The Weather Fronts

The term “weather front” refers to the barrier that arises when one air mass displaces another air mass with a different density. Note that weather fronts are the fundamental cause of weather phenomena. These meteorological conditions can include blizzards, heat waves, and thunderstorms. As a result, the first section of this essay concentrates on how cold fronts, warm fronts, and occluded fronts arise and behave.

When a less dense, warmer, moist air mass displaces a more dense, colder air mass at the ground level, a cold, dense, dry front results. In this regard, the warm air is pushed up by the cold dense air in a process known as undercutting as evident from Figure ­1. In most cases, the cold front forms at the edges of the cold advection pattern following the wake of an extratropical cyclone (“Cold Front”, 2010). The leading edge of the cold front is characterized by a trough of low pressure when viewed on a surface analysis chart. Moreover, it is represented by a blue line with triangles that point in the direction the front is moving. For instance, in the northern hemisphere, the cold front usually moves from northwest to southwest.

Pressure Changes

The movement of a cold front causes a significant change in the pressure of a particular region. Therefore, the atmospheric pressure of an area will steadily decrease when a cold front approaches which is attributed to the cold dense airlifting the less dense warm air as it moves. This phenomenon is also referred to as wedges. At the leading edge of a cold front, a low-pressure trough forms leading to a decrease in pressure. However, a sharp rise in atmospheric pressure will be observed when the leading front passes over an area. The steady increase in temperature will stabilize the normal atmospheric pressure once the front has passed.

Temperature Changes

In front of a cold front, the air temperatures are ordinarily warm; it is this warm air that is displaced by a mass of cold associated with the cold front. In this regard, there is a sudden transition of temperature from warm to cold air as the cold front transverses through an area. According to David and Roth (2006), the drop in temperature may be approximately 50 degrees Fahrenheit. Moreover, the temperature will continue to decrease after the cold front has passed and this can last for few hours or as long as a few days.

Wind Shift.

The shift in winds as a Cold front passes over an area will differ depending on whether an individual is in the northern or southern hemisphere. People in the northern hemisphere experience a relatively calm wind blowing from either the south or south-west. In contrast, the winds originate from either the north or northwest in the southern hemisphere. On the contrary, in both regions, the winds will always be shifting and gusting especially at the leading edge of the cold front. It should be noted that the winds along the leading edge can be mighty especially during the spring season when the pressure gradient is tighter than usual (Ahrens, 2013). Also, the gusting winds at the edge of a cold front can act as an indicator of the passing cold front in an area. However, after the passing of a cold front, winds in the northern hemisphere will be observed from the west or northwest while in the southern region from the south or southwest.

Weather and Precipitation

Several indicators can be used to predict the occurrence of a cold front at a given place. For instance, a light patchy rain occurs before the passing of a cold front, and if a squall line forms ahead of the front thunderstorms in the summer, then a cold front can occur (David and Roth, 2006). At the leading edge of a Cold is where the most severe rain and weather will be found. Nevertheless, the heavy rain, thunderstorms, hail, and tornadoes take place along the leading edge. This narrow precipitation band is caused by the warm moist air that is being forced up where it eventually condenses forming large cumulonimbus clouds. Once the cold front has passed, a light precipitation continues to be seen after which the weather will begin to clear.

Warm Front

In this type of front, a mass of less dense warm air displaces a mass of colder denser drier air by riding over the top of it in a process called overrunning. This causes the cold air to recede as shown in Figure 3. The slope between these two colliding air masses is more gradual than the slope of a cold front. Also, a warm front moves at a slower rate than a cold front because the overrunning process takes a lot of time as compared to the wedging process. There is the presence of an extensive cloud coverage caused by the rising and condensing water vapor as the less dense cold air is overrun along the leading edge of a warm front (“Warm fronts”, 2010). These clouds usually start high up in the sky as Cirrus clouds and to nimbostratus clouds as they gradually decrease in altitude.

On a surface analysis chart, a warm front will be shown by a large wide trough of low pressure followed by a ridge of high pressure. Moreover, they are visualized by a red line with semicircles extending in the direction the front is moving as seen in Figure 4. In the northern hemisphere, this direction will often be from the southwest to the northeast.

Pressure Changes

The atmospheric pressure decreases steadily when a warm front approaches an area. The area in front of a broad warm front will be a trough of low pressure much wider than what is seen on a cold front. However, the pressure levels off at the edge of the front as it passes. During the front, the pressure slightly rises, and it gradually begins to fall after the warm front has passed.

