Weather instruments are used to take measurements of various atmospheric parameters at local weather stations.
Why do I care? By understanding patterns of temperature, wind, and rainfall as measured by instruments, we can describe our local weather and climate.
I should already be familiar with: Temperature, Humidity, Pressure
Weather instruments are used to take measurements of temperature, wind, humidity, and rainfall, as well as other atmospheric factors which describe the local weather and climate. Different types of instruments are used to measure different parameters and there are many types to choose from. The variables measured with these types of instruments are wind speed and direction, pressure, humidity, temperature, and precipitation, including rain and snow.
Thermometer
Figure A: Electronic Temperature Sensor (Image from NOAA)
A thermometer is an instrument used to measure temperature. Thermometers are used to measure outside and inside temperature, body temperature, oven temperature, and food temperature. Most thermometers measure by direct contact with the air, although infrared thermometers use sensors to detect infrared radiation coming off of surfaces and estimate temperature that way (similar to the way night-vision goggles work). A common thermometer is the mercury thermometer used outside residential areas. The volume of the mercury changes as the outside temperature changes. The volume of the liquid expands as it heats up, representing an increase in temperature, and the liquid contracts when it cools down, representing a decrease in temperature. At modern weather stations an electronic temperature sensor is used to measure the outside air temperature. The temperature sensor on this device is contained within a vented unit which allows air to flow freely across the sensor and measure the temperature while keeping the thermometer shaded from the direct heating of the sun.
Anemometer
An anemometer is a type of weather instrument that measures wind speed. Some of these instruments measure both wind speed and wind direction. Anemometers are common at weather stations. A cup anemometer is a type of instrument that uses three or four hemispherical cups mounted on horizontal arms on a vertical rod. The wind pushes the cups and causes the arms to rotate at a rate proportional to the wind speed. A windmill anemometer is a common instrument used at weather stations to obtain the wind speed. A wind vane is used as part of the anemometer to determine the wind's direction. As the wind flows over the windmill, the speed and direction of the wind can be measured with this instrument. Some scientific anemometers use the speed of sound to measure the wind speed more precisely in three dimensions. Wind direction is always given by where the wind is coming from, so that a west wind is blowing from the west and going towards the east.
Figure B. Cup Anemometer. (Image from NOAA).
Hygrometer
Figure D. Sling psychrometer. (Image from NASA).
A hygrometer is an instrument used to measure relative humidity. Humidity is the measure of the amount of moisture in the air. A psychrometer is an example of a hygrometer. A psychrometer uses two thermometers to measure relative humidity; one measures the dry-bulb temperature and the other measures the wet-bulb temperature. (When you come out of your shower in the morning, your skin cools to the wet-bulb temperature and you feel a chill until the water evaporates.) The wet-bulb thermometer contains water in the base that evaporates and absorbs heat which decreases the temperature reading. To determine the relative humidity, the temperatures are taken from the dry-bulb thermometer and the temperature difference between the wet and dry bulb thermometers. From these measurements, a table is used to find the relative humidity at a certain location. A sling psychrometer is a common instrument used by meteorologists to determine the relative humidity. This instrument is swung around while being held. There are also a variety of other humidity sensors which work automatically to measure the water content and relative humidity of the atmosphere.
Rain Gauge
Figure E. Heated tipping bucket rain gauge. (Image from NOAA).
A rain gauge is an instrument used to measure the amount of liquid precipitation over a certain length of time. In its simplest sense, a rain gauge is nothing more than a can which collects water which falls from the sky as rain. The depth of the rain can be measured with a ruler. In the United States precipitation is usually measured and reported in hundredths of inches. Rain gauges are placed in open areas where there are no obstructions. Rain gauges do have limitations. During hurricanes, high winds make liquid measurements in rain gauges impossible. Also, when the temperature approaches freezing (0°C), liquid may freeze around the rain gauge and block the opening. A common type of rain gauge used at weather stations is the heated tipping bucket. This rain gauge melts frozen precipitation around the opening and keeps the precipitation in liquid form when it enters the bucket. As rain enters the funnel of the tipping bucket rain gauge, the rain drips into one of the two buckets that are balanced on a pivot below the funnel. When the bucket tips, it triggers a reed switch which sends data back to the weather station on the amount of precipitation in the bucket. However, the heating element can cause evaporation of small amounts of rain before it gets to the measuring funnel. Also, the tipping bucket can jam or overflow in high-intensity rain like thunderstorms, which can cause errors in the precipitation amount.
Barometer
Figure F. Modern Aneroid Barometer. (Image from Wikimedia Commons).
Atmospheric pressure is measured by barometers. An aneroid barometer, one of the most common types, uses a sealed can of air to detect changes in atmospheric pressure. As the atmospheric pressure goes up, it pushes in on the can, and the can is slightly reduced in volume, moving an indicator needle towards higher pressure. If the atmospheric pressure goes down, the can expands slightly and the needle indicates lower pressure. Some barometers in the past used special graph paper to track changing pressure over time; now, they report electronic signals to a computer, which plots the trends of pressure on computer monitors.
Links to National Science Education Standards:
5th grade science: 5.E.1.2 : Predict upcoming weather events from weather data collected through observation and measurements.
Activities to accompany the information above:
Activity: Building a Weather Instrument
Instructor Set-up Instructions
Student Activity: pdf document
Description: This activity will assist students in learning how to build their own instruments.
Relationships to topics: Pressure