June 12, 2006

Basic Facts about Meteorology

By Dr. Frank J. Collazo

Introduction:

The blanket of air around the earth is called the atmosphere.  All our weather happens in the bottom layer of the atmosphere called the troposphere, which is six to ten miles thick.  Meteorology is the study of the changes in temperature, air pressure, moisture, and wind direction in the troposphere.   

The Sun Starts it All:

There is one basic reason we have weather, and that is the sun.  Weather systems start because the sun's energy heats up some parts of Earth more than others.  Most of the time the sun shines most directly on the middle of Earth, with less heating at the north and south poles.  Earth is tilted on its axis at exactly the right angle to have seasons, with different parts of Earth being heated more or less during different times of the year.   Land heats up faster than water, setting up temperature differences between oceans and continents.  This unequal heating creates variations in temperature and air pressure, winds, and ocean currents. 

Air Masses and Fronts:

The winds move heat from the tropical regions toward the poles in a never-ending effort to reach a temperature balance.   In the process, fronts set up between warm and cold air masses and a stream of fast-moving air high up in the sky called the "jet stream" forms.   Disturbances develop along the fronts and in the jet stream that trigger centers of low and high air pressure.  The fronts and low pressure areas make air rise to form clouds and precipitation.

Earth's Amazing Water:

Earth is also unique in that all three forms of water--liquid, solid and gas--exist naturally.  Heating from the sun helps evaporate water from the oceans as a source for clouds and precipitation.  The sun-powered circulations of evaporation, condensation and precipitation move Earth's water from the oceans to the atmosphere to land and back between these three forms.

What else does the Sun do?

Heating from the sun can also trigger thunderstorms.  Clusters of thunderstorms over warm ocean waters can turn into hurricanes.  The sun is behind all the changes in our weather, and if the sun were to suddenly go out, our weather machine would stop too.

Precipitation:

In cold air way up in the sky, rain clouds will often form.  Rising warm air carries water vapor high into the sky where it cools, forming water droplets around tiny bits of dust in the air. Some vapor freezes into tiny ice crystals, which attract cooled water drops.  The drops freeze to the ice crystals, forming larger crystals we call snowflakes.  When the snowflakes become heavy, they fall.  When the snowflakes meet warmer air on the way down, they melt into raindrops. In tropical climates, cloud droplets combine together around dust or sea salt particles.  They bang together and grow in size until they're heavy enough to fall.

Look at the figure above. Sometimes there is a layer of air in the clouds that is above freezing, or 32 degrees F.  Then closer to the ground the air temperature is once again below freezing. Snowflakes partially melt in the layer of warmer air, but then freeze again in the cold air near the ground. This kind of precipitation is called sleet.  It bounces when it hits the ground. 

If snowflakes completely melt in the warmer air, but temperatures are below freezing near the ground, rain may freeze on contact with the ground or the streets. This is called freezing rain, and a significant freezing rain is called an ice storm. Ice storms are extremely dangerous because the layer of ice on the streets can cause traffic accidents.  Ice can also build up on tree branches and power lines, causing them to break and our lights to go out. There is another kind of precipitation that comes from thunderstorms called hail.

Thunder and Lighting:

Have you ever seen tall, dark puffy clouds forming on a hot humid afternoon?  These are called cumulonimbus clouds, sometimes nicknamed "thunderheads."  They can actually form any time of day when the temperature falls rapidly higher up in the sky.  These tall dark clouds are full of moisture and contain strong up and down air currents. Cumulonimbus clouds may tower more than 50,000 feet, and cover from just a few square miles up to two hundred square miles.

What is Lightning?

To put it simply, lightning is electricity.  It forms in the strong up-and-down air currents inside tall dark cumulonimbus clouds as water droplets, hail, and ice crystals collide with one another.  Scientists believe that these collisions build up charges of electricity in a cloud.  The positive and negative electrical charges in the cloud separate from one another, the negative charges dropping to the lower part of the cloud and the positive charges staying ins the middle and upper parts. Positive electrical charges also build upon the ground below.  When the difference in the charges becomes large enough, a flow of electricity moves from the cloud down to the ground or from one part of the cloud to another, or from one cloud to another cloud. 

