Common section

Chapter 9

Warming Up and Chilling Out: Why Climates Happen

In This Chapter

bulletFeeling the heat

bulletTilting along

bulletChanging seasons

bulletObserving current affairs

H ave you ever felt people were staring at you in a discreet sort of way? I certainly have. And I can give an example that relates directly to this chapter.

I was in a Swiss village on a sunny and warm July afternoon. The temperature was in the mid-eighties, which is why I was wearing shorts and a short-sleeved shirt. I got on a cable car to go way up into the mountains to take in the view and go for a hike. There were about 20 other people on board, and the higher we went, the more I felt their eyes. So I looked back, and in so doing quickly understood their stares.

Nobody else was dressed like me. Not even close. They had on heavy pants and heavy shirts, with sweaters and jackets at the ready. And for good reason. When we got to the top and the doors opened, a blast of cool air rushed in. I’m guessing the temperature was in the high forties. My first reaction was to feel totally stupid. Like, duh, shouldn’t a geographer have been prepared for this? My second reaction was to cancel the hike and keep warm until the cable car made the return trip.

That brief journey began and ended on the side of the same mountain under the same sun. But something happened to the air temperature that made all the difference in the world. The same is true globally. You can travel the world and experience all kinds of climates. Together they constitute a vast array of atmospheric characteristics that concern temperature, precipitation, and seasonal change. But no climate “just happens.” While all are products, directly or indirectly, of the same sun, factors are afoot that give each of them characteristics that really do make all the difference in the world.

The factors that determine climate in different parts of the world are of central interest to geography and are the subjects of this chapter. The characteristics, locations, and consequences of climate occupy the next chapter.

Getting a Grip on Climate


Like climate, weather is concerned with atmospheric conditions. The difference between the two is a matter of time. Weather refers to day-to-day conditions and changes in Earth’s atmosphere. Climate refers to the average of weather conditions at a location over a long period of time — 30 years, as far U.S. government climatologists are concerned. Climate, therefore, is the more appropriate topic for this book because it concerns general characteristics of a location or region. Accordingly, among the things you will not read about here are tornadoes, hurricanes, thunder, lightning, hail, and other forms of short-term atmospheric mayhem that fall within the purview of weather. (For more information on weather, see Weather For Dummies [Hungry Minds, Inc.].)

So what factors cause the different kinds of climates to occur? (Drum roll, please.) Six determinants, which may act singly, in combination, or in opposition to each other, make climate occur:

bulletThe angle at which solar energy strikes the Earth

bulletTilt of the Earth on its axis

bulletAltitude with respect to sea level

bulletSolar absorption properties of land and water

bulletOcean currents

bulletHigh and low atmospheric pressure belts

The following sections show how each bullet point creates climate.

Playing the Angles


Parts of Earth receive different amounts of solar energy — heat from the sun. The dosages are greatest in the equatorial realms and progressively diminish as one approaches the Poles. For this reason, rather warm climates generally dominate the low latitudes and give way to cooler and cooler climates in the mid-latitudes and polar regions.

These differences are due to the angles at which solar energy strikes the Earth at different latitudes. Figure 9-1 shows three “bundles” of sunshine whose widths are the same, so it can be assumed they contain equal amounts of solar energy. But if you examine the amount of Earth that each impacts, a key difference appears. Bundle A illuminates a much smaller area than Bundle B, which in turn illuminates a much smaller area than Bundle C.

Making hot and cold


The differences among the Bundles in Figure 9-1 are determined by the curvature of the Earth. Bundle A contains vertical rays, which strike Earth perpendicularly. Due to curvature, however, Bundle B strikes Earth at a sharper angle. As a result, its solar energy is spread over a larger area than is Bundle A’s. Sharper still, thanks to curvature, is the angle at which Bundle C strikes Earth. And as a result, its heat is spread over the largest area of the three examples.

