Let's briefly discuss some common ways we measure various variables, and more importantly some of the considerations you should make when using these data. Considerations do not include the universal concern which is that a sensor could be improperly installed, connected or calibrated, making it inaccurate. That is always a concern, particularly the farther a station is from a professional maintainer.
Air temperature is the most common variable for a surface weather station. In-Situ measurements (thermometers) often measure the electrical characteristics of metals at different temperatures. Different amounts of electricity can be measured from the metal and converted into a temperature based on how the sensor is designed.
Limitations & Considerations
Depending on design, the ability to sense extreme temperatures or fast changes in temperature may be hampered. Over time the characteristic of the metals will change, and without re-calibrating the sensor, it may become less accurate. It is uncommon for commodity thermometers to be calibrated after they are deployed.
Moisture can be measured many ways, and there are several moisture variables that can be sensed. Most common are Relative Humidity (RH) and dew point. The most common way for commodity sensors to measure RH is by assessing variations in the capacitance of a material exposed to the air. When the RH changes, this capacitance, which can be measured, changes in a relatively consistent fashion. To measure dew point directly, techniques exist to directly cool a surface until dew forms, and using a thermometer (above) to detect what temperature that was. This is very costly, and as a result is only found on very expensive observing platforms. In many instances the RH sensor is in the same physical package as the thermometer.
There is another moisture measurement, called the wet bulb temperature. This can be measured by exposing water to flowing air and determining where the two temperatures reach equilibrium. This is generally hard to do automatically, and can mathematically be extracted from temperature and another moisture observation.
Limitations and Considerations
Similarly to thermometers, over time the ability of the material to represent the ambient conditions will decrease. Particularly, extreme cold can negatively impact the ability of an RH sensor to be accurate. Generally they can be recalibrated, but not necessarily repaired if found to have very low precision.
Wind is another of the most common measurements, and a wind sensor (anemometer) is one of the most identifiable components of a classic weather station. Wind speed historically was measured with some kind of spinning cup and wind vane (prop-vane) or propeller device (aerovane). The rotation of the cups or propellers is measured as electrical pulses, which can then be related back to a speed. Wind direction in these instruments is measured with a 'variable resistor' most of the time, which has different electrical properties depending on where the direction sensor is along a circle.
However, in the last decade (and long before then) sonic wind sensors have become more common thanks to their decreased maintenance, lack of moving parts, and increased precision and accuracy. Sonic anemometers measure both the speed and direction of wind by sending small sound signals across a space, and measuring the doppler effect on the sound as it crossed that space. By measuring at 2 crossing directions, the speed in both angles, and thus the total speed and direction can be computed. Using a third pair of sensors, and separating the sensors both laterally and vertically 3-D wind speed and direction can be measured.
Limitations and Considerations
Besides normal wear-and-tear that degrades performance over time, anemometers are most popularly damaged by extreme weather conditions, which can damage them very visibly. Beyond that, ideally regular calibration should be performed on a mechanical anemometer for accurate measurement. prop-vane and aerovane sensors are not usually able to measure very low or variable wind speeds, and cannot measure turbulence directly.
Sonic anemometers historically were very expensive and delicate, but that has changed recently. Today, many reasonably priced sensor packs utilize the solid-state and compact nature of sonic anemometers for their wind sensing. Sonic anemometers require an accurate observation of temperature and pressure to correctly determine wind speeds, and as a result can be victim to any of the issues those sensors face. Sonic anemometers also can have issues in cold weather, as ice can build up on the transducers, reducing or eliminating the ability to sense.
Siting is one of the biggest issues with wind measurements. Though there is debate about what exactly representation even means, many stations are deployed close to obstacles which will change the wind speeds or directions from their free-field components which we would like to sense. Location concerns can render an anemometer completely useless depending on how close and significant nearby influences are.
Barometric pressure is both a very interesting and valuable observation, and it is one of the easier observations to make. Pressure represents the force the air at a location would push on a vacuum. Thus, to measure pressure a vacuum is created, and sealed with something that will appear different depending on the force the atmosphere is exerting. Mercury was historically popular for barometers because in addition to its eagerness to change volume with temperature, it was very heavy, and therefore over a short vertical distance the atmospheric pressure could be measured (water, if used the same way would take over 5 meters where mercury requires around 30 inches).
Since mercury is dangerous, a better way to automatically determine the pressure is to use sealed containers which are a vacuum inside, and whose exterior or lid is engineered so it can deflect slightly depending on increases or decreases of pressure. That deflection is then measured using several techniques, and converted into a pressure measurement. These canisters can be extremely small today, and barometers can be deployed in may environments as a result (most smartphones contain them).
