Variation in Climate Part One: The Tropics

Usually, when we go on a vacation to a place we have never been before, we find out what the climate is like first before we start packing. Finding out the climate before we go on vacation helps us to know what kind of clothes to pack, whether to bring umbrellas or not, whether to bring tents or tarps, and so much more.    But what exactly IS a climate?  Climate is the average weather of any given location, including how much precipitation it usually has during a given period of time, and what kind of extremes to expect at this location.

As most of you know, the sun controls the weather, therefore it also controls the climate of a location. For example: the farther north you travel (or if you live south of the equator, the farther south you travel) the less the sun angle is. When the sun angle is more acute to the ground (a lesser sun angle), it is usually colder than a place that has a higher sun angle. With this in mind, the higher the latitude, the colder it gets. But why then, is a city such as Eureka, California, that is at the same latitude as New York City, warmer then New York during the winter and colder in the summer, or why is a southern city such as Tampa Bay, Florida warmer then some cities at the equator, which has a higher sun angle than Tampa Bay? In the next few paragraphs, I will attempt to answer these questions as best as I can, seeing as that some of the climate questions are still puzzling meteorologists and climatologists alike.

The first climate we will start with is the Tropics. Usually climatologists think of the Tropics as the climate that controls the rest of the climates around the world. One of the main proponents in a tropical climate is the intensity of the sunlight hitting the earth. The intense sunlight causes intense heating of the surface and warm water temperatures. The intense heat from the surface rises, cools and condenses producing towering thunderstorms that produce heavy rainfall in a band that stretches around the world, called the Intertropical Convergence Zone (ITCZ). Thus, since maximum surface heating occurs where the most intense sunlight radiates the earth, the ITCZ moves with the sun and the seasons. When the heated air rises, expands, and condenses, it causes upper level divergence of the air, and as I stated in my last article, upper level divergence encourages surface convergence which further aids the development of thunderstorms. This is an example of a thermal low, one that is produced by heat and not by the jet stream. These are usually much weaker than storm systems in the middle latitudes. Usually the ITCZ, is located over or just south of the equator during January and is usually located around 23-25 degrees north during July, but extends much farther north over Asia during the summer. The reason for this is because land is heated more intensely than water and since Asia is the biggest continent it absorbs much more sunlight than in the western hemisphere at the same latitude and causes the air mass over this area to be very hot, humid and unstable. Because of this, there is a very large difference in the position of the ITCZ over Asia from the summer to the winter season, and is the main reason for the wet and dry season over southeast Asia, or the monsoonal seasons. Where the Intertropical Convergence Zone is located, is where the most rainfall will occur. (usually over the rainforests of South America, the Caribbean, Africa, and Southeast Asia) Also where most of the rainfall occurs, the summertime temperatures are usually colder then at a location that is located in the subtropical zones or even at a location in the middle latitudes. Not only does the ITCZ affect the weather in the tropics, but it also affects the weather of the subtropics and middle latitudes. When the ITCZ extends in between latitudes of 15 and 25 degrees north, the Coriolis force is now enough to turn the normally southeast winds on the south side of the boundary, when it is at the equator, to southwesterly winds. When the northeast winds on the north side of the boundary converge with the southwesterly winds on the south side of the boundary, they produce cyclonic rotation and vorticity, and when a tropical wave interacts with the area of vorticity, the tropical wave becomes organized and could develop into a hurricane if wind shear and dry air are not involved. The organized tropical wave then travels along the ITCZ until a subtropical high begins to steer the system northward into the United States.

This is a model that simplifies the basic climate variations in the world.

When sailors from the Old World first sailed in this region of the world (to get to the tropics they used the trade winds, the northeast and southeast winds that converge along the ITCZ) they called the area around the ITCZ the doldrums, because of the light winds and violent thunderstorms that were not reliable for wind power. This is because within the tropics, there is very little difference in sea level pressure. Without much wind, sea breezes and land breezes are much more noticeable in the tropics than in the rest of the world.
The circulation of the tropics is  involved with a circulation cell in the atmosphere, which is why I mentioned before that it is the driving force of most of the other climates of the world. This circulation cell is called a Hadley Cell, named after Sir George Hadley, who was using his theory to explain world circulation patterns in 1735, such as the trade winds.  His idea is today used as an idealistic model in tropical atmospheric circulation.
This is the model that is used today: the hot air mass over the tropics rises and condenses producing heavy thunderstorms over the ITCZ.  When this air hits the tropopause it spreads laterally. One: because the stratosphere (the layer above the troposphere) is stable, and because there is so much upward motion in this humid air mass, it spreads laterally toward the poles; and two: the pressure gradient force between the poles and the tropics causes air to move towards the poles. This air then begins to cool due to radiation and it begins to sink at about 30 degrees latitude on both sides of the equator, causing large high pressure cells to dominate the climate of the subtropics, which I will continue to explain in next week’s article.
Next week, I will talk about how the air flow in the tropics influences the climate of the subtropics, which is where the world’s largest deserts are located such as the Sahara Desert and the Atacama Desert in Chile—the driest desert in the world. Back to the weather around here, temperatures will definitely start to warm to a hotter drier pattern over the next several days, as the monsoonal trough develops over the American Southwest deserts. When this trough develops, a large subtropical high will move over the central plains, causing a major and intense heat wave to develop over the Midwest and East. According to long range forecasts, this heat wave could last to Independence Day or longer, before a cold front moves through temporarily ending the heat, only to return again after a few days.  The recent heat and the heat that is forecasted to come has lead me to change my own summer forecast to a very warm one. Other than that, have a great week!