Absent other factors that affect temperature, Area A has the warmest climate because it has the greatest concentration of solar energy. That is, the heat in Bundle A is brought to bear on a relatively small area. In contrast, Area C has the coldest climate because the heat it receives is spread over the largest of the three areas. Intermediate conditions are present in Area B.

Let’s look at some real locales and compare the climates of Manaus, Brazil, and Churchill, Manitoba (Canada). Manaus is located at about Latitude 3° South and therefore exemplifies Area A on the diagram. Churchill is located at Latitude 58° North and therefore has the characteristics of Area C.

In Manaus, the annual average temperature is 79° F. (That means that if you recorded the temperature every hour of every day for a year, the average would be 79° F). In Churchill, it’s 19° F. The difference is 60° F. Manaus is much warmer.


Beware the reason! Is it because Manaus is closer to the equator? Nope. Proximity to the equator per se is not the explanation. Instead, the answer lies in the angle. The sun is more directly overhead at Manaus than at Churchill throughout the year. Thus, Manaus experiences a greater concentration of solar energy throughout the year and is therefore a warmer climate.

Making rain and snow

Climate is about precipitation as well as temperature. Regarding wet stuff, Manaus receives 82 inches of precipitation on average each year. Churchill, in contrast, receives 15 inches. Thus, the precipitation difference in inches is even greater than the temperature difference in degrees Fahrenheit. Again, sun angle plays a major role in these particular cases.

In Chapter 8, I discuss how solar energy causes water to evaporate and plants to transpire (sweat), producing atmospheric vapor, the building blocks of raindrops and of other forms of precipitation. Generally, the greater the solar energy, the greater the amount of evaporation and transpiration, which result in vapor in the atmosphere. And the greater the amount of atmospheric vapor, the greater the likelihood of precipitation. Given that Manaus experiences much more intense concentrations of solar energy than does Churchill, its atmosphere is much more humid and therefore has a higher rainfall potential. Indeed, the very heat that produces that vapor may also create an atmospheric upwelling, or convection current, which carries vapor to a high altitude where it cools, condenses, and falls as rain (see Figure 9-9 later in the chapter).

Tilt-a-World: The Reasons for the Seasons


Earth’s axis is not, in reality, “straight up and down” as indicated by Figure 9-1. Instead, it’s tilted by 23 1/2° from the perpendicular. As a result, the vertical rays do not “stay put” at the equator as Earth orbits the sun. Instead, they “migrate” north and south of the equator at different times of year, bringing with them patterns of seasonal change (as illustrated by Figure 9-2). Take, for instance, the United States. During summer, the vertical rays move into the Northern Hemisphere, increasing its “dosage” of solar energy and producing the warm temperatures that are associated with that time of year. During winter, however, the vertical rays move into the Southern Hemisphere, greatly increasing that region’s solar dosage but at the same time greatly decreasing the dosage that hits the United States.

Special lines of latitude

The inclination of Earth on its axis accounts for four special lines of latitude that appear on most world maps and globes (see Figure 9-3). Two of them are the Tropic of Cancer (23 1/2° N) and the Tropic of Capricorn (23 1/2° S). The area between them may properly be called The Tropics. The other two lines are the Arctic Circle (66 1/2° N), and the Antarctic Circle (66 1/2° S). The area north of the Arctic Circle may properly be called The Arctic or The Northern Polar Region. Similarly, the area south of the Antarctic Circle may properly be called The Antarctic or The Southern Polar Region. But why are these lines there in the first place? For now we will concern ourselves with the Tropic lines because they have much to do with defining seasonal change.

Because of the angle of Earth’s tilt, during summer in the Northern Hemisphere, the vertical rays move north of the equator as far as Latitude 23 1/2° North. Conversely, during winter, the vertical rays migrate south of the equator as far as Latitude 23 1/2° South. Thus, the Tropic of Cancer marks the most northerly latitude that is struck by the sun’s vertical rays at some point during the year. Conversely, the Tropic of Capricorn marks the most southerly occurrence.

