NEW YORK — This year's summer solstice arrives as a dangerous heat wave affects millions across the country, with early-season extreme heat impacting parts of the Midwest and Northeast.

But the longest day of the year -- which begins Thursday at 4:51 p.m. ET -- is not typically associated with the hottest temperatures of the season in the U.S. The most scorching conditions typically occur much later in the summer, records show, with different regions of the country experiencing their warmest average temperatures at varying times. Brutal summer heat, however, will increase in frequency and duration for much of the country over the coming weeks and months.

The summer solstice marks the beginning of astronomical summer in the Northern Hemisphere, according to NASA. Astronomical seasons are determined by the Earth's tilt on its rotational axis and its orbit around the sun and do not take into account temperature data, which is what determines meteorological seasons.

Solstices occur when the planet's tilt is most pronounced. The summer solstice occurs when Earth's tilt toward the sun is at a maximum, bringing the longest day of the year.

On the first day of astronomical summer, the sun is at its highest point in the sky and so delivers the most direct solar radiation to the hemisphere experiencing the summer season. However, it takes time for that solar radiation to begin warming the Earth's atmosphere and surface, especially the oceans.

This time difference creates a lag between the first day of summer and when the Earth experiences the hottest temperatures of the year.

For the contiguous U.S., records show July is the hottest month of the year, on average, with the majority of the country -- including much of the Northeast, Midwest, and West -- experiencing the hottest temperatures of the year during the second half of July and first half of August. However, some regions of the country don't typically experience their warmest average temperatures until August, or even September.

Much of the South typically experiences its peak average temperatures during the second half of August, while for some regions of the West Coast, the warmest temperatures of the year usually don't occur until September.

For this summer, overall above-average temperatures are forecast across much of the country, with several significant heat waves likely for parts of the Northeast and Southwest.

Earth has experienced 12 months of record high temperatures, according to reports published earlier this month by the World Meteorological Organization and Copernicus, Europe's climate change service.

The record-breaking trend is expected to continue through June and could potentially last through the rest of the summer, forecasts show.

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