In this article, we look into the wind, and what causes it to blow. Motivation for the study of this theoretical section is that the more we know about the forces that cause the wind, the better prepared we are to understand what is happening, and the ability to make knowledge-based judgment about what will happen in the future.
Staying away from several important influences shore, the speed and direction of the wind can be predicted depending on exactly where we are within a particular weather systems (SYNOPTIC situation). Very often the best indicator of our location in the weather system is the direction of the current of the wind and change direction over time. To understand this requires an understanding of how the wind moves around systems with high and low atmospheric pressure, which we will call "highs" (atmospheric system with high pressure in the center is "High" or "H") and a cyclone (low pressure system is "Low" or "L"), to which we will come on after a brief overview of the atmospheric pressure as such.
Wind is simply the horizontal movement of air. 10-knot wind means moving air mass to 10 nautical miles per hour. Air is a fluid environment with his weight, like water, only 800 times lighter than water, i.e. less dense and more compressible. Understanding the fluid nature of air and its interaction with water, as a liquid, and water vapor, is key to the explanation of most of the weather and sea conditions.
The fluid-fluid interaction between air and sea surface causes the waves; a similar interaction between high-altitude wind and water vapor in the clouds makes waves in the clouds. Visible clouds show us the direction of the wind at height as well as waves on the water show the direction of the wind over the sea surface. Very valuable to know the wind direction at the height, as it informs us about the direction of the storm. And storms and frontal systems in the atmosphere behave like eddies and waves in the aquatic environment.
The air is held to the Earth by gravity); if the Earth was not heavy enough to provide this attraction, the air would be vented into space by the centrifugal force from the rotation of our planet. Because air is compressible and held his gravity decreases with height, most of the air mass is compressed in a relatively thin layer at the earth's surface and this layer has the greater density, the closer to the Earth's surface. Roughly 80 percent of the air of the atmosphere is below 40,00 feet and about 50 percent below 18,00 feet above the surface. This height, which is half the height of the Earth's atmosphere is an important demarcation line in meteorology, because most of the atmospheric phenomena that we experience as weather on earth occur in this part of the atmosphere. Wind blowing above this line are significantly stronger and more stable than above the surface; it's also changing the state of water in the clouds - from vapor to ice crystals occurs at this altitude.
In addition to reducing the density of the atmosphere with altitude, the air becomes colder because less and less "atmospheric blanket, keeping warm, stays above it. On average, the air temperature drops to 4° F per 100 feet of elevation, if you climb through the atmosphere. Clear the bottom or top edge of the clouds are at altitudes where the temperature drops to the dew point.
Although the weight of air per unit volume (density) is very small, the total weight of it is very impressing. For example, in a regular room contains about 100 pounds of air, while over every square mile of the earth's surface we have 30 million tons of air, and this huge mass is not always distributed evenly over the surface. An uneven distribution of air above the earth is what causes the wind. If the wind is blowing across an entire state, for example from the North with a force of 20 knots during the whole day, then, there, in the North, was very much more air than in the South. The amount of air at any place is determined by measuring the atmospheric pressure at that location. To characterize the atmospheric pressure, you just need to install, what is the weight of the column of air above a certain surface area. A typical value of 14.7 pounds per square inch, knowing that the air column cross-section in one inch extending from the surface to the upper boundary of the atmosphere weighs 14.7 to pounds. Atmospheric pressure generally ranges from 14.4 to 14.8 pounds per square inch.
However, atmospheric pressure is usually expressed in millimeters of mercury or in millibars (14.7 pounds per square inch = 1013.25 mb). Millimeters of mercury, as a unit of pressure measurement, taken from the mercury manometer is a device for measuring pressure. The height of the mercury column rises or falls in accordance with the atmospheric pressure exerted on an exposed surface of mercury. These units are used in aviation and radio and television weather forecasts.
On the other hand, in weather forecasts for the sea and all weather maps made to reflect the atmospheric pressure in millibars (mb), and this is the best unit for the study of marine weather. Millibar - a metric unit equal to the pressure of one kilogram of weight per area of one square meter. You can convert inches to millibars: mb = 33.864 x inches or start with 30.00 inches = 1016mb and modify accordingly to 1mb for each 0.03 inches.
