Category Science

          During high winds, some bridges may be closed for safety reasons. The structure of the bridges is rarely in doubt, although there have been cases of bridges collapsing in strong winds. The chief concern is for the safety of the vehicles that cross the bridge, particularly high-sided lorries and trucks. Those bridges in especially high positions are most prone to closure.

           When the wind reaches speeds of 65 miles an hour, the bridge closes to traffic.

          A category one hurricane starts at 75 miles per hour.

         The Mackinac Bridge Authority says there was a gust up to 72 miles per hour on Sunday.

          It caused both a camper and a boat on a trailer to tip over on the bridge.

         Typically the bridge would be closed at that wind speed, but that storm front only took a couple of minutes to go from calm winds to hurricane gusts.

         “We’re expected to get some pretty bad storms this week so everybody should take their time driving across the bridge,” said Bob Sweeney from the Mackinac Bridge Authority. “If you’re driving across the bridge during a high wind event, even if it’s only 20 miles per hours, or the winds only 20 miles per hour, drive slow, drive 20 miles per hour and you’ll safely get across the bridge.”

         At 35 miles per hour, the bridge authority starts escorting high profile vehicles like trucks or trailers.

        At 55 miles per hour, it is closed to those high profile vehicles.

        To keep an eye on the wind, the bridge authority watches weather reports online, and they have their own wind meter on the bridge.

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          The Beaufort scale was devised for use by sailors. By observing the wind’s effect on the ship’s rigging and the waves, sailors would know how much sail should be carried or stowed in order for the ship to sail efficiently and safely. The 12 levels of wind strength have since been adapted for use on land.

          Beaufort scale, in full Beaufort wind force scale, scale devised in 1805 by Commander (later Admiral and Knight Commander of the Bath) Francis Beaufort of the British navy for observing and classifying wind force at sea. Originally based on the effect of the wind on a full-rigged man-of-war, in 1838 it became mandatory for log entries in all ships in the Royal Navy. Altered to include observations of the state of the sea and phenomena on land as criteria, it was adopted in 1874 by the International Meteorological Committee for international use in weather telegraphy.

          The Beaufort scale as originally drawn up was calibrated to Beaufort’s assessment of the various effects of the wind on a full-rigged man-of-war. Somewhat arbitrarily, he identified 13 states of wind force on his vessel and ranked them 0 to 12. The scale, however, made no reference to the speed of the wind, and various attempts, particularly during the 20th century, have been made to correlate the two. An attempt made in 1912 by the International Commission for Weather Telegraphers was interrupted by World War I. In 1921 G.C. Simpson was asked to formulate equivalents, which were accepted in 1926 by the Committee. In June 1939 the International Meteorological Committee adopted a table of values referring to an anemometer at a height of 6 metres (20 feet). This was not immediately adopted by the official weather services of the United States and Great Britain, which used the earlier scale referring to an anemometer at an elevation of 11 metres (36 feet). The Beaufort force numbers 13 to 17 were added by the U.S. Weather Bureau in 1955.

          The scale is now rarely used by professional meteorologists, having been largely replaced by more objective methods of determining wind speeds—such as using anemometers, tracking wind echoes with Doppler radar, and monitoring the deflection of rising weather balloons and radiosondes from their points of release. Nevertheless, it is still useful in estimating the wind characteristics over a large area, and it may be used to estimate the wind where there are no wind instruments. The Beaufort scale also can be used to measure and describe the effects of different wind velocities on objects on land or at sea.

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          The strength of the wind varies between gentle breezes and destructive storms. Knowing the strength of the wind and its effect is important for the safety of people and property, particularly for those at sea. In 1805, Sir Francis Beaufort devised a scale by which the strength of the wind could be determined by observing its effect on the environment. This is known as the Beaufort scale.

          Wind has both speed and direction. Anemometers measure wind speed and wind vanes measure wind direction.

          A typical wind vane has a pointer in front and fins in back. When the wind is blowing, the wind vane points into the wind. For example, in a north wind, the wind vane points northward.

A cup anemometer is a common tool to measure wind speed. The cups catch the wind and produce pressure difference inside and outside the cup. The pressure difference, along with the force of the wind, causes the cups to rotate. Electric switches measure the speed of the rotation, which is proportional to the wind speed.