In typical lightning these are down-flowing negative charges, and when the positive charges on the ground leap upward to meet them, the jagged downward path of the negative charges suddenly lights up with a brilliant flash of light. Because of this, our eyes fool us into thinking that the lightning bolt shoots down from the cloud, when in fact the lightning travels up from the ground. In some cases, positive charges come to the ground from severe thunderstorms or from the anvil at the very top of a thunderstorm cloud. The whole process takes less than a millionth of a second.

Kinds of Lightning:

There are words to describe different kinds of lightning. Here are some of them:

In-Cloud Lightning: The most common type, it travels between positive and negative charge centers within the thunderstorm.

Cloud-to-Ground Lightning: This is lightning that reaches from a thunderstorm cloud to the ground.

Cloud-to-Cloud Lightning: A rare event, it is lightning that travels from one cloud to another.

Sheet Lightning: This is lightning within a cloud that lights up the cloud like a sheet of light.

Ribbon Lightning: This is when a cloud-to-ground flash is blown sideways by the wind, making it appear as two identical bolts side by side.

Bead Lightning: Also called "chain lightning," this is when the lightning bolt appears to be broken into fragments because of varying brightness or because parts of the bolt are covered by clouds.

Ball Lightning: Rarely seen, this is lightning in the form of a grapefruit-sized ball, which lasts only a few seconds.

Bolt from the blue: A lightning bolt from a distant thunderstorm, seeming to come out of the clear blue sky, but really from the top or edge of a thunderstorm a few miles away.

What Puts the Thunder in the Thunderstorm?

Lightning bolts are extremely hot, with temperatures of 30,000 to 50,000 degrees F. That's hotter than the surface of the sun! When the bolt suddenly heats the air around it to such an extreme, the air instantly expands, sending out a vibration or shock wave we hear as an explosion of sound. This is thunder. If you are near the stroke of lightning you’ll hear thunder as one sharp crack. When lightning is far away, thunder sounds more like a low rumble as the sound waves reflect and echo off hillsides, buildings and trees. Depending on wind direction and temperature, you may hear thunder for up to fifteen or twenty miles.

Clouds:

You have only to look up into the sky to try your luck at weather forecasting. Clouds give us a clue about what is going on in our atmosphere and how the weather might change in the hours or even days to come. Each type of cloud forms in a different way, and each brings its own kind of weather. 

Cool Condensation:

Clouds are water. As you probably know, we can find water in three forms: liquid, solid and gas. Water as a gas is called water vapor. Clouds form when water vapor turns back into liquid water droplets. That is called condensation. It happens in one of two ways: when the air cools enough, or when enough water vapor is added to the air. You’ve seen the first process happen on a summer day as drops of water gather on the outside of a glass of ice tea. That’s because the cold glass cools the air near it, causing the water vapor in the air to condense into liquid. Unlike the drops on the side of your glass though, the droplets of water in a cloud are so small that it takes about one million of them to form a single raindrop. Most clouds form this way, but the cooling comes not from ice in a glass, but as the air rises and cools high in the sky. Each tiny cloud droplet is light enough to float in the air, just as a little cloud floats out from your breath on a cold day. 

Too Clean for Clouds?

Our air has to be just a little bit dirty for clouds to form. That’s because water vapor needs a surface on which to condense. Fortunately, even the cleanest air has some microscopic particles of dust, smoke or salt for water droplets to cling to, so the air is rarely too clean for clouds to form. 

Cloud Classifications:

Meteorologists name clouds by how high in the sky they form and by their appearance. Most clouds have two parts to their name. Usually the first part of the name has to do with the height and the second part refers to the appearance.  If clouds form at the highest levels, they get the prefix “cirro” as the first part of their name. Middle clouds get the prefix “alto.” Low clouds don’t get a prefix. 

There are two cloud appearance types: cumulus and stratus, which are also the basic names of the low clouds. Sometimes they appear higher in the atmosphere and get a combination name with a prefix. For example, middle cumulus clouds are called “altocumulus” and high stratus clouds are “cirrostratus.” If a cloud produces rain or snow it gets either “nimbo” at the beginning or “nimbus” at the end. 

