Climate versus Weather

Climate versus Weather What is climate? Climate is the average weather usually taken over a 30-year time period for a particular region and time period. Climate is not the same as weather, but rather, it is the average pattern of weather for a particular region. Weather describes the short-term state of the atmosphere. What is our climate system? Atmosphere Oceans Land Ice Biosphere The atmosphere covers the Earth. It is a thin layer of mixed gases which make up the air we breathe. This thin layer also helps the Earth from becoming too hot or too cold. Oceans cover about 70 percent of Earth's surface. Their large size and thermal properties allow them to store a lot of heat. Land covers 27 percent of Earth's surface and land topography influences weather patterns. Ice is the world's largest supply of freshwater. It covers the remaining 3 percent of Earth's surface including most of Antarctica and Greenland. Ice plays an important role in regulating climate, because it is highly reflective. The biosphere is the part of Earth's atmosphere, land, and oceans that supports any living plant, animal, or organism. It is the place where plants and animals, including humans, live. What is weather? The weather is just the state of the atmosphere at any time, including things such as temperature, precipitation, air pressure and cloud cover. Daily changes in the weather are due to winds and storms. Seasonal changes are due to the Earth revolving around the sun. P1

CLIMATE ZONES A climate zone is an area that has similar average temperatures and precipitation throughout. Three of Earth s climate zones are tropical, temperate, and polar. The tropical, temperate, and polar climate zones are based on distance from the equator. The equator is the imaginary line that divides Earth into its northern and southern hemispheres, or halves. An area s latitude, or distance from the equator, determines what climate zone it is in. POLAR ZONE 90 o -60 o latitude Cool summers, cold year-round Dry TEMPERATE 60 o -30 o latitude True Seasons Variety of climate patterns Moderate precipitation (rain/snow) TROPICAL 30 o - equator No winter, warm year-round High temp, rainfall, humidity P2

What causes weather? Because the Earth is round and not flat, the Sun's rays don't fall evenly on the land and oceans. The Sun shines more directly near the equator bringing these areas more warmth. However, the polar regions are at such an angle to the Sun that they get little or no sunlight during the winter, causing colder temperatures. These differences in temperature create a restless movement of air and water in great swirling currents to distribute heat energy from the Sun across the planet. When air in one region is warmer than the surrounding air, it becomes less dense and begins to rise, drawing more air in underneath. Elsewhere, cooler denser air sinks, pushing air outward to flow along the surface and complete the cycle. Air Masses Air masses are large bodies of air. Their temperature and humidity are affected by the areas over which they form. Humidity is the amount of water vapor in the air. For example, the Gulf of Mexico is a warm sea. An air mass that forms over this area is warm and wet. Air masses can cover huge areas. Air masses affect the weather of an area. Maritime air masses are humid (hold lots of moisture), because they form over the ocean. Continental air masses are dry because they form over land. Tropical air masses are warm because they form over the tropics. Polar air masses are cold because they form over Arctic areas. Air masses are named according to their temperature and humidity. An air mass that forms over the Gulf of Mexico is warm and humid, so it is called a tropical maritime air mass. P3

Gravity pulls the gasses in the atmosphere towards Earth, causing the air to push down on Earth s surface. Air Pressure is the weight of air pressing down on a unit of area. Differences in air pressure at Earth s surface are caused by unequal heating of Earth. Warm air masses are less dense and have lower air pressure than cold air masses. Because they are less dense, warm air masses will rise and cold air masses will sink. A change in air pressure is a sign that a different air mass is moving into an area, which will cause changes in the weather. Weather Fronts As air masses move across the surface of Earth, they may collide. These collisions usually happen halfway between one of the poles and the equator. The two air masses don t mix much when they collide. Each keeps its own temperature, humidity, and air pressure. As a result, a boundary forms between the two air masses called a front. When the two air masses collide at a front, the warmer, less-dense air will always be pushed up over the cooler, denser air. The result is that the weather changes. Warm Fronts: When a warm air mass moves into an area of cooler air. The warmer, less-dense air slides up over the cooler air, and usually brings steady rain. Cold Fronts: When a cold air mass moves into an area of warmer air. The colder, denser air slides under the warmer air and forms clouds that bring precipitation. Stationary Fronts: When two air masses are not moving against each other. Some mixing of the air masses occurs and we can get light rain or snow that lasts for a long time. P4

