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Why Meteorological and Astronomical Seasons Don’t Line Up

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For many Americans, summer essentially starts after Memorial Day weekend. The school year's wrapping up, offices seem emptier, and jorts re-emerge from the depths of our closets. Yet the calendar says differently.

Technically, summer doesn’t start until after the summer solstice, usually around June 21, when most of us are already well into backyard barbecue season. But meteorologists define summer as the season running between June 1 and August 30. Why the disconnect?

There’s a difference between meteorological summer—shorts weather—and astronomical summer, which is based on where the Sun is positioned in relation to the Earth, as Weather Underground explains.

Over the course of the year, the tilt of the Earth means that one hemisphere is closer to the Sun than its counterpart for several months at a time, marking the summer season. When the Northern Hemisphere is closer, from late June to late September, the northern part of the world experiences summer, while the Southern Hemisphere—which is tilted farther away from the Sun—experiences winter. During summer months, the Sun takes a longer path across the sky, resulting in longer daylight hours. The equinoxes mark the days where the ratio of day-to-night stands at exactly 12 hours each, because the Sun is lined up with the equator.

Because the Earth doesn’t take exactly 365 days to travel around the Sun each year, the days that equinoxes and solstices fall on vary slightly year-to-year. Still, they typically take place around March 21 (spring equinox), June 21 (summer solstice), September 22 (autumnal equinox), and December 22 (winter solstice).

That variability makes it difficult to pin the seasons to calendar dates, so we have meteorological seasons. These are the times we normally think of as summer, fall, winter, and spring—the three-month chunks of time that correspond to the changes in the weather. Meteorological summer runs from June 1 to August 31, corresponding to how most people envision the season, running from about Memorial Day to about Labor Day. Fall goes from September 1 to November 30, winter from December 1 to February 28, and spring from March 1 to May 31.

The firm dates of meteorological seasons allow weather forecasters to better observe and predict weather patterns year-to-year, since they’re based on the annual temperature cycle, rather than the exact timing of the Earth’s orbit. Even if daylight hours aren’t yet at their peak in early June, temperatures are still more akin to summer than spring, so it makes sense to call it summer from a weather perspective. When it comes to compiling statistics on temperature and weather patterns for agricultural planning and business, working around the static calendar is a lot easier than trying to deal with the variability of the Sun’s position in the sky.

So yes, even though summer doesn’t technically start until June 21 at 12:24 a.m. Eastern Time, you and your jorts were onto something after all.

[h/t Weather Underground]

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Britain Is in the Midst of a Rare ‘Wind Drought’
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Generating renewable energy in Britain is a little less than a breeze these days: A “wind drought” is halting the country’s wind turbines.

This month’s wind energy output is down 40 percent from the same time last year, New Scientist reports. On average, about 15 percent of Britain’s electricity comes from wind power. Data starting from July 1 of this year put the monthly average closer to 6.9 percent. Last month, turbines were producing less than 2 percent of Britain’s electricity—the lowest output in two years.

That’s with even more wind turbines being installed over the course of the past year, New Scientist says. The data aren’t entirely surprising, though. The jet stream tends to make the UK’s weather drier and calmer during the summer and wetter and stormier during the winter. But the high pressure the jet stream has brought with it this year has been unusually prolonged, scientists say.

“It’s like a lid, it keeps everything still,” UK Met Office spokesperson Grahame Madge told New Scientist. “From the forecast looking out over the next couple of weeks, there doesn’t seem to be any significant change on the way.”

The wind drought shouldn’t cause too many problems in the short term. Electricity demand is low during the summer (very few British homes have air conditioning), and the country’s been able to compensate for the lack of wind by burning more natural gas. If the wind drought continues to persist, though, UK residents may begin to see an increase in utility fees. Natural gas prices have already risen with the increased demand.

“As we continue to transition to a low-carbon energy system, managing the intermittency of renewable power an important role in balancing supply and demand,” a National Grid spokesperson told New Scientist. “However, we have planned for these changes and [are] ready to play our part.”

The wind drought comes about eight years after British politicians vowed to reduce the UK's dependence on fossil fuels. Last year was the first year that electricity generated from low-carbon energy sources like solar power, wind power, and nuclear power outpaced high-carbon energy sources like coal and natural gas. This summer’s wind drought may make it difficult to improve on last year’s numbers.

[h/t New Scientist]

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Why Does the Sky Look Green Before a Tornado?
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A common bit of folklore from tornado-prone parts of the U.S. says that when the skies start taking on an emerald hue, it's time to run inside. But why do tornadoes tend to spawn green skies in the first place? As SciShow's Michael Aranda explains, the answer has to do with the way water droplets reflect the colors of the light spectrum.

During the day, the sky is usually blue because the shorter, bluer end of the light spectrum bounces off air molecules better than than redder, longer-wavelength light. Conditions change during the sunset (and sunrise), when sunlight has to travel through more air, and when storms are forming, which means there are more water droplets around.

Tornadoes forming later in the day, around sunset, do a great job of reflecting the green part of the light spectrum that's usually hidden in a sunset because of the water droplets in the clouds, which bounce green light into our eyes. But that doesn't necessarily mean a twister is coming—it could just mean a lot of rain is in the forecast. Either way, heading inside is probably a good idea.

For the full details on how water and light conspire to turn the sky green before a storm, check out the SciShow video below.

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