Limitations & Considerations
Barometers, like all sensors, do require regular calibration for ideal performance. However, unlike most atmospheric sensors, with relatively small error, the siting of a barometer is not critically important. Normally, outside of a sealed environment the barometric pressure is the same indoors and out, and certainly everywhere on a weather station the pressure should be the same. One concern is in wind, where a condition called dynamic pressure (pushing from the wind) can create momentary high and low pressure periods. This concept is used by aircraft to measure their airspeed via a Pitot tube.
Precipitation is a critical and conceptually simple atmospheric variable. Precipitation measurement is actually very complicated. We will discuss the several most common precipitation measuring methods we see:
A rain gauge is a bucket which quantifies how much water has entered it over a period of time. There are two variations: tipping bucket, and weighing. Tipping bucket use a teeter-tottering bucket device to discretely measure small amounts of liquid precipitation, reporting each tip of the bucket. A weighing gauge contains a pre-set amount of liquid, and a pressure sensor at the bottom, which reflects the weight of the contained fluid. When precipitation occurs the weight of the fluid increases, and that change is reflected as a precipitation amount.
Other Rain Sensors
There are a number of other ways to measure rain, some of which are frequently employed in all-in-one weather sensors. Sonic rain sensors employ what are essentially microphones to hear the impact of a raindrop on a clear surface. These care carefully tested and calibrated to do their best at estimating drop sizes and such. Hot plate anemometers maintain a metal surface at a high temperature (above water's boiling point), and the amount of energy required to keep the plate at that temperature is measured, as each molecule of water on the plate would have to be boiled. These techniques have various considerations for cost, power consumption and accuracy.
Limits & Considerations
Gauge methods of measuring precipitation are often accurate in the presence of liquid rain, but become variable in the presence of cold, snow, debris or wind. Weighing gauges can be filled with antifreeze instead of water so in winter months they can generally still produce accurate liquid water measurements of snow and frozen precipitation. In cold weather, tipping buckets both can mechanically freeze as well as will fill with snow which does not drip through the buckets until it melts, up to several days later.
There is lots of discussion about how well a single point precipitation observation represents anywhere else, but we won't go into that.
Stations with the proper equipment can determine the height of one or more cloud levels directly or indirectly overhead using laser ceilometers. A ceilometer is a large laser range finder which sends a pulse of laser light, and records how long it takes for the reflection off a cloud to return. If a cloud is thin enough, responded pulses from a second or third cloud can be received. These return periods are timed, and pulses quickly analyzed to determine distances where clouds were encountered. By analyzing the measured clouds over time (since they move over a fixed point) the coverage of the clouds in a location can be determined.
Because it can only see multiple cloud layers when either the lower layers are thin or the clouds are moving and exposing the multiple layers, reports of cloud height can vary widely from one to up to three layers. Ceilometers cannot assess the depth of a cloud, only the bottom. Ceilometers, like all lidars, are capable of seeing through a modest amount of rainfall, however this can disrupt the patterns the algorithms use to determine cloud heights, so generally in rain they are not fully functional.
Horizontal visibility is affected by either fog (condensed water) or pollution, and is an important factor in aviation and road weather. Visibility is measured by placing a transmitter and receiver a short distance apart, and passing a light signal through the space. This light signal, usually a near-visible laser, will be impacted by the contents of the air, and the difference in strength between the emitted light and received light is measured. This difference is computed to represent an approximated visible distance, though the measurement itself was only made at a single point.
Because the measurement is only made at a single point, it may not actually represent how the visibility changes over the given distance, so if you have 10 mile visibility, but there is fog 3 miles away, you will clearly not be able to see 10 miles. However, at the point of the measurement, the air was sufficiently clear. Airports frequently employ several of these sensors along a runway for this reason.
Many surface weather stations are operated by road agencies to monitor the travel conditions in their representative areas. In addition to standard weather above, road weather stations frequently include sensors measuring conditions in the road. These include
The road temperature can be measured either in-situ or remotely. In-situ observation involves placing a sensor block into the road pavement, ideally matching as many conditions as the pavement so its measured temperature reflects that of the surrounding pavement. Remotely, passive infrared emissions, measured by a special camera looking at the road, can measure the road skin temperature as well. Both sub freezing or extremely hot conditions are common on road surfaces, and have implications for maintenance and treatment.
Road condition is often measured using in-situ sensors or optical techniques. They are designed to determine wetness or ice on a given road surface sample.