Parenthetically, “Tropic” comes from the Greek tropos, to turn. The ancient Greeks observed that during their summer the sun’s vertical rays moved northerly until they reached Latitude 23 1/2° North, whence they “turned” back to the south. Cancer and Capricorn refer to stellar constellations that were prominent in the Greek sky when the vertical rays struck one or the other tropic lines.

Defining the seasons


Four days each year, vertical rays strike either the equator or one of the Tropic lines. On two of those dates, called equinoxes, the vertical rays strike the equator. On the other two dates, called solstices, the vertical rays strike one of the Tropics. The significance of these dates is that they mark the beginnings of the four seasons of the year. The following sections show the annual cycle as it relates to the Northern Hemisphere.


Keep in mind that seasons are relative. Summer never happens everywhere at once. Ditto fall, winter, and spring. Instead, summer occurs in one hemisphere while winter is happens in the other, and vice versa. Likewise, spring occurs in one hemisphere while autumn falls in the other, and vice versa. Thus, when the “Summer Olympics” were held in Sydney, Australia (Southern Hemisphere), it wasn’t summer as far as the locals were concerned, but instead late winter.


Sometime around March 21, the vertical rays strike the equator, marking the spring equinox. This is the first day of spring in the Northern Hemisphere and the period of daylight and darkness are the same. Everyday for about the next three months, the vertical rays strike the Earth at progressively more northerly latitudes. The daylight hours get longer while night gets shorter.


On or about June 21, the vertical rays strike the Tropic of Cancer, marking the summer solstice. This is the first day of summer as well as the date that has the longest period of daylight and the shortest nighttime. From this point, the vertical rays then “turn south.” Daylight hours lessen while nighttime hours increase.


On or about September 21, the vertical rays again strike the equator, marking the fall, or autumnal, equinox. This is the first day of fall and again the period of daylight and darkness are equal. The vertical rays then move into the Southern Hemisphere, striking ever more southerly latitudes each day for about the next three months. In the Northern Hemisphere, nighttime now exceeds daytime by a margin that increases each day.


December 21 is the approximate date of the winter solstice. The vertical rays then strike the Tropic of Capricorn, marking the first day of winter. Also on that date, the Northern Hemisphere experiences its longest period of darkness and shortest period of night. The vertical rays then “turn northward.” For about the next three months, nighttime periods continue to exceed daytime periods in the Northern Hemisphere, but by a difference that diminishes daily. Finally, on or about March 21, the vertical rays are back at the equator, marking the spring equinox. Day and night are again equal and the seasonal cycle is complete.

Special lines of latitude revisited

Before leaving this section, discussion is in order of those other two “special lines of latitude” — the Arctic and Antarctic Circles. Each demarcates parts of the world where something peculiar happens. Specifically, every location north of the Arctic Circle and south of the Antarctic Circle experiences at least one continuous 24-hour period of daylight, and at least one continuous 24-hour period of darkness during each year. Moreover, the farther north and south one goes with respect to those two lines, the greater is the number of days of complete daylight and darkness. The extreme cases occur at the two poles, where the year is divided into one six-month-long period of daylight followed by one six-month-long period of darkness.


To help understand this, look back at Figure 9-3. The North Pole is 90 degrees’ worth of latitude from the equator. On the first day of fall, the sun is directly overhead at the equator, but will appear to an observer at the North Pole to be on the horizon (90° from overhead). Everyday for the next six months, the vertical rays strike Earth below the equator. From the perspective of the North Pole, the sun is below the horizon all the while, so darkness ensues. Every other latitude between the North Pole and the Arctic Circle, Latitude 66 1/2° North, also experiences at least one continuous 24-hour period of darkness. The significance of Latitude 66 1/2° North is that it’s exactly 90° from the Tropic of Capricorn, which marks the farthest southerly point that feels the sun’s vertical rays.

The opposite occurs during spring and summer, when the North Pole has continuous daylight. As the summer solstice approaches, more and more latitudes south of the North Pole experience similar conditions. Finally, on the summer solstice, the experience of a continuous day of sunlight reaches its most southerly locale — the Arctic Circle.

