The warm, sunny days in early June made the last part of spring feel like summer. Summer technically doesn’t begin until the summer solstice on June 21 but we’ve enjoyed summer-like weather for a few weeks.
The summer solstice marks the day when the sun is at its farthest north position in the sky in the northern hemisphere north of the Tropic of Cancer just as the winter solstice marks the day when the sun is at its lowest position in the sky. On the summer solstice at the Tropic of Cancer (23.5 degrees north latitude), the sun is directly overhead at its highest point in the sky, such as in the Bahamas.
For many locations the summer solstice isn’t the day with the earliest sunrise or latest sunset due to the tilt of the Earth and the Earth’s elliptical path around the sun.
The earliest sunrise and latest sunset actually occur for several days. In Bonners Ferry, the earliest sunrise is at 4:42 a.m. from June 12 to June 20 while the latest sunset is at 8:52 p.m. from June 20 to June 30.
Therefore in Bonners Ferry, the longest day occurs on June 20 when daylight hours peak at 16 hours and 10 minutes. On June 20, the sun rises at 4:42 a.m. and sets at 8:52 p.m. whereas on June 21 (summer solstice), the sun rises at 4:43 a.m. and sets at 8:52 p.m. The summer solstice officially begins at 3:51 a.m. Pacific Time on Saturday, June 21 and the days start to become shorter.
While the first day of summer may be the longest day of the year north of the Tropic of Cancer, it definitely won’t be the hottest. Both April 12 and August 31 are the same number of days away from the summer solstice but a significant amount of snow can be on the ground on April 12 while a 90 degree heat wave can persist on August 31.
The difference in temperatures is that the northern hemisphere is still warming up in April and releasing heat in August. The Earth’s oceans and atmosphere act as heat sinks and absorb the sun’s warm rays. During the longest days of the year, they absorb the most heat and then retain that heat. This retention makes July and August the hottest months of the year.
After a certain point, more heat is lost at night than is gained during the day, causing the northern hemisphere to cool. This cooling trend continues until after the winter solstice when the days become longer and more heat enters the atmosphere. Similarly, the shortest day of the year isn’t the coldest, January and February are usually the coldest months.
The Earth takes longer to warm up than it does to cool down–on a yearly scale and a daily scale. After sunset, the coolness of night quickly rushes in while the warmth of the morning sun slowly warms the land. Just as the coolness of autumn rushes in and cools the land faster than the warmth of spring warms the land.
With the summer solstice occurring on June 21, the hottest days of summer aren’t far behind.