitude - around the middle of Mexico. On that day, the Northern Hemisphere experiences more direct exposure to solar radiation than it does on any other day of the year—and for more hours, since day length is also longest on the solstice.
On the face of it, this suggests that this should be the hottest day of the year. So why isn’t that the case? For the same reason that an oven doesn’t reach its highest temperature as soon as you turn the flame up to the highest setting. At any given moment, the temperature is the result of not just the heat being imparted at that instant, but also any stored heat from before.
Each day, as the Earth rotates, it goes through a heating and cooling cycle. During the day, as the Earth basks in the Sun’s radiation, it is warmed. Then, as night falls, that part of the Earth that moves to darkness radiates that heat back into space – causing the surface of the Earth to cool.
On the first day of spring (the Vernal Equinox), the entire planet experiences approximately equal periods of night and day – so the heating and cooling cycles are more or less the same. After that, as the Earth’s orbit around the Sun orients the Northern Hemisphere more directly toward the Sun, the days (warming periods) get longer, and more intense, while the nighttime cooling periods become shorter.
The effect of this heating and cooling is both immediate and cumulative. Directly beneath the Sun’s rays, the immediate effect of the radiation is more intense. But the cumulative effect is the net result of the heat that is picked up during the daytime, minus that which is radiated out to space at night. Though the Northern Hemisphere actually nets the most heat gain on June 21, the warmth of the surface of the planet is the net result of the days and weeks leading up to it. The decrease in day length after the solstice is gradual, and the heat which is added through the end of June, through July and August is also substantial, and the cooling periods of nighttime remain relatively brief – adding to the heat accumulation.
Depending upon where one lives, it may be early to mid August before the net effect of these cycles reaches its peak – and temperatures reach their highest. Somewhere between late August and mid-September, the shift in heating/cooling cycles begins to cause temperatures to noticeably decrease. At the Autumnal Equinox—around September 21—the periods of day and night are again approximately equal. After that point, the periods of warming (daylight hours) diminish further, and the periods of cooling (nighttime) increase.
This effect is not uniform everywhere, and many other factors (the Jet Stream, ocean currents, local geography, etc.) influence the climate of any given location. This is really just a 'broad-brush' explanation.
For example, this lag between the time of the greatest solar radiation (June 21), and the warmest time of the year is most pronounced near large bodies of water, such as oceans. Water has a tremendous ability to store heat, and oceans act as ‘thermal banks’, accumulating and storing solar energy. It takes longer for the oceans to warm than it does for land surfaces, but once heated, the oceans—and those areas whose climates are strongly influenced by the sea—remain warm longer than areas isolated from their influence.
This is why coastal areas are subject to significantly smaller temperature swings than inland areas. It is also why the coast often experiences the warmest weather in the late summer to early autumn – a time when inland areas have already begun to cool.
The hurricanes that plague the US Atlantic and Gulf coasts illustrate this point. These storms are caused in part by the evaporation of warm tropical waters in the Atlantic, and are sustained and intensified as the storm’s wet air mass passes over the warm waters further north in the Atlantic, the Caribbean, and the Gulf of Mexico. Though the Atlantic hurricane officially begins June 1, the really powerful hurricanes rarely occur prior to mid or even late summer - when these waters have been heated enough to contribute the energy necessary to sustain and intensify these storms; and it doesn’t officially end until November 30, when the heat in these waters has dissipated to the point where they can no longer do so.
