Meteorology Sensors Overview

Weather conditions are often an integral element in water resource and irrigation management applications. Weather parameters are frequently used in understanding and modelling environmental applications that are dependent on water resources. Stevens provides top quality and industry leading weather sensors that are easily deployed into an integrated environmental monitoring and control system.

Here is an overview of the different types of instrumentation for monitoring weather conditions.

Precipitation

Rain sensors are used to record the cumulative precipitation at a location for a given time. Buildings, landscaping and trees, wind and height placement of the rain sensor can influence the amount of rain being measured. Placing a rain gauge in an open area protected from the wind is best (usually two to six feet above the ground). The use of a windscreen around the rain sensor will help to improve the accuracy in windy conditions.

Tipping Bucket Rain Gauge

The worldwide standard for measuring rainfall, the tipping bucket rain gauge consists of a collector funnel with stainless steel leaf filter, an integrated siphon control mechanism, an outer enclosure with quick release fasteners, and a base which houses the tipping bucket mechanism. It operates on an internal tipping mechanism that tips back and forth each time a pre-set amount of rain collects in the unit. Each tip is then recorded by a data logger, allowing users to calculate how much rain has fallen in a given time period.

Non-Mechanical Rain Sensors

While not nearly as accurate and with a much higher power draw, solid-state rain sensor technology is offered in the compact multiparameter Lufft WS-800. Precipitation is measured by interpolating the quantity from the impacts of rain or snow on the top of the sensor.

Wind

Stevens provides a number of different wind sensors (known as anemometers) for a variety of application needs.

Cup and Vane Sensors

This is the most common type of instrument for the measurement of wind speed and direction. Wind speed is measured independent of the direction of the wind, providing fast response to changes in wind direction and speed. Lightweight cups and vanes provide low threshold measurements of wind speed and accurate determination of wind direction and direction variance. Wind speed is determined from the rotation of cup assembly converted to switch closure or optical chopper providing a frequency output vs. wind speed. Wind direction is measured by a potentiometer or direct voltage equivalent to 0-360 degrees of rotation.

Propeller Anemometer

Sometimes referred to as a “windmill” anemometer, this is a wind speed measurement device with a propeller on one end, which looks and functions much like the propeller on an airplane. The opposite end of the anemometer has a vertical fin to ensure the sensor always points into the wind, to provide accurate measurements. As the wind turns the propeller blades, an AC sine wave in proportion to the wind speed is generated, allowing wind speed and direction to be logged.

Ultrasonic Anemometer

Ultrasonic anemometers are becoming more popular in environmental monitoring applications due to the fact that they contain no moving parts, making them more resistant to harsh weather conditions and unattended operation.
Ultrasonic anemometers measure wind speed by timing ultrasonic pulses sent between transducers on the device, which are also positioned in such a way to measure wind direction and compensate for turbulent conditions. Time of flight of the sonic pulse slowed down or speeded up by the wind is electronically converted into a wind speed and direction.

Solar Radiation

A pyranometer is a type of sensor measuring the heating power of radiation and broadband solar irradiance on a planar surface and is a sensor that is designed to measure the solar radiation flux density (in watts per metre square) from a field of view of 180 degrees.

The World Meteorological Organization defines direct irradiance from the Sun measured on the ground of at least 120 watts per square meter. Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux. Bright sunlight provides illuminance of approximately 100,000 lux or lumens per square meter at the Earth’s surface.

Sunlight is the key factor in photosynthesis. Photosynthesis is the process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. This process allows plants to create their own food—a process that is a crucially important for life on Earth.

Pyranometers are frequently used in meteorology, climatology, solar energy studies and building physics. They can be seen in many meteorological stations—typically installed horizontally and next to solar panels—often mounted with the sensor surface in the plane of the panel.

Air Temperature and Humidity

Digital temperature sensors have the advantage of being able to send and record data automatically. Often the temperature sensor is combined with a humidity sensor—both of which are shielded from solar radiation but accessible to conditions in the air and at a height to avoid influence for the ground temperature (at least 5 feet).

Humidity is defined as moisture in the air—commonly referred to as “relative humidity”. Relative humidity is the ratio of the quantity of water vapor in the air required for saturation at the same temperature. Saturation point is the point at which condensation forms. By knowing the percentage of humidity in the air along with the current temperature, dew point temperature and heat index can be calculated. These factors can be important to those who work and play outside. They are also important to farmers and other agricultural concerns with regard to stress in livestock or plants, or in properly irrigating crops.

If at all possible, keep the humidity sensor away local sources of heating and cooling, and from nearby obstructions by a distance of at least four times their height. Be at least 100 feet (30 meters) from large areas of concrete and/or asphalt. Temperature and RH sensors should not be installed under the shade of trees or vegetation. The use of a motor or wind aspirated solar radiation shield will minimize the effects of solar radiation on the measured temperature or humidity.

Barometric Pressure

Atmospheric pressure (or air pressure) is the weight of the Earth’s atmosphere on the surface at a given location. It is generated by the downward force of Earth’s gravity. Atmospheric pressure depends on the amount of air above the location where the measurement is taken, consequently the pressure drops as you go higher. Air pressure decreases by about 1 inch of mercury for each 1,000 feet of elevation (or 1 hectopascal for each 8 meters) above sea level. Atmospheric pressure is also known as barometric pressure because barometers are used to measure it.

The National Weather Service reports air pressure in both inches of Mercury (inHg) and hectoPascals (hPa), also called millibars (mb). Inches of mercury refers to how high the air pressure pushes the mercury in barometers. In the metric system, the unit of pressure is the hectoPascal—a direct measurement of atmospheric pressure, like pounds per square inch. The Weather Service typically provides a “corrected” pressure level, which has been adjusted for the barometric value at sea level. A standard barometer will provide local barometric pressure at that location and is not corrected for sea level.

Barometric pressure changes as low and high pressure systems move across the Earth’s surface. Barometric pressure measurement is used to predict future weather conditions. Surface pressure measurements are used along with temperature, humidity, and wind observations in a vast network of weather stations, meteorologists are able to develop a complete picture of the location and movement of weather systems.

Barometric pressure sensors convert absolute atmospheric pressure into a linear, proportional voltage, which may be used in any meteorological program.

All-in One Multiparameter

Multiparameter weather sensors involve the cutting edge of miniaturization technology, offering a full set of common weather parameters in one instrument in a very compact size.

As the instrument has no moving parts, maintenance is minimal. A small network of these weather stations can often provide better area-wide weather accuracy than a single high-end weather station in one location.

The Lufft WS-800 is a complete, high performance weather sensor with a fully integrated design with ventilated radiation protection. It’s also the first and only all-in-one compact weather sensor with lightning detection.

It measures air temperature, relative humidity, precipitation intensity, precipitation type, precipitation quantity, solar radiation, lightning detection, air pressure, wind direction and wind speed. One external temperature sensor is connectable.

We also offer the Lufft WS-500, an all-in-one compact weather sensor combining 5 measurement parameters (no lightning detection).

Evaporation

Evaporation data can be calculated with a real time data system using an evaporation pan, a level sensor, and a rain gage. Additionally, various weather parameters such as solar radiation, wind, and relative humidity can help you correlate the evaporation data to environmental events. Evaporation is calculated based on the amount of water that is lost throughout a day in an evaporation pan. Rainfall is used as a parameter to account for the amount of water that is added to the pan due to rain.

The Novalynx Class A Evaporation Pan is a standard National Weather Service Class A type for measurement of water evaporation. It is normally installed on a wooden platform set on the ground in a grassy location. The pan is filled with water and exposed to represent an open body of water