Wednesday, February 17, 2010

Difference between snow, sleet, hail, rain, frost and dew

Here in Allentown, PA we have experienced either snow or rainfall everyday. This made me wonder how rain or snow or sleet or hail or dew and frost form? Did some Googling and here is what I got.

Rain: Water vapor condenses around condensation nuclei (such as dust) and falls when the droplet is heavy enough.

Sleet: Same formation as rain, but it freezes somewhere along its path from the clouds to the ground. Sleet is a mixture of snow and rain

Snow: Water droplets form and then freezing occurs slowly, allowing for the development of uniquely designed ice crystals know as snow. Snow is formed when water vapour is deposited in the higher reaches of the atmosphere at a temperature less than zero degrees Centigrade, and then falls to the ground. Snowflakes are typically symmetrical, hexagon-ally-shaped groups of ice crystals that form while falling in and below clouds. Simply put - Snow forms if the air in a cloud is below freezing. The water vapour then turns to ice instead of rain and the tiny ice crystals stick together until they form snowflakes. When they get heavy enough to fall, they drop out of the clouds. At this point though, we still don’t know whether they will end up as rain or remain as snow. This depends on the temperature of the air they travel through on the way down to the ground. If it gets warmer, they turn into rain, but if the air stays close to freezing all the way down, then the snowflakes will make it without melting and so fall as snow. If this occurs in a mountain area, it is possible for snow to be falling on the mountaintop while lower down in the valley the air is warmer and so it is raining instead.

Hail: Water droplets are carried high into the atmosphere by thunderstorm updrafts, which cause them to freeze. Multiple drops tend to freeze together, which is why the diameter of hail can be large.

Dew: Water vapor on the ground condenses on objects such as blades of grass when the surface temperature is equal to the dew point.

Frost: Dew forms and then it freezes. This commonly occurs when nighttime radiational cooling drops the ground temperature down enough. There are three kinds of frost, delicate crystallized ice on windowpanes is called hoar frost. Glazed frost consists of thick coatings of ice on cold surfaces. Rime frost is formed when super-cooled water droplets freeze on contact with cold surfaces. It often occurs when freezing fogs or drizzle blanket the ground.

Freezing rain: Forms and falls as rain. At the surface or near the surface, the temperature is at or below freezing, which causes the rain to freeze on contact.

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