Convergence Of The Easterlies
Near the equator, the belts of tropical easterly winds meet. This is the intertropical convergence zone ITCZ , which was introduced in Chapter 2. It is a region of low pressure and light winds. Thundershowers often form in this zone. In ancient times, this region was called the doldrums, because sailing ships passing through it on their way from the northern hemisphere to the southern, or from the southern hemisphere to the northern, were often becalmed for lack of wind. In the Atlantic, the...
Axial Tilt
The seasons are caused mainly by the tilt of the earth's axis. The earth's orbit lies in a single plane, known as the ecliptic plane. The planetary axis is not perpendicular to the ecliptic plane. If that were the case, the sun would always be above the horizon for 12 hours each day and below the horizon for 12 hours each night, except at the poles, where the sun would follow the horizon, making a complete circle around the compass every 24 hours. In all the inhabited parts of the world, the...
Anatomy of a Hurricane
Although hurricanes are low-pressure systems, they differ from the lows of the temperate latitudes. The hurricane is more symmetrical than the cyclonic storms familiar to inhabitants of Europe, the interior United States, and other temperate regions. The hurricane, in its tropical stage, does not contain frontal systems. The isobars are almost perfect circles, especially in a well-developed storm Fig. 6-4 . The central pressure is usually lower and sometimes much lower than that of a cyclone in...
Radar
The term radar is an acronym derived from the words radio detection and ranging. Electromagnetic EM waves, having frequencies in the ultra-high or microwave range, reflect from certain types of objects and particles, including aircraft, missiles, raindrops, ice pellets, and snowflakes. By ascertaining the direction s from which radio signals are returned, and by measuring the time it takes for an EM pulse to travel from the transmitter location to a target and back, it is possible to locate...
Orbital Eccentricity
In addition to the foregoing, the eccentricity of the earth's orbit changes with time. Sometimes, as now, the orbit is nearly a perfect circle. But at some times in the past, the earth's orbit has been more elongated, far from being a perfect circle. This will occur again at intervals in the future. When the earth's orbit is the most elongated, the difference between perihelion and aphelion is such that the amount of solar energy reaching the surface varies by about 25 instead of the current 6...
Spring Winds
March, like November, is famous for its wind. Winter begins to lose its grip, and people look forward to sunny skies and gentle breezes. The skies are relatively sunny in March in many parts of the world, but the breezes aren't always gentle. The jet stream begins its retreat in earnest by the end of February, and brutal cold waves are not as likely or as frequent as before. It seems that everywhere in the United States, from Maine to California, and from Seattle to Miami, March is a windy...
Blizzard Watches And Warnings
Meteorologists issue winter storm watches and winter storm warnings in much the same way as they do for tropical storms, hurricanes, severe thunderstorms, and tornadoes. If a hurricane, severe thunderstorm, or tornado watch is posted, the forecasters do not mean to imply that any specific location will be directly hit. The situation is different in the case of a winter storm. Winter storms are much larger than any other type of storm in terms of the sheer geographic areas they strike. A winter...
Hurricane Breeding Zones
Hurricanes form just north or south of the equator in many parts of the world Fig. 6-3 . At the equator, there is no Coriolis force, and the big whirlwinds cannot develop the latitude must normally be greater than 5 either north or south. Hurricanes almost always mature in the tropics, but a few mature in the temperate zones. The key is ocean temperature, which must be above approximately 27 C 80 F in order for a storm to reach hurricane strength. Hurricanes that threaten the eastern United...
The Weather Stick
Long before Europeans came to North America, Native Americans in the region now known as New England discovered that the branches of certain trees bent in one direction in fair weather, and in the opposite direction in foul weather. These branches, measuring about 30 cm 1 ft long, were de-barked, mounted in plain sight, and used as weather-forecasting instruments. They can still be obtained today under names such as Maine Weather Stick. You can find some of these for sale on the Internet by...
Birds Roost Before A Storm
The notion that birds roost sit around and don't fly much when a storm is approaching has a sound basis, assuming birds can sense weather changes. During times of adverse atmospheric conditions, the informed small-plane pilot stays on the ground. Before a storm comes in from the sea, small craft remain in port. Do birds really have a built-in sense of the weather to come Many people, especially rural folks, will tell you they do. If they do, then they apparently heed it. The next time a big...