Rain How does rain form? Water droplets form from warm air. As the warm air rises in the sky it cools. Water vapor (invisible water in the air) always exists in our air. Warm air holds quite a bit of water. For example, in the summer it is usually very humid. When enough of these droplets collect together, we see them as clouds. If the clouds are big enough and have enough water droplets, the droplets bang together and form even bigger drops. When the drops get heavy, they fall because of gravity, and you see and feel rain. What causes rain? When clouds develop or rain occurs, something is making the air rise. Several things can make this happen. Mountains, low-pressure areas, cold fronts, and even the jet stream. How big are raindrops? Raindrops are much smaller than we think! They are actually smaller than a centimeter. Raindrops range from 1/100 inch (.0254 centimeter) to 1/4 inch (.635 centimeter) in diameter. Clouds What are clouds? A cloud is a large collection of very tiny droplets of water or ice crystals. The droplets are so small and light that they can float in the air. How are clouds formed? All air contains water, but near the ground it is usually in the form of an invisible gas called water vapor. When warm air rises, it expands and cools. Cool air can't hold as much water vapor as warm air, so some of the vapor condenses onto tiny pieces of dust that are floating in the air and forms a tiny droplet around each dust particle. When billions of these droplets come together they become a visible cloud. P5

Winter Storms How do winter storms form? Winter storms derive their energy from the clash of two air masses of different temperatures and moisture levels. Winter storms usually form when an air mass of cold, dry, Canadian air moves south and interacts with a warm, moist air mass moving north from the Gulf of Mexico. The point where these two air masses meet is called a front. If cold air advances and pushes away the warm air, it forms a cold front. When warm air advances, it rides up over the denser, cold air mass to form a warm front. If neither air mass advances, it forms a stationary front. Thunderstorms What is a thunderstorm? A thunderstorm is a storm with lightning and thunder. Its produced by a cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail. What causes a thunderstorm? The basic ingredients used to make a thunderstorm are moisture, unstable air and lift. You need moisture to form clouds and rain. You need unstable air that is relatively warm and can rise rapidly. Finally, you need lift. This can form from fronts, sea breezes or mountains. P6

Global Wind Patterns Wind is the movement of air from an area of higher pressure to an area of lower pressure. Recall that these differences in air pressure are caused by the uneven heating of the Earth. As air cools, air pressure increases. As air warms, air pressure decreases. What causes the wind to blow? As the sun warms the Earth's surface, the atmosphere warms too. Some parts of the Earth receive direct rays from the sun all year and are always warm. Other places receive indirect rays, so the climate is colder. Warm air, which weighs less than cold air, rises. Then cool air moves in and replaces the rising warm air. This movement of air is what makes the wind blow. A jet stream is a band of very strong wind that blows from east to west high in the atmosphere, usually above 6000 m (20,000 ft). Jet streams form along the upper boundaries of large air masses when warm air from the tropics meets cold air from the poles. The sudden temperature change causes a huge difference in air pressure. This results in wind speeds of up to 498 km/hr (310 mi/hr). Global winds are winds that blow across long distances in predictable patterns. These winds carry air masses from one region to another. Each of these winds always blows in the same direction. Global winds and ocean currents do not follow a straight path as they flow from the poles toward the equator or from the equator toward the poles. They curve clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere because of Earth's rotation on its axis. This curving of the winds and ocean currents is called the Coriolis effect. P7

Global winds and jet streams can affect local weather. For example, the prevailing westerlies move from the Pacific Ocean along the Pacific Coast of the United States. They bring moist, mild weather. The westerlies bring most of the weather changes across the country from west to east. During the winter, a jet stream often dips south into the United States, bringing cold weather from Canada Oceans and Climate Oceans cover nearly three-fourths of Earth's surface. Because of this, oceans have important effects on Earth's climate. In fact, without oceans, Earth's climate would be too harsh to support life. This is because water takes much longer to heat up and cool down than land does. Because water takes longer to heat up, it helps keep land cooler during the summer. And because water takes longer to cool down, it helps keep land warmer during the winter. This is why seasons are more extreme in the middle of continents. The differences in water and land temperatures also cause winds to form. During the day, land temperatures rise faster than water temperatures. As the air warms, it becomes less dense. The denser, cooler air over the ocean moves toward the land and the warm air moves upward. This is how a sea breeze forms. The air on land stays cooler during the day because of the sea breeze. The opposite situation occurs at night. The land cools down more quickly than the ocean water. The warmer air over the ocean moves upward and the cooler air over land moves in to take its place. This movement of air is called a land breeze. P8