          At wind speeds below about 3 mph, the cup anemometer is prone to error because friction keeps the cups from turning. At wind speeds above 100 mph, cup anemometers often blow away or give unreliable measurements. In freezing rain, the anemometer can literally freeze up and stop turning.

          Propellers also can measure wind speed. The propeller blades rotate at a rate proportional to the wind speed.

          A windsock often is used at airports. A windsock is a cone-shaped bag with an opening at both ends. When it is limp, winds are light; when it is stretched out, winds are strong. Pilots can quickly determine the wind direction and speed along a runway just by observing the shape and direction of a windsock.

          Sonic anemometers use sound waves humans cannot hear to measure wind speed and direction. The instrument determines the wind velocity by measuring the time between when the instrument sends a sonic pulse and when it is received.

          An anemometer looks like a weather vane, but instead of measuring which direction the wind is blowing with pointers, it has four cups so that it can more accurately measure wind speed. Each cup is attached to the end of a horizontal arm, each of which is mounted on a central axis, like spokes on a wheel. When wind pushes into the cups, they rotate the axis. The faster the wind, the faster the cups spin the axis.

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          Windmills usually face into the prevailing wind, but they can also be adjusted should the wind direction change. Some types of windmills can be completely rotated according to the wind direction; in others, the angle of the sails can be adjusted to receive the maximum amount of wind power. Some wooden sails have spring shutters that open and close according to the wind strength. If the wind gusts, the shutters open up, if it drops, they close. In this way, a constant wind force is maintained on the windmill sails.

          Up until recently, people still only had visual impressions of what a windmill is, often associating it with the past and particularly before the industrial revolution. Today, things have come full circle, if you will and there is now a growing demand for large, technologically advanced windmills across the world. The term wind energy or wind power describe the process through which wind turbines convert the kinetic energy in the wind into electrical energy by the use of generator.

         What this introductory guide seeks to do is describe the apparatus in layman’s terms and also outline how they work and what they were intended for originally and the purposes for which they are used today. We begin with a brief definition of what a windmill is.

          It was originally a structure with sails, much like that on pre-industrial ships, and was originally used to produce flour from corn. In order to do this, the wind would have to prompt the sails to turn. They were also originally built by master craftsmen.

         A dictionary definition explains it thus; it is a machine which is propelled by the wind from a horizontal shaft which extended onto sails. Windmills still used today, mainly in parts of the world which have traditionally relied on them, are powered by electricity or water.

       The dictionary expounds this definition further by relating it to a human physical exercise technique which replicates the symbolism and movement of the original windmill. It is also famously symbolic in Cervantes’ classic of Don Quixote. This definition reminds readers that the original mill was also used to pump water and generate power.

       In modern terms, the advanced windmill operates with just three blades mainly to generate sustainable sources of electricity and energy. Today, these windmills are also referred to as wind turbines.

      Winds are produced due to uneven heating of the atmosphere by the sun, the rotation of the earth and the irregularities of the earth’s surface. Wind flow patterns differ from place to place and are modified by bodies of water, vegetation, and differences in terrain. This next section explains briefly but accurately how windmills work. Sourcing more extensive information, readers will learn that understanding technical processes initiated in wind turbines will be easy to follow because the manner in which windmills work follows a simple process. Here we continue to rely on layman’s terms.

       A number of different options were tried when modern wind turbines were first built. Today, the universal mechanizing principle is to operate the turbine by using just three blades placed around a rotor which is connected to a shaft. Note that numbers of variations have been tried, two blades and even one blade. But, three blades work the best.

       As its name states, the windmill’s only source of energy is derived from the wind. The wind turns the blades which spins a shaft, in turn, prompt a generator to produce electricity. These blades are connected to a generator, sometimes through a gearbox and sometimes directly. In both the cases, the generator converts the mechanical energy into electrical energy. Interestingly, most modern turbines turn in a clockwise direction. Depending on wind speed, most modern turbines can operate at speeds from as little as four meters per second to as much as 15 mps.

       Quite a number of green energy advocates and NGO’s describe the wind-generator process more succinctly by correlating it closely with the environmental sustainability initiatives.

       Once the turbine’s blades turns a shaft located inside of a box placed on top of the turbine, gearbox mode is propelled and more speed rotation is given off. A transformer within the turbine then converts electricity into a voltage suitable for distribution to a national grid.

