What Does A Professional Or Home Weather Station Measure?
From reading or watching weather forecasts, you will be quite familiar with some terms that are regularly used during these forecasts. If you were always wondering what any weather station measures, which instruments are used and how the different facets of weather are measured, read on.
When you think about how weather is measured, it is actually not just one, but a combination of variables that are measured. Combined, these variables form the atmospheric conditions we experience as a specific type of weather at any specific time.
The most common elements measured by a home weather station are temperature, humidity, wind speed and direction, rainfall and air pressure.
To better understand them, we will look at each of these different elements that make up the weather. Under each category, we also look at the different instruments used to measure it, as well as how it measures the specific aspect of weather.
Temperature is measured by a thermometer. You get different types of thermometers, but but to keep things simple lets just focus on the two more familiar ones.
The traditional thermometer consists of a tube filled with a liquid that expands or retracts in response to changes in temperature. One of the most well-known examples is the mercury filled thermometer. As the temperature rises, the mercury expands and push higher up in the tube giving you a measured reading. As the temperature drops, it contracts and the level indicator in the tube drops accordingly.
The majority of modern thermometers however, including the ones used in weather stations, use thermocouples. They measure temperature by the amount of electrical resistance that is flowing between two different metals. Let me explain...
Two different alloys are joined together. Each alloy's electrical resistance responds differently to any changes in temperature. An electrical current flowing through these two metals will be influenced directly by this difference in resistance created by these alloys. By measuring the the changes in the current, an accurate reading of temperature can be obtained.
Humidity is the amount of moisture present in the air, and is measured by a hygrometer. A variety of different hygrometers exist, but again I will just focus on the two most important ones to stay on point.
The psychrometer is a well-known earlier example which uses two thermometers (one being covered with a wet cloth and the other to open air.)
The bulb of the thermometer covered with the cloth will give a lower reading due to evaporation of the moisture on the cloth, which rate of evaporation in turn depends on the amount of moisture present in the air. The different readings between the two thermometers are then used to determine the relative humidity of the air.
Clearly this is not a very accurate way of measuring humidity, but luckily the invention of the electronic hygrometer made everything a lot easier and more accurate.
As with thermocouples (used for measuring temperature), electrical resistance is again used to make an accurate reading, but in this case humidity is the target.
A capacitive and resistance hygrometer operate slightly different, but are both based on the same principle. A material able to absorb moisture, is used to measure the amount of electricity flowing through it.
The more moisture a material contains, the better it is able to carry an electrical current. The amount of humidity in the air will determine how much moisture the material is able to absorb. As a result the relative humidity can be determined by measuring the strength of the electrical current flowing through the material.
Rainfall is the amount of precipitation measured in a specific area over certain period of time and measured by a rain gauge.
Professional weather stations use a cylindrical funnel with an aperture of 8 inches (203mm) which is placed about 12 inches (300mm) above ground level. It can normally hold up to 1 inch (25mm) of rainfall.
After a 24 period the content is collected and poured into a calibrated glass for accurate measuring, while the empty cylindrical funnel is repositioned in place to record the next 24 hours of rainfall.
Naturally, not all rainfall gauges can be manned and emptied regularly, especially when it comes to personal weather stations and weather stations placed in remote areas. As a result, almost all modern home and remote weather stations use Tipping Bucket Rain Gauges.
Tipping Bucket Rain Gauges have the advantage of recording rainfall automatically and never needs to be emptied. It consists of two buckets that is balanced on fulcrum (and works in a see-saw action).
A funnel collects the rainfall which is collected in one bucket. Once the bucket reads a certain predetermined amount of rain, it tips and empties its contents into a runoff area. As it tips, it triggers an electronic signal that is send to the base station to record the reading. At the same time the empty bucket on the opposite side is lifted into position to record the same amount of rainfall before it also tips, the measurement is recorded and the process is repeated.
Wind speed is simply the rate at which air is moving at any specific location and is caused by a difference in air pressure, as air flows from an area of high pressure to an area of low pressure. The instrument used for measuring wind speed is called an anemometer.
The most well-known and common used anemometer is the 3-cup anemometer (or 4-cup), where cylindrical cups are attached to horizontal arms that pivots around a center pole.
As the wind speed increases, the cups start rotating faster. By measuring the speed of this rotation, the wind speed can be calculated. The actual measuring can be done in more than one way, but are all based on the speed at which the cups are rotating.
There are other ways of measuring wind speed, like using a hot-wire anemometer or tube anemometer, but are not important for normal everyday wind speed measurement.
Wind direction obviously refers directly the direction of air movement (wind). It may not be as obvious as you think though. Wind direction refers to direction the wind is coming from, and not the direction it is blowing in. (This means when the weather reports refers to a southerly wind, it means it is coming from southern direction. Many people understand this the other way around.)
Wind direction is measured predominantly by a windsock or wind vane. Windsocks are often seen at airports, but the weather vane is used on a regular basis in professional and personal weather stations.
Most of you know what a weather vane looks like. Very often seen on top of tall buildings, it consists of a horizontal pole spinning freely around a central axis with an arrow (or similar pointing device) on the one side, and a big flat vertical surface on the opposite side that responds to any changes in wind direction.
Air Pressure (also called barometric or atmospheric pressure) is the amount of molecules present in the air. To put it another way, the air around us has weight, which is determined by the number of molecules present in the air.
Air pressure is closely related to gravity, which means that the air closer to the ground is exposed to stronger gravitational forces, making it much heavier while air higher up in the stratosphere are exposed to very little gravity and is much lighter as a result.
(This is the reason jet airliners fly at such high altitudes of around 30 000 feet. The low air density allows it to fly much faster and economical through the thinner air, keeping fuel usage much lower than it would having to fly at low altitudes and having to push through dense air. It keeps them out of more turbulent air closer to the earth's surface as well.)
The instrument used for measuring air pressure is called a barometer. There are basically two kinds of barometers. The Mercury and Aneroid barometers.
Mercury barometers date back centuries. It consists of a glass tube partially filled with mercury. It is placed upside down in a container (reservoir) also containing mercury. The container is exposed to the surrounding air.
As air pressure increase (gets heavier), it pushes down on the mercury in the reservoir, forcing the mercury inside the glass tube to rise. This change in mercury levels is measured and air pressure can be calculated as a result. Naturally, as air pressure drops the opposite occurs.
As mercury is very poisonous and the setup requires a fairly controlled environment, this kind of barometer is not very safe and practical solution.
Aneroid barometers solve all these problems and is used in almost all modern weather stations (and even in your smartphone!) It consists of a partially vacuumed sealed metal container. This is attached to a spring and levers, which is connected to a needle or electronic measuring device.
As the air pressure increases, it causes the container to contract, causing the attached spring/levers to expand or move, which is registered the measuring device and either displayed or transmitted to the base station. When the air pressure decreases, it causes the container to expand, causing the opposite reaction which is also measured and recorded.
Now you have a clear understanding what variables a weather station measures, as well as which instruments are used to measure them and how they are measured.
Also take not that not all home/personal weather stations can measure all these variables. Normally they can contain any combination of measuring tools. (The more comprehensive weather stations come standard with all of these measuring capabilities though.)
Some personal weather stations even read more variables than the ones mentioned in this post, like solar radiation and soil moisture. These are advanced functions and not a necessity for the majority of your weather measuring requirements.
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Until next time, keep your eye on the weather!