[MUSIC] Climate and weather are terms that are often used interchangeably, but they mean different things. Weather is the hour to hour and day to day variability in atmospheric conditions. This includes temperature, precipitation, humidity, air pressure and wind at any specific location and these conditions can change rapidly. In contrast, climate is the long-term average of weather and its extremes. While the climate of a region is generally predictable, weather is much more variable. For example, tropical climates tend to be hot and wet, but the weather on any particular day could be cold and dry. In this lesson, we'll focus on some of the forces that influence mountain climate. Mountain climates are often characterized by extremes in temperature, precipitation, wind and radiation. Consequently, climate is a huge influence on human activities as well as plant and animals that live in mountains. To understand how mountains and climate interact, we need to examine the forces that influence climate at two very different scales. First, we'll explore a large scale climate drivers, which are fundamentally determining how mountain climates vary around the globe. At this global scale, the three primary forces determining and regulating mountain climates are latitude or distance from the equator, elevation or height above sea level and continentality, which is the proximity or distance to large water bodies such as oceans. Later in this lesson, we'll consider the influence of smaller scale climate drivers which can determine local conditions within a mountain range or even on a single peak. And finally, we'll examine a biological manifestation of mountain climate, the alpine treeline. Mountain regions that are located closer to the equator have warmer climates than those of higher latitudes, but why are temperatures higher near the equator? Remember that the Earth is heated by absorbing solar radiation emitted by the Sun. The spherical nature of the Earth causes the amount of solar radiation that's received at the Earth's surface to differ across latitudes. Climates are warmer near the equator, because the Sun's ray hit the Earth directly concentrating solar radiation. In contrast, at higher latitudes, the Sun's rays hit the Earth at an angle causing the same solar radiation to be spread over a larger area. In addition to control mean temperature, latitude controls how much temperatures fluctuate within a year. This is called seasonality. At higher latitudes, seasonality is greater. A common misconception is that the Earth is closer to the Sun during the summer and further away from the Sun during the winter. Interestingly, the elliptical orbit of the Earth does cause the distance of the Earth from the Sun to vary seasonally. However, seasonal changes in the distance between the Earth and the Sun are small relative to how far away they are in space. In fact, in the northern hemisphere, the Earth is closer to the Sun during the winter and further away during the summer. Greater seasonality at higher latitudes is caused by three predictable factors, the tilt of the earth on its axis, the revolution of the Earth around the Sun and the greater variation in solar radiation. Let's explore how these three factors interact in more detail. Relative to the Sun, the Earth is tilted on its axis at an angle of 23.5 degrees. So as Earth completes its annual orbit around the Sun, the Northern and Southern Hemispheres are angled towards the Sun at opposite times of the year. Whichever hemisphere, North or South that is tilted towards the sun receives solar radiation most directly and this results in higher temperatures. In addition, this hemisphere will have more hours of daylight. And therefore, more hours during the day for absorbing solar radiation. The opposite pattern is true for the hemisphere that is angled away from the Sun. So for example, when the Northern Hemisphere is angled towards the Sun, locations in this region experience the long, warm days of summer. At the same time, locations in the Southern Hemisphere experience shorter and colder days of winter. Therefore, the tilt of the Earth causes higher latitudes to experience large variations in the amount of solar radiation received throughout the year. This results in large seasonal temperature fluctuations. In contrast, the amount of solar radiation received at the equator is less variable over the course of the year. So, seasonality is weaker. There is an exception to the general rule that temperatures decrease with increasing latitude. During summer in the Northern Hemisphere, solar radiation strikes the Earth most directly near the Tropic of Cancer around the latitude of 23.5 degrees North. Here, temperatures are higher during the summer than in the tropics near the equator. The opposite pattern occurs during the summer in the Southern Hemisphere where temperatures are often warmest near the Tropic of Capricorn. So, the main point to remember is that there is a very predictable temperature imbalance over the surface of our planet created by uneven solar radiation across the latitudes. The temperature imbalance created by uneven solar radiation across latitudes also controls global patterns of wind and precipitation. To understand what drives large scale movement of air on the surface of the Earth called atmospheric circulation, you need to be familiar with the relationships between air temperature, air density and atmospheric pressure. Of course, air temperature is just a measure of how hot or cold the air is. Air density is the compactness of molecules in air. Hot air is less dense than cool air. Because when molecules are heated, they move faster and bump into each other more frequently. This movement expands the space occupied by each molecule. The lower density of hot air causes it to float upward, this is how hot air balloons work. Heating the air within the balloon causes it to become less dense than the surrounding air and so it raises upward. In contrast, cooler denser air tends to sink. Atmosphere pressure is the downward force of air in the atmosphere cause by gravity pulling molecules of air towards the air. Lower air density leads to lower atmospheric pressure, because the more spread out air molecules