Cumulus clouds are low individual billowy globs that are low, have flat bases and look a little like cauliflower. They are at least as tall as they are wide and form on sunny days from pockets of rising air. Their constantly changing outlines are fun to watch because they can take the shapes of almost anything, including animals and faces. Cumulus clouds usually signal fair weather.  If they build into the middle or high part of the atmosphere they get the name cumulonimbus. A cumulonimbus cloud is tall, deep and dark and can bring lightning, heavy rain and even severe weather such as hail, damaging winds or tornadoes. It is a sign of rapidly rising and sinking air currents.

Stratus clouds are layered and cover most of the sky. They are much wider than they are tall. If you see them in broken or puffy layers, they are stratocumulus clouds.  If you see them in thin high layers that turn the sky solid white, they're cirrostratus clouds. The tiny prisms of ice in a cirrostratus layer can bend the sun's light.  As a result, often you can see a halo or veil of rainbow colors around the sun. When stratus clouds are very thick, they become dark nimbostratus clouds, which can produce rain, drizzle or snow.

Cirrus clouds are high and thin and made entirely of ice crystals. Forming above 20,000 feet in the atmosphere, they often look like wisps of white hair. Cirrus clouds, which are a sign of warm moist air rising up over cold air, are sometimes an early signal that thickening clouds could bring light rain or snow within one or two days. Try to learn the names of the different clouds, and the next time you look up into the sky, take notice of what kind of clouds you see. And if you try, you might be able to guess what kind of weather they will bring.

Snow:

Snowflakes form when water vapor freezes into ice crystals in cold clouds.  The ice crystals attract cooled water droplets to form various shapes.  They get heavy and fall.  If the air is cold enough, the snow falls all the way to the earth without melting.  If the ground is freezing, the snowflakes stick to the ground.

No Two Alike?

Have you noticed that there are many different shapes of snowflakes?  That is because a snowflake is usually made of many different kinds of snow crystals, and the shape of a snow crystal depends a lot on the temperature at which it forms.  Table I illustrates the shape of the crystals corresponding to temperature:

Table I Temperature/Snow Flakes Relationship

Temperature Range	Crystals Shape
25-32 degrees F	Thin Plates
20-25 degrees F	Needles
15-20 degrees F	Hollow columns

Usually the colder the temperature, the smaller the crystals.  As the crystals fall from the cold clouds, they bump into other crystals and freeze together, making even more shapes. This is one reason why it's so hard to have two snowflakes exactly alike.  In fact, in air right at the freezing mark, several snowflakes may stick together, forming large clumps of flakes that may melt as they hit the ground.

Snow photographs by Wilson A. Bentley ("Snowflake Bentley") of Jericho, VT

Snow is one kind of weather that people can duplicate.

The Sun:

The sun causes all our weather because it heats the earth unevenly. The contrast between the hot parts and the cold parts of the earth turns our atmosphere into a powerful engine.  The engine keeps cold and warm air moving and makes changes in air pressure.  Those air pressure changes cause wind.  The heat of the sun also helps moisture to rise and form clouds, bringing rain, snow, or thunderstorms.  So all the changes in our weather come, at least indirectly from the sun. 

Blanket of Air:
As the sun warms up the earth, the ground absorbs the heat, and reflects some of it back into the air.  That's one reason why it's usually warmer near the ground and cooler on the higher hills and mountains.  The atmosphere acts like a big blanket over the earth, holding in the warmth and reflecting it back to earth.

Air Pressure:

The wind blows because air has weight.  Cold air weighs more than warm air, so the pressure of cold air is greater.  When the sun warms the air, the air expands, gets lighter, and rises.  Cooler, heavier air blows to where the warmer and lighter air was, or in other words, wind usually blows from areas of high air pressure to areas of low pressure. If the high pressure area is very close to the low pressure area, or if the pressure difference (or temperature difference) is very great, the wind can blow very fast.

High or Low:

Although wind blows from areas of high pressure to areas of low pressure, it doesn't blow in a straight line.  That's because the earth is rotating.  In the northern hemisphere, the spin of the earth causes winds to curve to the right.  This is called the coriolis force.  So in the northern hemisphere, winds blow clockwise around an area of high pressure and counter-clockwise around low pressure. See Note 1.