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          Land breezes occur at night, as the land cools down more quickly than the sea. The cold air sinking over the land pushes out to the low-pressure area over the sea. Land breezes tend to be lighter than sea breezes, as the difference in temperature between the sea and the land during the night is only slight. Land and sea breezes help make a coastal climate very different from that inland.

          Land breeze, a local wind system characterized by a flow from land to water late at night. Land breezes alternate with sea breezes along coastlines adjacent to large bodies of water. Both are induced by differences that occur between the heating or cooling of the water surface and the adjacent land surface. The land breeze is typically shallower than the sea breeze since the cooling of the atmosphere over land is confined to a shallower layer at night than the heating of the air during the day. Since the surface flow of the land breeze terminates over water, a region of low-level air convergence is produced. Locally, such convergence often induces the upward movement of air, fostering the development of clouds. Therefore, it is not uncommon to see clouds lying off the coast at night, which are later dissipated by the daytime sea breeze.

          The air over the ocean is now warmer than the air over the land. The land loses heat quickly after the sun goes down and the air above it cools too.  This can be compared to a blacktop road. During the day, the blacktop road heats up and becomes very hot to walk on. At night, however, the blacktop has given up the added heat and is cool to the touch. The ocean, however, is able to hold onto this heat after the sun sets and not lose it as easily. This causes the low surface pressure to shift to over the ocean during the night and the high surface pressure to move over the land. This causes a small temperature gradient between the ocean surface and the nearby land at night and the wind will blow from the land to the ocean creating the land breeze.

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          On a hot and sunny day, coastal areas will experience sea breezes. The land and the sea heat up and cool down at different rates, producing moving currents of air. The land heats more quickly than the sea, producing an area of low pressure, into which the cooler sea air moves. This breeze may move in a completely different direction from the prevailing wind and can blow up to 30km (18 miles) inland.

           A sea breeze or onshore breeze is any wind that blows from a large body of water toward or onto a landmass; it develops due to differences in air pressure created by the differing heat capacities of water and dry land. As such, sea breezes are more localized than prevailing winds. Because land absorbs solar radiation far more quickly than water, a sea breeze is a common occurrence along coasts after sunrise. By contrast, a land breeze or offshore breeze is the reverse effect: dry land also cools more quickly than water and, after sunset, a sea breeze dissipates and the wind instead flows from the land towards the sea. Sea breezes and land breezes are both important factors in coastal regions’ prevailing winds. The term offshore wind may refer to any wind over open water.

          Wind farms are often situated near a coast to take advantage of the normal daily fluctuations of wind speed resulting from sea or land breezes. While many onshore wind farms and offshore wind farms do not rely on these winds, a near shore wind farm is a type of offshore wind farm located on shallow coastal waters to take advantage of both sea and land breezes. (For practical reasons, other offshore wind farms are situated further out to sea and rely on prevailing winds rather than sea breezes.)

Cause

          The sea has a greater heat capacity than land, so the surface of the sea warms up more slowly than the land’s. As the temperature of the surface of the land rises, the land heats the air above it by convection. The warming air expands and becomes less dense, decreasing the pressure over the land near the coast. The air above the sea has a relatively higher pressure, causing air near the coast to flow towards the lower pressure over land. The strength of the sea breeze is directly proportional to the temperature difference between the land and the sea. If a strong offshore wind is present (that is, a wind greater than 8 knots (15 km/h)) and opposing the direction of a possible sea breeze, the sea breeze is not likely to develop.

Effects

          A sea-breeze front is a weather front created by a sea breeze, also known as a convergence zone. The cold air from the sea meets the warmer air from the land and creates a boundary like a shallow cold front. When powerful this front creates cumulus clouds, and if the air is humid and unstable, the front can sometimes trigger thunderstorms. If the flow aloft is aligned with the direction of the sea breeze, places experiencing the sea breeze frontal passage will have benign, or fair, weather for the remainder of the day. At the front warm air continues to flow upward and cold air continually moves in to replace it and so the front moves progressively inland. Its speed depends on whether it is assisted or hampered by the prevailing wind, and the strength of the thermal contrast between land and sea. At night, the sea breeze usually changes to a land breeze, due to a reversal of the same mechanisms.

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