Environmental Issues

In simple terms, atmospheric pressure is the weight of air at any given point on earth. It is affected by elevation and temperature (Tarbuck and Lutgens, 2004). At higher locations, the pull of gravity is weaker hence the air is less dense and lighter. Towards the center of the earth, atmospheric pressure increases because of the growing pull of gravity.

Temperature also affects atmospheric pressure. Air molecules expand when heated, therefore there is less air per unit volume at higher temperatures. Conversely, air contracts when it is cold so there is more air per unit volume.

Air rises when it is heated, and fast rising air leaves behind a low pressure area near the ground. Hot air can hold more water, and as it rises, it cools and contracts. The water condenses and precipitates. This is a low pressure system characterized by hot and wet weather. In contrast, high pressure systems create the opposite. As heavier and colder air descends to the ground, it gets warmer and expands, creating dry and clear skies.

Wind is caused by the differences in atmospheric pressure. At sea level, air pressure is around 1013.25 millibars. When it drops radically, the surrounding air rushes in to fill the impending vacuum. This can cause storms, tornadoes, and hurricanes. In 2005, hurricane Katrina fell to 920 millibars, the third lowest in US history (NOAA, 2008). The greater the discrepancy in air pressure, the faster the air moves.

The airs composition also affects its weight. According to NASA (2009), air is composed of 80 Nitrogen and 20 Oxygen, weighing approximately 1.26 grams per liter. Water vapor, on the other hand, weighs 0.8 grams per liter. Substituting water vapor (or humidity) for some of the air actually makes it lighter.
In a single measurement, atmospheric pressure takes into account the airs temperature, elevation, and composition. That is why it is the best indicator of current weather conditions. And because air always moves from high to low pressure areas, atmospheric pressure also points to how the weather will change.