CBSE Class 7 Social Science Atmospheric Pressure, Winds and Rainfall Worksheet Set 03

Answer: (b) gravitational pull. Gravity pulls air towards the Earth, so most air is packed near the surface.

Teacher's Note:
1. Density of air is greatest at sea level.
2. Gravity holds the layers of air close to the Earth.

Answer: (b) rotation. Rotation is the turning on the axis; revolution is the movement around the Sun.

Teacher's Note:
1. Rotation causes day and night.
2. Do not confuse it with revolution, which takes a year.

Answer: (a) millibars. Millibar (or hectopascal) is the unit for atmospheric pressure.

Teacher's Note:
1. Average sea level pressure is about 1013 millibars.
2. The instrument used is a barometer.

Answer: (c) barometer. A barometer measures atmospheric pressure.

Teacher's Note:
1. Hygrometer measures humidity and rain gauge measures rainfall.
2. The word barometer comes from the Greek word for weight.

Answer: (b) doldrums. Strong heating makes the air rise at the equator, leaving calm, low pressure doldrums.

Teacher's Note:
1. Sailing ships often got stuck in the doldrums.
2. Winds are weak and variable in this belt.

Answer: (b) summer. It blows from June to September, in the summer season.

Teacher's Note:
1. It brings most of India's annual rainfall.
2. The north-east monsoon is the winter monsoon.

Answer: (b) relief (orographic). Air forced up a mountain slope cools, condenses and rains on the windward side.

Teacher's Note:
1. Relief rainfall is also called orographic rainfall.
2. The Western Ghats get this type of rain.

Answer: (a) Dew. Water vapour condenses into dew, fog, frost and clouds.

Teacher's Note:
1. Dew forms on cool surfaces at night.
2. Evaporation is the opposite change, liquid to gas.

Answer: True. Air moves from a high pressure area to a low pressure area, and this movement is wind.

Teacher's Note:
1. Greater the pressure difference, stronger the wind.
2. Vertical air movement is called an air current.

Answer: False. In the northern hemisphere the winds spiral anticlockwise into a cyclone.

Teacher's Note:
1. The Coriolis effect deflects winds to the right in the north.
2. In the southern hemisphere a cyclone turns clockwise.

Answer: True. Warm air expands and rises, so it exerts less pressure on the surface.

Teacher's Note:
1. Cold air is heavier and creates high pressure.
2. This is why the equator has low pressure.

Answer: True. Heat energy turns water from oceans, rivers and lakes into vapour.

Teacher's Note:
1. Evaporation is faster in warm, dry and windy conditions.
2. It is the first step of the water cycle.

Answer: 100. The air then holds the maximum water vapour it can at that temperature.

Teacher's Note:
1. Relative humidity compares the vapour present with the most the air can hold.
2. Cooling saturated air leads to condensation.

Answer: Cyclonic (frontal). The warm air is lifted over the cold air, cools and condenses.

Teacher's Note:
1. Frontal rainfall is common in cyclones.
2. The boundary between the two air masses is a front.

Answer: Atmospheric pressure is the weight of the column of air pressing on a unit area of the Earth's surface. It is highest at sea level and falls as we go up.

Teacher's Note:
1. Air has weight because it is made of gases.
2. Pressure is measured with a barometer.

Answer: Dew point is the temperature at which air becomes saturated and its water vapour starts to condense into tiny drops. It is lower for dry air and higher for humid air.

Teacher's Note:
1. Dew forms when surfaces cool below the dew point.
2. Meteorologists use it to judge how humid the air is.

Answer: They are belts of high pressure near 30°N and 30°S where air sinks and winds are light and calm. They lie between the trade winds and the westerlies.

Teacher's Note:
1. Sinking air gives dry, clear weather.
2. Many of the world's hot deserts lie near these latitudes.

Answer: The three types are convectional, relief (orographic) and cyclonic (frontal) rainfall. They differ in how the moist air is made to rise.

Teacher's Note:
1. Rising air cools and forms clouds in all three cases.
2. Convectional rain falls in the tropics, usually in the afternoon.

Answer: Dew and fog are two forms; frost and clouds are others. All form when water vapour cools and turns to liquid or ice.

Teacher's Note:
1. Fog is a cloud close to the ground.
2. Frost forms when the dew point is below freezing.

Answer: Humidity is the amount of water vapour present in the air. It is measured by a hygrometer.

Teacher's Note:
1. Absolute humidity is the actual mass of vapour in a unit volume of air.
2. Warm air can hold more vapour than cold air.

Answer: The Coriolis effect is the deflection of winds caused by the Earth's rotation. Winds turn to the right in the northern hemisphere and to the left in the southern hemisphere.

Teacher's Note:
1. Without rotation, winds would blow straight from high to low pressure.
2. It is named after the French scientist Gaspard Coriolis.

Answer: Sea breeze (day): 1. Land heats faster than the sea. 2. Air over land warms, rises and creates low pressure. 3. Cooler, higher pressure air from the sea blows towards the land.
Land breeze (night): 1. Land cools faster than the sea. 2. Air over the warmer sea rises, creating low pressure. 3. Air from the cooler land blows towards the sea.

Teacher's Note:
1. Both are local winds caused by unequal heating of land and water.
2. The wind always moves from high to low pressure.

Answer: 1. Altitude: pressure decreases as we go higher because less air lies above.
2. Temperature: warm air rises and gives low pressure; cold air sinks and gives high pressure.
3. Water vapour: moist air is lighter than dry air, so humid areas have lower pressure.

Teacher's Note:
1. Altitude is the most important factor for a small area.
2. These factors explain the world's pressure belts.

Answer: Planetary winds blow throughout the year in the same direction over large areas, for example the trade winds and the westerlies. Seasonal winds change direction with the season, for example the monsoon, which blows from the south-west in summer and from the north-east in winter.

Teacher's Note:
1. Planetary winds depend on the pressure belts and the Coriolis effect.
2. Seasonal winds depend on the changing heating of land and sea.

Answer: A cyclone is a system of low pressure with winds spiralling inwards, usually bringing clouds and rain. An anticyclone is a high pressure system with winds spiralling outwards from the centre and it brings clear, dry weather. In the northern hemisphere a cyclone turns anticlockwise and an anticyclone clockwise.

Teacher's Note:
1. Pressure at the centre is the key difference.
2. Tropical cyclones can cause heavy rain and strong winds on coasts.

Answer: The Sun's rays fall almost vertically at the equator, so the land and sea are heated strongly all year. The air warms, expands and rises, leaving a low pressure belt at the surface. The rising air cools, condenses and brings heavy rain.

Teacher's Note:
1. Rising warm air always reduces surface pressure.
2. This belt of calm winds is called the doldrums.

Answer: Moist winds from the Arabian Sea rise along the western (windward) slope, cool and give heavy relief rainfall. After crossing the crest, the air descends on the eastern (leeward) side, warms up and has little moisture left. So the eastern side lies in a rain shadow.

Teacher's Note:
1. Rain shadow areas are on the sheltered side of mountains.
2. Pune and the Deccan plateau are examples.

Answer: The Sun's heat evaporates water from seas, rivers and plants. The vapour rises, cools and condenses into clouds. When the droplets grow heavy, they fall as precipitation (rain, snow or hail) and flow back to the sea.

Teacher's Note:
1. Precipitation is any form of water falling from clouds.
2. The cycle keeps the water on Earth in constant circulation.