Kevin Rose | Smithsonian Environmental Research Center
What is a meteorological sensor?
Meteorological sensors are used to understand and measure climate and weather. These sensors are typically deployed on a buoy or on shore nearby to record average lake conditions. Meteorology regulates many key characteristics of lakes and other water bodies, such as temperature, wave height, and dissolved oxygen. These sensors are usually physical sensors, and work on a variety of different principles and methods, often measuring internal voltage changes in response to changing meteorological parameters.
What do meteorological sensors measure?
While many meteorological measurements are possible in aquatic environments, several important, commonly measured meteorological parameters include:
Other meteorologically related characteristics such as temperature and transparency are commonly used. Information on these parameters can be found in the physical sensors section. Several meteorological parameters can often be measured together on a sensor package.
Wind speed and direction
Wind regulates a number of lake characteristics. For example, high wind lakes are usually colder and mix deeper than similar low wind lakes (see Figure 1). Wind also regulates evaporation and gas movement (for example, oxygen and carbon dioxide) between water and the air. Wind speed and direction are often measured together on an anemometer.
Wind speed is typically measured with cup, wind-mill, or sonic anemometers (see Figure 2). Cup and wind-mill anemometers measure wind speed by calculating the rotating speed of cups or a wind-mill. Sonic anemometers measure wind speed by sending ultrasonic pulses of sound among small towers on the sensor. Changes in wind speed affect the length of time it takes for these pulses to reach opposing towers in the same way a tail-wind or head-wind affects the duration of an airplane flight. By using three or more towers, sonic anemometers can triangulate wind direction. Wind-mill anemometers can be coupled with compass or GPS measurements to provide wind direction. Typically, wind-mill anemometers are the most accurate, cup anemometers are the most cost-efficient, and sonic anemometers have the highest sensitivity to low wind speeds.
Relative humidity measures the amount of moisture in the air. Just like sweat builds up on your skin on humid days, high relative humidity also reduces evaporation rates from lakes. In areas where water balance is critical, like reservoirs in hot and dry regions, relative humidity can be a very important measurement.
Relative humidity sensors typically work by using materials that change their resistance or capacitance, depending on the relative concentration of moisture in the air. These sensors are typically inexpensive and reliable, but must be calibrated every few years to ensure accuracy.
In many regions, lakes and reservoirs are water supplies. In these water bodies, precipitation sensors can be used to gauge changes in water availability. Precipitation also affects water temperature, mixing, and gas movement between water and the atmosphere.
Because precipitation occurs in many forms, it can be measured in several different ways. Precipitation measurements are often made by measuring water volume, often in a tipping bucket sensor. Rain is measured directly by volume and heated funnels and collection cups melt sleet, hail, and snow to measure their water equivalent. The bucket will automatically tip to pour out its contents regularly to avoid overfilling and reset itself. Precipitation sensors that measure water equivalent are usually very accurate and relatively inexpensive.
Different forms of precipitation can be measured in several ways. Two of the most common ways include radar sensors and pressure sensors. Pressure sensors measure precipitation by counting the number and impact of drops hitting a pressure plate. These sensors can usually distinguish between rain and hail by impact duration. Pressure sensors require very little maintenance but are not as accurate as other sensor types. Radar sensors [r3] measure precipitation quantity, intensity and speed. Volume is calculated by correlating quantity and intensity and speed is used to estimate precipitation type.
Atmospheric pressure regulates the solubility of gasses in water and can be used to forecast changes in weather conditions. There are many different technologies available and many companies that manufacture sensors to measure barometric pressure. Often these sensors are included as part of a sensor package.
The intensity of solar radiation is frequently measured at lakes for management and research. Light drives most aspects of lake ecosystems from the temperature to the amount of photosynthesis. However, not all light is the same or has the same intensity (see Figure 3 and Table 1).
Just like there are different colors of light, there are different types of light outside of the visible range the human eye can see. These include ultraviolet, visible, infrared, shortwave and longwave radiation (Table 1) and ranges can overlap. The most common type of light measurement on lakes is often visible or shortwave radiation.
Light sensors can vary in price and therefore measurement. Inexpensive sensors often do not distinguish between different ranges of light (e.g., ultraviolet vs. visible vs. infrared) while more expensive sensors are very sensitive and accurate. Figure 4 shows an example sensor response for a common light sensor.
Table 1: Types of light and their wavelength ranges.
|Tye of Light||Wavelength Range (nanometers)|
There are many different types of meteorological sensor measurements made on lakes. Some sensor packages are available which can facilitate many measurements in a single unit. These include, for example, the Vaisala WXT520 and the Lufft WS501 . These packages can measure, for example, air temperature, barometric pressure, relative humidity, wind speed, wind direction, and precipitation or solar radiation. Sensor packages are often an economical way to measure many meteorological parameters, but sensor accuracy is usually lower than if individual research grade sensors were purchased separately.