Tekct 6. Cumulonimbus

Задание 1. Прочесть, перевести.

The thunderstorm cloud is the cumulonimbus which often extends to the tropopause.

Cumulonimbi have a duration of at least 1 h, compared to 10 to 15 min for the smaller cumuli.

Some thunderstorm cloud conglomerations with diameters of 50 km (30 miles) and more have been tracked for a number of hours.

«Conglomeration» is appropriate: a cloud mass so large contains several updraft areas which, after some minu­tes, are replaced by others, all within the same massive en­velope.

Shifting of active centers can be observed clearly at night from an airplane.

There is hardly a finer weather sight than that provided by traveling in quiet air under a starry sky beside a boiling thunderstorm mass and watching the lightning illuminate first one set of turrets and then another in different parts of the huge, dark cloud.

Some sections of this cloud suddenly come to life: they contain new and active updrafts of air.

In other parts the lightning display dies out, indicating that the air motion has turned downward. Cells with updrafts and downdrafts often closely adjoin one another.

As the nighttime spectacle suggests, the cells in a thun­derstorm which may be several miles in extent, go through a life cycle.

The downdrafts, often violent, carry the conden­sation product to the ground.

After some minutes, a downdraft may encompass a whole cell, which then dies.

A large thun­derstorm can be expected to have cells in various stages of the life cycle.

To picture how the downdraft begins, consider a mass of water (or water and ice mixed) carried upward in an updraft and then falling out into nearby air whose temperature is typical of the thunderstorm's environment.

The water evapo­rates into this unsaturated air, which cools, acquires down­ward buoyancy and starts to sink.

In a typical summer situation, the downdraft air will arrive at the surface with relatively cold temperatures of about 22° C (72° F).

The drag of the falling rain also accelerates the down-draft.

Задание 2. Перевести предложения:

1. The troposphere is well-mixed, often moist, and contains all the clouds and precipitation of the atmosphere.

2. The tendency for cumuli to align in streets, or rows, has been observed for many years.

3. For large-scale processes the pressure can be regarded as a measure of the weight of the atmosphere above a given level.

4. The temperature distribution is used to classify the va­rious layers of the atmosphere.

5. The circumstances that produce the charge separation have not been fully understood.

6. Circulation systems are produced by changes of eleva­tion, differences between land and water, differences in ther­mal properties of the ground.

Tekct 7. Thunderstorms

Задание 1. Прочесть, перевести.

Lightning and Thunder, We do not know exactly what cau­ses lightning.

It is thought to be part of a mechanism which conducts an electric current from earth to ionosphere.

Nor­mally, i. e., during fair weather, the atmosphere conducts a current from the positively, charged ionosphere to the ground, which carries a negative charge.

This leakage would wipe out the earth's charge in about ten minutes unless a return or supply current, from earth to ionosphere constantly restored the prevailing charge distribution.

Observations of the electric field strength between the earth and the ionosphere, made at widely separated places, have shown that the field strength varies daily in its intensity, reaching a peak simultaneously over the whole earth.

This peak, referred to Greenwich Mean Time (GMT), which is the local time at longitude 0°, occurs during the afternoon and early evening.

This is precisely the time that thunderstorm activity is greatest over the earth as a whole, because of afternoon maxima over equatorial Africa and South America.

Because the earth and the ionosphere are both conductors, the peak occurs all over the earth simultaneously, irrespective of local time in other areas.

The correla­tion suggests that thunderstorms may furnish the mechanism for the return current.