Sunday , February 28 2021

4 Questions Answered About Weird Winter Weather



Editor's note: Extreme cold weather can produce unusual phenomena, from the so-called ocean smoke to dirty ocean waves. As atmospheric scientist Scott Denning explains, these striking events are mainly caused by the behavior of water at very low temperatures.

Why do lake and ocean waters seem to steam during cold snaps?

There are three phases, or states, of water: solid ice, liquid water, and gaseous water vapor. Even in extremely cold weather, liquid water can not be colder than the freezing point – about 32 degrees Fahrenheit – so the surface of the ocean is much warmer than the air above it.

Lots of water evaporates from the warmer ocean into the cooler dry air above. As this invisible gas rises only a little above the relatively warm water, it reaches the air which is much colder and can not contain much steam, so the vapor condenses into microscopic droplets of liquid water in the air.

Some people call the thin clouds caused by condensation just above the winter ocean or lakes "smoke from the sea". This is a better term than steam. The actual steam is very hot water vapor – that is, the water in its gaseous phase, which is invisible.

Lake Michigan at 39th Street Harbor in Chicago, January 30, 2019.
AP Photo / Nam Y. Huh

Time watchers seem very excited by the thunder. What is it and why is it rare?

Thunder is a sound boom created when a lightning bolt causes air to expand faster than the speed of sound. The beam is formed by sparks of static electricity between the clouds and the ground. The friction that forms this static is usually caused by the rapid rise of the "thermals" of floating air on hot summer days, which is why storms are common in summer.

The air can not rise from the icy ground of winter because the cold air is dense, so winter is quite unusual. Thundersnow happens when the really cold air blows from the north. This cold air is denser than the air on the surface, so it literally drops, pushing air from the surface up. This can create exactly the same type of static charge as a summer storm, and BOOM – thundersnow! This only happens with a really dramatic change in temperature, such as approaching a cold Arctic front.

Thundersnow Episodes in New England, March 8, 2018.

How common is it for the oceans to freeze outside the polar regions?

Salt water has a lower freezing point than fresh water, which is why we put salt on our streets and sidewalks to melt the ice in the winter. Sea water is salty enough to be frozen – about 28 degrees Fahrenheit. It is very unusual for sea water to freeze in the continental United States, although this happens all the time in the Arctic winter.

When sea water freezes, most of the salt is pushed into the ocean water beneath it. That's why people in the Arctic can melt sea ice to drink water. As small pieces of freshwater ice form on the surface of the ocean, the remaining water becomes saltier and saltier, so it becomes harder and harder to freeze.

But sometimes, when it is extremely cold, small ice flakes form on the surface of the ocean. The waves break them, so that the surface can become like a ripple. For anyone willing to face the cold, it is wild to stand by the shore and watch the muddy and smoky sea with its slow waves. At the poles, it is so cold that floating ice crystals eventually converge and solidify in the sea ice.

Muddy surf in Nantucket, Massachusetts, January 5, 2018.

Scientists have discovered that Mars also has blizzards. How are they different from the snow on Earth?

The atmosphere on Mars is almost pure carbon dioxide, which we know as the main greenhouse gas that drives climate change here on Earth. But the atmosphere on Mars is much thinner than ours, so it does not trap much heat. On a beautiful Martian summer day, temperatures can reach 70 F and fall to less than 100 F on the same night.

Winters are even colder. It gets so cold in the polar winters on Mars that the air itself freezes, forming tiny flakes of carbon dioxide the size of red blood cells that accumulate enough to form polar ice caps.

A table of carbon dioxide frozen in the south pole of Mars.
NASA

During the long polar night, about a third of the entire atmosphere of Mars falls like snow. This makes a partial vacuum sucking the winds of the planet's summer hemisphere into your winter hemisphere to make up the difference. In the spring, these planetary-scale winds reverse direction as dry ice turns into gas and begins to fall at the other end of Mars.

Farther into the solar system, "giant ice" planets and many of their moons have enormous amounts of water and ice of carbon dioxide – much larger quantities than all of our oceans. But on Earth, dry ice can not form above zero F. So there will never be carbon dioxide snow on our planet – just frozen water in all its forms.


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