Category Science

          The wind can make the air temperature feel colder than it actually is. A thin layer of warm air normally surrounds your body, creating an insulating “blanket” of air. If the wind is strong, this warm air gets blown away, making you feel a lot colder. This is known as the wind-chill factor. In a breeze blowing at 9km/h (5.6mph), an air temperature of 0°C (32°F) will feel like —3°C (27°F). If the breeze increases to around 15km/h (9.3mph), the wind-chill factor will make it feel like —10°C (14°F).

Meteorologists call this phenomenon the wind chill factor.

   Wind chill is what the air temperature feels like on our exposed skin due to wind. It’s always lower than the actual air temperature.

   For example, even though the thermometer may indicate it’s 35° F (1.6° C) outside, a 25-mile-per-hour wind will make it feel like it’s only 8° F (-13.3° C)!

   The opposite effect can occur at temperatures above 50° F (10° C). At higher temperatures, humidity on the skin can make the air temperature feel hotter than the actual temperature. Meteorologists call this effect the heat index.

   It’s important to note that wind chill is a prediction of what experts believe humans will perceive the temperature to be because of the wind. No matter how fast the wind blows, the air temperature is what it is and can be measured by a thermometer.

     The wind chill factor, on the other hand, is calculated using various formulas. There is no one formula that all scientists agree on. Most meteorologists in the United States use a standard formula accepted by the National Weather Service.

    What causes the wind chill effect? It’s a result of the fact that the human body loses heat through a scientific process called convection.

     During convection, heated air molecules rise into the air and are replaced by cooler air molecules. How quickly your body loses heat by convection depends on air flow around your body.

     Your warm body usually loses heat slowly. When it’s windy, though, the wind carries the warm air molecules away from your body more quickly, making you feel colder than the actual air temperature around you.

     The faster the wind blows, the faster your body loses heat by convection. As the air temperature around you falls, the effect of wind is magnified, making the wind chill effect greater the colder it gets.

     If you’ve ever blown on a hot bowl of soup to cool it down before eating, you’ve created your own wind chill effect on your soup!

     Even though the air temperature stays the same, the presence of wind makes us feel like it’s colder outside than it actually is. The wind chill effect isn’t all mental, though.

     Since wind chill speeds up heat loss by convection, our bodies experience heat loss and react as if the temperature were as low as it feels…even if the actual air temperature is much higher than the wind chill factor.

     Wind chill factors are calculated under the assumption that a person is properly dressed and dry. If you’re not wearing suitable outdoor clothing, if your clothes are wet, or if you’ve been outside for an extended period of time, the wind chill factor will be magnified even further.

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          Prevailing Winds are those that blow constantly in certain parts of the world. They are produced by hot air moving north and south from the Equator and by cold air moving away from the poles. The prevailing winds are the Polar Easterlies, found in the extreme north and south; the Westerlies, blowing between 30° and 60° north and south of the Equator; and the Trade winds, which blow north-east and south-east, either side of the Equator.

          Since the atmosphere is fixed to the earth by gravity and rotates with the earth, there would be no circulation if some force did not upset the atmosphere’s equilibrium.  The heating of the earth’s surface by the sun is the force responsible for creating the circulation that does exist.

          Because of the curvature of the earth, the most direct rays of the sun strike the earth in the vicinity of the equator resulting in the greatest concentration of heat, the largest possible amount of radiation, and the maximum heating of the atmosphere in this area of the earth.  At the same time, the sun’s rays strike the earth at the poles at a very oblique angle, resulting in a much lower concentration of heat and much less radiation so that there is, in fact, very little heating of the atmosphere over the poles and consequently very cold temperatures.

          Cold air, being more dense, sinks and hot air, being less dense, rises.  Consequently, the rising warm air at the equator becomes even less dense as it rises and its pressure decreases.  An area of low pressure, therefore, exists over the equator.

          Warm air rises until it reaches a certain height at which it starts to spill over into surrounding areas.  At the poles, the cold dense air sinks.  Air from the upper levels of the atmosphere flows in on top of it increasing the weight and creating an area of high pressure at the poles.

          The air that rises at the equator does not flow directly to the poles. Due to the rotation of the earth, there is a buildup of air at about 30° north latitude. (The same phenomenon occurs in the Southern Hemisphere).   Some of the air sinks, causing a belt of high-pressure at this latitude.

          The sinking air reaches the surface and flows north and south.  The air that flows south completes one cell of the earth’s circulation pattern.  The air that flows north becomes part of another cell of circulation between 30° and 60° north latitude.  At the same time, the sinking air at the North Pole flows south and collides with the air moving north from the 30° high pressure area.  The colliding air is forced upward and an area of low pressure is created near 60° north.  The third cell circulation pattern is created between the North Pole and 60° north.