How Fast Is It?

You can estimate wind speed with the Beaufort Scale.  It was developed in 1805 by a Navy admiral to measure wind at sea.  But we can also use it to measure wind on land.

Miles Per Hour Effects:   
<1  Smoke rises straight up, no motion

1-3 Smoke drifts slowly, tree leaves barely move

4-7 Leaves rustle, wind felt on face

8-12 Leaves and twigs move, dust raised from ground

13-18 Small branches move, dust and paper blown away.             

19-24 Small trees and large branches sway

25-31 Big branches move a lot, wind whistles, umbrellas
turn inside out.

32-38 Whole trees sway, hard to walk

39-46 Tree twigs break, very hard to walk

47-54 Branches, roof tiles blown down

55-63 Trees uprooted, severe building damage

64-72 Widespread building damage

73+ Severe destruction

Wind Pump:

Use a bicycle pump to show how air pressure makes wind.  When you push down on the pump, the air pressure inside the pump is greater than the air pressure outside the pump.  So the air flows from the high pressure inside the pump to the lower pressure outside.  The harder you pump or the greater the difference in air pressure, the faster the air blows out. 

How Cold Do You Feel?

Wind makes you feel cooler, because it causes your body to lose heat faster.  The stronger the wind, the more heat is lost, and the colder you feel.  Below is a brand new table so you can figure out what temperature your body feels, called the Wind Chill temperature.

The Four Seasons:

The earth is slightly tilted on its axis.  As the sun shines on the earth, it shines more directly on the northern hemisphere in June, and more directly on the southern hemisphere in December.  That's why the seasons are different  in each hemisphere.  In the spring and fall, the sun shines fairly straight on the equator, giving both hemispheres equal warming.   

Experiment:

Global Lighting:

Take a globe and a light bulb or flashlight.  Imagine the globe is the earth and the light is the sun.  In the summer, what would be the position of the earth?  What is the earth's position in each of the other seasons? 

Spring:

You don’t have to look far to see signs of spring. From the budding of the trees and the warming of the temperatures to the animals coming out of their winter hideouts, there seems to be a promise of new birth and color in the springtime air.

The first day of spring is around March 20 or 21, depending on what day the vernal equinox occurs. This is when the sun sits directly above the equator on its apparent trip northward. Of course this sun isn’t moving; the Earth is. As Earth revolves around the sun, the top half, called the Northern Hemisphere, becomes tilted more toward the sun as winter turns to spring. Meanwhile the bottom half, the Southern Hemisphere, becomes tilted more away from the sun. The beginning of spring for us is the beginning of autumn for people in Australia and the southern parts of Africa and South America.

Unequal Equinox:

The word “equinox” comes from Latin and means “equal nights.” Around March 20, sunrise and sunset are about twelve hours apart everywhere on Earth. Because of that, a lot of people think that day and night are of equal length on March 20. But actually the day is a little longer than the night on this date. There are a few reasons for that. Sunrise occurs when the top of the sun (not the center) is on the horizon. But the sun actually appears to be above the horizon when it is in fact still below it. That’s because Earth’s atmosphere refracts or “bends” light coming from the sun, so we see the sun a couple of minutes before it actually rises over the horizon. If you add the daylight that persists after sunset, you’ll find the day on the equinox is several minutes longer than the night.

Variety in Spring Weather:

In the United States, spring is a time of transition not only for plant and animal life, but for the weather too. It can mean weather extremes from very cold and snowy days to humid and stormy days. Some of the country's biggest snowfalls have occurred in March, and the period from March to May is the time of year when much of the south is most likely to get severe thunderstorms with hail and even tornadoes. This is why the beginning of spring is a good time to put together a plan for what you and your family would do in case of a severe thunderstorm or tornado.

Summer Heat:

It's summer and the heat is on.  The summer solstice in the Northern Hemisphere, or top half of Earth, is around June 21.  That's when the sun reaches its most northern point over our planet.  The days around June 20 are the longest of the year, and the rays from the sun beat almost directly down on use and warm us up.

