"What is clear is that the atmosphere is a continuous mass resting on the earth and the sea, and that these two react upon each other. Any disturbance which appears at any one point must make itself felt at very considerable distances from that point. We shall often have to seek for the cause of a certain phenomenon in another which has taken place perhaps in another hemisphere … we have found interesting simultaneous relations between the barometrical pressure and the rain at different centers of action.-Hugo Hildebrandsson (1838-1925)
Although most of Earth is made of rocks and metals, these materials do not have as much effect on our daily lives as the air and the water around us. The weather determines what you wear each day, whether you stay inside or go out and sit in the sun at lunchtime, how much you will pay for heat this winter. Not only that, weather gives us something to talk about during those awkward silences.
Temperature and precipitation, the two main components of weather, are part of the movement of energy around the planet. Energy from the sun is absorbed and converted to heat. This heat sets wind and water into motion, redistributing energy from warm places to cool places—and making the weather. A worldwide system of wind and water currents has been described as a conveyor belt, transferring energy from one place to another.
Why does the jet stream flow from west to east?
Next time you fly between the coasts, check the schedule of your flights. It is very likely that the east-west flight is at least an hour longer than the west-east flight. The reason is that the flights get into the jet stream. Going westward, the plane is fighting a strong headwind; going eastward, it is pushed along by the same wind—the jet stream. What is the jet stream and why does it blow from west to east?
There are actually four jet streams in Earth’s upper atmosphere, two in the Northern Hemisphere and two in the Southern Hemisphere. The jet streams are like broad rivers of air that move at a higher velocity than the air around them. These air flows are caused by a combination of differences in solar heating at different latitudes and the rotation of the planet.
The strongest jet streams occur at roughly 50 to 60° north and south of the equator (in the Northern Hemisphere this is around the U.S.-Canada border) and a weaker jet stream at roughly 30° from the equator (about the U.S.-Mexico border).
Because of Earth’s tilt, the amount of heating of the atmosphere, ocean, and land decreases with distance from the equator. The air that is warmed in lower latitudes tends to rise in the atmosphere (here’s that connection between temperature and density again). It then moves northward as cooler air moves from areas closer to the poles. The jet streams occur where warm and cool air masses come together.
Jet streams form where the warm and cool air masses meet in the upper atmosphere. Their average speed is about 90 mph, but they can exceed 300 mph. The winds are strongest in the winter when the temperature difference between the equator and polar regions is the greatest. Although their channels can extend for thousands of miles in an east-west direction, the jet streams are generally only a few hundred miles wide (north-south) and they extend from about 4 miles to about 8 miles above the surface. They do not run in a constant channel but can extend far north or south of their average location.
Jet streams are an important part of the wind system that distributes solar energy on Earth. They tend to push weather systems around, having a great effect on local weather. Jet streams are also believed to play an important role in the paths followed by hurricanes and other tropical cyclones.
Despite the observations of Lisa Simpson, the Coriolis effect does not determine the direction of the whirlpool in a sink. While it does play a role in tropical cyclones and large ocean currents, the effect is much too small to be noticeable on the scale of the drain. The direction of the drain whirlpool is determined instead by the shape of the basin and by currents in the water, and it is equally likely to be clockwise or counterclockwise in either the Northern or Southern Hemisphere.
Why is it usually cooler at the top of a mountain than at the bottom?
When you think of the tops of tall mountains, you tend to think cold. Even mountains near the equator and mountains that rise from bases in hot deserts can have a year-round cap of ice and snow. Why are mountain tops so cold?
Remember that heat is transferred from one material to another by collisions between atoms and molecules. In air, nitrogen and oxygen molecules are in constant motion, colliding with one another and with surfaces with which they make contact. If the molecules are very energetic and the collisions are very frequent, the air feels warm. On the other hand, if the molecules have less energy and there are fewer collisions, the air is cooler.
Death Valley in California, with an elevation of about 300 feet below sea level, is the hottest place in the Western Hemisphere. The average summer high temperature is 98°F. Meanwhile, at the 14,497-foot summit of nearby Mount Whitney, hikers have to cross snow and ice, even i n August, to reach the summit.
At the base of a mountain, the air is heated as the ground absorbs solar energy as light and radiates it into the air as infrared radiation. Air molecules absorb this radiation, gain energy, and move faster. The temperature rises. As the air becomes warmer, it becomes less dense and rises.