Why is Christmas celebrated on December 25th?

Scripture is silent about the date of Christ’s birth, which only began to be celebrated several centuries after the event. In pre-Christian Northern Europe, there was widespread belief in a sun god whose birth date was celebrated on the winter solstice. On that date, the sun is lowest in the daytime sky in the Northern Hemisphere, but rises somewhat everyday thereafter until the summer solstice. Thus, the date on which the sun began to rise symbolized the sun god’s birth. As Christianity spread into those lands, mixing of religious beliefs occurred, one of which was to substitute Christ’s birth for the sun god’s on the winter solstice. Subsequent refinement of the calendar resulted in the few-day’s lapse that now separates Christmas and the winter solstice.

Hot or Cold? Adjust Your Altitude


Altitude has an important impact on climate. The rule of thumb is that temperature and elevation are inversely related. Or in every day speech, “the higher you go, the cooler it gets and vice versa.” Thus, highland areas have cooler climates than lowland areas.

Consider this example: Fewer than 200 miles separate Guayaquil and Quito, the two largest cities in Ecuador. But the annual average temperature is 77° F in Guayaquil and only 56° F in Quito. The explanation is that Guayaquil is virtually at sea level while Quito is up in the Andes at an elevation of about 9,250 feet. Despite being nearly on the equator, the city does not experience the warm climate one would normally expect at that latitude, due to the altitude factor. How does that work? Glad you asked.

Warming the atmosphere

Part of the answer concerns how the atmosphere obtains heat. A portion of the solar energy that reaches the Earth (18 percent) is absorbed directly by the atmosphere, while an even larger percentage (32 percent) reflects back into space. The largest portion by far (50 percent), however, is absorbed by the Earth, which then re-radiates that heat into the atmosphere. Thus, solar energy turns Earth’s surface into something like a giant frying pan that heats the atmosphere above it. Generally, therefore, air that is at or near the Earth is relatively warm, while increasing elevations above “the frying pan” brings progressively cooler temperatures.

Weighty matter

Another reason why “the higher you go, the cooler it gets” is the fact that the atmosphere has weight. Gravity is constantly “pulling down” on it. Thus, as altitude increases, the amount of air decreases. And because the atmosphere holds heat, less air means colder temperatures. Therefore — and to return to our opening examples — Quito, which is way up in the Andes, has a colder climate than Quayquil, which is down by sea level.

Seeing (and feeling) is believing

Because relatively cool temperatures characterize high elevations, precipitation in highlands and mountainous regions is apt to be snow rather than rain for a good portion of the year — if not for all of it in the case of really high mountains. That results in what many regard as one of nature’s most aesthetically pleasing sights — a snow capped mountain (as you can see in Figure 9-4). But the effects of elevation may be felt as well as seen. Go up a high mountain, and not only does it get colder as you go higher, but also breathing becomes increasingly difficult. It makes sense. Air has weight. It wants to sink towards sea level. So the higher you go, the less air is available to help warm things up and to help you breathe easier. The condition is called thin air.

The lapse rate


The numerical relationship between temperature change and elevation change is called the lapse rate. It works out to about 3.5° F per 1,000 feet, or 6.4° C per 1,000 meters. That is, if you have two hikers on a mountain separated by 1,000 feet of vertical distance, then the person higher up is experiencing a temperature that is about 3.5° F colder than the person down slope. Parenthetically, I say “about” because humidity — the amount of vapor in the air — can and does tamper with these formulae. Therefore, think of the above numbers as average figures.

Windward slope, leeward slope


In addition to temperature, altitude can have a profound effect on patterns of precipitation. An air current is forced to rise when it meets a mountain (see Figure 9-5). As it does so, its vapor cools and condenses, forming raindrops (or snowflakes). Precipitation that is produced in this manner is called orographic (from the Greek oros, mountain), meaning that it is mountain-related. The heaviest rains and snows tend to occur on the windward slope, which is the side of the mountain from which the wind is blowing.