Temperature Changes

The area in front of an advancing warm front will contain cold air. Along the edge, the temperature will rise quickly extending out over the top of cooler air as the warm front overruns the cold air. Therefore, the temperature will gradually level off after the warm front has passed. According to Ahrens (2013), the area behind a warm front is known as the warm sector, and a cold front will always follow it.

Wind Shift

In the northern hemisphere, the winds before a warm front will be backing from either the south or southeast. On the other hand, the winds will be veering from either the north or northeast in the Southern hemisphere. During the warm front, the winds passing over will be variable from various directions. After the warm front has passed, the winds in the northern hemisphere will be veering from either the south or the southwest, but in the southern hemisphere, the winds will be backing from either the north or northwest.

Weather and Precipitation

The area before an advancing warm front is often characterized by a light to moderate rain, snow and light showers especially during the summer months. During the period of a warm front, the environment is covered with thick fog leading to poor visibility. As aforementioned, the rain is caused by the uplifting of warm moist air which condenses to form clouds. In this regard, when the front is passing, the rain will begin to taper off to a drizzle or stop completely. After the warm front, researchers agree that there will be little rain with patchy light showers or no rains at all.

Occluded Fronts

Occluded fronts occur when a slowly moving warm front is overtaken by the fast moving cold front. This happens when a warm front becomes perpendicular to a cold front on the trailing edge of an extra-tropical cyclone, and the cold front begins to overtake the warm front. This is the point where the most turbulent weather will be found as seen in Figure 5. These occluded fronts commonly form near the Pacific and Atlantic oceans as well as around the great lakes (“Occluded Fronts”, 2010). Occluded fronts are categorized into two groups; the warm and cold type occluded fronts. A cold type occluded front forms when a mass of cold dense air overtakes a warm front and undercuts the warm air behind the warm front. Figure 6 shows the cooler air in front of the warm front. Also, the cold type occluded fronts are the most common fronts along the Pacific coast, and they mostly move towards the interior of North America (Ahrens, 2013).

A warm type occluded front can occur when a cold front of milder maritime polar air overtakes a warm front that is preceded by much colder continental polar air. However, both types of the occluded front are shown by the purple line of alternating triangles and circles that point on the direction the font is moving on a visualized on a weather chart.

Pressure Changes

Both types of occluded front share the same characteristics regarding the pressure changes in an area. Just like on the cold front, when an occluded front approaches a region, the atmospheric pressure gradually falls. The decrease in pressure reaches its lowest when the front is passing over the area, but once the occluded front moves, the pressure begins to rise. In this regard, the occluded fronts share a significant number of characteristics with the cold front concerning pressure.

Temperature Changes

The two types of occluded, that is, the cold and the warm types exhibit differences in their characteristics concerning the changes in temperature. The temperatures of an area will be cold or decrease when a cold occluded front approaches. Moreover, the temperatures will continue to decrease even after the occluded cold front has passed. But in a warm occluded front, the coldest air will be found ahead of the front because the milder air of the colliding front pushes the colder air forward of it. As the warm occluded front passes over an area, the temperature will begin to rise and once the front has passed the temperature will be warmer and milder than the air ahead of the front.

Wind Shift

Cold type and Warm type occluded fronts both have common wind patterns. As these fronts approach, winds will be coming from the east, southeast or south. During this period, when the front is directly overhead, the winds originate from various variable directions. Once the front has passed the winds will begin to come from the west or northwest. In both the two fronts, the strongest wind can be expected to be found at the triple point.

Weather and Precipitation

The weather associated with occluded fronts closely resembles that of an approaching cold front (Occluded fronts, 2010). As an occluded front approaches precipitation, it moves from being moderate to heavy. When the cold front collides with the warm front, the temperature greatly changes, and the weather becomes more severe with heavy rain and a possibility of thunderstorms. Once the front has passed, the weather clears up with some slight chances of light showery rain.

High-Pressure Systems

A high-pressure system is an area of sinking air in which the atmospheric pressure is higher than that of the surrounding air. The key characteristics of a high-pressure system also known as an anticyclone include the converging air aloft and diverging air at the surface. For instance, the process of pushing air down causes the high pressure as depicted on the right side in Figure 8. The formation of high-pressure systems is known as anticyclogenesis and takes place when the cold air aloft in the upper regions of the troposphere begins to compress sink. The winds in a high-pressure system are light and will move from the center outwards just like air in a high-pressure system. This can be seen from Figure 10 which shows a net outflow at the surface.