Longest, Not Hottest:

You might think that the longest days of the year would be the hottest.  But they are not.  The hottest weather, on average, comes about a month after the summer solstice.  This is because the amount of heat from the sun continues to accumulate during the long hot days, and the short nights don't allow as much heat to leave.  The days start to cool down only after the days grow short enough to allow more heat to leave Earth's surface than arrives.

Humidity:

It is not the heat, they say, but the humidity. Humidity is the amount of moisture in the air. When the air has so much moisture in it, our bodies don’t do a good job of cooling us down because sweat doesn’t evaporate as quickly from our skin. That is why we feel so hot on a humid day. In fact, the temperature we feel may be warmer than the actual air temperature. This is called the heat index. Look at the figure below. If the temperature is 100 degrees and the relative humidity is 50 percent, the heat index, or the temperature we actually feel, is 120. Hot humid days can be dangerous for humans and animals, so stay out of the sun and drink lots of water when the humidity is high.

Figure 1 - Heat Index

The Sun Starts It All:

The changes start with the sun and the earth’s orbit around it. Often people think that our temperatures get cooler in autumn because the earth moves farther from the sun. But this is not true. What actually changes is not the distance of the earth to the sun, but the angle of the sun’s rays on the earth. In our summer, the sun shines more directly on the top half of the earth, or northern hemisphere. That’s when we see long warm days. In our winter, the sun shines more directly on the bottom half of the earth, or southern hemisphere. With less and less direct sunlight on the top half, the days here get shorter and temperatures get colder as winter nears. Those shorter days and cooler temperatures act as a signal to the trees and plants around us to get ready for winter.

Chlorophyll (The Magic Chemical):

All summer long, a tree’s leaves have been making food for the tree so it can grow. An amazing chemical in the leaves, called chlorophyll, uses sunlight to turn carbon dioxide and water into the tree’s breakfast, lunch and dinner. Chlorophyll gives the leaves their green color. Also in the leaves are other chemicals with yellow and orange colors—the same chemicals that give color to some flowers, carrots, and even bananas. You don’t see those colors in summer because there is so much green chlorophyll in the leaves hiding the other colors.

In the winter, the short days don’t provide enough sunlight for the trees to make their food, so the trees live off the food they stored during summer. Before winter arrives, the shorter days make the trees slow down their food making, and the green chlorophyll disappears from the leaves. That’s when the yellow and orange colors take over. At the same time, other chemical changes may happen in the leaves, forming other colors such as red or purple. Like an artist mixes paint for his canvas, the chemicals mix to form different colors in different trees. Eventually some trees, but not all, lose their leaves.

How Bright Can It Get?

You may have noticed that in some years, the leaves seem more brightly colored than in others. Again, weather is the reason. The leaves are brightest when the late summer is warm and dry and autumn has a lot of sunny days and cool nights with temperatures in the upper thirties or low forties. The sunny days will help the leaves make some food, but the cool nights will keep the food from moving out of the leaves. Under those weather conditions, the trapped food will form brilliant purple and red chemicals in the leaves.

On the other hand, if the autumn days are cloudy and the nights are warm, the leaves won’t produce as much trapped food and so they will not be as colorful. If temperatures go below freezing at night, the frost will dull the leaves’ colors. Sometimes leaves on the same tree may have slightly different colors from one another. That’s because different leaves receive different amounts of sunlight, so some leaves produce more red and purple chemicals than others. On some trees, leaves that receive a lot of sunlight may turn red, while the leaves in the shade may be yellow. Again, it is the sun that is responsible for nature’s wide palette of colors.

Look Around:

As you are playing outside or riding in your car this fall, take a closer look at the trees around you. Take a moment to think about the colors you see and how they got there. And as you observe the changes in the season and the changes in the weather, you just might be able to forecast how bright this year’s autumn colors will be!

Winter:

When we think of winter, we often imagine sledding, icicles and snowball fights. But think about this: winter is when earth’s north pole is tilted farther away from the sun than at any other time of the year. This is called the winter solstice, and the days just before Christmas are the shortest of all. During this time, the sun shines more directly on the lower half of the earth, or Southern Hemisphere. While we in the United States have winter, Australia, South Africa and the southern parts of South America will have summer. Because the weather continues to cool for about another month after the shortest days, the coldest weather actually arrives after the winter solstice. For most of the Northern Hemisphere, January and February are typically the coldest months.

