Wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the Sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space. Winds are commonly classified by their spatial scale, their speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in the Solar System occur on Neptune and Saturn. Winds have various aspects: velocity (wind speed); the density of the gas involved; energy content or wind energy. Wind is also an important means of transportation for seeds and small birds; with time things can travel thousands of miles in the wind.
Winds can shape landforms, via a variety of aeolian processes such as the formation of fertile soils, such as loess, and by erosion. Dust from large deserts can be moved great distances from its source region by the prevailing winds; winds that are accelerated by rough topography and associated with dust outbreaks have been assigned regional names in various parts of the world because of their significant effects on those regions. Wind also affects the spread of wildfires.
Factor Affecting Wind Motion
- Horizontal Pressure (Gradient)
- Rotation of the Earth (The coriolis Force)
- Frictional Forces
- Centrifugal action of wind
Horizontal Pressure (Gradient)
Pressure gradient is the change in the air pressure per unit of distance travelled along a certain line. In another words, it is an average change in barometric pressure per unit of distance along a certain linear direction in a given region.The pressure gradient determines the direction as well as the intensity of the wind blow. The winds always blow from the high pressure areas to the low pressure areas.Pressure gradient can be calculated by the change in the isobaric values (barometric pressure generallyin milibars per km (distance)).
Rotation of the earth - The Coriolis Force
Coriolis, practically speaking is not a force but it an effect which is observed on a mass of body in a rotating system. It results from the rotational movement of the earth and the movement of air in relation to the earth. It acts perpendicular to the axis of the earth.It is determined by the mass of the body and its rate of rotation. The earth rotates from west to east on its axis. Hence, the Coriolis force operates in north-south direction. The Coriolis force is zero at the equator and maximum at the poles. This concept was first explained by French engineer G. G. Coriolis in 1835, thus it is known by his name . One rotation takes about 24 hours. The velocity of the rotation of the earth is 1670 km/hour (the circumference is about 40000 km) along the equator which comesdown to half (835 km/hour) along 600north and south latitudes.We know that the earth is not a non-rotating body but, it rotateson its axis. When the air moves equatorward on the spinning earth in the northern hemisphere, the air is deflected to the right as the surface itself moves forward. The same thing also happens when the air is moving from equator to pole, i.e., rightward turning in the northern hemisphere.
air friction is the resistance to motion of air in relation to the surface roughness and irregularity through which the wind is blowing. Frictional force reduces the velocity of the wind near the ground. Therefore, wind is most affected by the friction andit is maximum near the ground. Above the ground, the friction is reduced very drastically.The surface irregularity and undulation is impacting the blowing winds. Within a height of about one km from the ground, the wind is creating eddies and straight blow is heavily affected. After that the velocity is increasing. Right moving arrows shows the direction as well as the velocity of the winds.
Centripetal force operates at right angles to the blowing wind. It is the inward pulling force i.e. towards the centers of rotation. Vertically above about a height of around 5.5 km, the air pressure is approximately 500 mb. By this height, the friction force is almost terminated except over the highlands and mountains. Coriolis force is equal to centripetal force plus pressure gradient force. But in case of high pressure where anticyclone is developed in upper troposphere, pressure gradient force is equal to Coriolis force plus centripetal force. In these conditions, the winds are blowing parallel to the isobars. It is called geostrophic winds by which the jet stream is developed.
Scales of Atmospheric Circulation
The atmospheric circulation happens at a varied scales of operation starting from a very small local scale to the global one. The circulation is classified on the basisof spatial extension as well as its temporal duration.
- Micro scale
- meso scale
- synoptic scale
- Planetary scale
The micro scale events are still at further minute level and they are mostly the product of the local variations.
The meso scale weather events and movementsare associated with further small areas and their occurrences are of less than a day. Greater degree of variation is associated with these events. Some of them are directly associated with the heat received from the sun and are most commonly found at a regular diurnal interval. Some are not very predictive and appear randomly, but require fulfillment of certain conditions.
The synoptic scale movements like frontal development, cyclones etc. have a certain duration of occurrence but not very definitely pinpointed one.Their variations are sometimes randomlyassociated with certain conditional fulfillment. That variation might be of hundreds of km. They develop in a specific belts in both the hemispheres.
The planetary scale atmospheric as well as ocean circulation are redistributing the thermal energy on the earth surface and atmosphere. The atmospheric circulation varies from one year to another, butthe large scale patterns remainalmost alike.