A weather station is a collection of individual and specific instruments which, when looked at together will give you some idea of the local conditions. When many individual weather stations spread out over a wide area are looked at, and this weather station data is observed over time, patterns emerge and with that comes the ability to more accurately forecast the weather.
While it would take a book to describe all of the names of instruments used to measure weather, what we can look at is the weather instruments most commonly available in a full featured home weather station .
A thermometer is the foundation instrument for any weather measurements.The word “thermometer” basically means “heat meter”. A thermometer will give you an indication of the relative energy (as heat) in the air around it. “Relative to what?”, you ask. Weather stations use thermometers that are calibrated relative to the freezing and boiling point of water at sea level.
What is important is that everyone is using the same reference points. That way not only can you watch your temperature over time, but you can look at everyone else’s temperature also and compare.
It wasn’t always like this. When Daniel Gabriel Fahrenheit first came up with his temperature scale, his reference points were created using a special brine solution that froze at a much colder temperature than water, and at the other end he used his body temperature. This was later changed to use freezing and boiling reference points, but it left us with the rather odd scale we use now.
Anders Celsius developed the centigrade temperature scale using the modern reference points but initially had boiling water at 0° and freezing at 100°. Isn’t that strange? Converting from F° to C° and back is easy now, but what if one scale ran up and the other down?
Different kinds of thermometers used in weather measurements
The most well known thermometer is the mercury thermometer. This is the same kind of thermometer that Fahrenheit invented 300 years ago. This is an excellent and precise tool and is often still used for weather measurements. As a matter of fact, it is often suggested that you calibrate your fancy modern digital weather station thermometer against a high quality mercury thermometer.
Another common thermometer is one with a bimetallic spring movement. Theses are commonly seen on porches and gardens. These are not precision instruments but they are useful to see the temperature at a glance.
Electronic thermometers are the most common type in digital home weather stations. They produce a signal that is easy to digitize and transmit to an indoor display. Thermistors are the most common electronic component used to measure temperature electrically . Thermistors change their electrical resistance as the temperature rises and falls so a increase or decrease in the voltage across a thermistor correlates with a rise or fall of temperature.
Another common electronic thermometer uses silicon diodes. The voltage through them changes with temperature. Silicone diodes can be included in a silicon integrated circuit.
Another consideration is that infrared solar radiation will heat up anything left exposed to the Sun, including thermometers. While not a weather measuring tool itself, without a solar radiation shield your temperature measurements will be junk if your thermometer is in direct sunlight.
Solar shields come in two basic forms; passive and active. Examples of passive solar shielding would be putting the thermometer in the shade of the house, shade of a tree, a solar shield made for purpose.
House shade problem: Doesn’t stay shady and the house has a thermal inertia. If your putting the thermometer next to a structure, you are taking the temperature of that structure, which changes temperature slower than the surrounding air.
Tree shade problem: Trees, through transpiration, create a cool down flow of cool air and the temperature under the tree is not representative of the general temperature of the area.
There are plenty of solar radiation shields made for the purpose from ones that slide over the temperature sensor to a small ventilated box like a Stevenson screen.
Active solar shielding involves installing a fan on or in the shield to move fresh air over the temperature sensor. This is sometimes known as FARS, or fan aspirated radiation shield. This is the best solution in my opinion.
A hygrometer measures the water vapor content of the air. We usually refer to this as humidity. Without humidity the weather would be pretty boring, wouldn’t it?
In the late 1700’s a hygrometer was made using a ten inch or so piece of human hair. Hair has the property of being hygroscopic, that is it attracts and holds water. As hair holds more water, it gets longer, and of course it get shorter as it drys. Using the hygroscopic property of hair, and a clever mechanism came the first hygrometer. Nobody does this anymore, but the name stuck.
A mechanical hygrometer that is used today is called a psychrometer, also called a wet bulb-dry bulb hygrometer. Basically this is consists of two thermometers together with one having it’s bulb covered with a wet fabric. When the psychrometer is swung around in the air the wet one is cooled by evaporation and the relative humidity is looked up on a psychrometer chart by using the different readings on both thermometers.
Measuring humidity has always been tricky. Unlike thermometers, hydrometers have no easy frame of reference like freezing and boiling at sea level. Most of the hygrometers that we use in personal weather stations detect relative humidity and relative humidity is dependent on the temperature and total water content of the air. For this reason a hygrometer and thermometer are paired up and is called a thermo-hygrometer.
One calibration method that should be good enough for most people is the saturated salt method. To use this method, you put some table salt (sodium chloride) in a small container and add distilled water to make a “wet sand” consistency. Then put the container and your hygrometer in a zip lock bag and let it set for 12 hours or so. (Don’t put your hygrometer in the salt!) It’s the nature of this closed system to reach an equilibrium of 75% relative humidity.
Types of hygrometers used in home weather stations
The way these work is that the material in between the plates of a capacitor picks up water vapor and thereby changes the capacitance. When capacitive hygrometers are calibrated properly they can be accurate to ±2% RH. These hygrometers have a nice linear response to changing relative humidity. Davis weather stations use this type of hygrometer. These capacitors react to their electrical surroundings and must be calibrated that way.
Resistive hygrometers work by measuring the resistance across a hygroscopic substance; the more water vapor absorbed, the lower the resistance. This type of hygrometer has to be corrected for temperature variations, making the circuitry more complex, however, they are not sensitive to their electrical environment. This make replacing one easier. Accurite and Oregon Scientific weather stations and others use this type of hygrometer. I know this because I took mine apart…
Barometers are used to measure atmospheric pressure, also called barometric pressure. Changes in pressure is a leading indicator of a weather change. By watching the direction, rate of change , and magnitude of the pressure change, you can get a good idea of what kind of weather to expect.
Most people have at one time or another pulled a straw out of their drink with a finger over the top and have noticed that they could keep fluid in the straw. This is the idea behind Torricelli’s barometer. Big deal, right? But back in Torricelli’s day, the early 1600’s, this phenomenon was impossible to explain. Settled science said that a vacuum was impossible and that air had no weight. Even Toricelli’s teacher Galileo believed this, after all the Church said it was true and Galileo by this time had learned not to argue with them.
By acknowledging the concept of a vacuum was real and at the same time giving the atmosphere some weight, Torricelli advanced science on a scale on par with Galileo himself. This, of course, is dangerous business. Torricelli died three years later at the ripe old age of 39.
The Torricellian barometer or more commonly called mercury barometer can be an interesting looking weather instrument, but it is not always practical to cart a jar of mercury around.
The aneroid barometer
This is a fluid-less barometer that uses the expansion or contraction of an evacuated and sealed metal box called an aneroid cell. The small movements of the cell are leveraged up to move a meter. These can be made quite small and are easy to move around. Another nice thing about aneroid barometers is that they are not electrical; they just power themselves.
Electronic barometers resemble aneroid barometers in concept. Instead of a mechanical connection to an aneroid cell, an electronic strain gauge is on a very small aneroid cell. As the strain gauge gets distorted by flexing, the resistance across it changes and the reading is converted to our atmospheric pressure. Electronic barometers can be made small, very, very small! They are sometimes called mems barometers as in ‘microelectromechanical systems‘ barometers. These are the most commonly used barometers in home weather stations.
As you might see, barometers make for good altimeters; just set it at zero, then drive up a hill and you’ll be able to figure out the hill’s height.
That makes for a crappy barometer though. To make a barometer good for weather forecasting everyone has to be on the same page. The frame of reference used is the average global pressure at sea level of 29.92 in hg. So you have to add back in the pressure drop from your altitude. Since air pressure also changes with temperature and humidity, this must be figured in also.
There are calculators on the net for this purpose, or, if you have a local airport at your same elevation you can adjust your barometer to match theirs. Acurite has a patented self calibrating barometer. It gets your average pressure by taking a measurement every 12 minutes for 33 days. Then it is just a matter of adding the average difference to your barometric reading.
Anemometers and Wind Vanes
Anemometers measure wind speed and wind vanes detect wind direction. Both of these weather instruments are usually found together.
Anemometers used in modern home weather stations come in three forms:
- Spinning cup anemometer uses 3 specially designed cups and can measure wind from any direction.
- A vane anemometer uses a propeller mounted to a wind vane to keep it aimed into the wind. They can resemble a windmill.
- A sonic anemometer uses distortions in sound waves to detect both speed and direction.
To determine speed and direction, these wind sensors use a magnet passing a reed switch or a light beam getting interrupted to count revolutions or position. Some wind vanes use a potentiometer to sense position. On my LaCrosse unit the propeller uses a magnet and the wind vane uses an optical sensor. I had trouble with the wind vane at a certain time of day at a certain time of year. Weird. Seems that when the Sun was in a very particular position it interfered with the optical sensor. So I wrapped some electrical tape around the area I thought the sensor was at, and that fixed it.
With a sonic, or ultrasonic wind sensor you have no moving parts. Two or three receiver -transmitter pairs are arranged such that when wind blows across the array the sonic signals are changed more on one pair than another. From this the wind speed and direction can be calculated.
A very common and old measure of wind is the Beaufort scale, or BFT. This measures the observed force of the wind. Originally coming from sailors observations of the effects that the force of the wind had on their sails and on the ocean. The Beaufort scale runs from 0, meaning dead calm, to 12 meaning hurricane force winds. On land the observations are of how smoke rises and how the the wind affects tree leaves and branches.
A rain gauge is used to measure the amount of precipitation that occurs over a period of time. An electronic rain gauge can also determine the rate of rainfall.
Modern rain gauges used in digital weather stations are most commonly of the tipping bucket, or sometimes called tipping spoon design.
The tipping bucket design is a rain collection container with a small hole in the bottom. Water goes through the hole and collects in a small calibrated receiver. Imagine two spoons with the handles cut off and the two spoon’s handle stubs connected together. This is setup as a lever or a pivot so that as one spoon gets full of water it tips downward and the water spills out, meanwhile the other spoon is lifted up and is filling up.
Rainfall is measured by electronically counting how many tips the buckets made and adding up the capacity of the buckets.
Rainfall rate is determined by how fast the buckets tipped back and forth.
As you can see, there are some limitations to this device. For instance, a high precipitation rate can overwhelm the capacity of of the gauge and the water just spills out. And the tipping bucket arrangement has to be fed water at a reasonable rate to keep water from splashing out. The other side of that is you cannot measure less than a full bucket.
Another problem is that crud, bird dropping, bugs, and leaves can get in either plug the hole or keep the tipping mechanism from working properly. Here I have blowing dirt and sand that plugs up the works.
So these rain gauges must be situated to make for easy periodic maintenance.
Another rain gauge is a graduated cylinder that hold rain water so you can go out and read it. These are very accurate but require manual reading and emptying. This would be a good supplement to a personal digital weather station to check for calibration and to measure both very high and low rainfall.
Solar Radiation Sensors
These are also called pyranometers. A solar radiation sensor measures the light energy from the Sun and gives you a measure in watts per square meter. Light from as much of the sky and from as much of the visible spectrum possible is captured.
This is an objective measure of solar brightness or energy an is useful in setting up and designing solar energy systems.
Solar radiation sensors also provide the available evaporative energy inputs when calculating evapotranspiration for farms, forests, lakes and more.
Ultra violet sensors detect and report the sunburn causing radiation from the Sun. For people with health concerns in regards to Sun exposure, a UV sensor is a good idea.
Farmers and gardeners can use this measurement to aid in plant production. Many plants benefit from the high energy light of ultra violet, as those that use grow lights can attest. Some fruits, like apples require UV exposure to ripen and turn red.
And of course there is just the fun of having another weather measurement to play with!
Lightning detectors pick up the electromagnetic pulse, (EMP) put out by lightning. The strength of the received signal gives you some indication of how far away the strike occurred. By uploading your data to a lightning monitoring network, you can get an indication of where the strike was by triangulating using the signals from the other detectors.
Well, those are the names and some information on the most commonly used instruments used to measure weather. We covered temperature, humidity, wind speed, wind direction, rainfall, solar radiation, UV radiation, and lightning.
While no home weather station that I know of will give you all eight weather instruments, there are two that will give you seven: Ambient Weather has a 7 in 1 weather sensor, and Davis instruments has an upgrade system or an add-on with solar and UV to the station I reviewed here.