What's Up with Winter?

The type of precipitation we get depends on the temperature inside the clouds and the temperature between the clouds and the ground. In clouds that are cold enough for ice crystals to form, we can get snow. Those cold clouds aren’t hard to find. Even in the summer, most of our rain actually starts out high in the clouds as snow. But in winter, the temperature of the air is sometimes cold enough all the way from the clouds to the ground, so snowflakes don’t melt into raindrops. They stay in crystal form and we see snow pile up and schools close.

Figure 2 - Temperature Rain Relationship

Look at the figure above. Sometimes there is a layer of air in the clouds that is above freezing, or 32 degrees F. Then closer to the ground the air temperature is once again below freezing. Snowflakes partially melt in the layer of warmer air, but then freeze again in the cold air near the ground. This kind of winter precipitation is called sleet. It bounces when it hits the ground. If snowflakes completely melt in the warmer air, but temperatures are below freezing near the ground, rain may freeze on contact with the ground or the streets. This is called freezing rain, and significant freezing rain is called an ice storm. Ice storms are extremely dangerous because the layer of ice on the streets can cause traffic accidents. Ice can also build up on tree branches and power lines, causing them to break and our lights to go out.

The Water Cycle:

The rain that falls from the sky today has been around for thousands of years.  The molecules of water in today's rainfall might have been in yesterday's cloud or last week's dew, or in a lake or ocean.  Although water takes three basic forms (liquid, solid and gas) we see in many forms of it, such as frost, snow, rain, and clouds.  The cycle of evaporation, condensation, and precipitation never ends.

Triple Waters: Water comes in three forms.  As a liquid, we see it in the lakes and oceans, we see it falling as rain, and we see it come into our home for drinking or washing.  As a solid, we see water as ice and snow.  As a gas, water vapor is always floating in the air.  Water changes from liquid to solid by freezing.  It changes from liquid to gas by evaporation.  It changes from gas to liquid by condensation.  Can you guess how it changes from solid to liquid?

Air Full of Water:
You can't see it, but the air contains a lot of water.  Warm air can hold more water vapor than cold air.  When air gets cold, the water vapor condenses into clouds.  And when warm air holds a lot of water vapor, the air can feel sticky and damp.  The amount of water in the air is called humidity.

Rainbows, Halos and Coronas:

Throughout time, people have been fascinated with rainbows. Their arched splashes of color have been the subject of songs and poems, stories and mythology. In the Bible, the rainbow is seen as a sign of God's promises, and most of us are familiar with the legend of the pot of gold at the end of the rainbow. To unlock the rainbow's secrets, let's explore how water and light work together to produce a colorful work of art.

What makes rainbows so mysterious is this simple but often puzzling fact: Rainbows are light. You can't touch them. You can't reach around behind them. They exist only in the eyes and sometimes the photographs of the people who see them. Light, or more specifically visible light, includes every color we can see, with violets and blues on one end of the spectrum and oranges and reds at the other end. A rainbow is visible light broken into what we see as seven colors: red, orange, yellow, green, blue, indigo and violet.

What Does It Take?

Three things must happen for you to see a rainbow's colors. First, the sun must be shining. Second, the sun must be behind you, and third, there must be water drops in the air in front of you. Sunlight shines into the water drops, which act as tiny prisms that bend or "refract" the light and separate it into colors.

Actually, the rays of light bend twice. As they enter the drops, the rays of light bend, then reflect off the back of the drops. Then they bend again, this time while exiting the drops. That's when the light appears before our eyes. Each drop reflects only one color of light, so there must be many water drops to make a full rainbow. You'll see the brightest rainbows when the water drops are large, usually right after a rain shower.

The rainbow is circular because when a raindrop bends light, the light exits the raindrop at an angle 40 to 42 degrees away from the angle it entered the raindrop. The violets and blues bend at a 40-degree angle, and the oranges and reds bend at a 42-degree angle. As a result, the only beams of light you see are from raindrops that are 40 to 42 degrees away from the shadow of your head. This gives the rainbow its curved appearance.