Ingredients Of A Blizzard
A cyclonic bend in the jet stream a turn from right to left in the northern hemisphere can cause winter storm systems to intensify to the point where blizzard conditions develop. The counterclockwise flow of air enhances the circulation of a low-pressure system, increasing the speed of the winds. Also, a cyclonic bend is likely to be accompanied by atmospheric divergence, which causes the barometric pressure to drop more than it otherwise would. Cyclonic bends in the jet stream are common over...
Cloud Seeding
Two substances have occasionally been dropped or hurled into clouds in the hope of causing precipitation to fall. Dry ice and silver iodide, in powder form, have been sprayed from airplanes or fired from ground-based cannons. Dry ice chills the air, and silver iodide provides nuclei on which water can condense or freeze. This activity is called cloud seeding. It is done in some areas in the hope of increasing rainfall to alleviate drought, and in other places in an attempt to increase snowfall...
The Effects Of Human Activity
During the 20th century, the 1930s drought was much worse than any of the others. The reason is not certain, but perhaps improper farming techniques had something to do with it. People found out that New England farming methods did not work in the Great Plains. When the soil out west was plowed, it dried out during the summer months, and the almost constant winds blew it away. After the 1930s, trees were planted as windbreaks, and much of the land was irrigated. Droughts are correlated with...
Santa Ana Winds
In the south or southwest portion of a wintertime high-pressure system in the western United States, a Chinook-like effect takes place over the mountains in southern California. The primary difference is that the wind is warmer and drier because of the more southerly latitude. As the wind descends into the valleys near the coast, the speed and temperature increase. Gusts of 70 kt 80 mi h are not unusual in the Santa Ana area, and for this reason, the wind has been called the Santa Ana wind. A...
Clouds Warn Of Storms
Certain clouds, or combinations of clouds, indicate the approach of foul or severe weather. Other clouds reassure us that the weather will stay fine for awhile. You can usually get a good idea of what to expect, simply by observing clouds. Small, puffy cumulus clouds, with little or no clouds at higher altitudes, generally mean that the weather will remain fair for the next day or so. If the clouds begin to build up during the late morning or early afternoon, however, you should take your...
Are Droughts Cyclic
Temporary droughts, such as the one that took place during the 1930s, occur when the high-pressure belt is closer to the pole than normal. The conditions that produce a temporary drought are the same as those that cause a heat wave a persistent ridge in the jet stream over a continent. Then, in the northern hemisphere, the desert belts shift or expand northward. Summer rainfall decreases while the temperature increases. There were other droughts during the 20th century besides the dust bowl of...
What Qualifies As A Blizzard
Meteorologists define blizzards according to the temperature and the wind speed. Blizzards are characterized by blowing and drifting snow, limited visibility, and cold temperatures. When the temperature drops below -7 C 25 F , accompanied by a wind of 30 kt 35 mi h or more, along with blowing snow, blizzard conditions exist. If the temperature falls to less than -12 C 10 F with winds of 39 kt 45 mi h or more, severe blizzard conditions exist. Although the meteorologist does not rigorously...
The Fahrenheit Scale
In much of the English-speaking world, and especially in the United States of America, the Fahrenheit temperature scale F is used by lay people. A Fahrenheit degree is the same size as a Rankine degree. However, the scale is situated differently. The melting temperature of pure water ice at sea level is 32 F, and the boiling point of pure liquid water is 212 F. Therefore, a temperature of 32 F corresponds to 0 C, and 212 F corresponds to 100 C. Absolute zero is represented by a reading of...
Waves Depressions And Storms
All hurricanes begin as tropical waves. These are first observed as irregularities, or bumps, in the tropical isobars Fig. 6-2 . Showers and thunderstorms develop on the eastern, or trailing, side of the disturbance. Tropical waves, also called easterly waves, are common during the summer months, and they usually move from east to west without intensifying. But when conditions are favorable for intensification, an easterly wave can develop a cyclonic circulation. When some of the isobars become...
