Problems with certain wireless thermometers


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Wireless thermometers and other radiosonde instruments, can have little peculiarities. Quirks that scientists interpreting the data should be aware of.

Wireless Thermometers

The wireless thermometers we deal with here are called thermistors. They measure temperatures using an electrical resistor whose resistance varies with the temperature.

However some factors can affect one's performance.

Calibration - These things conform to a curve within about 0.2°C. They work best at temperatures between -90 and 60 degrees.

Time lag - It takes up to a few seconds for the weather instrument to fully respond to a sudden change in temperature. Manufacturers define lag as the time it takes for the sensor to show a change amounting to 63.21% of the original temperature change.

Moisture - When you get wet, you feel colder. Same thing happens to thermometers when they get wet. What happens when it moves from a wet layer into a dry zone? A special kind of heat, latent heat of evaporation, exits the themistor. Then it can actually read a colder temperature than what is correct.

Wireless thermometers and humidity calibration

We can include a device to gauge humidity. The electrical hygristor has resistance that changes with the surrounding relative humidity. What hitches do we encounter here?

Calibration - within about 2% relative humidity is typical.

Time lag - This variable depends somewhat on temperature. Maybe a second for the same 63% feedback as used to define thermistor delay above at 20°C. It can take up to a couple of minutes for the same response at 40 below.

What if the balloon is moving quickly? Then this error can become quite significant. Consequently, all moisture data at high elevations becomes dubious.

Overload - also known as saturation. It can get waterlogged. Then we have a less accurate humidity gauge as it moves into dry air above.

Cloud Information

Relative Humidity

The radiosonde converts relative humidity into dewpoint temperature before it transmits the data back to the station.

How do clouds affect the humidity readings?

Inside a cloud, dewpoint is slightly less than temperature if above freezing. This indicates high relative humidity or even saturated air, as would be expected. After all, water is all around it.

What about a cloud that becomes supercooled, below freezing but still containing liquid water droplets? The dewpoint will be nearly the same as the temperature.

And if the cloud is made entirely of ice crystals instead? The temperature roughly matches the frost point, which is slightly higher than dew point. Up to about 3 degrees higher.

What the wireless thermometers leave us with then is a larger gap between the temperature and dewpoint trace on the tephigram or skew T plot than should normally be expected. This error makes the cloud appear thinner than it really is on the trace.

It's not perfect, but it is good. Go back from Wireless Thermometers to the Chasing Storms web page, or visit the Stuff in the Air homepage.


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