Areas of high and low pressure (called "highs" and "lows") shown in G255 and G251, which shows schematically the distribution of air over one thousand miles of ocean. Anticyclone - a huge hill air cyclone - depression (depression).
Figure G251 shown that where either high (1024 mb) pressure is higher than anywhere else in the cyclone (996 mb), as well as above it more air.
Since air is a fluid, it should be a natural tendency to flow down from the height into the cavity, flowing by gravity. Therefore, the air in an anticyclone is always moving out of the high pressure and falls, seeking inside the cyclone to the center of low pressure, and rises. Have the wind! At point "B" may not be the wind because the air is distributed evenly and the pressure is equal on all sides of the region. The circle above the cyclone and anticyclone show how these plots are represented on weather maps. The circles are contours of equal pressure are called isobars, is usually marked by the last two digits of the value of the pressure which they represent (24 means 1024mb) and painted with 4-mb interval. Isobars can be considered by analogy with the height contours on topographic maps, and on this schematic picture of the distribution of air, as if they reflect the actual height of the "hill" or "depression", although the actual pressure also depends on the density (temperature) of air and altitude.
And now we come to another problem - why the wind blows in a circle. The driving force of gravity is the same in all horizontal directions, but the actual motion of the air must be described taking into account the spherical shape of the earth rotating beneath it. The result is called the Coriolis effect: anything that moves in a straight line above the rotating surface describes Kuroda on this surface. Without delving into this topic note: because of the Coriolis effect on all the earth (wind, current, ballistic missiles) in their motion in the Northern hemisphere are deflected to the right and in the southern hemisphere to the left. This deviation depends on the speed and geographic latitude. The influence of the Coriolis effect is the key in meteorology and Oceanography. As this effect influences the movement of air masses, his part is called the Coriolis force. As the air in the Northern hemisphere is forced to move from areas of high pressure inside the cyclone under the influence of gravity, the Coriolis force causes it to deviate to the right (see Fig. G255). At high latitudes in the atmosphere where there are no other forces acting on the air, the wind continues to steer right up until Coriolis force will not equalized with the force of gravity, which pushes the air out. In the interaction of these forces air circulates clockwise around a high and counterclockwise in the cyclone.
This is the basic rule in the movement of air in the cyclone and the anticyclone is fundamental in predicting the wind, but before we continue with this, it is necessary to clarify some points. Because of a difference of density of the air, the anticyclone may contain more than located next to the cyclone, even at the same height of the air column. Cold air is heavier than warm, so in the region with the cold air, the pressure will be higher than in the same warm air at the same height. Therefore, the wind from the anticyclone in the cyclone and rotation it will be the same. Therefore it is better to think of the pressure gradient as the main force that causes air to move. (see Fig. G255a)
The greater the pressure gradient (i.e. the pressure difference over the adjacent areas), the greater the force acting on the air and the stronger the wind. The magnitude of the pressure gradient or slope is shown on weather maps, the distance between isobare: the closer the isobars are together, the greater the gradient. (Fig. G254)
Now, when we have considered these questions, let us note another important factor. When the wind blows over the earth's surface, there is another force - the force of air friction on this surface, and it affects the speed and direction of the wind. Based on the fluid nature of air, you can make a comparison with the water flowing along the inclined surface, such as glass and covered with sandpaper. It is clear that sandpaper creates more friction and the flow slows down more than the glass. And with the wind. As soon as he starts to blow from high and Coriolis force deflects it to the right, the friction of air on the earth resists this deviation and reduces the wind speed in comparison with the wind at height where there is no friction. As a result, the wind speed near the ground is less than the height and the direction we should no longer strictly along the isobars, and strives slightly outward from the center of the anticyclone. The same happens in the cyclone, only on the contrary, the surface wind is deflected to the inside of the cyclone, to its centre. Typically, the angle of deviation of the wind from the Isobar out of a high and inside the cyclone is 15° - 30° and depends on sea conditions and bending of the Isobar. Above ground, where the friction is large, the angle can reach values of 30° to 45°. (Fig. G255)
Although it may not seem important, but the difference in speed and direction of wind over the earth, and the height is very important when monitoring weather and forecasting. To better explain these points, we must agree on the terms. The wind called upon the direction from which it blows. The North wind is blowing from North to South, South-westerly wind blowing from the South-West, etc. Also, the sea breeze blows from the sea to the shore, I land breeze from the shore into the sea. Changes in wind direction are also given names. Turning winds to the right is called "veer" (Wii), and left backing shift" or "back" (back), as shown in figure G135.
