Measuring the atmosphere above us

Published 7:00 am Tuesday, April 11, 2017

By Skip Rigney

In this column I often discuss patterns of wind, pressure, and temperature that are miles above us in the atmosphere.

Observations of the upper atmosphere are crucial input for computer weather models and for forecasters charged with issuing predictions and severe weather warnings for their area.

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Have you ever wondered how we know what is going on miles above the earth’s surface? Sensors onboard satellites are able to provide some of that information. However, the most important measurements come from systems called radiosondes, which have been used in the U.S. since the 1930s.

Imagine a small box weighing less than a pound, which you can easily hold in your hand. Sticking out of the box are probes that measure temperature and humidity. Inside the box is a pressure sensor, a GPS receiver to measure location, a radio transmitter and batteries to provide power.

Those are the basic components of the radiosonde system.

How does it get to the upper atmosphere? The answer is the weather balloon.

Twice every day at a universally agreed upon time, which happens to be noon and midnight in Greenwich, England, observers at weather stations around the world fill white, latex balloons with helium or hydrogen until their balloon is about four feet in diameter.

The weather observer attaches the radiosonde to the bottom of the balloon. When the observer lets go of the balloon, it floats upward taking the radiosonde with it.

As the radiosonde ascends it transmits temperature, humidity, and pressure measurements back to a receiving station at the weather office.

Radiosondes with GPS sensors also transmit precise location and time data, which allow accurate wind speed and direction to be computed. Radiosondes for which winds can be calculated are known as rawinsondes.

As the balloon, rises the pressure and density of the surrounding atmosphere decrease. In response the air in the balloon expands, and the balloon stretches becoming larger.

Eventually, just before the balloon bursts at altitudes up to 20 miles high, the balloon swells to a diameter of 15-20 feet.

When the balloon finally bursts a parachute deploys and the radiosonde drifts down to the ground. About 80 percent of the sensors launched in the U.S., each of which costs several hundred dollars, are never seen again.  The other 20 percent are found by the public, returned to the National Weather Service (NWS), and refurbished for reuse.

It is certainly possible that some of us in Pearl River County might find a rawinsonde. The NWS Office in Slidell is part of the network of approximately 600 stations around the world that release radiosondes twice each day.

NWS meteorologists in Slidell post to the Internet a brief description of balloon flights and the data received. An example is their discussion on May 23, 2015, which states that the balloon “burst 17 miles downrange around 21.5 miles up near Picayune, one mile east of I-59 near Highway 43.”

The rawinsondes launched this week from Slidell will probably find a rather warm and dry atmosphere above us. The NWS is forecasting no rain with high temperatures in the middle 80s. The only exception might be a few widely scattered summer-like showers over the weekend.