However, the pressure of the atmosphere decreases as the elevation increases. That is because there is less air above it pushing down on it and compressing it. As air rises in the atmosphere, its pressure decreases, and according to the gas laws, so does its temperature. As you climb a mountain, the temperature, on average, drops about 5 to 6°F for each thousand feet of elevation change. Of course, that means that temperature increases as elevation decreases. If you descend into the Grand Canyon, you will find that it is 20 to 30°F hotter at the bottom than at the top.
Why do some clouds look white and others look gray?
Clouds are made of drops of water or crystals of ice that form when water vapor cools and condenses in air that rises from the surface. Clouds do not always have the same appearance when you look at the sky. Some clouds are thin, white wisps; others form a gray layer that covers the sky from one horizon to the other; in between, there are the big, fluffy puffs that are often white on top and gray on the bottom. Why do some clouds appear white while others appear gray?
As water evaporates from the surface of rivers, lakes, oceans, and even the soil, water molecules mix with the nitrogen and oxygen of the air. When a mass of air is heated by the sun or has contact with the warm ground, it begins to rise above the denser, cooler air around and above it. The air mass rises, expanding with the dropping pressure, and the entire mass cools. As the water molecules lose energy, they begin to condense and form tiny droplets or crystals.
The water droplets and ice crystals in a cloud reflect and scatter the sunlight that strikes them. The scattered light looks white, which is why the fluffy clouds that spread across the sky appear white. In fact, all clouds look white in the daytime when you look down on them from an airplane due to the refection and scattering of the light.
Fast Facts
Cloud seeding is the process of spraying tiny particles of silver iodide in the upper part of a cloud to induce rain. The idea is that ice crystals grow around a particle, such as dust (or silver iodide) and then fall, melting to form rain. Unfortunately, even if it does rain, it is hard to be certain that it would not have done so without the seed.
The amount of light that passes through a cloud depends on the density of the water droplets in it. High, wispy cirrus clouds contain very little water, so most sunlight passes through them. In some clouds, however, such as the nimbus clouds often associated with thunderstorms, the density of water droplets is much higher. In addition, from top to bottom, these clouds are usually much thicker. As a result, they reflect much more sunlight upward, away from the observer on the ground. The thicker the cloud and the more condensed water it holds, the darker the cloud appears. That is why the darkest clouds are usually associated with rain.
Why don’t hurricanes form near the North and South Poles?
The 2005 hurricane season was one of the worst on record. Altogether, there were 28 tropical storms that year. Four of them topped the scale of hurricane strength at category 5, including the devastating hurricanes Katrina and Rita. With just a few exceptions, these storms started in a band of the Atlantic Ocean between the latitudes 10°N and 30°N, roughly the northernmost extent of South America and the boundary between Florida and Georgia. Why don’t hurricanes form farther north?
A tropical cyclone is an intense circular storm that originates over warm tropical oceans. Cyclones demonstrate low atmospheric pressure, high winds, and heavy rain. Some tropical cyclones are called hurricanes or typhoons, depending on where they originate.
Hurricanes are tropical cyclones, which form over the warm waters of the Atlantic Ocean or the eastern part of the Pacific Ocean. Almost all hurricanes start out in a band of water within about 2,000 miles of the equator because that is where the specific conditions exist that lead to hurricane formation. Cyclones also form in the Pacific and Indian oceans. They are not called hurricanes, but they are the same type of storm.
The main ingredients that go into the birth of a hurricane are solar energy and the rotation of Earth. During the summer, the ocean waters of the tropical and subtropical oceans absorb a lot of energy from the sun. This energy heats the water near the surface of the ocean to temperatures of 80°F or more. Cyclones form only in areas where the top 150 feet or so of the water is heated to at least 80°F. As the molecules of water gain energy, more and more of them escape from the water’s surface to become water vapor in the atmosphere above the water.
As this warm air rises, it begins to cool and condenses to form clouds. The condensation process releases a lot of energy, heating the air again and increasing its pressure. As the higher-pressure air moves outward, winds begin to build. Wind can move out in any direction. Because of the motion of the rotating Earth, air that is flowing north tends to be deflected eastward. The result is a counterclockwise circular motion of the air around the center of evaporation and condensation. The Coriolis effect causes a clockwise motion in the Southern Hemisphere.
As evaporation and condensation continue, a wind pattern develops, forming a circular flow around a cylinder of cooling air. As long as the wind pattern is over warm water and no other winds disrupt their flow, the cycle builds as more energy is added and the wind spirals faster and faster. When the wind speed reaches 74 mph, the storm is considered to be a cyclone. These storms reach at least 50,000 feet into the atmosphere and can be hundreds of miles across.
