Atmospheric pressure and factors affecting it

The correct answer is B: air pressure generally decreases as altitude increases. At higher altitudes, there is less air above a location, so the weight of the overlying air—and therefore the pressure—is lower. Pressure does not normally become zero and then increase.

Correct answer: A. Air pressure is caused by the weight of the air above a surface. At greater heights, less atmosphere lies overhead, so the air column above is lighter and exerts less pressure. The temperature is not always zero, and soil does not increase upward through the atmosphere.

Correct answer: B, near Earth’s surface. Earth’s gravity holds atmospheric gases close to the planet, and the weight of the air above compresses the lower layers. As a result, air molecules are packed more closely near the surface, so density is greatest there. Density decreases with altitude as the air becomes thinner.

The correct answer is A, the troposphere. It is the lowest atmospheric layer, nearest Earth’s surface. As altitude increases through this layer, less air remains above a location, so air pressure decreases rapidly. The other listed layers are much higher in the atmosphere.

Correct answer: A. Air pressure is mainly the force exerted by the weight of the air above a unit area. As altitude increases, less air remains overhead, so the weight of the air column above decreases and the pressure falls. Changes in ozone, meteors, and ocean depth do not explain this regular decrease.

The correct answer is A. Air pressure decreases with altitude, so the air becomes less dense. Although oxygen remains about the same proportion of the air, each breath contains fewer oxygen molecules. This can cause breathlessness, tiredness, or altitude-related illness in mountaineers.

The correct answer is A. Air pressure at a location is caused by the weight of the air above it. As we go higher, there is less air overhead, so that air column weighs less and exerts less pressure.

Atmospheric pressure is the force exerted by the weight of the air column on a unit area of a surface. Air has mass, so gravity pulls it downward and creates pressure on the Earth and on objects around us. Pressure is commonly measured with a barometer and is often expressed in hectopascals or millibars. Therefore, option B gives the correct definition.

Atmospheric pressure generally decreases as altitude increases because the amount and density of air above a high location are smaller. Consequently, the air column exerts less weight on each unit area. Pressure does not normally become zero suddenly, because some atmosphere remains even at great heights. Thus, mountain regions usually have lower atmospheric pressure than places near sea level, making option C correct.

Near sea level, a surface has a larger and heavier column of air above it than a surface at high altitude. Gravity causes the weight of this air column to press downward, producing greater atmospheric pressure. As altitude rises, part of the air column is left below the observer, so the pressure decreases. Therefore, the heavier air column above sea level makes option A correct.

A barometer is the scientific instrument used to measure atmospheric pressure. It may use a column of mercury or an aneroid mechanism, and its readings are commonly expressed in millibars or hectopascals. A thermometer measures temperature, a rain gauge measures rainfall, and an anemometer measures wind speed. Hence, option B is the only correct instrument-pressure pair.

When air is heated, its molecules move faster and spread farther apart. The same mass of air therefore occupies a larger volume, so its density decreases. Warm, less-dense air tends to rise and is commonly associated with lower surface pressure. Thus, option D is correct.

Cold air has a lower temperature, so its molecules move more slowly and remain packed relatively close together. This makes cold air denser and heavier than warm air. The heavier air exerts greater downward force on the Earth’s surface, producing a region of high atmospheric pressure. Therefore, high pressure is generally associated with cold air, although local conditions can also influence pressure.

A low-pressure area has relatively less air mass at the centre than its surroundings. Air near the surface therefore moves horizontally inward, or converges, toward the centre. This converging air is forced to rise, which can support cloud formation and unsettled weather. Option A is correct.

In a high-pressure area, air descends from higher levels and reaches the surface. At the surface it diverges, moving outward from the pressure centre toward surrounding areas. Descending air usually suppresses cloud development, so high pressure is often linked with settled weather. Option B is correct.

Isobars are lines drawn on a weather or pressure map to join places having equal atmospheric pressure, usually measured in hectopascals or millibars. Their spacing also helps indicate the pressure gradient: closely spaced isobars show a steep gradient and stronger winds. Option C is correct.

Water-vapour molecules have lower molecular mass than the nitrogen and oxygen molecules they replace in moist air. At the same temperature and pressure, humid air is therefore less dense than dry air. Increased humidity can consequently contribute to lower pressure, although pressure also depends on temperature, altitude, and air movement. Option D is correct.

Altitude directly changes the weight of the air column above a place. As elevation increases, there is less air overhead, so atmospheric pressure generally decreases. At lower elevations, the overlying air column is heavier and pressure is greater. Therefore, altitude is the correct factor and option A is correct.

High pressure is commonly associated with air that is relatively dense and heavy. Such air exerts greater downward force on the surface and tends to subside. Subsiding air reduces cloud development, whereas warm, light air is more often linked with lower pressure. Therefore, option B is the best answer.

In a low-pressure system, air converges near the surface and rises. As rising air expands and cools, water vapour may condense into clouds and precipitation. The exact weather depends on moisture and atmospheric stability, but cloud and rain are common possibilities. Thus, option C is correct.

High pressure is usually accompanied by sinking air. As air descends, it becomes warmer and relatively drier, which discourages condensation and limits cloud growth. Surface divergence also reduces upward motion. Consequently, high-pressure systems commonly produce clear, dry, and stable weather. Option D is correct.

Atmospheric pressure is produced by the weight of the air column pressing on the Earth's surface and on objects below it. Although air is invisible, it has mass and therefore weight. The greater the amount of air above a place, the greater the pressure exerted there. Thus, the basic cause of atmospheric pressure is the weight of air.

Atmospheric pressure is commonly reported in millibars in meteorology; the equivalent SI-related unit used today is the hectopascal, with 1 millibar equal to 1 hectopascal. Kilometres measure distance, degrees Celsius measure temperature, and percent expresses a ratio, so millibar is the only suitable pressure unit here.

Air pressure varies from place to place because the conditions controlling air density and the weight of the air column are not uniform. Temperature changes the expansion and density of air, altitude changes the amount of air above a location, and humidity also affects density. Therefore, pressure is not equal everywhere.

During the day, intense solar heating of a desert surface warms the air near the ground. Warm air expands, becomes less dense, and rises by convection. As air moves upward, the amount of air pressing on the surface decreases, which can produce a relative low-pressure area. Therefore, option D is correct.