Lesson 1. Meet the Tropics

This is a sample lesson page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: Steve Seman


Photograph of a sunny, tropical beach
Who doesn't like the thought of crystal clear waters along a quiet tropical beach? Great vacation spots are just one aspect of the tropics, however.

When you think of the tropics, or the word "tropical," you might picture white, sandy beaches, and perhaps sipping on a refreshing, fruity beverage (complete with a tiny umbrella in your glass, of course). Besides being a favorite vacation destination for many people, the tropics are home to some fascinating meteorology. Coming into this course, you should have a good overall grasp of weather in the middle latitudes and how mid-latitude cyclones work. Some of that foundational knowledge will serve as a stepping stone for concepts we'll cover in this course, but we're about to find out that the tropics are quite different than the middle latitudes!

First off, what exactly are "the tropics?" Good question! Actually, folks can't seem to agree on a single definition of the tropics. The definition from the AMS Glossary, for example, is pretty vague! Other definitions are based in geography, and define the tropics as the area between certain latitude lines in each hemisphere. Some definitions actually consider the tropics to be the area between 30-degrees North latitude and 30-degrees South latitude, which is exactly half of the Earth's surface! This large low-latitude region will be our focus throughout this course.

Regardless of what specific definition of the tropics one uses, this large area is characterized by weather that's quite different than that in the middle latitudes. Consider these contrasts between the tropics and the middle latitudes for starters:

  • Seasonal swings in temperature across the tropics are typically small compared to the large swings that occur in the middle latitudes from summer to winter. In fact, temperature swings during the year in the tropics can be so small that the seasons are determined more by dramatic changes in clouds and rainfall.
  • Wind directions in the tropics tend to be much less variable than they are in the middle latitudes (at many tropical locations, a single particular wind direction tends to dominate).
  • Weather systems in the tropics often move from east to west -- exactly the opposite of the typical west-to-east movement of weather systems in the middle latitudes. For example, check out this animation of water vapor images spanning a full year. The movement of weather systems in the tropics clearly goes against the grain of the movement in the middle latitudes.
  • Tropical cyclones (the generic name for intense low-pressure systems like hurricanes that form in the tropics) tend to form over warm, tropical seas with weak horizontal temperature gradients. Meanwhile, you've learned that mid-latitude cyclones thrive off of strong horizontal temperature gradients.

Intrigued? The tropics and middle latitudes can be as different as night and day, and we'll explore many of these contrasts in this lesson, and throughout the remainder of the course. Also in this lesson, we'll cover some important basics, such as the map projections commonly used by tropical forecasters, computer guidance, and various forecast products issued by the National Hurricane Center (NHC). If you're eager to learn about tropical cyclones, learning about these basic tools now will help you follow along with developments in tropical weather throughout the semester.

Indeed, if you're ready to "Meet the Tropics," let's get started!