Tropical And Subtropical Tornadoes
Large, violent tornadoes do not take place in the tropics and subtopics as often as they occur at temperate latitudes. This is mainly because the battles between warm and cold air masses are less common, and generally less intense, in the tropics and subtropics as compared with the temperate zones, especially over North America. The atmosphere is comparatively free of frontal cyclones at low latitudes, particularly in the summer. Small and moderate tornadoes are common in the tropics and...
Chinook Winds
Strong, cold westerly or northwesterly winds, after passing over the mountains at a high altitude, become warmer as they descend into valleys and onto the foothills or prairies. The winds can get gusty and, once in a while, even violent. Fig. 7-9. A persistent, strong high-pressure system can cause Chinook and Santa Ana winds in the deserts and mountains of the western United States. Fig. 7-9. A persistent, strong high-pressure system can cause Chinook and Santa Ana winds in the deserts and...
Lightning Rods
If you have ever taken a drive on a back road in the Great Plains, you have seen lightning rods. A lightning rod is a metal conductor, usually steel, erected verti- cally on top of a building or house. The lightning rod is electrically connected to an earth ground by a heavy wire. When a lightning rod is installed, the conductor running to the ground should be as heavy as possible, preferably American Wire Gauge AWG No. 4 or larger. Copper tubing is also good. The conductor should follow the...
Cloud To Ground
A common type of lightning discharge between a cloud and the earth occurs as a flow of electrons from the base of a thundershower to the surface. The base of the cloud carries a negative charge, inducing a localized positive voltage on the Fig. 4-7. Four types of lightning stroke intracloud A , cloud-to-ground B , intercloud C , and ground-to-cloud D . Fig. 4-7. Four types of lightning stroke intracloud A , cloud-to-ground B , intercloud C , and ground-to-cloud D . surface beneath at B in Fig....
Quiz Pyj
This is an open book quiz. You may refer to the text in this chapter. A good score is 8 correct. Answers are in the back of the book. 1. Silver iodide is a chemical that is believed to increase the likelihood of a rain or snow, when it is scattered in clouds. b global warming, if it accumulates in the atmosphere. c global cooling, if it accumulated in the oceans. d the formation of amino acids in bodies of water. 2. Which of the following statements a , b , or c , if any, is false a The earth,...
Layers Of The Atmosphere
The lowest layer of the atmosphere, rising from the surface to approximately 18 km 11 mi altitude, is the troposphere. This is where all weather occurs most of the clouds are found here. In the upper parts of the troposphere, high-speed rivers of air travel around the planet. The strongest of these rivers, called jet streams, blow in a generally west-to-east direction between 30 degrees north latitude 30 N and 60 degrees north latitude 60 N , and between 30 degrees south latitude 30 S and 60...
Symbols In General
The temperature data on a weather map gives an indication of the locations of fronts, where air masses having different characteristics come together. Most severe weather occurs near fronts. The isobars are kinked along the line of an intense or fast-moving front. Stormy weather tends to occur near these zones. Weather fronts are plotted as lines with bumps or barbs that indicate the type of front and the direction of movement. There are four major types of fronts. In a cold front, a cold air...
Thunder
When a return stroke occurs in a lightning bolt, the air is heated almost instantaneously to a temperature of several thousand degrees Celsius. The heating causes the air to expand with great force, generating a shock wave that we hear as thunder. The sound can be heard for varying distances, depending on the wind direction and speed, the presence or absence of favorable sound propagation conditions, and the amount of background noise. Usually, thunder is audible up to about 8 km 5 mi from the...
Basic Observation Tools
These days, we have precision instruments that can measure parameters such as temperature and humidity. Some of these devices operate on principles similar to those of the Maine Weather Stick. Others are more sophisticated. Here are some examples of devices that can measure temperature, humidity, barometric pressure, and other variables. All of these should be included in a home weather station, if you're interested in observing the weather, recording daily conditions, or conjuring up your own...
Gases In Outer Space
Outer space was once believed to be a perfect vacuum. But this is not the case. There is plenty of gaseous matter out there, and much of it is hydrogen and helium. There are also trace amounts of heavier gases, and plenty of solid rocks and ice chunks as well. All the atoms in outer space interact gravitationally with all the others. The motion of atoms in outer space is almost random, but not quite. The slightest perturbation in the randomness of the motion gives gravitation a chance to cause...
