Measuring Atmospheric Weather

This page compares methods to measure weather as background reading to Windsond.

Official weather reports

All nations cooperate through the World Meteorology Organization WMO to make daily measurements of the weather. The resulting weather models give us the official weather predictions available to us all as well as more specialized reports and predictions. These observations and predictions work well at altitudes above a thousand meters, but below that the weather is largely affected by the terrain. A lake, hill or field can have real effects that large-scale prediction models can’t encompass. Furthermore, weather conditions can change rapidly which is hard to predict accurately. The screenshot below shows how an evening breeze by a large lake completely changes the wind patterns in little more than an hour.

The traces of two Windsonds released 80 minutes apart are completely different.

The traces of two Windsonds released 80 minutes apart. Within this timespan and within a range of a few hundred meters of altitudes, every possible wind direction is encountered. In this case, a large lake triggered a phenomenon when evening came.

Radiosondes

A radiosonde typically weighs 200-300 grams. It’s insulated against the harsh environment at 10-30 kilometers altitude where temperatures are in the range -80 to -50 degrees C and the air pressure is a fraction of that at sea level. They are transported by big latex balloons that themselves weigh 100-350 gram, filled with 1-3 cubic meters of helium or hydrogen gas.

These are commonly used at fixed sites by national weather services, military and upper-air research programs. The complexity of launching a big weather balloon limits the usefulness for field use and the cost of helium and one-time-use sondes is prohibitive for some applications. While hydrogen brings down the cost somewhat, hydrogen is an explosive gas that requires rigorous safety measures.

Windsond offers a new angle on radiosondes by focusing on the lower parts of the troposphere, where most of human activity and weather phenomenon occurs.

Here is a fascinating online collection of radiosondes: radiosondemuseum.org.

Theodolite

Winds close to the ground can be estimated by filling a small balloon (pibal) with helium and tracking it by hand using a binocular-like instrument called a theodolite as it rises. The changes in horizontal and vertical angle towards the balloon can be translated into a wind speed and direction, assuming the rise speed is constant. The materials cost of a measurement is very low but severe disadvantages limit the usefulness: When there are thermal winds or weather fronts, a constant rise speed can not be guaranteed, casting doubts over the final data (see comparison between theodolite and Windsond in German). A theodolite is also unable to report temperature and humidity. Another disadvantage is the amount of manual work; setting up the instrument and about fifteen minutes of non-interrupted concentration to keep the theodolite aimed at the balloon. Low clouds, darkness and ground obstructions also limit the use.

Dropsonde

An instrument similar to a radiosonde is carried aloft by an airplane, then dropped to measure the air column while falling. These are often used to analyze hurricanes.

Windsond can be used as dropsonde with a few firmware adjustments.

Weather tower

Fixed installations for measurement up to 50-100 meters.

Wind profiler

A ground-based doppler radar is used to measure movement of the air. These are expensive and only give an approximate view of wind speed and direction. From Japan Meteorological Agency:

A wind profiler is a device that continually measures wind speed and direction in the upper air using radio waves. It plays an important role in monitoring atmospheric phenomena and providing initial values for numerical prediction models. Operational wind profiler networks are used around the world, including in the United States, Europe, and Japan. They have worked to improve the corresponding data quality management and advance the technologies that use this data.

Links

For a thorough introduction to meteorology, for example see this lecture series from Yavapai College, Arizona: youtube channel

Swedish weather radar images: http://se.baltrad.eu/

 

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