It is evident that Coriolis effect deflects the winds in high-pressure systems. Therefore, the winds will rotate in the anticlockwise in the southern hemisphere and an opposite direction in the northern hemisphere. The high-pressure systems are responsible for the hot, dry weather with the possibility of some haze and nighttime fog (“Glossary of Meteorology”, 2009). The hot weather of a high-pressure system causes the sinking and compression of air which initiates the adiabatic heating through compressional heating. This process also causes most of the water vapor in the air to evaporate leading to the clear skies of a high-pressure system. Due to the lack of cloud cover in a high-pressure system, the days will be hot and nights will be cold. Thick hazes may also form in a high-pressure system if the sinking air motions pull down and trap particles in the air near the surface; this effect is prevalent in urban areas due to the large pollution (Tokay, 2009). Lastly, even though high-pressure systems might be severe at times, they also responsible for giving people warm weather and bright day.

Low-pressure systems

Low-pressure systems are an area of rising air in which the air inside is at a lower atmospheric pressure than the surrounding air. Low-pressure systems are also known as cyclones; they have converging air at the surface and diverging air aloft. In essence, this air movement creates a vacuum effect causing a drop in pressure in a low-pressure system as shown in Figure 8. The formation of a low-pressure system also called cyclogenesis. Cyclogenesis happens when the cold surface air rises after it is warmed. The warming of air is associated with the latent heat of thunderstorm activities or when high water temperatures warm the air in the atmosphere (Ahrens, 2013). Low-pressure systems will have winds that move from the edges of the system inward and converges as the surface is pulled aloft. This air would move in straight lines towards the center of the system, however, due to the effects of the Coriolis effect, it is reflected in the direction of the earth’s rotation. Implying that in the northern hemisphere the winds will move counterclockwise as seen in Figure 9 and the southern hemisphere, the wind will to lead in the opposite direction.

One type of low-pressure system that does not have rotation is the Monsoons. Monsoons from along the ITCZ where the Coriolis effect is too weak to start a rotation (Ahrens, 2013). Low-pressure systems bring with them cold stormy weather with heavy precipitation and high winds. The main cause of this weather is the adiabatic cooling of warm moist air as it rises. This cooling causes the air to condense and form clouds causing precipitation to occur. The cloud coverage also means that low-pressure systems will have cooler days because less sun heat can reach the earth’s surface and warmer nights because the cloud coverage traps heat from the day. However, low-pressure systems have been known to cause some of our most severe storms such as hurricanes, monsoons, and blizzards.

Part Two

Wind Systems of the World

There are primary circulation cells in the different parts of the globe which cause large weather systems. Various parts of the earth have specific weather systems which are characterized by either the presence or absence of the prevailing winds. It should be noted that the weather systems can be categorized into five elements which include; polar easterlies, the horse latitudes, doldrums, prevailing westerlies, and trade winds. Each of these weather systems is experienced in both the northern and the southern hemisphere. Therefore, this section categorically focuses on analyzing the five systems about their location and boundary’s pressure, direction and strength of the wind, and the weather associated with each of the systems.

Doldrums

Doldrums are equatorial regions that have low pressure and are found within the Inter-Tropical Convergence Zone (ITCZ). The doldrums are characterized by lack of winds and presence of long calms. In this regard, these regions acquired the name doldrums from the early sailors who could wait for sailing winds to fill their ships to facilitate their movement. The primary cause of low pressure and long calms in the doldrums is the meeting of two major circulation cells called Hadley cells. These cells power the trade winds as shown in figure 12. At the meeting point of these two cells is where the airlifting happens leading to the low pressure and calm winds of the doldrums. On the other hand, there is unpredictable weather and violent weather in the doldrums. The weather varies from long calms, short, intense rainfall, and large thunderstorms. However, the most common weather of the doldrums are the monsoons which are large rainstorms that form in a trough of low pressure leading to the intense rainy season around the ITCZ (What are the doldrums? 2016). Doldrums have been known globally by sailors as the most dangerous and unpredictable zones to be when sailing.

