The North and South Poles have two seasons: summer and winter. Each season lasts for six months. The difference between the two seasons is more a matter of daylight than of temperature. Although both seasons are cold, the temperature is lower in winter than in summer at the poles. In summer, there is 24 hours of sunlight (no nights), whereas in winter there is 24 hours of darkness (no daylight).
The axis of rotation of the Earth is tilted, and this can be seen clearly in a globe. Around June 21, the axis points clearly towards the sun. As a result, a large part of the northern hemisphere is illuminated by a strong sun and daytime remains long till the rotation of the Earth causes this area to move away from the direct glare of sunlight. The place, thus, has long hours of sunlight and the days are long. In fact, the sun shines over the North Pole continuously for several weeks without setting even once. This period is known as the ‘polar day’. During Christmas, the axis of the Earth points away from the sun. During these days, the northern hemisphere remains dark for a longer period of time and a ‘polar night’ sets in at the North Pole. On the other hand, the southern sphere enjoys long hours of sun.
One rotation of the Earth takes one day. If at 12 noon the sun is at the highest point in the sky, then after 24 hours it will come back again to the same position. It was the Babylonians who first divided the day into 24 hours. They could have counted the hours from 1 to 10, then to 20, and so on, but instead they calculated from 1 to 60, then to 120, and so on. For this reason, time is divided into parts of 60 (1 hour has 60 minutes, 1 minute has 60 seconds) or in parts of 6 (one day has 24 hours, i.e., 4 X 6 hours).
The rotation of the Earth on its axis causes the alternation of day and night. The Earth also revolves around the sun. As a result of both these movements, the Earth does not get continuous light from the sun. This means when countries such as India faces the sun and has a bright day, the areas on the other side of the globe, such as the United States, are immersed in complete darkness. Sunshine duration per day depends on the season, i.e., the distance between the Earth and the sun.
Water evaporates when the sun shines on oceans, seas, or wet grounds or when wind blows over them. The water vapour rises up and gets converted again into tiny droplets on reaching the cold heights of the sky.
The reason for this is that light travels faster than sound. The speed of light is 300,000 km per second. On the contrary, sound is slow and covers only about 330 m in one second, i.e., one kilometre in three seconds. If we count the seconds till we hear the thunder after seeing the flash of lighting in the sky and divide this value by three, we will know how many kilometres the storm is away from us.
Accumulation of moisture in the atmosphere causes a thunderstorm. This occurs mostly in summer when the sun is hot and keeps soaking up water from the ground and water bodies. The moist air rises and forms small clouds that pile up gradually to form huge cauliflower-shaped clouds as high as 10 km. Inside the clouds, large water drops and ice crystals form due to extreme cold conditions. These remain afloat, being supported by powerful winds. When they collide and rub against one another, an electrical voltage is produced between the upper and lower layers of clouds. This voltage, once sufficiently high, is discharged in the form of lightning.
When we talk about cyclones, we do not usually mean the small destructive tornadoes, but instead the large tropical storms, which the coastal inhabitants face. In America, these are known as ‘hurricanes’, in East Asia ‘typhoons’, and in India ‘cyclones’. Hurricanes occur in early spring in the warm waters around Africa. During this season, copious amounts of water evaporate rise upwards, and forms clouds. This sharply reduces the pressure directly above the sea, allowing more moist air to flow in from the surrounding areas. Due to the rotation of the Earth, bundles of cloud start rotating and an air vortex is created. This vortex moves fast over the sea and wrecks widespread havoc on reaching the land.
The weather is predicted by meteorologists—scientists who monitor weather conditions. They study the factors that lead to weather phenomena in different parts of the world. They observe and record atmospheric temperature, humidity, pressure, and precipitation and the direction and strength of winds. Weather balloons and weather stations give out information regarding the state of the atmosphere. On the basis of their findings, they make forecasts about future weather conditions.
Universe, cosmos, and space mean the same - the space all around us and what it contains as far as we can see and think. We do not know whether the space is endless or is finite. If there were a border, what would be behind it? The astronomers are still dealing with the question of the origin of the universe and its end. An important factor to support the Big Bang theory of the origin of the universe is the ‘cosmic background radiation’, which was discovered in 1964. We still wonder whether there are intelligent civilizations somewhere in space - and are looking for them.
Unlike climate, weather depends on the seasons and can also change anytime. Meteorologists predict in the evening how the weather will be the next day. The weather is driven by the sun, which heats up the ground and the water surfaces. Evaporated water creates clouds from which thunderstorms build up, and the ensuing bursts of thunder and lightning make us flinch. Every time a cyclone hits the tropical regions, lashing rain sends people running for cover.
There have been many ice ages in the entire history of the Earth. During these cold phases, the average temperature of the Earth dropped 5°C below the normal level and the Polar Regions were completely covered with ice. The last big cold period, known as ‘ice age’, ended around 10,000 years ago, with the thawing of the large glaciers that were several kilometres thick. It is not yet clear what causes an ice age. Researchers believe that the sudden change in the Earth’s climate may be a result of some sort of a disturbance in the orbit of the Earth or a reduction in the heat of the sun reaching the Earth. Another theory suggests that, prior to the ice age; the continents might have changed positions and blocked or reduced the flow of warm currents from the equator to the poles, and thus allowed ice sheets to form in the region.
Hot or cold ocean currents affect the climate of the landmasses along their course. The Agulhas current, for instance, long prevented Portuguese sailors from reaching India. Ocean currents also affect temperatures all across the world. The current that brings warm water up the North Atlantic Ocean to the north-west European shores stops ice from forming near the coasts and blocking ships from entering and exiting ports. In northern Canada, cold ocean currents freeze the soil all through the year. The cold Benguela current that runs off the coast of Namibia has created one of the driest deserts of the world - the Namib Desert. This current does not allow the sea water to evaporate, and thus prevents the formation of clouds in the local sky.
The trade wind is a wind that blows throughout the year in the tropics, a broad belt around the equator. In the northern hemisphere, it blows from the northeast to the equator, and in the southern hemisphere, it blows from the southeast to the equator. The sun, being directly overhead, is very strong and heats up the air in the equatorial region. The warm air automatically rises up, reducing the atmospheric pressure and allowing the surrounding air to flow in towards the equator. This flow of air is called the ‘trade wind’.
While the weather changes every day and a hot and humid day can be followed by a rainy one, the climate of a region remains the same for a long time. Thus, the summer season at the North Sea is generally cooler than in Freiburg and the winter season is rather cold and damp and not as snowy as in the Alps. The climate is driven by the sun, which heats up the air and gives rise to winds such as the trade wind. The oceans and the ocean currents also affect world climate, an instance of which can be seen in the way things turn topsy-turvy due to occurrence of ‘El Nino’. The climate also undergoes long-term changes.