After cresting the summit, the air descends. As it does so, it warms, which is the opposite of what air needs to do in order to condense and form rain. But the air also has little vapor remaining, so the prospects for precipitation are just about zilch. The result is a dry leeward slope, whose droughty environs are said to be in a rainshadow.

Instead of a lone mountain, as in Figure 9-5, imagine a lengthy mountain range. The windward side has a rather wet climate. The leeward side, being in the moisture-deprived rainshadow, is a desert or semi-desert. For example, the Himalayan Mountains lie perpendicular to seasonal rain-bearing winds that come from the Indian Ocean. The result is a virtual tropical rainforest climate on the southern windward side, and a vast expanse of arid and semi-arid conditions (including the Gobi and Takla Makan Deserts) on the northern leeward side.

Similarly, the Coast and Cascade Ranges in Northern California, Oregon, and Washington State, intercept moisture-bearing winds that enter the mainland from the Pacific Ocean. The result is a very moist coastal fringe (noted for its tall trees — the redwoods — and lush forests) on the windward side. But the lands to leeward are arid and semi-arid.

Applied Geography: Locating an island resort

Many islands have distinctive windward and leeward sides. On those that do, predominant wind direction is often an important consideration in choice of location for a resort. On the windward coast the wind comes off the ocean, often resulting in rough surf conditions and blowing sand that is picked up from the beach. And if highlands are inland, rain will also result as air is forced to rise over the higher elevations. The leeward side, in contrast, is apt to experience much less rain, calmer waters, and absence of wind-blown sand. So if you’re in the market for island real estate on which to build a resort, chances are good that you will opt for land on the leeward coast.

Gaining Heat, Losing Heat


Locations in the middle of continents tend to have hotter summers and colder winters than do locales at similar latitudes by the sea. This condition is called continentality. It occurs because land and water have very different characteristics when it comes to absorbing and retaining solar energy. To illustrate this point, take a look at Figure 9-6 and assume it shows a sandy beach and an adjacent lake receiving equal amounts of solar energy.

Earth and sand are not transparent, so most of the solar energy that strikes them is absorbed by and concentrated in the top-most inch or half-inch of surface material. As a result, the beach becomes super-hot. If you have ever experienced scorched feet while walking barefoot on dry sand on a sunny summer day, then you know exactly what I am talking about.

In contrast, because the lake water has a certain transparency, solar energy penetrates the surface and, depending on the clearness of the water, spreads itself out over the depths. Also, wave action and other flow mix the upper layers of water, and thereby transport the absorbed heat away from the surface. As a result, a much greater volume of water, compared to the beach, absorbs heat. Therefore, the temperature of the lake on a sunny summer day tends to be somewhat cooler than the temperature of the beach.

This difference has significant implications for an outing at the beach, as explained below. The same is true regarding the tendency for mid-continent locales to have warmer summers and cooler winters than locations by the coast.

Afternoon versus evening

Assume it’s a boiling hot summer afternoon. You and some friends are on a beach blanket at Point A on Figure 9-6. The shoreline is at Point B. You decide to take a swim. The sand is terribly hot against your feet as you run for the water and take the plunge. That water sure feels cool!

Now assume it’s 9 p.m. and you are still hanging out by the lake. The sun has gone down. The temperature has cooled considerably, and the sand is now somewhat cool against your feet. Someone suggests you all go for a swim and you decide to join in, however reluctantly. So you run for the water and take the plunge. That water sure feels . . . warm!?

How can the same body of water that cools you off at midday warm you up at 9 p.m.? As we saw in the discussion of Figure 9-6, the beach gets super-hot on a sunny summer because solar is absorbed and concentrated in the top veneer of surface material. In contrast, the lake heats up more slowly because the solar energy it absorbs is spread out over a much larger volume of matter. Thus, you experience a cooling sensation when you jump into the lake in the middle of the afternoon.