          Because of the rotation of the earth and the coriolis force, air is deflected to the right in the Northern Hemisphere.  As a result, the movement of air in the polar cell circulation produces the polar easterlies.   In the circulation cell that exists between 60° and 30° north, the movement of air produces the prevailing westerlies.  In the tropic circulation cell, the northeast trade winds are produced.  These are the so-called permanent wind systems of the each. 

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          The moving Air that produces the winds tries to take the most direct route possible between the different areas of pressure. However, it is deflected by the rotating movement of the Earth. This is known as the Coriolis effect. In the Northern Hemisphere, the winds are deflected to the right of the direction in which they are headed; in the Southern Hemisphere, they are deflected to the left.

          The Coriolis effect describes the pattern of deflection taken by objects not firmly connected to the ground as they travel long distances around  Earth. The Coriolis effect is responsible for many large-scale weather patterns.

          The key to the Coriolis effect lies in Earth’s rotation. Specifically, Earth rotates faster at the Equator than it does at the poles. Earth is wider at the Equator, so to make a rotation in one 24-hour period, equatorial regions race nearly 1,600 kilometers (1,000 miles) per hour. Near the poles, Earth rotates at a sluggish 0.00008 kilometers (0.00005 miles) per hour.

          Let’s pretend you’re standing at the Equator and you want to throw a ball to your friend in the middle of North America. If you throw the ball in a straight line, it will appear to land to the right of your friend because he’s moving slower and has not caught up.

          Now let’s pretend you’re standing at the North Pole. When you throw the ball to your friend, it will again to appear to land to the right of him. But this time, it’s because he’s moving faster than you are and has moved ahead of the ball.

          Everywhere you play global-scale “catch” in the Northern Hemisphere, the ball will deflect to the right.

          This apparent deflection is the Coriolis effect. Fluids traveling across large areas, such as air currents, are like the path of the ball. They appear to bend to the right in the Northern Hemisphere. The Coriolis effect behaves the opposite way in the Southern Hemisphere, where currents appear to bend to the left.

          The impact of the Coriolis effect is dependent on velocity—the velocity of Earth and the velocity of the object or fluid being deflected by the Coriolis effect. The impact of the Coriolis effect is most significant with high speeds or long distances. 

Weather Patterns

          The development of weather patterns, such as cyclones and trade winds, are examples of the impact of the Coriolis effect.

          Cyclones are low-pressure systems that suck air into their center, or “eye.” In the Northern Hemisphere, fluids from high-pressure systems pass low-pressure systems to their right. As air masses are pulled into cyclones from all directions, they are deflected, and the storm system—a hurricane—seems to rotate counter-clockwise.

          In the Southern Hemisphere, currents are deflected to the left. As a result, storm systems seem to rotate clockwise.

          Outside storm systems, the impact of the Coriolis effect helps define regular wind patterns around the globe. 

          As warm air rises near the Equator, for instance, it flows toward the poles. In the Northern Hemisphere, these warm air currents are deflected to the right (east) as they move northward. The currents descend back toward the ground at about 30° north latitude. As the current descends, it gradually moves from the northeast to the southwest, back toward the Equator. The consistently circulating patterns of these air masses are known as trade winds.

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          The wind is created by differences in air pressure and temperature —winds blow from areas of high pressure to those of low pressure. Rising warm air creates a low-pressure area, and the gap created is filled by high pressure produced by cooler air. The greater the difference in pressure, the stronger the wind.

          Bob Dylan says, “You don’t need to be a weatherman to know which way the wind blows”. I say to Bob, “But, it doesn’t hurt!”

          Wind is a rather elusive meteorological variable, especially since we can’t really see it, like we can clouds or precipitation. Wind, during a storm, is something we expect. Wind can be an unpleasant nuisance though, especially on a bluebird day, to cyclists, sailors, paragliders, climbers, etc.

          The atmosphere is constantly adjusting itself, trying to balance the changes in temperature and humidity from one part of the planet to the other. This leads to different areas of high and low pressure that encircle the globe, and the bigger the difference in temperature, and/or humidity, from one area to another, the bigger the difference in pressure, and the faster the wind blows.

          That’s what gets it started in motion, always moving from high pressure towards lower pressure. Friction at the surface, mountains, buildings, etc. can slow the wind down and alter its direction. In the upper levels of the atmosphere, the wind starts moving from high to low, but it gets re-routed, and turned to the right in the northern hemisphere, because the earth is rotating. This is known as the Coriolis Effect.

          When we observe stronger winds, it means that there is a big difference in pressure across the region, or sometimes across the entire country. A big low-pressure center over the mid-western U.S. and a big area of high-pressure along the West Coast, for instance, could result in strong winds in-between, over the Rockies.