Trade Winds

The trade winds are strong winds that lie between the ITCZ and the Subtropical highs on either side of the equator, roughly from the ITCZ to 28 degrees N/S. The trade winds were first charted by Portuguese sailors who gave them the name “trade winds” because they were used to help the ship move quickly along the established trade routes. The traders moved across the Atlantic and the other parts of the ocean. The early chart of the trade can be seen in Fig 13.

The trade winds are a strong easterly wind blowing from the Southeast in the southern hemisphere and northeast in the northern hemisphere. These trade winds lie within the Hadley cell in which the air rises at the equator flow to the horse latitudes and sinks moving parallel across the surface back towards the equator. It is this advecting air at the surface that combines with the westerly deflection of the Coriolis Effect that causes the trade winds. Weather in the trade-winds can be varied as they cover such a large expanse of ocean. However, one of the common weather experienced within them is a temperature inversion known as a trade wind inversion where the warmthandry air of the trades settles on top of a maritime tropical air mass causing an inversion. (“Glossary of Meteorology”, 2009) The trade-winds are one of the biggest and most well-known wind systems and have been helping sailors for years to cross the oceans.

Horse Latitudes

The Horse latitudes are an area of high pressure and calm winds that is found between 30 and 38 degrees of latitude on both sides of the equator. The naming of the horse latitudes could have many possible origins; it may be attached to the killing of horses as ships became stuck for weeks in the calm winds or from the term horsed in which a ship in the face of no wind would follow with the current. Although the true reason has never been found about the origin, all the arguments have to do with sailors being stuck in the calm winds. (What are the Horse Latitudes?) The horse latitudes form in an area of high pressure known as the subtropical highs where two circulation cells meet the Ferrell cell and Hadley cell. The high pressure of the horse latitudes is responsible for the major deserts of the world with all of them sitting within their band; these desserts can be seen in Figure 14. Weather of the horse latitudes has similar characteristic to that of other high-pressure systems such as hot and dry with calm winds and large anticyclone formations. Like the doldrums, the horse latitudes area also posed a great problem to the early sailors.

Westerlies

The westerlies are strong winds that lie between the Subtropical highs and Polar lows, approximately between 30 to 60 degrees of latitude on either side of the equator. The westerlies lie within the Ferrell circulation characterized by sinking air at the tropics which then moves across the surface towards the poles where it rises at the polar front. The Coriolis Effect deflects this air along the surface like the trade-winds in the direction of the earth’s rotation. The westerlies blow from the southwest in the northern hemisphere whereas in the southern hemisphere from the northwest. The winds of the westerlies are much stronger than those of the trade. This is because of the greater effect of the Coriolis since it is close to the pole. In the southern hemisphere that has less land mass to interrupt the wind flow of the westerlies are at their strongest, so much, so they have earned special names, such as the roaring 40’s the furious 50’s and the shrieking 60’s. These high-speed winds near the poles were useful especially during the age of sailing ships to make fast voyages around the Cape of Good Hope to Australia and the East Indies. These fast winds are still used today by sailors when circumnavigating the world. The westerlies also affect ocean currents causing those in the southern hemisphere to be stronger than the northern hemisphere and making currents on the western boundaries of ocean basins such as the Gulf Stream in a process known as western intensification. This current can be seen in Fig 16. (Ahrens, 2013) This effect is caused by the westerlies blowing in the same direction as the current hence amplifying it. Weather in the westerlies like in the trade winds varies greatly. They are responsible for the recurving of storms that pass into them, causing these storms to change direction forming a curve. An example of this effect can be seen in Fig 15. They help feed mid-latitude cyclones a low-pressure system which can bring with them rain and thunderstorms. The westerlies are some of the fastest winds that have helped sailors for years quickly circumnavigate the world.

Polar Easterlies

The easterlies are irregular light winds located from 60 degrees north and south latitude to the north and south poles. The easterlies lie within the polar cell, where high pressure sinking air is found. Like previous winds, the flowing air is deflected by the Coriolis effect in the direction of the earth’s rotation implying that in the northern hemisphere, the easterlies come from the northeast while in the southern hemisphere, they come from the south-east. The weather of the easterlies is mostly cold and dry at the poles with rain experienced in areas where the easterlies terminate with the low-pressure air of the polar front. The easterlies are also responsible for bringing down cold Cp and Ca air masses into lower latitudes bringing with them extremely cold weather. Of all the winds of the Easterlies are considered the weakest low-pressure, and during the age of sail, they were unreliable to be of any use to sailors.