are, the less force they exert. When a pocket of hot air rises up, an area of low atmospheric pressure is created beneath it. Differences in atmospheric pressure between areas create pressure gradients. Air tends to move along pressure gradients from areas of high pressure to low pressure, forming wind and driving global circulation patterns. We know that wind is generated by air moving along pressure gradients from areas of high to low pressure. Given this information, what direction do you expect winds to be blowing along latitudes immediately north and south of the equator? Near the equator, high temperatures create an area of low atmospheric pressure called the intertropical convergence zone. In contrast, lower temperatures north and south of the equator create areas of high pressure. Following the high to low pressure gradient, air flows along the surface of the Earth from higher latitudes towards the equator. At the same time as air moves across the surface of the Earth towards the equator, warm air that rises near the equator moves towards the poles, forming circulation cells. Circulation cells are belts that encircle the Earth in which prevailing winds occur. If the Earth did not rotate, there would only be two circulation cells. One in each hemisphere. However, the rotation of the Earth on its axis influences global circulation patterns causing there to be three circulation cells within each hemisphere. Warm air that rises at the equator and moves poleward sinks towards the ground around 30 degrees latitude, creating high pressure zones. These high pressure zones around 30 degrees cause air to diverge. Winds that flow back towards the equator complete the Hadley cell. It's easy to forget that these physical processes that affect the Earth's climate system have been known for almost 300 years since first being described by the English meteorologist George Hadley in 1735. Hadley cells are dynamic features of atmospheric circulation, where rising air near the equator flows towards the poles, 10 to 15 kilometers above the surface. And then, descends in the subtropics before returning to the equator closer to the surface. In contrast, winds that flow poleward from 30 degrees latitude form the circulation cells at midlatitude. The final circulation cells form above 60 degrees latitude, because cold winds flowing towards the equator meet warm winds flowing poleward. In addition to causing the formation of multiple circulation cells, the rotation of the Earth deflects winds so they do not flow directly north and south. This phenomena is called the Coriolis effect and it provides a simple explanation of why objects curve on the Earth when they should move straight. To understand the Coriolis effect, imagine you're standing in the middle of a merry-go-round or a carousel with a friend standing directly across from you. If the merry-go-round is stationary, you can throw a ball directly at your friend and it will reach them. However, if the merry-go-round is rotating, a direct throw will miss. Even though the path of the ball does not change, the position of your friend in space has moved. From their perspective, it appears that the ball's movement has been deflected from a straight trajectory. Similarly, as the earth rotates, the path that the winds are moving around the surface of the earth appear to be deflected. However, and this is the tricky part, the direction in which the winds appear to be deflected depends on which hemisphere you're in. Let's consider how this works. The Earth rotates from east to west. So the Earth spins counterclockwise when viewed from the North Pole and clockwise when viewed from the South Pole. In the northern hemisphere winds appear to be deflected to the right. And in the southern hemisphere winds appear to be deflected to the left. Let's look back to our map showing global circulation patterns using our knowledge of the Coriolis effect. With winds deflecting to the right in the northern hemisphere and to the left in the southern hemisphere. The winds in the Hadley cell that tend to blow west and towards the equator are called trade winds. Prevailing winds in the other circulation cells are named based on the direction the winds come from. At midlatitudes, the Westerlies blow from the west towards the east and upwards towards the poles. At high latitudes, the polar Easterlies blow from the east towards the west and towards the equator. In addition to determining the direction of prevailing winds, global circulation patterns influence the water cycle. The movement of water between the Earth's oceans, atmosphere, and land. Let's consider the link between circulation cells and patterns of precipitation across latitudes. Water is converted from its liquid state on the Earth to a gaseous state in the air through a process called evaporation. Evaporation increases at higher temperatures because, as we learned earlier, water molecules move faster when heated. Which allows more water molecules to escape into the atmosphere. Therefore, rising pockets of warm air in low pressure areas tend to contain a lot more moisture. However, as altitude increases, atmospheric pressure decreases because the weight of the overlying air is reduced. As a result, a rising pocket of air will expand and the heat it contains will be spread out causing it to cool. At lower temperatures, air molecules move slower causing many of them to convert back into a liquid state in a process called condensation. Condensation leads to cloud formation and precipitation. Therefore, the rising of moist, warm air causes areas of low atmospheric pressure to be associated with cloudiness and high levels of precipitation. In contrast, the sinking of cooler, drier air in areas of high atmospheric pressure causes clear skies and dry conditions to prevail in these regions. So now we can start to think about mountains again and examine the rule of elevation, which is the second factor that accounts for large scale controls on mountain climates. First, let's take a look at a map showing mean annual temperatures around the world. You should now be able to explain how latitude acts as one of the three main drivers of mountain climates at a global scale. The Himalayas, for example, located