Lecture 2 - Meteorological Instruments

Instruments are an important part of Meteorology. Instruments perform measurements which describe the current conditions and can be ingested into a computer for modeling purposes. There are many different variables that a meteorologist uses so he or she must be familiar with the instruments that measure them. We will look at a few now and later we'll look at some instruments more in-depth.

Networks

The most common way of obtain mass quantities of observational data is through the use of meteorological networks. Most networks are stationary, which allows detailed plots of data to be created easily. However, the use of mobile networks is becoming more useful in research studies. Mobile networks allow researchers to place instruments in a particular position relative to a moving object, like a supercell thunderstorm.

The two most common stationary networks that will be discussed today and in the future are the Automated Surface Observation System (ASOS) and the Oklahoma Mesonet. Automated Surface Observation System (ASOS) towers are installed nationwide to measure weather conditions and run by the National Oceanographic and Atmospheric Administration (NOAA). In Oklahoma, there are between 30-35 ASOS towers. Every tower records data every hour.

The Oklahoma Mesonet is a dense network of stations in Oklahoma that reports weather data every 15 minutes. The Oklahoma Mesonet is run by the Oklahoma Climatological Survey, which is based here at the University of Oklahoma. Please visit the Oklahoma Mesonet website and read more about their weather observation network.

Thermometer

A thermometer is an instrument used to measure temperature. There are four main types of thermometers that will be discussed: Liquid-in-Glass, Bi-metallic, Infrared and Thermoelectric.

Liquid-in-Glass:
A liquid-in-glass thermometer is a glass tube with a bulb at one end filled with a liquid and has a scale fastened either on or next to the tube. The tube has a inner tube in which the liquid rises as the temperature increases, and falls as the temperature decreases.

The liquid is usually either mercury or alcohol. Alcohol is sometimes preferred, because mercury is toxic to humans. Also, mercury can only be used above temperatures of -39°C (at which point it freezes), while alcohol can be used down to -62°C.

The liquid-in-glass thermometer is the most common type of thermometer in everyday use (medicine, cooking, etc.). The largest downside to a liquid-in-glass thermometer for meteorologists is that it can not be automated very easily. This led to the development of other types of thermometers.


Bi-metallic:
A BI-metallic thermometer uses a coil of two different types of metals attached to one another. Since different metals expanded at different rates as the temperature increases, this causes the coil to bend. The temperature can then be determined by how much the metal has bent.

Outdoor thermometers (like shown on the left) use this type of a device.

Like the liquid-in-glass thermometer, the BI-metallic thermometer can be very difficult to automate.

Infrared:
Infrared thermometers are being used more and more everyday. Infrared thermometers measure the infrared radiation emitted by an object that is beyond the sensitivity range of the human eye. It essentially detects the amount of energy an object has and calculates a temperature from it.

As seen on the left, doctors use this technology to instantly determine a patients temperature through their ear. Where the previous two types of thermometers must "heat up" or "cool down" until an accurate measurement is detected, an infrared thermometer is instantaneous.

Also unlike the previous two types of thermometers, the infrared thermometer can easily be automated; therefore, making it very useful for automated stations. (However other aspects can affect accurate measurements such as ground color, and thus infared thermometers are used in conjunction with other thermometers.)


Thermoelectric:
A thermoelectric thermometer is the most commonly used thermometer on automated stations. There are many forms of the thermoelectric thermometer.

An electrical resistance sensor is one whose resistance varies as a function of temperature. A known voltage is passed through a metal wire (usually platinum) and the resistance is measured at the other end of the wire. The temperature can then be calculated from this measurement.

These types of thermometers have a wide useful temperature range, are rugged, reliable, inexpensive, and have a very fast response. They are also extremely easy to to automate.

Here is more information on thermometers. You can read these sites to answer some frequently asked questions pertaining to thermometry.

Anemometer

An anemometer is an instrument used to measure wind speed and direction.

A wind vane (seen on top in the picture on the left) is directed into the flow of wind by the force of the wind on the flag. By putting a rotatable sensor inside, the direction of the wind can easily be automated.

The cups (seen on bottom in the picture on the left) catch the wind (as the air particles hit the cup) and rotate at the speed of the wind. This to can easily be automated by using electrical sensors.

Nearby objects such as trees and buildings can cause false measurements of the true wind speed. As a rule, an anemometer must be placed 10 times the distance away from the tallest nearby object. For example, if there was a 15 meter tree, then the anemometer must be placed 150 meters away from the base of the tree.


Barometer

A barometer is an instrument used to measure atmospheric pressure. It works somewhat like a weight scale for the atmosphere. Like the thermometer, there are many forms of the barometer. A common type, most likely the one seen in the photo, uses a sealed container filled with a gas. As the atmospheric pressure changes (either rises or falls), the container will either expand or contract. Gears and dials can then be attached to read out the atmospheric pressure.

(Have you ever bought a bag of potato chips and then gone to a higher elevation like the mountains? Did you notice that the bag expanded? A barometer works the same way.)

Tipping Bucket Rain Gage


A tipping bucket rain gauge is an instrument used to calculate the amount of rain that has fallen over a given amount of time. Two buckets are on a seesaw type of lever. As one bucket fills up with water, the weight forces the lever to switch and the buckets dumps the water out. The other bucket then repeats the process. This is repeated over and over for each bucket.

An electrical switch counts the number of times the "seesaw" has flipped back and forth. Since the volume of the bucket is known, then the total amount of rainfall can be calculated.

For example, if the bucket size was .01" and the switch counted 157 flips, then 1.57" of rain had fallen in that time period.


Other Instruments

There are many other instruments that are used to record meteorological information, such as a hygrometer which measures humidity. You will learn in great detail how these and other instruments work, how to calibrate them, and how to detect errors in their measurements in METR 3613 "Meteorological Measurements". You will take this course during your Junior year as a meteorology student.

Other methods of collecting data that can be thought of as instruments like a radar, satellite, and rawindsondes will be discussed in later labs. These are so important (and much more detailed) that separate labs have been constructed for them.

That is all we are going to cover in this lab. Please go to the Homework #2 page and retrieve your homework. Print out the assignment, complete it, staple the pages together (if necessary) and turn it in at the beginning of class on 9/17.

Written by: Brad Illston