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A Weather Counting Book_Cvr_SI.i2-3 2-3 15/3/06 4:03:22 PM Y s' L er ON ch n ea tio r t ec Fo sp in A Weather Counting Book_Txt_SI.i3 3 7/2/06 2:17:26 PM Y s' L er ON ch n ea tio r t ec Fo sp in Introduction Weather is the day-to-day change in the blanket of air that surrounds Earth. This blanket of air is called the atmosphere. Changes in the atmosphere bring about changes in the weather. Changes in temperature, wind, rain, and sunshine are all part of the weather of a place. We describe the weather in many different ways, such as sunny and warm, rainy and windy, or snowy and cold. At any time, the weather is different at different places on Earth. It may be rainy in one place, but sunny in another. Weather affects our lives every day. To know what clothes to wear or whether to stay inside or go outdoors, we need to know what the weather will be like. Scientists who study the weather try to forecast or predict what the weather might do next. Weather takes place in the lowest layer of Earth’s atmosphere. A Weather Counting Book_Txt_SI.i4 4 6/2/06 4:57:46 PM Y s' L er ON ch n ea tio r t ec Fo sp in One Sun Our planet Earth moves around one sun. The sun is a huge, glowing ball of fire. It is the nearest star to Earth and gives it heat and light. The temperature at the centre of the sun is about 15 million degrees Celsius. The weather on Earth is caused by the heat it gets from the sun. As Earth moves around the sun, different parts get different amounts of heat. Because Earth’s surface is curved, the sun’s rays do not hit all parts of Earth in the same way. The sun’s rays hit some parts of Earth more directly than others. The sun is a huge, glowing ball of burning gas. A Weather Counting Book_Txt_SI.i5 5 6/2/06 4:58:01 PM Y s' L er ON ch n ea tio r t ec Fo sp in Earth can be divided into two equal parts by an imaginary line. This line is called the equator. The equator runs across Earth’s centre. Places on Earth closest to the equator are the warmest. This is because these places get the most direct rays from the sun. It gets colder the further away a place is from the equator. Along the equator, days and nights are always the same length. Earth’s Poles North Pole Axis Equator South Pole A Weather Counting Book_Txt_SI.i6 6 6/2/06 4:58:15 PM Y s' L er ON ch n ea tio r t ec Fo sp in Two Poles Earth has two poles, called the North Pole and the South Pole. They are the two ends of an imaginary line called Earth’s axis. Earth is always spinning, or rotating, on this axis. The axis passes through the centre of Earth and ends at either pole. The north end of the axis is the North Pole and the south end of the axis is the South Pole. The two poles are the coldest places on Earth. This is because the sun’s rays do not hit Earth directly at the poles. So, very little heat from the sun reaches Earth at the poles. Land around Earth’s poles is covered with ice. A Weather Counting Book_Txt_SI.i7 7 6/2/06 4:58:31 PM Y s' L er ON ch n ea tio r t ec Fo sp in Three Steps in the Water Cycle The water on Earth moves in a cycle from the rivers, lakes, and oceans to the air, to the land, and back to the rivers, lakes, and oceans again. This is called the water cycle. There are three main steps in the water cycle. In the first step, the sun’s heat causes water to rise, or evaporate, into the air as moist vapour. The water changes from liquid water to invisible water vapour. This is called evaporation. The sun’s heat also causes plants to lose water through their leaves. This is called transpiration. In the second step, the moisture meets colder air above. The vapour grows cooler and condenses to form a cloud of water droplets. This is called condensation. When the droplets in a cloud are too heavy, the water falls back to Earth as rain or snow. This step is called precipitation. It may fall on land, or in the rivers, lakes, and oceans. If it falls on land, it will either run back into the rivers, lakes, and oceans, or soak into the soil to become groundwater. Because water moves in a cycle, the amount of water on Earth is always the same. A Weather Counting Book_Txt_SI.i8 8 14/3/06 4:03:17 PM