But it’s a different matter at 9 p.m. Because nearly all of the land’s heat is contained within a thin veneer of surface sand, it tends to radiate back into the atmosphere rather fast after the sun goes down. But the lake is different. Only a small portion of its solar inventory is at the surface — exposed to the atmosphere. After the sun goes down, the lake’s heat is radiated back into the atmosphere at a slower rate. Thus, it retains its absorbed energy longer than land, resulting in a warming evening swim.

Summer versus winter

The fact that land and water heat up and cool down at different rates has significant implications for climate. Look again at Figure 9-6, but instead of a beach by a lake, assume it portrays a continent next to an ocean. Also assume that Point A is a city in the middle of that continent and that Point B is a coastal city a thousand or so miles away. Time-wise, consider summer versus winter as opposed to mid-afternoon versus 9 p.m.

During the summer, the city in the middle of the continent is likely to be warmer. That is because, just like in the beach example, solar energy will concentrate at Earth’s surface and heat the atmosphere overhead. In contrast, the coastal city is likely to be less warm during summer. That is because its atmosphere will be warmed by heat that radiates off both the land around it and the water offshore. But because the surface of the water contains so much less heat than the land, the total amount of heat that is radiated into the atmosphere is far less than occurs in the middle of the continent. In the parlance of climatology, the water body has a modifying or mitigating effect. That is, it results in the atmosphere being less warm than would be the case if there were land all around. The net result, whatever the vocabulary, is that the city in the middle of the continent will experience a warmer summer than the one by the sea.

Winter is another matter, however. Regarding the mid-continent city, the heat that was absorbed by the surrounding land during the summer — being concentrated at the surface and now exposed to long winter nights — radiates into the atmosphere rather rapidly, contributing to cold temperatures. These land-related conditions also apply to the coastal city. But something else of significance also affects the temperature of the atmosphere in the latter locale. Because the heat absorbed by the water body is distributed over a certain depth — and is therefore not concentrated at the surface and exposed to long winter nights — it is radiated back into the atmosphere at a much slower rate. In effect, it may serve as a source of atmospheric warmth for a significant portion of the winter, and result in a warmer winter for the coastal city.

Consider a comparison of Pierre, South Dakota and Portland, Maine. The former is in the middle of a continent, the latter is on the coast, and their latitudes are nearly the same. In Pierre, the coldest month of the year averages 17° F and the warmest month of the year averages 75° F. That makes a 58-degree annual temperature range (the difference between the coldest and warmest month). In Portland, the coldest month averages 23° F and the warmest month averages 68° F, making its annual average temperature range 45° F. So winters are harsher and summers are hotter in the mid-continent city.

Going with the Flow: Ocean Currents


The oceans have warm and cold surface currents that act like a global heating and air-conditioning system. They bring significant warmth to high latitude areas that would otherwise be much cooler, and significant coolness to low latitude areas that would otherwise be much warmer.

The currents also play a major role in determining the global geography of precipitation. The sun can more easily evaporate warm water than cold water, and thereby produce the atmospheric vapor that results in rain. Therefore, lands that get sideswiped or impacted by warm currents tend to have abundant precipitation in addition to a comparatively warm climate. Conversely, lands impacted by cold currents tend to receive very little precipitation in addition to a comparatively cool climate.

Generally, surface currents exhibit circular movements (see Figure 9-7). North of the equator, the flow is usually clockwise. South of the equator, the flow tends to be counter-clockwise. These movements are principally products of prevailing winds that “push” the ocean’s surface. On the map you can see occasional exceptions to the general rules of circulation. They are the results of deflections caused by the angle at which a current strikes a land mass or the continental shelf, or by the direction of prevailing sea level winds at particular latitudes.

Warm currents, cold currents

The warm and cold portions of these circulatory systems have rather predictable geographies. As ocean currents move westward along the equator, they absorb lots of solar energy, heat up, and become warm currents. As they turn away from the equator, they generally continue to absorb about as much heat as they dissipate, at least while they remain in the Tropics — that is, the region between the Tropic of Cancer and the Tropic of Capricorn.