Factors That Affect Hurricane Tracks
Upon analysis of past hurricane paths and their relationship to the surrounding weather systems, a correlation can be found between hurricane tracks and the conditions in the temperate zone. When a large area of low pressure exists to the west of a hurricane, the storm will at first be drawn toward this low-pressure region. Then the hurricane will be steered around the eastern edge of the low, where the winds come from the south. The hurricane moves northward into the belt of prevailing...
Easterlies And Westerlies
Between the equator and about 30 N, the air at the surface flows generally toward the equator, from a zone of relatively high pressure to a zone of relatively low pressure. The same thing happens between the equator and about 30 S. As the air reaches progressively lower latitudes that is, it gets closer to the equator , the tangential speed of the earth's surface, resulting from the rotation of the planet, increases. Therefore, air flowing toward the equator is deflected toward the west the...
The Extratropical Hurricane
The hurricane of the tropics is a symmetrical storm, having isobars that form almost perfect circles. It is somewhat more intense, and covers a slightly wider area on the side in which the wind blows in the same direction as the storm moves. In the northern hemisphere, this is the righthand half of the whirlwind in the southern hemisphere, it is the lefthand half. There are no frontal systems associated with a tropical hurricane. As a hurricane travels farther from the equator, it gets less...
The AzoresBermuda High
At all times of the year, but especially during the summer in the northern hemisphere, the prevailing winds over the North Atlantic revolve clockwise around a persistent region of high pressure. Because this system is usually centered near the Azores or the island of Bermuda, it is often called the Azores-Bermuda high, or simply the Bermuda high. This system is responsible for much of the weather that occurs along the east coast of the United States in the summer. In other parts of the world,...
The Watch
When a severe thunderstorm watch is issued, it means that conditions are favorable for the development of such storms. Severe thunderstorm watches and tornado watches are often given together, because severe thunderstorms are more likely than less intense showers or thundershowers to spawn tornadoes. Geographically, severe weather watch areas are nearly always shaped like rectangles or parallelograms. Heavy thunderstorms tend to develop along the most intense parts of a cold front. It is common...
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Tornadoproofing A Building
It is doubtful that large buildings, gymnasiums, arenas, or shopping malls can be constructed with the reinforcement necessary to resist tornadic winds. The massive roofs would be torn off by airfoil effects. Small homes might be designed to withstand F3 or perhaps even F4 winds, but special construction methods and precautions would be necessary. Experience has shown that the windows are usually the first part of a house to be damaged by high winds. They are either blown out by the force of...
Synoptic Forecasting
The term synoptic means generalized, or taking a view of the whole. Synoptic weather forecasting is done by assembling weather maps of large regions from observed and reported data at numerous stations station models . The defining tool of synoptic forecasting is the weather map. Synoptic forecasting evolved before computers were available to analyze weather data in high detail. A meteorologist might look at a sequence of weather maps showing conditions in the United States at intervals of a...
Quiz Ste
This is an open book quiz. You may refer to the text in this chapter. A good score is 8 correct. Answers are in the back of the book. 1. A looping-path tornado is especially dangerous because a it contains higher winds than other types of tornadoes. b it is larger in diameter than other types of tornadoes. c it contains multiple mesocyclones. d it can strike a specific point more than once. 2. The winds in tornadoes can blow at speeds of up to about 3. Suction vortices in a tornado system a...
The Atmospheric Capacitor
The earth is a fairly good conductor of electricity. So is the upper part of the atmosphere known as the ionosphere. The air between these two conducting regions, in the troposphere where most weather occurs and in the stratosphere, is a poor conductor of electric current. When a poor conductor is sandwiched in between two layers having better conductivity, the result is a capacitor. A capacitor has the ability to store an electrostatic charge. This produces a potential difference voltage...
Tornadoes And Microbursts
The wind damage from a hurricane results mainly from the prolonged, violent blow in the eyewall. Few hurricanes retain deadly wind speeds farther than about 160 km 100 mi inland, although this depends to a large extent on the terrain, and considerable damage can still result from sustained winds of 50 to 60 kt about 60 to 70 mi h . Hurricanes can produce tornadoes, however, and these have higher wind speeds than those in the general circulation of the eyewall. Tornadoes are most frequently...
