Meredith holds a B.S. in marine science with a minor in philosophy, as well as a master's of aeronautical science with a space science emphasis. She has taught subjects including marine science, biology, astronomy, math, and reading to students from kindergarten through high school.
Pressure in the Atmosphere
We have all seen a weather map before at some point, most likely on the morning news. The meteorologists talk of 'high' and 'low' pressure systems moving across the country, sometimes bringing severe weather with them. These areas indicate changes in atmospheric pressure, the force exerted on the ground by the weight of the air above it.
If you are standing at sea level, you are feeling the greatest amount of possible atmospheric pressure, since there is the most possible air sitting on top of you. Someone standing on the top of a very tall mountain would be experiencing lower air pressure - less air on top. But air pressure is not necessarily so simple; it routinely fluctuates throughout the atmosphere and across the surface. Pressure is affected both by factors that frequently change, temperature, for example, and factors that do not change, the presence of land and ocean.
Temperature and Pressure
As you know, we live on a spherical planet. Earth's shape has a major effect on how it is heated which, in turn, has a major effect on how air moves across it. Because the equator absorbs more solar radiation than the poles, the atmosphere is warmer there. If you've ever worked with a convection oven, then you know that convection occurs when warm air rises and cool air sinks.
The same thing happens in Earth's atmosphere: that warm, equatorial air rises. Due to the Coriolis Effect, air is pulled with Earth's rotation and deflects away from the Equator, moving towards the poles and creating cells of updrafts and downdrafts, where warm air rises and cool air sinks.
When warm air rises, it creates a low pressure system where air has risen upwards, leaving less air pressure at the surface. Likewise, when cool air sinks, it creates a high pressure system where cool air has dropped down to the surface, bringing increased pressure. Because air tends to have water vapor in it, especially warm equatorial air, low pressure systems tend to include lots of precipitation and stormy weather when that warm vapor cools and condenses. High pressure systems, likewise, are usually associated with fair weather, as that cool air from way up in the atmosphere tends to be drier. So, next time you watch weather on the news, see if you recognize this connection between pressure systems and what kind of day can be expected.
Land and Sea
Now that you've seen how temperature can affect pressure in the atmosphere, we can also take into account the role of land in affecting the movement of air. If you've ever spent a nice, relaxing summer by a swimming pool under a hot sun, you've likely noticed how the concrete around the pool can become scorching hot and even burn the bottoms of your feet. And yet, even though it is being heated by the same sun, the water in the pool never gets quite that hot.
This is because different materials have different specific heat capacities, the amount of heat necessary to change the temperature of the material by one degree. Water has a very high specific heat capacity, almost five times that of concrete. That means that water can absorb a lot of heat before it actually changes temperature, whereas concrete requires much less heat in order to heat up. The same applies for soil, grass, rock, and basically all other materials that make up land. So air sitting on top of land will also heat up a lot quicker than air sitting on top of water. That warm air will rise, creating lower pressure areas over land than the areas over water.
The placement of continents and oceans, therefore, have a big impact on the movement of air in the atmosphere by affecting the pressure distribution within the atmosphere. Glider pilots have to understand this concept very well if they are to fly safely. They know that flying over bodies of water likely means a downdraft, followed by an updraft once they fly over land again. They essentially surf the waves in the air that we can't see. It's all about how air rises and sinks based on its temperature!
Changes in atmospheric pressure are greatly influenced by the distribution of land and oceans across the planet. While the Coriolis Effect combined with convection cycles moves atmospheric heat from the Equator to the poles, high pressure systems and low pressure systems can be generated due to differences in specific heat capacity between land and water. Air warms quickly over land and rises, creating low pressure areas, while air over water will tend to stay cooler and not rise, creating an area of relatively higher pressure. These differences, though not visible to the human eye, drive weather patterns that affect our everyday lives!
Following this lesson, you should be able to:
- Explain how land and oceans affect atmospheric pressure
- Describe how high and low pressure systems are related to specific heat capacity
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