Full Circle Bows:

In the song "Moon River," Georgia songwriter Johnny Mercer wrote, "We're after the same rainbow's end." Actually, though, rainbows have no end. We usually don't see the full circle because the horizon of the Earth is in the way. But if the sun is very low in the sky, either just before sunset or just after sunrise, we can see a half circle. The higher the sun is in the sky, the less we see of the rainbow. The only way to see the full circle of a rainbow in the sky is to be above the raindrops and have the sun behind you. You would have to look down on the drops from an airplane.

Double the Pleasure:

Often you can see a second rainbow above a brighter one. This is caused by extra reflection inside the raindrops. Because the secondary rainbow is formed from two reflections instead of one, it has a wider radius than the first rainbow and its colors are reversed, with red on the bottom and indigo on the top.

The secondary rainbow also isn't as bright as the first because it's a secondary reflection.

A third or a fourth rainbow also can occur because light can be reflected more than two times within a raindrop. But we rarely see them because they are even fainter than a secondary rainbow.

A Few Oddities:

Because rainbows are light and because light rays strike everyone's eyes a little differently, the rainbow you see will be a little different from the one someone else sees, even if he or she is standing right beside you. Someone else a short distance away or looking from a different angle may see a much different rainbow -- or no rainbow at all.

Another oddity is that you might see a rainbow's reflection in a lake without seeing any rainbow above it in the sky! That's because you may be at the wrong angle to see the rainbow but are in the perfect spot to see its reflection in a lake.

How To See A Halo:

As any avid sky watcher knows, there are many other examples of color in the sky. Have you ever seen a single ring of color around the sun or moon? It is a halo, and it occurs more frequently than a rainbow. To see a halo, don't look directly into the sun. Instead, block the sun from your view with your hand, a car visor or other object so you can just see the clouds around it. Sunglasses also may help you see a halo -- but even with sunglasses, you'll need to block the sun from your eyes.

Sometimes the halo is white. Sometimes you can see red and orange in the middle, with yellow and blue at the outer edges. A 22-degree halo is the most common. It is formed when light refracts -- or bends -- around the edges of long ice crystals at a 22-degree angle. If a thin cloud's ice crystals are in the right position, you might see arcs just above or below the halo. The arcs form when light refracts inside long pencil-shaped ice crystals.

Flat ice crystals can produce an effect high above the halo called a circum-zenithal arc - or an upside-down rainbow. Also when light refracts through flat horizontal ice crystals, you might see bright spots of light along the right and left sides of a halo. These bright spots are commonly called sun dogs; their scientific name is parhelia.

Coronas:
They are similar to halos but are formed from a cloud's water drops rather than from ice crystals. Light, which shines through cloud droplets, curves around the circular drops and gets diffracted. This means it spreads out, creating an area of light larger than the sun or moon. This "crown" of milky light around the sun or moon is the corona. When all the cloud droplets are about the same size, the diffracted light can make the corona separate into colors. These colors may repeat themselves.

Hurricanes:

Summer also brings hurricanes. From late spring to early fall, weather conditions come together to form swirling tropical cyclones over the Atlantic and Pacific oceans. These develop from areas of low pressure and thunderstorms over the warm seawater. The thunderstorms give off heat that warms the atmosphere. Air rises and the barometric pressure falls even more. As the air pressure drops, winds increase, and a tropical depression may form. When steady winds reach 39 miles an hour, the cyclone is called a tropical storm and it gets a name. If winds reach a speed of 74 miles an hour inside the tropical cyclone, we call it a hurricane. Near the hurricane’s center will be an area with very few clouds where the air sinks. This is the “eye” of the hurricane.  Most hurricanes never reach the United States coastline, but those that do can bring high waves, coastal flooding and destructive winds.

Typhoons vs. Hurricanes:

Typhoons are the same thing as hurricanes. That is the name used in the western Pacific Ocean. In the Indian Ocean, they are called tropical cyclones.

Summary:

The blanket of air around the earth is called the atmosphere. The sun is the main reason why we have weather. There is one basic reason we have weather, and that is the sun. Weather systems start because the sun's energy heats up some parts of Earth more than others. Most of the time the sun shines most directly on the middle of Earth, with less heating at the north and south poles. Earth is also unique in that all three forms of water--liquid, solid and gas--exist naturally. Heating from the sun can also trigger thunderstorms.