After leaving the Tropics, the reverse starts to happen: the currents radiate more heat than they gain — but slowly for the reasons you read about in the previous section concerning the ability of water to store and retain heat. Thus, the currents remain comparatively warm longer after they have left the tropics. The Gulf Stream, for example, is a warm-water current that moves up the Eastern coast of the United States and then becomes the North Atlantic Current (see Figure 9-7). Although it loses a fair amount of heat as it moves eastward across the mid-Atlantic, the North Atlantic Current reaches Europe with a considerable amount of stored heat remaining. As it continues to radiate that heat, it contributes to the climate of Northwestern Europe a degree of warmth that is unusual for those latitudes, and also abundant rainfall.

Take a look at an example of an area that is affected by the North Atlantic Current. Bergen, Norway (Lat. 61°N) has an annual temperature of 45° F and receives 77 inches of precipitation per year. Compare that to Churchill, Manitoba (Lat. 58° N), which, as mentioned earlier in the chapter, has an annual average temperature of 19° F and only 15 inches annual precipitation. Bergen is significantly warmer — despite its high latitude — and much wetter. The difference is partly a matter of Churchill’s continentality, and partly a matter of the relatively warm current that sideswipes Bergen.

But the Gulf Stream-North Atlantic Current is not yet finished. After impacting Western Europe, the current turns south towards the equator, now as the Canaries Current, to complete its circulatory cycle. By that time, however, it has lost most of the heat it once had. As a result, the Canaries Current that sideswipes Northwest Africa is quite cool.

Casablanca, Morocco (Lat. 33° N), for example, has an annual average temperature of 63° F, which is comparatively cool for a country on the fringe of the Saharan realm. It also receives only 17 inches of precipitation per year. That is a paltry sum compared to the 77 inches that the same circulatory systems dumps on Bergen, and the 46 inches of precipitation that Charleston, SC, receives by being located near the Gulf Stream directly across the Atlantic.

Coastal deserts

Casablanca, Morocco, highlights one of the world’s most provocative geographic juxtapositions: places where oceans border deserts. Indeed, a couple of coastal deserts exist. What most have in common is a neighboring cold water current that makes evaporation difficult and rainfall unlikely.

Going against the norm: El Niño and La Niña

You should remember that climate is an average of yearly conditions, but that in any given year very “un-average-like” events can occur. El Niño and La Niña, which happen every so many years, provide good examples. (Niño and niña mean boy and girl in Spanish.) As you can see in Figure 9-8, during an El Niño, the surface waters become unusually warm in the tropical portion of the Pacific. The reasons for this are not fully understood; but because the conditions occur around Christmas in the waters off western South America, the local populace call it El Niño, referring to the Christ child. During La Niña, the opposite happens (“girl” being the opposite of “boy”) — the water is unusually cold.

Because the affected ocean water circulates, and also influences the behavior of atmospheric pressure belts (which you can read about in the next section), the impact can be substantial and widespread. Just what that means varies from place to place and year to year. Sometimes, for example, rainy seasons become extremely stormy and dry seasons become prolonged droughts. On the other hand, the effects are not always bad, as may be evidenced perhaps by a normally harsh winter that turns up mild. Generally, the media have cast “the boy” and “the girl” as climatological brats. In some times and places, however, they are the most pleasant kids you’d ever want to have around.

Living Under Pressure


You’re under pressure all the time — atmospheric pressure, that is. Just about everybody has seen a weather map with big “H’s” and “L’s” here and there. And just about everybody knows that they respectively stand for: high pressure and low pressure. But just about nobody understands what exactly they mean, except maybe that lows are associated with cloudy, rainy (or snowy) days, and highs usually are associated with pleasant, sunny days.

A low-pressure system is an area of relatively warm, moist ascending air. A high-pressure system is an area of relatively cool, dry descending air (see Figure 9-9). In general, therefore, you can think of low pressure as being a rainmaker, and high pressure as a drought-maker.