close to 30 degrees north and the Central Andes, located close to 30 degrees South, are in high pressure zones where many of the world's deserts are found. These mountains have more arid climates then, for example, Mount Kilimanjaro in East Africa and Mount Kinabalu in Borneo. Which are located close to low pressure zones near the equator where many of the world's tropical forests are located. However, remember that the amount of solar radiation received in the Himalayas and the Central Andes varies seasonally. For example, during summer in the northern hemisphere, latitudes around the Himalayas will receive the most solar radiation causing global circulation patterns and associated weather systems to shift north. As a result, precipitation is much higher in the Himalayas during summer resulting in wet monsoons. However, if you look closely, you may notice that some regions are cooler than others at the same latitude. Take a moment to inspect the map to see if you notice a pattern in these locations. You may recognize the regions that have cooler than expected climates at their latitude as mountain ranges. For example, the Himalayas and the Tibetan plateau, the Andes, and the American Rockies have lower temperatures than the surrounding regions. Cooler climates and mountain ranges occur because temperatures decrease with elevation. As elevation increases, atmospheric pressure decreases. Causing rising parcels of air to expand and cool. Heat is not lost to the atmosphere, it's simply spread out over a greater area. Another reason why temperatures are lower at higher elevations involves solar radiation. However, as you'll discover, solar radiation is not lower at high elevations. In fact, these locations actually receive more solar radiation than lower elevations. The atmosphere acts as a filter, screening out some types of incoming radiation including and most importantly ultraviolet radiation which can be harmful to life on Earth in high concentrations. Particles in the atmosphere also cause scattering of incoming radiation, so the amount of solar radiation that reaches the earth's surface is substantially reduced. This means that higher elevations receive more solar radiation including ultraviolet or UV radiation. So why doesn't greater exposure to solar radiation result in higher temperatures with elevation? [SOUND] The answer is that very little of the atmosphere is heated directly by absorbing solar radiation. Instead, most incoming solar radiation is either scattered in the atmosphere or passes through it and is absorbed by the earth. This is why the ground is often warmer than the air surrounding it. However, not all incoming solar radiation is absorbed by the earth. Some of it is radiated back into the atmosphere where it's then absorbed by gases. Therefore, the planet is the source of heat that warms the atmosphere and temperatures decrease with distance from the earth's surface. Although mountains also absorb solar radiation, the ability of the earth to modify the climate depends on land area. Relatively small, isolated mountain peeks have little influence on the temperature of the atmosphere. Nonetheless, large mountains that are massed together do influence regional climates, a phenomenon called the mountain mass effect. The third large-scale control on mountain climates is their proximity to large bodies of water such as oceans. This is called continentality. The further inland you are, the more continental your location. Water bodies moderate climate by curbing temperature extremes because water heats and cools more slowly than land. As a result, daily and seasonal temperature differences are smaller in coastline mountain ranges relative to landlocked ones. Mountains also act as barriers that intercept air masses, forcing them upward, leading to cloud formation and increasing precipitation relative to low-lying areas. Precipitation that occurs when topographic barriers force air to rise are called orographic precipitation. Mountains in maritime and coastal areas receive more precipitation because air masses coming off oceans contain more moisture than air masses moving over land. Therefore, continental mountains experience greater temperature extremes, less cloudiness, and less precipitation than coastal mountains. Fewer clouds in continental interiors also permit more solar radiation to reach the surface in these locations. Continentality also influences climate within a mountain range because mountains connect as barriers, blocking the flow of air masses. The side of a mountain that faces the prevailing winds, called the windward side, receives a lot of orographic precipitation. In general, precipitation increases as you move up a mountain because decreasing temperatures lead to greater condensation. However, beyond a certain elevation, precipitation tends to decrease because most of the moisture has already condensed. Once the moisture content of the air becomes exhausted, clouds begin to break up causing mountain peaks to be quite dry. Consequently, by the time air reaches the opposite side of the mountain that's sheltered from the wind, called the leeward side, most of the moisture has been lost. Therefore, the leeward side of a mountain receives much less rain, producing a rain shadow. Also, as the air descends on the leeward side of the mountain, it encounters higher atmospheric pressure and compresses, causing it to warm. The resulting dry, warm winds that flow downslope are called foehn or Chinook winds. For example, the Sierra Nevada Mountains in California prevent the spread of the marine climate inland, allowing the continental climate to extend closer to the coast than it otherwise would. The extension of the continental climate in this region has led to the formation of the Mojave desert. However, the potential for mountains to block the spread of marine climates inland depends on their orientation relative to the prevailing winds. The Sierra Nevada Mountains block the spread of marine climate inland because their north-south orientation runs perpendicular to the prevailing Westerlies. In contrast, the east-west orientation of the Alps runs parallel to the Westerlies, allowing the marine climate to extend further inland in Europe.