The North wind, which varies on the North-East is to the right (veered)if LF is changed to the Northwest, then to the left, counterclockwise (backed). These definitions apply to all wind directions and in all areas in the Northern hemisphere, and southern. Directions relative to the wind also have their own names. “Windward” (windward) means wind in your face. “Leeward” (leeward) - the wind in the back. “Lee shore” - (Lee-choe) - Lee, you are looking at, looking downwind (not against the wind!)
To determine where the center of the cyclone, you become a face to the wind, and then the low-pressure center will be on your right side, slightly behind (about 30°).(see figure G168)
If we have a North wind, say, 30 knots at a height of Cumulus clouds, the surface is only a 20-knot wind from the North-West. The reason is that the friction of air on the earth's surface. In other words, winds aloft turns to the right, relative to the wind at ground level, and this is true for all winds in the anticyclones and cyclones. The result is the motion of Cumulus clouds. Because they run in the winds, the direction of their movement does not coincide with the direction of the wind near the earth. If you face to the wind, you will see clouds moving to the right you. Usually, their rate deviates by 30° to the right. Wind is another example of this effect. By reason of the vertical instability in the air. They are usually just large patches of wind with height, which are thrown down to the earth. And because they come from the top, where there is no friction, wind gusts stronger and goes to the right relative to the wind, keeping its "upper" direction. In short: when it comes gust of wind, he turns to the right.
Another consequence of friction is his contribution to the typical change in wind direction at low coast. Since water creates less friction than land, over the sea the wind is usually stronger than on the shore. Is not uncommon that the wind over a large water space twice stronger than on the nearest shore at the same time is an important practical tip for facing the sea of harbours or coastal anchorages. When the friction of the water is replaced by rubbing the ground when crossing the coastline, the wind direction also changes, and the wind turns to the left, at some distance from the shore. (G267).
The General trend is such that when approaching the shore, the wind blows more perpendicular to the coast couple, than in the open sea.
Along with the fact that wind circulate in different directions around the highs and lows, there is a much more important difference between these two weather systems: Cyclone always brings bad weather, and high pressure usually good. Moreover, winds in cyclones always stronger. And anticyclones is known for its weak winds. To understand why this is happening, let's take a closer look at what forces are involved here. Knowing that two forces cause wind - gradient (or difference) of pressure, and the Coriolis force, by simple vector decompositions forces, we can conclude that for winds in an anticyclone resultant force = Coriolis force is the pressure difference. In the cyclone, this resultant force that causes the wind to be = Coriolis force + force pressure difference, which is of course more than in the first case. Therefore, the wind in a cyclone stronger.
As a rule, the faster moving cyclone, the more intense his wind. Summer cyclones are traveling with a speed of 10 to 20 knots; in winter, their speed increases to 20 to 30 knots. At any time, the cyclone can stop (the so-called "stationary cyclone"), and in winter its speed can reach 60 knots. Throughout the mid-latitudes (30° - 60°), the cyclones move eastward across the ocean. In the tropics the same - more in a westerly direction.
All bad weather is the cyclones. The storm is a large recirculating cyclone; hurricanes and other tropical disturbances are small, intense cyclones; frontal systems is a long and narrow region of low pressure; and squalls are very localized cyclones under high Cumulus clouds. Along with strong winds, cyclones bring clouds and rain: as the air in cyclones always rises, it cools with height up to the dew point, and when it becomes saturated with moisture or ice crystals, it starts raining. In anticyclones, air drops, and any clouds that appeared in the anticyclone will evaporate when lowering down and heated.
In the next article we will look at the fronts and frontal systems.
From the "Weather trainer" by David Burch, published in the journal of the Fairway