Happy Monday and welcome to another edition of Beyond The Forecast!
The winter so far, as you may have guessed, is a little out of the ordinary. At this point in the year, afternoon temperatures tend to be in the upper 40s, and through the first half of January, the average high was 57 degrees.
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The determining factor in day length is our position in orbit around the sun. Because the Earth is titled at an angle (23.4 degrees from vertical) and stays at that same tilt, the sun shines in different ways at different times of the year.
The Equinoxes, Vernal meaning spring and Autumnal meaning fall, represent the most balanced spread of day length across the globe. The whole planet gets about 12 hours of daylight, with the sun directly overhead at the Equator at noon.
The Solstices, summer and winter, on the other hand, represent the most extreme variations in day length. When the Earth makes its orbit and keeps its tilt, the North Pole is tilted 23.4 degrees toward the sun on the day of the Summer Solstice. For the Winter Solstice, the South Pole is pointed at that angle toward the sun, keeping the northern hemisphere further away.
The Summer Solstice is the longest day of the year in the Northern Hemisphere, and the difference from day length on an Equinox is more drastic as you get closer to the North Pole.
At the Equator, the day is still close to 12 hours since the angle toward the sun is a smaller difference. Meanwhile, at the North Pole (and anywhere above the Arctic Circle), the sun stays above the horizon for the duration of the Solstice.
The coldest average temperatures of the year in Roanoke are from the 16th of January to the 27th of January (the average low for those days is 28 degrees).
Days over that stretch go from 9 hours, 54 minutes of daylight to 10 hours, 11 minutes. The actual shortest day of the year was December 21st with 9 hours, 35 minutes of sun.
So why do we have our coldest temperatures a few weeks after the shortest day of the year?
The heating variations are most noticeable on the Solstices themselves, but they last through the entirety of a season. The Northern Hemisphere’s summer days are longer than in the Southern Hemisphere, and the heat in the atmosphere is absorbed by the planet. The ground and plant life absorb a good portion, but the oceans store a massive amount of solar energy.
Even as days start to shorten after the summer solstice, the fact that we have so much extra heat stored in the oceans keeps temperatures warmer than what we would have on a planet without water. Maximum temperatures tend to be in July or August because of this heat delay.
On the other hand, even though the shortest day of the year is the start of winter, the coldest days tend to be a few weeks after the Solstice. Water is great at storing heat, but it also takes time to add new heat to it. Days are so short that it’s hard for the planet to absorb enough heat to bring temperatures up until days are sufficiently long in the later parts of January or February.
One big difference between the hemispheres is the percentage of land and water. The Northern Hemisphere has a split of 40% land to 60% water; in the Southern, the split is 20%/80%.
Since there is so much more water in the Southern Hemisphere, temperatures are more moderate. Water is slow to absorb new heat from the sun, but it’s also slower to release heat than land is.
The most extreme hot and cold temperatures tend to be in the Northern Hemisphere, where the land can more easily absorb and release heat.
The warm stretch lasts through this traditionally cold time of year well into the end of the month. Know what to expect for temperatures each day; you can download our weather app and get Meteorologist Chris Michaels’ latest updates online.
You can always get specific forecast details for your zone, whether it’s the Roanoke Valley, the Lynchburg area, the New River Valley or elsewhere around Southwest and Central Virginia, anytime at WSLS.com/weather. Know your zone!
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-- Marshall Downing