Kinds Of Precipitation

Precipitation may occur in other ways, apart from rain. For instance, there are snow flakes with their prism-like or plate-like forms in beautiful

Figure 9.3 Rainfall in Tasmania during 103 months of seeding the shaded area, as a percentage of the long-term average rainfall at each point. The increase of rain in the north-east is attributed to silver iodide carried to the mountainous country there by south-west winds.

Figure 9.3 Rainfall in Tasmania during 103 months of seeding the shaded area, as a percentage of the long-term average rainfall at each point. The increase of rain in the north-east is attributed to silver iodide carried to the mountainous country there by south-west winds.

tree-like (i.e. 'dendritic') patterns of hollow columns, needles and hexagonal plates, according to the temperature at which they grow. The flakes fall at a speed of about 1 m/s, irrespective of their size or shape. A consequence of so low a terminal velocity, within horizontal winds which are much faster, is that snow is blown sideways as it descends, and then drifts in the wind, accumulating only in places of relative calm.

Descending snowflakes lose their crystalline appearance when they collect supercooled droplets, as the latter freeze immediately upon contact. This process is known as riming. Eventually the original flake structure may become invisible, and such heavily rimed snowflakes are known as graupel. Graupel falls at 1-10 m/s, depending on the amount of riming. A hailstone is an extreme case of graupel (Section 9-8; Table 9.1).

Rain from cold clouds results from the melting of snow, but the raindrops may later become supercooled once more if there is an intense ground inversion, so that surface temperatures are below 0°C. Such rain freezes on contact with the ground, accumulating as ice on powerlines, trees, etc. The eventual weight of this ice may cause extensive damage. Fortunately, the problem of freezing rain occurs chiefly over land between 40 and 60 degreees of latitude in winter, and there is not much land at these latitudes in the southern hemisphere.

If a ground inversion is very strong and deep, then the falling rain may freeze in the air before reaching the ground, and the resulting precipitation is called sleet. Sleet consists of balls of clear ice up to 4 mm in diameter (the maximum size of rain) and is distinct from true hail (Section 9.8).

The various kinds of precipitation are shown on weather charts by the symbols in

Figure 9.4.

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment