For the uninitiated understanding what weather sensor does what, is daunting. Many sensors’ names are rooted in ancient languages, exacerbating things further. Fear not, ESS Earth Sciences has come up with this straight-to-the-point guide to help you understand what each weather sensor does.
Note: We will not be covering solar sensors here with exception of the sunshine duration sensor. We will be posting a separate article that covers different solar irradiance sensors.
We thought we would start easy. A thermometer takes the ambient air temperature.
It is important thermometers avoid direct sunlight and full shade. This ensures the temperature recorded is accurate and not too hot or cold. Radiation shields such as the Stevenson Screens or gilled cylinders, protect from direct radiation. These protect from direct sunlight and allow air to flow around the thermometer.
Modern thermometers used for meteorological observations are usually ‘electric’ and not the common mercury style. As a result of being electric, the thermometer can feedback real-time data into a computer database. This negates the requirement for regular human monitoring.
A hygrometer measures humidity of the surrounding air. Humidity is a measure of the moisture content in the air. Humidity is represented as a percentage from 0 – 100%. 100% humidity is high meaning the air contains more moisture. A tropical jungle will see humidity levels of 100%. deserts will be closer to 0%.
Humidity is a function of temperature. As a result, thermometers and hygrometers often come in a single instrument outputting the two measurements.
From the Greek word for wind, anemos, anemometers measure wind speed. Anemometers come in a few different formats. Cupped anemometers and ultrasonic anemometers provide the most accurate measurements. As a result, the meteorology sector primarily uses them. Cupped anemometers work by ‘capturing’ the wind in cups mounted onto a spindle, the speed the spindled spins correlates to the wind speed. Alternatively, as an extremely simple explanation, ultrasonic anemometers measure the wind’s effect on pulses of ultrasonic sounds between sensors.
Wind vanes record wind direction i.e which way the wind is blowing. The designs are not too dissimilar to the centuries-old Cockrell with an arrow design seen on top of houses. However, modifications of wind vanes over the years yield more accurate results and better stability to withstand gale-force conditions.
Wind vanes and anemometers are often combined in to a single unstrument simultaneously measuring both parameters.
Another easy one, rain gauges measure the quantity of precipitation (rain, snow, sleet, etc). However, they come in various different styles. For low-level applications where accuracy is not paramount, a simple funnel and collection jar is sufficient. Consequently, this type of rain gauge requires manual monitoring and recording.
For meteorological applications, where automated monitoring is preferable, tipping bucket rain gauges are ideal. Two buckets in a see-saw formation comprise the internals of the rain gauge. The gauge’s funnel captures water that feeds down and lands in a bucket. When the bucket fills it becomes heavier. Subsequently, at a certain weight the bucket drops (raising the opposite bucket), which sends a pulse to the datalogger (which automatically sends data via telemetry or wired connection).
Another form of rain gauge uses doppler radar. This method is able to accurately distinguish between precipitation types (rain, snow, hail, sleet, etc.) and calculate their intensity. Doppler radar rain gauges can detect a single drop of rain 0.3mm small.
Barometer is derived from the Ancient Greek word βάρος, romanized: báros meaning “weight”, and Ancient Greek: μέτρον, romanized: métron meaning “measure”. source https://en.wikipedia.org/wiki/Barometer. A Barometer measures air pressure. There are many types of barometers, ranging from mercury barometers to water barometers. Modern instruments use MEMS barometers which are accurate, sensitive and small enough to install in smart phones.
Sunshine Duration Sensor
We won’t insult your intelligence with this one. But one important note is that sunshine is only ‘officially’ (according to the WMO) sunshine when the irradiance is above 120 Wm⁻². Consequently, anything below this is not sunshine.
Traditional sunshine duration sensors such as the Campbell-stokes sunshine recorder focussed the sun’s rays onto a piece of card. When it was sunny the card would be burnt. At the end of the day, the length of burns would be added up giving the total amount of sunshine.
Modern-day sunshine durations sensors use thermopile or electrical sensors to record the sunshine duration. They can output either analogue or serial data. Hence, with modern sunshine duration sensors there is no need to make any interpretations.