Beneficial Effects
Lightning is not all bad. It generates nitrogen compounds that aid in the fertilization of the soil. Lightning limits the charge that builds up in the atmospheric capacitor. The lightning strokes that take place in thunderstorms are mere sparks compared to the discharge that would ultimately take place if the earth-ionosphere voltage were to rise to a high enough level. Some scientists think that lightning was a key event in the development of life on our planet. It has been suggested that the...
Thermal Inversion Cause
In stable air, the temperature decreases at a fairly uniform rate as altitude increases. This holds true for several kilometers upward. Frontal systems upset the uniformity, and this is partly responsible for the formation of rain clouds. In the vicinity of a frontal system, the temperature drops at first with increasing altitude, but when the boundary between air masses is reached, the temperature abruptly increases. As the altitude increases further, the normal pattern resumes. Fig. 4-2 is a...
Ridges And Troughs
The jet stream can change position and contour with amazing swiftness, moving north or south at any given longitude by hundreds of kilometers in a single day. But in some winter seasons, it seems to get locked in to a certain mode. A persistent winter ridge anticyclonic bend in the jet stream over the center of North America can produce a warm, dry season with little snow in the United States, but the risk of a subsequent summer drought. A persistent winter trough cyclonic bend in the jet...
Persistence Forecasting
One of the simplest methods of forecasting the weather is to assume that it will not change for a little while. This technique, called persistence forecasting, is often reliable over periods of minutes or hours. If a blizzard is blowing where you are right now, you can be pretty sure it will still be going 10 minutes from now, or 30 minutes from now. If the current barometric pressure is 29.65 inHg, you can be reasonably certain that it will be close to this value for the next few hours, unless...
Formation and Prediction
Large thunderstorms can, as we have seen, acquire a complex circulation. In a northern-hemisphere thunderstorm of this type, descending air rotates clockwise in the left-hand usually northern or northwestern part of the system, and rising air spins counterclockwise in the right-hand usually southern or southwestern part. Supercells develop a generally counterclockwise rotational circulation or mesocyclone. Mesocyclones are often associated with tornadoes. A mesocyclone can grow stronger and...
The Station Model
Today, every city has a weather station that is linked by wire, fiberoptic cable, terrestrial radio, and satellite systems to a central office. The meteorologist in Rochester, Minnesota, can find out in a matter of seconds what happening in Eureka, California. The data from a particular place includes temperature, dew- point, extent and type of cloud cover, type of precipitation if any , wind direction, wind speed, and barometric pressure. All of this information is fed into a computer, and the...
Meanders In The Jet Stream
The jet stream flows in an irregular path, not straight around the world at one latitude. An experiment can be conducted to demonstrate this effect on a small scale, using a rotating pan, ice, water, and a heating element. The pan is filled with water of a moderate temperature. A container of ice water is placed at the center of the pan, and the heating element is placed under the periphery of the pan, all the way around Fig. 2-5 . This sets up a battle between cold water and hot water....
Gases Near A Planet
Now imagine the shroud of gases that compose the atmosphere of a planet. Gravitation attracts some gas from the surrounding space. Other gases are ejected from the planet's interior during volcanic activity. Still other gases are produced by the biological activities of plants and animals, if the planet harbors life. On the earth, some gases are produced by industrial activity and by the combustion of fossil fuels. All the gases in the earth's atmosphere tend to diffuse into each other when we...
Semipermanent Pressure Regions
Near the 30 and 60 latitude lines, both in the northern hemisphere and in the southern hemisphere, the surface winds converge from, or diverge in, opposing directions. This gives the air an impetus to spin clockwise at 30 N and 60 S, counterclockwise at 60 N and at 30 S. The air also spins around the poles, clockwise in the arctic and counterclockwise in the Antarctic. The eddies near the 60th parallels tend to pull surface air inward, and the eddies near the 30th parallels and the poles tend...
Protecting Appliances
If you have had one or more lightning rods installed, do not become overconfident. If lightning strikes the rod, or even if it strikes in the neighborhood, the resulting EMP can damage electrical appliances and electronic equipment in your house. The chances of damage can be reduced by unplugging computers, Fig. 4-8. A lightning rod can provide a cone of protection within which the probability of a direct hit is low. Fig. 4-8. A lightning rod can provide a cone of protection within which the...