Doldrums are regions of the Atlantic and Pacific oceans that have little if any wind. Doldrums are caused by solar radiation from the sun, as sunlight beams down directly on area around the equator. This heating causes the air to warm and rise straight up rather than blow horizontally. The result is little or no wind, sometimes for weeks on end. The Doldrums are located a little north of the equator, but the effects can be felt from 5 degrees north of the equator to 5 degrees south of it. This was a particular problem for sailors in the past when they depended on the winds to propel their ships. It was a problem that could be potentially deadly.

Altimeter is an instrument that measures the height of the land surface or any object such as an airplane. The most common types of altimeters are barometric. They determine altitude by measuring air pressure. As altitude increases, air pressure decreases. This is because the density of air is lower at high altitudes. Altimeters are important navigation instruments for aircraft and spacecraft pilots who monitor their height above the Earth’s surface. Skydivers and mountaineers also use altimeters to pinpoint their location in the sky or on the ground.

• Both atmospheric pressure and  the density of air decrease with altitude.
  
• This is due to the fact that air molecules are constantly being pulled downwards (towards the center of the Earth) by Gravity.
•  So, air molecules are tightly packed near the surface of the earth, compared to some higher altitude.
  

Isobars are imaginary lines joining the places having equal values of pressure. The horizontal distribution of pressure is shown by isobars. For convenience all the local level pressures are converted to sea level. Closely spaced isobars indicate strong pressure gradient and widely spaced isobars indicate weak pressure gradient.

Anemometer is a device which is used for measuring the speed of wind in the atmosphere. It also measures wind pressure. Anemometers are important tools for meteorologists, who study weather patterns. They are also important to the work of physicists, who study the way air moves. Anemometer has three or four cups attached to horizontal arms. The arms are attached to a vertical rod. As the wind blows, the cups rotate, making the rod spin. The stronger the wind blows, the faster the rod spins. The anemometer counts the number of rotations, or turns, which is used to calculate wind speed.

At a high altitude pressure is low as at higher elevations, there are fewer air molecules above a given surface than a similar surface at lower levels.

Due to the intense heating from sun rays around the equator, the air in this region gets hot and hence ascends upwards, thus creating a low pressure in the region. Hence it is the equatorial low-pressure belt and is also called doldrums as it is the region of extreme calmness, not as much as a breeze.

Although air is well mixed throughout the atmosphere, the atmosphere itself is not physically uniform but has significant variations in temperature and pressure with altitude, which define a number of atmospheric layers i.e. when lower layers have atmospheric pressure and higher ones have low atmospheric pressures due to the altitude.

Air pressure increases when air descends. Lower pressure at higher altitudes cause the temperature to be colder on top of a mountain than at sea level. As air rises, the pressure decreases. It is this lower pressure at higher altitudes that causes the temperature to be colder on top of a mountain than at sea level.

The factors which affects in the distribution oof atmospheric pressure are, altitude, temperature and rotation of the earth. Atmospheric pressure decreases with increaseing altitude. Regional variation of pressure are produced by temperature. Movement of wind are also responsible for pressure channge, prevailing wind systems are caused by rotation of the earth.

The weight of water vapor per unit weight of air is called Specific humidity. It is usually expressed as grams of vapor per kilogram of air. The rate of evaporation of water from any surface is directly proportional to the specific humidity, the difference between the surface and the adjoining air. It is an extremely useful quantity in meteorology. Specific humidity does not change with temperature and pressure.

Wind
Air moves from the high-pressure areas to low-pressure areas, creating the wind.
Now with the difference in speeds of traveling wind, it may be called as the storm, hurricane, etc.

Low temperature causes air to contract and higher temperature causes air to expand.

As air warms up, the molecules in the air become more active and they use up more individual space. This leads to a rise in the air pressure. Similarly, when the air cools down the molecules remain compact, leading to a decrease in air pressure.

As we go up the pressure decreases with altitude.

This is because at higher altitudes number of air molecules present are low when compared to those that are present in the lower atmosphere.

Moreover gravitational pull which is responsible for atmospheric pressure is low on the molecules that are away from its surface than those that are closer to it.

• Bars are the unit of pressure hence C is the correct option.
• Calorie is the unit of energy
• Newton is the unit of force.
• Watt is the unit of power.

Atmosphere,

Air pressure always decrease with the increase of altitude but not always uniform. The decrease is caused by the compressional  effect of the air from above and its density. It is estimated that the pressure decreases at an average one centimeter for every 110 meters.

The heat zone of earth arises due to the temperature difference. The classification of heat zones of the earth was first done by Greek Scholar Aristotle. The different heat zones of earth are: - Torrid Zone, Frigid Zone and Temperate zone. All these zones have different temperature range. Torrid Zone is the hottest zone among the zones of the earth.

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The weight of humid air is less to than dry air.

Pressure gradient force,

Near Earth's surface the pressure decreases with height at a rate of about 1 millibars for every 10 metres. However, over cold air the decrease in pressure can be much steeper because its density is greater than warmer air.

There exists a negative relation between pressure and temperature. When temperature increases pressure will fall and vice versa. That is why, pressure increases from equatorial region towards the polar region.