Geoscience Reference
In-Depth Information
convenient name - the Beaufort scale. Although measured in wind speed units, not
force units, the numerical values of the scale are related to empirical observations.
The wind is classifi ed as
breeze
(light, gentle, moderate, fresh, and strong) up to
Beaufort number 6 (maximum 49 km/h).
Gale
starts at Beaufort number 7 as mod-
erate and ends at 9 as strong (up to 88 km/h).
Storm
is classifi ed at numbers 10 and
11. The number 12 and eventually higher is denoted as
hurricane
(for 118 km/h and
more).
The wind dries out the soil surface, and as water is lost from the soil aggregates,
the wind disintegrates the dry aggregates into elementary particles. The silt and clay
particles are then easily lifted by the turbulently blowing wind that is especially
strong when there is sparse vegetation. In arid zones where the land is virtually
without any vegetation, the lift of particles is extremely easy. Once the tiny particles
are in the air above the soil surface, they are simply carried away by the wind. If the
elementary particles are only weakly bonded together, their quasi-aggregates are
easily broken by the force of wind, and the air erosion is more vigorous. Not only
does wind damage the land by erosion, but the sedimentation of the dust particles,
usually at great distance, causes damage not only on highways but to vegetation
since the plants are suffocated by their stomata being plugged by clay and silt. And
when the process occurs in hot weather, the vegetation is even burned by hot parti-
cles. The higher are both wind speed and air temperature, the more intensive is wind
erosion with completely negative consequences. The higher the wind speed is and
the air temperature is higher as well, the more intensive is the wind erosion with all
negative consequences. In many dry areas around the world, when wind erosion
gradually converts fertile land into wasteland, we can truthfully speak about the
death of soil.
The process of wind erosion is recognized in three phases:
1. Movement of soil particles smaller than 0.1 mm suspended in the air. The sus-
pension is moved up to height above the land surface, and there, parallel to land
surface, the particles are carried far away like a large cloud. They return to earth
only when the wind subsides or when there is a high obstacle oriented crosswise
to the blowing wind. It is similar to the formation of a snowdrift. If the process
is repeated many times during thousands of years, a new layer of sediments is
formed. Our best example of such erosion and sedimentation is the 10-m thick
loess layers that originated from a product of wind erosion during the glacial
periods in the last 500,000 years of our recent Pleistocene. On the other hand, if
we observe only one erosion phase, we sometimes discover that the wind erosion
stops when the particles are carried downward with precipitation.
2. Movement of particles by a series of short bounces along the surface of the
ground and dislodging additional particles with each impact. The process is
denoted as saltation. The bouncing particles ranging in size from 0.1 to 0.5 mm
usually remain within 30 cm of the soil surface. Depending on conditions, this
process accounts for 50-90 % of the total movement of soil by wind.
3. The rolling and sliding of larger soil particles along the ground surface, known
as soil creep. The movement of these particles is aided by the bouncing impacts