          That difference in pressure from Point-A to Point-B is known as a pressure-gradient. A strong pressure-gradient equals strong winds. You can track that each day by looking at a surface weather map, and look for big highs and big lows, and lots of pressure contour lines in-between, as well.

          The other thing that can cause strong winds at the surface is when the jet stream is directly overhead.

Example: The air pressure is higher in an inflated balloon than outside it. If a hole is made in the balloon, the air streams out, creating a wind that blows from the greater pressure in the direction of the lower. The wind settles when the pressure is the same inside the balloon as outside.In the atmosphere the pressure at the earth’s surface reflects the weight of air above it, which in turn is determined mostly by its temperature, and as people generally know from everyday life, hot air is lighter than cold. This fits with the fact that depressions (low pressure systems) usually bring warm air.

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          A cold front is followed by an area of cold air. Thick, dark clouds, heavy rain and sometimes violent storms arrive immediately. If seen from the side, a cold front looks much steeper than a warm front. Cold air pushes beneath the warm air and rising water vapour condenses into clouds and then rain. Showers of rain will often follow as the front passes over.

          The cold front is depicted on a weather map as a blue line with triangles or as simply a blue line (Figure 9.28). A cold front, as discussed in the previous section, is the leading edge of colder air brought southward by winds around an area of low pressure. These fronts are most common during the active weather times of fall, winter, and spring.

          Winds ahead of the cold front are southwesterly in the warm sector of the mid-latitude cyclone. After the cold front passes a point, winds turn to the west, northwest, or north. Since the cold air is very dense it is very effective at displacing the warm air ahead of it. The dense cold runs under the warm air lifting it. The lifting of warm moist air usually causes cloudiness at the least. If the air is moist and unstable enough, rain and thunderstorms can accompany the passage of the front. Air pressure usually falls as a cold front approaches, rising rapidly after passage as the dense cold air moves in. The dew point falls indicating the change to a dry air mass. Usually there is little local observational evidence of a cold front approaching.

          A cold weather front is defined as the changeover region where a cold air mass is replacing a warmer air mass. Cold weather fronts usually move from northwest to southeast. The air behind a cold front is colder and drier than the air in front. When a cold front passes through, temperatures can drop more than 15 degrees within an hour.

          On a weather forecast map, a cold front is represented by a solid line with blue triangles along the front pointing towards the warmer air and in the direction of movement.

          There is usually an obvious temperature change from one side of a cold front to the other. It has been known that temperatures east of a cold front could be approximately 55 degrees Fahrenheit while a short distance behind the cold front, the temperature can go down to 38 degrees. An abrupt temperature change over a short distance is a good indicator that a front is located somewhere in between.

          Again, there is typically a noticeable temperature change from one side of the warm front to the other, much the same as a cold front.

          If colder air is replacing warmer air, it is a cold front, if warmer air is replacing cold air, then it is a warm front.

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          As its name suggests, a warm front has an area of warm, moist air behind it. The warm air rises above the cold air, and clouds are formed along the front. From the ground, the first sign of a warm front approaching is the sight of high, wispy cirrus clouds and maybe some light rain. When the warm front has passed, there is usually a short period of dry weather.

          A warm front is the transition zone that marks where a warm air mass starts replacing a cold air mass. Warm fronts tend to move from southwest to southeast. Normally the air behind a warm front is warmer than the air in front of it. Normally when a warm front passes through an area the air will get warmer and more humid. Warm fronts signal significant changes in the weather. Here are some of the weather signs that appear as a warm front passes over a region.

          First before the warm front arrives the pressure in area start to steadily decrease and temperatures remain cool. The winds tend to blow south to southeast in the northern hemisphere and north to northeast in the southern hemisphere. The precipitation is normally rain, sleet, or snow. Common cloud types that appear would various types of stratus, cumulus, and nimbus clouds. The dew point also rises steadily

          While the front is passing through a region temperatures start to warm rapidly. The atmospheric pressure in the area that was dropping starts to level off. The winds become variable and precipitation turns into a light drizzle. Clouds are mostly stratus type clouds formations. The dew point then starts to level off.

          After the warm front passes conditions completely reverse. The atmospheric pressure rises slightly before falling. The temperatures are warmer then they level off. The winds in the northern hemisphere blow south-southwest in the northern hemisphere and north-northwest in the southern hemisphere. Cloudy conditions start to clear with only cumulonimbus and stratus clouds. The dew point rises then levels off.

          Knowing about how warm fronts work gives a better understanding of how pressure systems interact with geography to create weather. Looking at warm fronts we learn that they are the transition zone between warm humid air masses and cool, dry air masses. We know that these masses interact in a cycle of rising and falling air that alters the pressure of atmosphere causing changes in weather.

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