As long as the hurricane is above warm water, the cycle of evaporation and condensation can continue to feed energy into the moving air. Hurricanes rapidly lose strength when they move over cool water or land.
Much of the damage of a major hurricane that hits land is caused by the storm surge. Although a storm surge is sometimes portrayed in movies as a giant cresting wave, slamming into tall buildings and knocking them down, in reality, the storm surge is a slow, building phenomenon. Winds blow the water against the shore, where it piles up and overruns the land like a huge tide. The water can rise more than 20 feet during the storm surge, stranding ships as much as a mile inland.
What is the "wind chill factor" that the meteorologist discusses on the TV weather report?
As you watch the weather forecast on a cold, windy day, the announcer warns you to bundle up because the wind chill temperature is -5°F. However, when you look at the thermometer, you see that the temperature is actually +20°F. Why are the two temperatures so different?
The wind chill factor is a measure of the effect of the apparent temperature felt by exposed skin. The principle behind it is that moving air carries heat away more effectively than still air. Whenever the temperature of the air is less than your body temperature, heat radiates away from your exposed skin. However, if the air is not moving, the heated air tends to remain close and provides some insulation.
The wind chill factor is not a measure of actual temperature, so it does not affect the measured temperature of the air itself or of objects such as water, ice, or a thermometer. It only applies to radiating bodies such as people and animals.
A wind chill temperature (WCT) of -5°F means that your body experiences the same rate of heat loss as it would in still air at -5°F. If the WCT is -18°F or lower, exposed skin can experience frostbite in less than 30 minutes.
The concept of wind chill was first developed in the 1940s. In 2001, the National Weather Service issued an improved method for determining WCT based on advances in science, technology, and computer modeling that have improved the ability to measure the effects of temperature and wind. The complex formula used today is based on a model of the human face and skin interactions.
National Weather Service Wind Chill Values
|
Temperature |
||||||
|
Wind (mph) |
30 |
20 |
10 |
0 |
-10 |
-20 |
|
5 |
25 |
13 |
1 |
-11 |
-22 |
-34 |
|
10 |
21 |
9 |
-4 |
-16 |
-28 |
-41 |
|
15 |
19 |
6 |
-7 |
-19 |
-32 |
-45 |
|
20 |
17 |
4 |
-9 |
-22 |
-35 |
-48 |
|
25 |
16 |
3 |
-11 |
-24 |
-37 |
-58 |
|
30 |
15 |
1 |
-12 |
-26 |
-39 |
-53 |
|
35 |
14 |
0 |
-14 |
-27 |
-41 |
-55 |
|
frostbite in 30 minutes |
frostbite in 10 minutes |
Fast Facts
The wind chill factor applies only in cold weather. A related concept used in hot weather is the heat index, which combines air temperature and relative humidity to determine an apparent temperature. High humidity decreases the rate of cooling by evaporation of sweat, so humid air feels hotter than dry air. The heat index is only used when the actual temperature is greater than 68°F.
Why is there often a strong wind just before a thunderstorm?
An oncoming thunderstorm often announces its approach. Rumbles of distant thunder may echo while the sky still looks clear. Then dark clouds build, towering high above you, with flashes of lightning. A very strong, cool wind often rushes just ahead of the storm, sometimes having enough force to knock down trees. How does this wind form?
A downburst is a strong wind that blows in all directions from a storm. In rare cases, downbursts have tornado-strength winds, up to 150 mph, and produce damage that is similar to tornados. However, the wind from a downburst differs from a tornado because it blows in one direction and it has a different cause.
A downburst is caused by the storm itself, as air flows downward from an area inside the storm.
In general, a downburst forms when there is an area of relative dry air within the storm. As rain falls into this air from above, it begins to evaporate rapidly. Evaporation is an endothermic process, which means that it absorbs energy, in this case from the air around the evaporating water. As the energy is absorbed, the air cools, making it denser. The density of the cold air causes it to descend toward the ground. If the air mass falls very quickly, it is diverted as it hits the ground, causing a strong wind that blows in every direction from the base of the storm.
Most downbursts extend less than 2V2 miles, creating a strong wind that lasts for only a few minutes. Because the downburst extends in every direction, it can also be experienced as a wind blowing away from the storm after it has passed. They are particularly dangerous in aviation due to the downward component, known as wind shear, that can cause an airplane to lose altitude very rapidly.
