Lesson 2: Remote and In-Situ Observations in 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


You already have some experience with both in-situ and remote sensing from your previous course work. In this lesson, we're going to broaden that experience so that you can better understand how meteorologists observe tropical cyclones . As a reminder, "in-situ" observations are taken by instruments that are in direct contact with the medium that they are "sensing." Everything from tossing blades of grass in the air to get a sense for the wind direction (blades of grass are in direct contact with the moving air) to thermometers, barometers, rain gauges and standard anemometers are considered in-situ observations. Indeed, many of the observations taken by the instruments that make up Automated Surface Observing System (ASOS) stations commonly located at airports, for example, are in-situ measurements.

But, meteorologists can't rely on in-situ observations alone, especially in the tropics. Given that oceans constitute a large part of the tropics, there is an insufficient number of traditional surface observations and upper-air observations to represent the current state of the tropical atmosphere. Fortunately, forecasters have access to some other sources of in-situ observations in the tropics, such as those from ocean buoys, ships, and aircraft (including aircraft flying into hurricanes to measure air pressure, wind speed and wind direction). We'll delve deeper into these alternative in-situ measurements in this lesson, but ultimately, there just aren't enough of them to provide a complete picture of tropical weather. There's undoubtedly a relative dearth of traditional in-situ observations in the tropics.

In order to fill in the gaps left by the available in-situ observations in the tropics, meteorologists turn to remote sensors, which make observations of a medium that they are not in direct contact with. For instance, the conventional satellite and radar images you've learned about in previous courses are an example of remote sensing. But, not all remote sensors are alike. We can further break down remote sensors into two basic types -- active and passive remote sensors. To really understand the capabilities of remote sensing instruments, it's important that you understand the difference between the two:

  • Active remote sensors emit electromagnetic waves that scatter back to the sensor when they strike "targets". Conventional radar is an example of an active remote sensor.
  • Passive remote sensors detect natural electromagnetic waves emitted or scattered by objects. Conventional visible, infrared, and water vapor satellite imagery are all examples of products from passive remote sensors.
Left: a six-meter NOMAD buoy containing in situ sensors. Right: A U.S. Air Force Reserve WC-130, which employs both in situ and remote sensors to monitor hurricanes
(Left) A six-meter NOMAD buoy contains in-situ sensors that measure atmospheric and sea conditions in its immediate environment. Credit: National Data Buoy Center (Right) A U.S. Air Force Reserve WC-130 aircraft (Hurricane Hunter) uses both in-situ sensors (a thermistor mounted on the aircraft, for example) and remote sensors (on-board radar, for example) to observe conditions inside a hurricane.
Credit: U.S. Air Force

In this lesson, we'll cover the in-situ sensors that we have at our disposal, as well as a wide array of active and passive remote sensors used to monitor conditions in the tropics (and elsewhere). We'll start with the in-situ observations we can get from tropical ocean buoys, and we'll delve into the variety of data collected by remote and in-situ sensors aboard United States Air Force and NOAA aircraft that fly into hurricanes. Finally, you'll learn that satellites can collect much more data than the conventional images you're already familiar with. Read on.