Some vapor freezes into tiny ice crystals that attract cooled water drops. The drops freeze to ice crystals, forming larger crystals we call snowflakes. When the snowflakes meet warmer air on the way down, they melt into raindrops. Snowflakes partially melt in the layer of warmer air, but then freeze again in the cold air near the ground. This kind of precipitation is called sleet. Cumulonimbus clouds may tower more than 50,000 feet and cover from just a few square miles up to two hundred square miles. Depending on wind direction and temperature, you may hear thunder for up to fifteen or twenty miles. Most clouds have two parts to their name. Usually the first part of the name has to do with the height and the second part refers to the appearance. Snowflakes form when water vapor freezes into ice crystals in cold clouds. The colder the temperature is, the smaller the crystals.

The sun causes all our weather because it heats the earth unevenly. The contrast between the hot parts and the cold parts of the earth turns our atmosphere into a powerful engine. As the sun warms up the earth, the ground absorbs the heat, and reflects some of it back into the air. Although wind blows from areas of high pressure to areas of low pressure, it doesn't blow in a straight line. That's because the earth is rotating. The Beaufort Scale is used to estimate wind speed. The Beaufort Scale is a system of recording wind velocity (speed) devised in 1806 by Francis Beaufort (1774–1857). It is a numerical scale ranging from 0 for calm to 12 for a hurricane.

As the sun shines on the earth, it shines more directly on the northern hemisphere in June, and more directly on the southern hemisphere in December. That's why the seasons are different in each hemisphere. In the spring and fall, the sun shines fairly straight on the equator, giving both hemispheres equal warming. Around March 20, sunrise and sunset are about twelve hours apart everywhere on Earth. Some of the country's biggest snowfalls have occurred in March, and the period from March to May is the time of year when much of the south is most likely to get severe thunderstorms with hail and even tornadoes.

The summer solstice in the Northern Hemisphere, or top half of Earth, is around June 21. June 20 is the longest of the year. Hot humid days can be dangerous for humans and animals, so stay out of the sun and drink lots of water when the humidity is high. What actually changes the temperature is not the distance of the earth to the sun, but the angle of the sun’s rays on the earth.

Chlorophyll, uses sunlight to turn carbon dioxide and water into the tree’s breakfast, lunch and dinner. Chlorophyll gives the leaves their green color. The leaves are brightest when the late summer is warm and dry. During the winter we see some rain, but sometimes we see other forms of precipitation too. Winter precipitation also includes snow, sleet, and freezing rain. The type of precipitation we get depends on the temperature inside the clouds and the temperature between the clouds and the ground.

In the Bible, the rainbow is seen as a sign of God's promises, and most of us are familiar with the legend of the pot of gold at the end of the rainbow. The rainbow is circular because when a raindrop bends light, the light exits the raindrop at an angle 40 to 42 degrees away from the angle it entered the raindrop.

The violets and blues bend at a 40-degree angle, and the oranges and reds bend at a 42-degree angle. Sometimes the halo is white. Sometimes you can see red and orange in the middle, with yellow and blue at the outer edges. A 22-degree halo is the most common. Coronas are similar to halos but are formed from a cloud's water drops rather than from ice crystals. When steady winds reach 39 miles an hour, the cyclone is called a tropical storm and it gets a name. If winds reach a speed of 74 miles an hour inside the tropical cyclone, we call it a hurricane. Typhoons are the same thing as hurricanes. That is the name used in the western Pacific Ocean. In the Indian Ocean, they are called tropical cyclones.

Bibliography

Introduction to Meteorology (Atm. Sci. 240). Instructor: Professor Jon Kahl. Course Information. General Information · Syllabus ... www.uwm.edu/~kahl/240/.

"Introduction to Meteorology and Related Sciences," web site: www.msc_smc.ec.gc.ca/education/imres/index_e.cfm.

The navigation men, this module is called "Meteorology" and the available modules are listed as menu items, beginning with this introduction. ...

ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/home.rxml.

Note 1: Coriolis Effects Report by Dr. Francisco J. Collazo, web site: www.fjcollazo.com.