"Part of the success of the invasion of the French Coast [in WWII] came by virtue of the fact that the weather forecast for that event, made by the American forces, was so inconceivably bad that the German meteorological experts, who were substantially better, simply couldn’t believe that we would be so stupid as to make so bad a forecast, and could not believe that we would act upon it, and therefore could not believe that the invasion would occur at the time when it actually did. So, rather curiously, we profited by the bad state of our meteorology at t hat moment."
Why is the ocean salty while many lakes are not?
Take a mouthful of water while swimming in the ocean and you know right away that there is a difference between ocean water and the water you find in rivers and most lakes. Ocean water has a strong salty taste. If you don’t rinse your swimsuit, it dries with a white crust of salt. Why is the ocean so salty when most other water is not?
Salts are compounds consisting of ions of a metal element and ions of a nonmetal element or a combination of elements. On average, about 3.5 percent of the weight of ocean water is salt.
Most of the salt in the ocean is sodium chloride, familiar as table salt, but altogether there are at least 72 elements found in seawater, most of them at very low concentrations.
The salinity of the ocean occurs because of a combination of water’s ability to form solutions of salt compounds and the cycling of water into and out of the atmosphere.
Salinity is a measure of the quantity of dissolved salts in water, generally expressed in parts per thousand (ppth: 1 part per thousand = 0.1 percent). Fresh water has a salinity that is less than 5 ppth; brackish water 5 to 29 ppth. On average, the salinity of the oceans is about 35 ppth. The salinity of the Dead Sea is about 315 ppth (31.5 percent salt).
When the oceans first formed, most likely during ancient volcanic eruptions releasing water from inside the planet, they were not nearly as salty as they are today. However, the atmosphere at that time, billions of years ago, contained a mixture of emissions from volcanoes, nasty things like hydrogen chloride (hydrochloric acid), sulfur dioxide, and hydrogen bromide. These chemicals all dissolve in water very rapidly.
Other materials were components of rock that dissolved in the acidic mixture after it fell from the sky. These materials include sodium, which combines with chloride ions to form sodium chloride. Other elements extracted from rocks and minerals include magnesium, calcium, and potassium.
Over time the ocean has continued to get more and more salty. Most of the salts that reach the oceans remain there while water constantly evaporates and recycles as rain. Rainwater dissolves more salts from minerals on land and carries them to the sea. Although the freshwater of rivers carries a low concentration of salt compared to the water of the sea, this salt then remains in the ocean, adding a bit more salinity.
Why are lakes less salty than oceans? Actually, while it’s true that the salinity of most lakes is less than that of the oceans, there are exceptions. The concentration of salt in the Great Salt Lake in Utah, for example, varies between 5 percent to more than 20 percent compared to the average of 3.5 percent for the ocean. The salinity of Lake Superior is much less than 0.01 percent, typical for many freshwater lakes. This low value can be explained by the flow of water through the lake and into Lake Huron. On average, the water in Lake Superior is replaced over a period of 200 years, so the salt does not accumulate over time.
If the salt could be removed from the oceans and spread across the land surface of Earth, it would form a layer more than 500 feet thick, almost half the height of the Empire State Building.
Why does Seattle get twice as much precipitation as Spokane?
In an average year, the city of Seattle, Washington, receives about 38 inches of precipitation. The annual precipitation in Spokane, about 250 miles to the east of Seattle, however, is only about 17 inches. Why is the amount of precipitation so different in two cities that are so close together?
If you drive between the cities, or even look at a good map of the state, you will find a prominent feature between Seattle and Spokane—the Cascade Mountains. This mountain range runs in a north-south direction through the entire state. The mountains also provide a divider between areas of high precipitation and areas of low precipitation.
The prevailing winds across North America blow from west to east. The air reaching Seattle contains a lot of water vapor due to evaporation of water from the Pacific Ocean. As a result, the climate is relatively rainy. However, as this moist air reaches the Cascade Mountains and is forced upward, it expands due to lower pressure and its temperature drops. (If this seems to be a familiar theme, that’s because it has such a major impact on the weather.) At the lower temperature, water condenses to form rain. The western slopes of these mountains tend to receive a lot of rain.
As the air passes beyond the mountains, it carries with it much less water vapor than it had before. In fact, as the cool, dry air drops closer to the ground, it warms, and water that is already in the ground tends to evaporate into the air, making the area even more arid.