High pressure is so-named because the atmosphere is pressing down on the Earth. In contrast, low pressure is so-named because, due to its upward-moving air, the pressure (or weight) of the atmosphere against the Earth is comparatively low. Both are linked in a three-dimensional pattern of atmospheric circulation as shown in Figure 9-9.

Solar energy sets this circulatory system in motion. Some parts of Earth heat up more rapidly than others. Over those areas that do, the air tends to warm, expand, and rise. The vapor in the air cools as it rises in a convection current, causing condensation and (in all probability) precipitation to occur. Thus, low pressure is associated with cloudy, rainy (or snowy) conditions.

After precipitating, air at the top of a low-pressure system is cool, dry (having “lost” its moisture) and heavy. It wants to sink back down to Earth, but can’t because of other air coming up from underneath. Air in the upper atmosphere therefore moves laterally until it finds a place where it can descend as a high-pressure system composed of comparatively cool, clear, and dry (low humidity) air.

Pressure belts

Because the equatorial latitudes receive a greater degree of solar energy than anyplace else on Earth, a global “belt” of low-atmospheric pressure characterizes them (see Figure 9-10). This phenomenon is called the inter-tropical convergence zone (ITCZ), because air from the tropics north and south of the equator is drawn into (converges on) this zone before it rises in a convection current.

The result is a warm, humid “rainmaker” that produces the tropical climates presented in Chapter 10. As implied by Figure 9-9, the air that rises in this low-pressure belt must fall to Earth elsewhere. Generally, this occurs in two sub-tropical high-pressure belts that roughly correspond to Latitudes 25-30 North and South. Given these belts of “drought-makers,” it’s not surprising to see desert and semi-desert conditions over much of these latitudes.


Now you know how and why the sun’s vertical rays migrate north and south of the equator during the year. (You may wish to refer back to “Tilt-a-World: The Reasons for the Seasons,” earlier in this chapter.) Because the equatorial low-pressure belt is a product of those very same rays, it migrates as well, and so do the related subtropical highs. During the year, therefore, tropical latitudes may be alternately dominated by the low-pressure belt, which brings a rainy season, and one of the high-pressure belts, which brings a dry season.

Wet season-dry season transitions happen in many parts of the tropical world, the most well-known example being the monsoons of South Asia. As the vertical rays move northward during summer, the low-pressure belt moves with it, drawing moisture-laden air from the Indian Ocean and producing a pronounced rainy season. During winter, the ITCZ moves south of the equator. At that time the subtropical high pressure belt moves over south Asia and surface winds blow from the interior of the continent to the Indian Ocean, resulting in a relatively rainless period (see Figure 9-11).

For example, Mumbai (Bombay), India (Lat. 19° N) receives 72 inches of rain during June-September. In contrast, it receives less than half an inch during December-April. For some reason, many people associate “monsoon” strictly with the rainy season. In reality, there are two monsoons, wet and dry, and each significantly impacts the region, albeit in entirely different ways.

The wettest place on Earth?

Cherrapunji, India, has the wettest recorded climate of any settlement on Earth. I include the question mark with the title because there may be wetter locales that go unrecorded. In any event, it may be near impossible to beat 425 inches of precipitation per year. That’s Cherrapunji — more than an inch of rain per day on average. But even that doesn’t tell the whole story. Check out the following table, and pay attention to the substantial monthly variation.


Cherrapunji has a dry monsoon season that runs from November through February, during which it receives about 7 inches of rain. But then things change rather dramatically. In an average June and July, the town receives more than 3 inches of rain per day, before things taper off to a mere 2+ inches per day in August, and an inch-plus in September. Cherrapunji exemplifies the extreme conditions that can occur when two climatic determinants “pull together.” In this case, a fortuitously located low-pressure system and the effects of altitude combine. The town is 4,300 feet above sea. Thus, when the wet monsoonal winds are forced to rise in and around Cherrapunji, the results are very wet indeed.

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