Answer: A biome is a plant and animal community that covers a large geographical area. The boundaries of different biomes on land are determined mainly by climate. Therefore, a biome can be defined as the total assemblage of plant and animal species interacting within specific conditions such as rainfall, temperature, humidity, and soil conditions. There are five major biomes i.e. forest, desert, grassland, aquatic, and altitudinal biomes. Their characteristics can be outlined as under:

• Aquatic biomes: These include both freshwater and marine biomes. Freshwater biomes are bodies of water surrounded by land-such as ponds, rivers, and lakes that have a salt content of less than one percent. Marine biomes cover close to three-quarters of Earth’s surface. Marine biomes include the ocean, coral reefs, and estuaries.
• Grasslands: These are open regions dominated by grass, having a warm, dry climate. They can be of two types: tropical grasslands/Savannas and temperate grasslands.
  • Savannas are found closer to the equator and can have a few scattered trees. Temperate grasslands are found further away from the equator. They do not have any trees or shrubs and receive less precipitation than savannas. For instance, prairies and steppes are two types of temperate grasslands.
• Forests: Forests are dominated by trees, and cover about one-third of the Earth. They contain much of the world’s terrestrial biodiversity, including insects, birds, and mammals. Based on latitudinal location and climate, the three major forest biomes are temperate forests, tropical forests, and boreal forests (also known as the taiga).
  • Tropical forests are warm, humid, and found close to the equator. Temperate forests are found at higher latitudes and experience all four seasons. Boreal forests are found at even higher latitudes and have the coldest and driest climate, where precipitation occurs primarily in the form of snow.
• Deserts: Deserts are dry areas where rainfall is less than 50 cm (20 inches) per year. They cover around 20 percent of Earth’s surface. Because of their extreme conditions, there is not as much biodiversity found in deserts as in other biomes. Any vegetation and wildlife living in a desert must have special adaptations for surviving in a dry environment. Here, wildlife consists primarily of reptiles and small mammals. According to their geographic location or climatic conditions, deserts can fall into four categories: hot and dry, semiarid, coastal, and cold.
• Altitudinal Biomes: These have extremely inhospitable conditions, with the lowest measured average yearly temperatures of any of the five major biomes ranging from -34 to 12 degrees Celsius. Precipitation is just 15-25 centimeters (six to ten inches) per year, as soil nutrients are of poor quality and summers are short. They can be classified into arctic and alpine.
  • The arctic biome is found north of boreal forests and the alpine biome is found on mountains where the altitude is too high for trees to survive. These do not have much biodiversity and vegetation is simple, including shrubs, grasses, mosses, and lichens. This is partly due to a frozen layer under the soil surface, called permafrost.

Answer: The Earth is surrounded by a thin layer of air called the atmosphere. The air in the atmosphere moves in response to differences in temperature at the equator (warm) and the poles (cold). This movement of air is called global atmospheric circulation. The movement of air across the planet occurs in a specific pattern that is dependent on:

• Latitudinal variation of atmospheric heating.
• The emergence of pressure belts.
• The migration of belts following the apparent path of the sun.
• The distribution of continents and oceans.
• The rotation of the Earth.

The whole system is driven by the equator, which is the hottest part of the Earth. Air rises at the equator, leading to the creation of low pressure and rainfall. When the air reaches the edge of the troposphere (tropopause), it cannot go any further and so it travels to the north and south. The air becomes colder and denser, and falls, creating high pressure and dry conditions at around 30° north and south of the equator. Large cells of air are created in this way. The following are the 3 major cells:

• The Hadley Cell: This is driven by warm rising air. The Coriolis effect causes the air that has risen at the equator and moving to the poles to deflect and become increasingly westerly high up in the atmosphere that the circulation breaks down and the air sinks back towards the ground in the subtropics (30° the ‘horse latitudes’). Once the air has descended back to the ground, it returns to the equator and is deflected to the east – they are known as the easterly trade winds.
• Polar Cell: The same thing happens in the polar regions, but it is driven by sinking cold air. The sinking, cold air at the poles moves back towards the equator, but because it is slow-moving, it becomes increasingly easterly as the underlying earth moves faster, and the circulation eventually breaks down at around 70° latitude. The air then rises again and returns to the poles, resulting in another atmospheric cell.

Ferrel Cell: This lies in between the Polar and Hadley cells, and is a little more complicated. Put simply, it is the net effect of air motions from all the storms or ‘depressions’ that occur in the mid-latitudes. Air sinks in the sub-tropics and rises around 60-70°. In this region, westerly surface winds occur. These cells drive airflow, atmospheric pressure, and rainfall. The rising and sinking of air cause high and low pressure at the surface, respectively. The large-scale winds of the atmosphere initiate large and slow-moving currents of the ocean. Oceans in turn provide input of energy and water vapour into the air. These interactions take place rather slowly over a large part of the ocean.