Nautical considerations are a very important part of the weather enterprise, and Synoptic is no exception, with thousands of publicly and privately owned buoy data sources available. Weather buoys often measure the same variables as mentioned above, as well as oceanic factors such as water information (temperature, turbidity, salinity, etc.) and dynamic information such as wave heights.
Water information is sensed by a pack of sensors attached to the buoy anchor line, and using similar techniques to the above. Temperature is measured the same way, comparing the electrical characteristics of metal exposed to the water at the given temperature. Frequently multiple temperature sensors will be placed at different depths.
Wave heights are measured by using the buoy's anchor line, which is capable of extending and retracting based on the bouncing of the buoy. The extension or retraction of this line is determined from some median state, and this is reported as the current wave height. This wave height information is critical for mariners and ocean safety organizations.
Limitations and Considerations
Due to their hardened nature, buoys are occasionally able to survive hurricanes and severe weather events better than land-based stations which are not engineered for the constant assault of ocean waves. In addition, the surface/in-situ observations to determine El Niño (ENSO) sea surface temperature patterns comes from a network of carefully gridded buoys in the Pacific Ocean.
Many weather stations are established to assist with agriculture, and one of the most important considerations for agriculture is the moisture and temperature in the soil. Soil temperature measurements employ the same techniques as above. Soil moisture is measured by determining how easily electrical current travels through a short distance of soil. The less moisture that is present, the harder it is for the electricity to travel. For this reason soil moisture sensors often resemble forks, where one tine produces a current and the other attempts to receive it. This can then be stabbed into the soil at the desired height to give a measurement from relatively undisrupted soil.
Applications and Considerations
Soil sensors can be placed at various depths depending on the requirements of the data recipients. Frequently 5cm and 10cm are common. Different stations may perform soil measurement differently. Soil sensors ideally will be located some distance away from the sensor, and might even include multiple ground coverings to represent the different forms of water processing that may happen.
Soil sensor electronics may be damaged by complete submersion in water, so flooding can result in permanent damage or disabling of a soil sensor set.
Weather stations can measure the amount of sunlight that reaches the ground. This is called shortwave radiation, and it is measured via a pyranometer, which is a device that directly converts incoming solar radiation into a small but measurable amount of electricity. It is also a radiometer which accepts only short-wavelength electromagnetic waves. In effect they are small, precise solar panels. The amount is expressed in watts per meter squared, representing a total amount of solar energy that arrives at the surface every second. These radiometers
An alternative form of solar sensor involves collocated black and white panels, each with thermometers. Using a calibrated system, the difference in temperature between the hot black panels, and cool white panels in direct sunlight represents the air temperature. These sensors are more expensive than the modern methods, however.
The opposite of the sun
For the atmosphere, the energy that comes from the sun is balanced by energy leaving. At the surface we can also measure that departing energy, as it reflects how the surface will warm up during the day, or cool at night. This is measured using a longwave radiometer, which filters out shortwave (visible) light, points down towards the ground, and only measures the emissions that are actually created by the ground (which happen 24 hours per day, not just when the sun is up).
Finally, there is a sensor that can combine all these into an interesting single number, called a net radiometer, which measures both longwave and shortwave radiation from both directions, and can subtract the energy going up from the energy coming down, and give a total energy balance for any time.
Air quality is an entire category of observations, but for our purposes here, it can be reduced to the sensing of particles or chemicals in the air. Particles can be measured either in a gross fashion, using techniques similar to visibility, above, or by passing air through careful filters and weighting them after a fixed period of time. In the case of PM2.5 (particulate matter less than 2.5 micrometers(µm) in size), two filters are employed, one filter rejects all particles over 2.5 µm in size, and within the enclosed chamber, either a second filter attempts to acquire all contained particles, or lasers are used to determine the number of contained particles.
Lasers can also be used to determine contents of specific chemicals, based on the tendency of chemicals to absorb specific wavelengths of electromagnetic energy. In this sense, a chamber can be built to hold sampled air, and a laser of specific frequency can be used to get a measure of how much of a certain molecule is in the air.
Fire weather stations keep track of a variable representing the moisture and temperature of fuels of a certain diameter. This is done (often) by attaching sensors inside a wooden dowel of the represented diameter, and measuring its temperature and moisture within, using methods similar to those above. This stick is then located in a representative location such that it can approximate the conditions experienced by an actual fallen stick.
Some airport weather stations will produce a coded description of the present weather at their station. These conditions are often computed by examining the other variables measured, including cloud heights, temperatures, winds, precipitation, visibility, and others, to make a guess at what is actually happening. Often times, for larger airports, this value can also be set manually.