Thursday, January 15, 2009

Frozen Clouds

When the weather gets cold we all see the frost that forms on our windows, of our homes and cars but what about the frost you see on plants outside during the winter?

Frost forms when water vapour freezes into ice crystals on cold surfaces. In winter, temperatures are usually low as the sun is low in the sky during the day and the nights are long. On clear nights, when there is no blanket of clouds to keep the warmth in, then any heat received during the day quickly escapes. The temperature will therefore drop considerably and as the moisture in the air freezes, the ground will be covered with frost.

There are three types of frost that occur in different ways:
Rime frost is ice formed when a damp, icy wind blows over flowers, branches and other surfaces. Rime frost looks like icing around the edge of petals and leaves, and only occurs when the temperatures are very low. In the mountains this also happens when the clouds settle over the high peaks for a length of time. The trees, bushes and plants get completely coated with a thick covering of frost, so much so it looks like snow or frozen clouds!

Hoar frost occurs when water vapour touches a very cold surface and freezes on it instantly. This can happen to the leaves and branches of plants, and will cover them with ice crystals that look like spiky fingers.

It can also occur on other freezing surfaces such as soil and metal, and so can often be seen on cars. Hoar frost can occur at higher temperatures than rime frost – usually when the air temperature is around 0°C (32°F). However, the ground is usually much colder, and the air must be moist for the ice crystals to form.

In particularly cold weather, fern frost may appear on windows. This happens when tiny water droplets (dew) first form on the cold glass. These then turn into ice and more moisture freezes on top. As this process continues, more ice crystals are formed and the frost develops into what looks like feathery fingers. Fern frost can create beautiful patterns of ice crystals, which often look leaf or fern like – hence the name.

So when it is really cold and cloudy look up to the tops of the mountains near you. What you see may not be snow - it is most likely a heavy covering of rime frost on the trees.

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