Earth's oceans also affect climate in another way. Suppose you placed a beach ball in the middle of the ocean. Even if there was no wind, would the ball stay in that same spot? Probably not. That's because ocean water flows in steady, streamlike movements known as currents. Currents are caused by many factors, including wind, gravity, heating from the sun, and Earth's rotation. Surface currents, or currents near the surface of the ocean, are produced by global winds. Global winds move in fairly regular patterns, so surface currents move in regular patterns, also. The movements of and also the temperatures of surface currents can be predicted easily. Because of the Coriolis effect, warm-water currents, such as the Gulf Stream, begin in tropical regions. The Gulf Stream flows northeast from the Caribbean Sea. It carries warm water across the North Atlantic toward Europe. During the winter, the warm Gulf Stream helps keep Europe s climate mild. P9

Currents can also cause warm climates to cool down. Cool-water currents, such as the California Current, form near the poles and flow toward the equator. This current then flows southward down the West Coast of the United States. The current helps keep summers along the West Coast cool. The current also keeps the ocean water on the West Coast much cooler than the ocean water on the East Coast of the United States. Sun and Atmosphere You may have noticed that when night falls, the outdoor air temperature usually becomes cooler. This is because the sun heats Earth's atmosphere just as it does the surface. However, some parts of Earth are warmer or colder than other parts. What causes these differences? The sun does not heat Earth evenly. Because Earth is round, sunlight strikes Earth at different angles. It strikes the equator from almost directly overhead, at a 90 angle to the surface. This concentrates the energy over a small area and results in warm temperatures. At the equator, the air is warmed more than the air to the north or the south. The sun's rays strike the poles indirectly, at a lower angle to the surface. This spreads out the energy over a larger area and results in lower temperatures. The level of energy hitting the equator and the poles is the same, but the energy is more spread out at the poles. P10

The uneven heating of Earth causes the movement of large bodies of air called air masses. The air masses over Canada are usually colder and drier than the air masses over the Gulf of Mexico. The movements of air masses cause changes in the weather. SEASONS Does the weather change much from season to season where you live? Why do seasons change from winter to spring, summer to fall? The temperature changes that take place from season to season are caused by a change in the way the sun s rays hit Earth s surface. When the rays hit a spot on Earth s surface directly, the surface absorbs a lot of energy. This heats the air above the surface, causing air temperatures to rise. As Earth revolves, the sun s rays begin to hit the same spot at an angle. When this happens, the surface absorbs less energy and temperatures drop. What changes the way the rays hit Earth s surface? It s the tilt of Earth s axis. As Earth revolves around the sun, this tilt is always the same 23.5 degrees. As a result, the Northern Hemisphere is sometimes tilted toward the sun or away from the sun. There are times during an orbit when both the Northern Hemisphere and the Southern Hemisphere receive light at nearly the same angle. P11

When the Northern Hemisphere is tilted toward the sun, the rays hit this part of Earth s surface more directly. As a result, there are more hours of daylight, and the surface absorbs more heat. This heat, in turn, warms the atmosphere, resulting in summer. As Earth continues revolving, it reaches a point where neither hemisphere is tilted toward or away from the sun it is spring or fall. Now days and nights are nearly of equal length. The sun s rays begin to hit the Northern Hemisphere at a greater angle, causing temperatures to start dropping and the season of fall to begin. As Earth keeps moving through its orbit, the Northern Hemisphere is tilted farther away from the sun. The sun s indirect rays provide less energy than when the sun shines more directly on an area, resulting in winter s low temperatures. The Northern Hemisphere receives fewer hours of sunlight each day, resulting in more hours of darkness. The sun appears to be lower in the sky. The year goes on, and Earth continues in its orbit, again reaching a point at which its hemispheres are not tilted either toward or away from the sun. Now the days and nights are of nearly equal lengths again. The sun s rays begin to hit the Northern Hemisphere more directly, warming the air, and spring begins. As Earth keeps following its orbit, the Northern Hemisphere has more hours of sunlight, and spring turns into summer. During all this time, the Southern Hemisphere experiences the opposite seasons. P12