What causes snow, sleet, and freezing rain?

If you’re from some God-forsaken outpost in Siberia, pretty much the only wintry precipitation you ever experience is snow. But, if you’re from the Southern US like me, your winters are often sprinkled with a few helpings of several different types of water falling from the sky. Why is that? What causes snow sleet and freezing rain? Let’s find out!

First things first

First of all, let’s start with some definitions, ’cause I’ve gotten into some pretty heated debates with people over what terms apply to what stuff falling from the sky.

  • Snow – We all know what snow is. The white fluffy stuff that comes in flakes.
  • Freezing Rain – This is the stuff that’s a liquid while it’s falling, but turns into a glaze of ice all over everything after it makes impact with the ground. It is not pellets of ice, as lots of people will try to tell you.
  • Sleet – This is little pellets of ice that turn into pellets of ice before they hit the ground. Sleet is not light snow or a snow/rain mix… unless you’re from England. It’s also not hail (more on that later).
Freezing Rain

The aftermath of some pretty serious freezing rain – note the thick glaze of ice over the branches.
(Image credit: Nicolas Perrault III on English Wikipedia.)

How does it form?

Whether water comes out of the sky in the form of rain, freezing rain, sleet, or snow all depends on the temperature of the air – and, more importantly, the temperature of various air pockets that often sit on top of each other… As the National Weather Service explains:

  • Snow – Snow forms when you have cold air from ground level all the way up to the clouds. Frozen water molecules in the clouds accumulate into snowflakes, then start falling to the ground. After a while, things start to pile up.
  • Sleet – Sometimes, snowfall doesn’t go quite that neatly. Suppose a big pocket of warm air comes between the cold air in the clouds and the cold air on the ground. Snow will form in the clouds, melt in the pocket of warm air on its way down, and then re-freeze in the cold air near the ground. By the time it hits the ground, it’s just a frozen water droplet.
  • Freezing Rain – Suppose you had a pocket of warm air (like in the sleet example) that started getting pretty close to ground level. Snow would form in the clouds, melt in the pocket of warm air, hit the cold air near the ground… but run out of time to re-freeze before impact. This causes freezing rain – the precipitation is still melted when it gets to the ground, but it quickly starts freezing up on whatever it lands on.
  • Plain Old Rain – You can get plain old rain when you’ve got warm air in the clouds and the water never freezes… or when that pocket of warm air underneath the clouds stretches all the way to the ground. In that scenario, snow will form, melt, and never re-freeze.

So what about hail?

Like sleet, hail is ice pellets that fall from the sky. But it’s slightly different, and it forms in a very different process. As we’ve already seen, sleet forms when water droplets freeze on their way down to earth – it’s one drop that freezes solid. Hail forms inside thunderstorms, freezes way up high, and has layers like an onion. In a hail storm, droplets of water get blown around by all the air inside a thundercloud. As they move around, they encounter cold air (which makes them freeze), then moist air (which adds another layer of water on the outside), then cold air (which freezes them again). This goes on for a while, until they fall down. Scientists are still debating exactly what the process is, but the results can be pretty terrifying…

Well, that’s all for now. Stay safe this winter!

In space, what would kill an unprotected human first?


Surely, there’s a good reason he’s wearing that suit…

This weekend, I was riding with some friends to a wedding about three hours from home. As the ride got longer, the conversation got more varied, and we eventually ended up wondering whether an unprotected human could survive in space for any period of time. We also wondered what would be the first thing to kill them (as opposed to the second thing to kill them) when they did eventually pass. You know, because that’s what you usually talk about on the way to weddings.

Several ideas were floated: perhaps the zero-pressure environment of space would make your body instantly explode. Perhaps you wouldn’t explode, but you would have tissue damage from expansion in your lungs, blood vessels, and pretty much anything else that traps air or liquid. Maybe you’d freeze? But then, there’s no matter to conduct heat, so maybe you wouldn’t? Alternatively, maybe you’d fry from all the radiation? There were plenty of theories.

So, what would actually happen to you? Read on to find out.

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What are those web like things on tree branches?


You know, these things. (Credit: Alison Hunter)

If you live just about anywhere in the US, you’ve likely noticed weird things that look like a really intense spider web or maybe some sort of cocoon on the tips of tree branches in the late summer or early fall. They often seem to be clumped together with several on one tree, and they show up all over the place at the right time of year. So what are they?

The culprit is the Fall Webworm, which is actually a type of moth. Moths will lay hundreds of eggs on the underside of tree leaves. About a week later, the eggs hatch into small, hairy caterpillars, which begin eating the leaves and other parts of the tree. As they do, they build a big, silken web, which encloses and protects them. This web catches bugs, leaf remains, and even caterpillar droppings which gives it its characteristically nasty look. By the time winter comes around, the caterpillars wrap themselves in cocoons in order to survive the winter. They hatch in the spring in order to breed, lay eggs, and start the cycle all over with more nasty-looking webs.

BONUS: The Penn State Agricultural Sciences Extension page on the Fall Webworm states, “Warning: Pesticides are poisonous.” I thought that was kind of the point…

How much dirt would it take to bury Cary, NC?

Cary Map

Cary bleeds over just slightly from Wake to Chatham County. (Credit Rcsprinter123)

A friend in a church group recently posed an interesting question: how much dirt would it take to cover Cary, NC (our hometown) in a foot of dirt? Ignoring all of the terrible human suffering this would cause, it’s actually an interesting question. So, I decided to go about figuring it out.

The Easy Way

So… “covering in a foot of dirt” could be interpreted two different ways. We’ll start with the easy way – putting one foot of dirt everywhere, kind of like a blanket of snow.

According to Wikipedia, Cary is 55.4 square miles, or 1,544,463,360 square feet. That makes for simple math.

1,544,463,360 ft^2 * 1 ft = 1,544,463,360 ft^3

So, it would take 1.5 billion cubic feet of dirt to cover Cary. But that’s not really a very interesting answer… let’s see if we can do better.

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Why does the moon turn red during an eclipse?


Credit: Anne Dirkse

This week’s eclipse got me wondering about something – we all learned in school that a lunar eclipse happens when the earth comes between the sun and the moon, putting the moon in the earth’s shadow. But nobody ever explained the red glow that the moon takes on during the eclipse. Why does that happen? It’s actually the same reason that the sky is blue and sunsets are orange- Rayleigh scattering. What’s Rayleigh scattering, you ask? Read on to learn more.

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