ICE, SNOW, HAIL, AND FROST

How does snow form?

Snow forms in clouds in much the same way as water. Water vapor in the clouds collects around nuclei made out of dust or other particles in a cloud. When the temperature within the cloud is cold enough, and the water molecules begin to make contact with one another, they form crystals; and when the crystals become sufficiently heavy, gravity takes over and they fall to the ground.

The distinctive hexagonal crystal pattern of snowflakes occurs because of the six-fold molecular symmetry of ice. Water is composed of one oxygen bonding with two hydrogen atoms in a kind of V-shape configuration. When the temperature is cold enough and the molecules of water are drawn together, they naturally form into hexagonal rings. This pattern continues as the snowflake grows until it is apparent to the naked eye.

Does water always freeze at 32°F (0°C)?

The standard that water freezes at 32°F, or 0°C, is only true when the air pressure is exactly one atmosphere and when the water is fresh water and not salt water or some other form of water with impurities in it. Salt water (depending on the percent of salinity) freezes at a lower temperature than fresh water, and when the air pressure is greater than one atmosphere the melting point of fresh water is lowered. However, it takes a lot of pressure to make a difference: to lower the freezing point of water to 30°F (1°C) you must exert a pressure equal to 134 atmospheres!

Because water droplets in clouds must condense around impurities such as dust, they do not necessarily freeze at 32°F but can remain in a liquid state at temperatures as low as -40°F (-40°C). In other words, the water droplets cannot form ice until they begin to make contact with each other around a nucleus so they can form a crystal lattice. On the other hand, icebergs in the salty oceans are frozen at about the normal freezing point because icebergs are actually formed of fresh water. This happens because ice in the oceans is often formed slowly, allowing the ice crystals to force out impurities such as salt. On the other hand, ice flows—sections of the arctic ice sheet that break away each summer—are composed of sea ice; sea water begins to freeze at 28°F (-2°C) when there is no surface turbulence.

Another interesting property of ice on frozen lakes or in skating rinks is how friction makes ice skating possible. It was once believed that the pressure of an ice skate blade against the ice surface changed the state of the water from solid to liquid, thus creating a slippery surface on which to glide. More recently, it has been concluded that it is the friction of the blade against the ice that melts it and makes ice skating possible.

Why don't lakes and ponds freeze solid in the winter?

When ice forms in lakes, ponds, and other bodies of water it floats to the surface because liquid water is denser than solid ice. As water cools below 38°F (3.3°C) it becomes less and less dense, and thus more buoyant. This is good news for fish and other plants and animals that live in small bodies of water, since they can live below the layer of ice that forms.

What is chionophobia?

Chionophobia is a fear of snow.

What is the difference between freezing rain, sleet, and hail?

Freezing rain is rain that falls as a liquid but turns to ice on contact with a freezing object to form a smooth ice coating called glaze. Usually freezing rain only lasts a short time, because it either turns to rain or to snow. Sleet is frozen or partially frozen rain in the form of ice pellets. Sleet forms when rain falls from a warm layer of air, passes through a freezing air layer near the Earth's surface, and forms hard, clear, tiny ice pellets that can hit the ground so fast that they bounce off with a sharp click. Hail is a larger form of sleet.

 
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