There is a good chance that in a science class, or even on a TV weather report, you have been told that it rains when air cools because cold air "holds less water" than warm air. Some textbooks have even shown the atmosphere as a sponge being squeezed to cause rain. This is not true because air does not "hold" water at all. It is not even correct to speak of air being saturated.
In reality, water in the atmosphere is constantly condensing and evaporating. When the temperature of the water vapor drops, the energy of the molecules decreases so there is more condensation than evaporation and water droplets form. The air itself does not play any part in the process besides carrying the molecules of water along with it.
Why can palm trees grow in Dublin but not in Portland?
The city of Dublin in Ireland is much farther north than Portland, Maine. Both cities are close to the Atlantic Ocean, yet Dublin’s climate is warmer. In fact, the average low temperature in January in Dublin is 38°F and in Portland the average is 9°F. It is even possible to grow palm trees in Dublin. Don’t bother trying that anywhere in Maine. Why are the climates of the two cities so different?
As strange as it may seem, the difference is attributed to the warm waters of the Gulf of Mexico. The Gulf Stream is an ocean surface current, sort of like a river in the ocean. A constant flow of water loops through the Gulf. A map of the ocean surface temperatures shows warm water, heated by the sun in the fairly shallow Gulf of Mexico, flowing out of the gulf and around the tip of Florida. Because this water is warmer than the water below it, it stays near the surface of the ocean.
The Gulf Stream current carries the water along the eastern coast of North America and it even passes Portland, Maine. However, the wind generally blows from west to east, so the warm waters off the coast do not affect the climate of Portland.
After leaving the coast of North America near Newfoundland, this still-warm water flows into the Atlantic Ocean and becomes the North Atlantic current, which flows near the western coasts of Ireland and Scotland. Here, however, it can affect the climate.
The winds blowing over the North Atlantic also tend to move from west to east, so they blow right across the surface of this warm current. The air is heated and moistened, carrying warmer, wetter weather to Dublin.
The Gulf Stream carries about 100 times as much water as all the rivers on Earth combined, flowing at speeds as high as 75 miles per day. The first person to describe and map the Gulf Stream was Benjamin Franklin, who was trying to explain why it took ships longer to travel from Europe to America than to travel from America to Europe.
The Gulf Stream is part of a system of ocean currents that surrounds the globe. Warm water flows across the top of the ocean in surface currents. In the polar regions, however, water loses energy to the colder atmosphere. As the fluid seawater cools, it becomes denser and sinks toward the bottom of the sea. Cold-water currents deep in the ocean cycle water from the poles toward the equator. Where these currents come back to the surface, they bring cool, nutrient-rich waters from the deep ocean. One example of this phenomenon occurs off the coast of northern California. Unlike the waters of the Gulf of Mexico, which can reach 90°F in shallow areas, the Pacific Ocean waters off the shore of northern California remain at a chilly 50°F or so year-round.
Do you get wetter by walking or running in the rain?
Our final weather question is an old one. You are getting ready to go across the parking lot to your car, when a sudden spring shower pops up. You can walk to the car or you can run (you only have a small purchase to carry). Which tactic is best for reaching the car with minimal drenching?
In a thoroughly unscientific study of a parking lot during a rain, you will probably find that most people run for the car. Is that, however, the best option? If you walk you are in the rain longer, so it would seem that you get wetter. However, if you run, you collide with raindrops more often, so you may get wetter by running.
A British study in 1995 determined that the worst option (in terms of getting wet) is standing still in the rain, but that neither walking nor running offers a significant advantage. However, two meteorologists at the National Climactic Data Center in North Carolina—Thomas Peterson and Trevor Wallis—were a bit skeptical. For one thing, the study was a mathematical exercise, not a real experiment, and besides, their own calculations gave a different result.
The only way to be sure was to do an actual experiment. On a rainy day, the two scientists dressed in identical clothing and went out to answer the question definitively. One of them dashed 100 meters in heavy rain, while the other took his time, walking to the finish line. Weighing the clothes afterward, they found that the runner had absorbed 40 percent less water than the walker. So the question is now answered. As any child heading from the school bus seems to know instinctively, you don’t get as wet if you make a fast dash for the door.
"Unfortunately no one can tell you with any scientific basis whether New Orleans will be hit this year, and if they are what strength the storm or hurricane would be. The best I can tell you is that New Orleans has always been an accident waiting to happen when it comes to hurricanes and flooding rains. This year is no different than in past years."
