Weather Instruments: Barometers, Anemometers & Hygrometers: Fifth Grade Science
Meteorologists monitor, record, and forecast atmospheric changes using precision weather instruments that measure temperature, pressure, wind, humidity, and rainfall.
The Need for Precision Weather Instruments
Human senses are notoriously unreliable for measuring atmospheric conditions. You can feel whether a day is warm or cold, but you cannot determine the exact temperature in degrees Celsius, detect a 2-millibar drop in barometric pressure, or accurately calculate wind velocity in kilometers per hour. To collect quantitative scientific data, meteorologists rely on an arsenal of specialized weather instruments installed at surface weather stations, carried aloft on weather balloons, or orbiting in space on satellites.
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| CORE METEOROLOGICAL MEASUREMENT TOOLS |
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| Instrument Name | Variable Measured | Standard Measurement Unit |
+---------------------+-------------------+--------------------------------+
| Thermometer | Air Temperature | Degrees Celsius (°C) / °F |
| Barometer | Air Pressure | Millibars (mb) / In. of Mercury|
| Anemometer | Wind Speed | Kilometers per hour (km/h)/ mph|
| Wind Vane | Wind Direction | Compass points (N, S, E, W) |
| Hygrometer / | Relative Humidity | Percentage (%) / |
| Psychrometer | and Dew Point | Degrees Celsius (°C) |
| Rain Gauge | Precipitation | Millimeters (mm) / Inches |
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The Barometer: Measuring the Weight of Air
The barometer is one of the most critical forecasting tools in meteorology because changes in air pressure directly forecast upcoming weather. Italian scientist Evangelista Torricelli invented the mercury barometer in 1643.
- Mercury Barometer: A glass tube filled with dense liquid mercury inverted into a reservoir dish. Atmospheric pressure pressing down on the open dish supports a column of mercury roughly 760 millimeters (29.92 inches) high at sea level.
- Aneroid Barometer: A modern, non-liquid barometer containing a sealed, flexible metal capsule called an aneroid cell. As atmospheric pressure increases, the metal capsule is squeezed inward; as pressure drops, the capsule expands. Mechanical levers translate this flex into a needle moving across a calibrated dial.
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| ANEROID BAROMETER MECHANISM |
| |
| Dial Indicator Needle |
| \ |
| +--------*--------+ |
| | | |
| Leverage Spring Linkage |
| | |
| +---------------+ |
| | Sealed Metal | <-- High Pressure compresses cell; |
| | Vacuum Cell | Low Pressure lets cell expand! |
| +---------------+ |
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The Anemometer and Wind Vane: Tracking Wind
Wind is tracked using two distinct instruments operating in tandem:
- Anemometer: Measures wind speed. The most common design is the three- or four-cup anemometer: wind catches the open hollow cups, spinning them around a central vertical spindle. An internal electronic sensor counts revolutions per minute, converting rotation into wind speed (mph or km/h).
- Wind Vane: Measures wind direction. A wind vane features an asymmetric arrow with a broad tail fin that pivots freely on a spindle. Wind pushes against the broad tail fin, forcing the arrow tip to point directly INTO the oncoming wind. Wind is ALWAYS named for the direction FROM WHICH IT BLOWS! A "north wind" blows from the north toward the south.
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| ANEMOMETER AND WIND VANE PAIR |
| |
| CUP ANEMOMETER (Wind Speed) WIND VANE (Wind Direction) |
| ( ) ( ) /| |
| \ / / | Tail Fin |
| ( )--+--( ) <==== [Arrow]----+ |
| | Wind pushes tail; arrow points |
| [ Digital ] INTO oncoming wind (Heading N!) |
| [ Counter ] |
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The Hygrometer and Sling Psychrometer: Moisture
A hygrometer measures the humidity (water vapor content) of the atmosphere. A classic, reliable scientific hygrometer is the sling psychrometer, which consists of two identical thermometers mounted side-by-side on a swiveling handle:
- Dry-Bulb Thermometer: Measures standard ambient air temperature.
- Wet-Bulb Thermometer: The bulb is wrapped in a wet cotton cloth wick soaked in distilled water.
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| HOW A SLING PSYCHROMETER OPERATES |
| |
| 1. Swing the psychrometer in circular loops for one minute. |
| 2. Water evaporates off the wet cotton wick into the air. |
| 3. Evaporation absorbs latent thermal heat, cooling the wet bulb! |
| |
| - If the air is DRY: Evaporation is rapid; wet bulb plummets low! |
| - If the air is 100% HUMID: ZERO evaporation occurs; wet bulb equals |
| dry bulb temperature! |
| 4. The difference between Dry and Wet bulbs reveals Relative Humidity! |
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Upper-Atmosphere and Space Meteorology
Modern meteorology extends far beyond surface ground stations:
- Weather Balloons (Radiosondes): Large helium or hydrogen latex balloons released twice daily from hundreds of global locations. As they climb 30 kilometers into the stratosphere, an instrument package called a radiosonde records vertical profiles of temperature, pressure, and humidity, transmitting data via radio.
- Doppler Radar: Emits microwave radio pulses that bounce off airborne raindrops and snowflakes. By measuring the frequency shift of returning echoes (the Doppler effect), meteorologists calculate precipitation intensity and track rotating winds inside tornadic supercells.
- Weather Satellites: Orbiting in geostationary orbit (36,000 km high), satellites capture high-resolution visible, infrared, and water-vapor images tracking hurricanes, typhoons, and global cloud systems across entire continents.
Exercises and Solutions
Exercise 1: Calculating Relative Humidity with a Psychrometer. A student whirls a sling psychrometer outdoors. The dry-bulb thermometer reads 24°C. The wet-bulb thermometer reads 16°C (a wet-bulb depression of 8°C). Using a relative humidity chart where a dry bulb of 24°C and a depression of 8°C corresponds to 44%, explain why the wet-bulb temperature dropped so far below the dry-bulb reading, and explain what it reveals about the humidity of the air.
Solution 1: The wet-bulb temperature dropped 8°C below the ambient air temperature because of evaporative cooling. As the psychrometer whirled through the air, water molecules evaporated off the wet cotton wick into the atmosphere; because vaporization is an endothermic phase change, the evaporating water absorbed latent heat directly from the thermometer bulb, lowering its temperature. The large temperature depression of 8°C proves that the surrounding air was relatively dry (44% relative humidity); dry air has a low moisture content and can easily accept additional water vapor, driving rapid evaporation and pronounced cooling. If the air had been saturated at 100% humidity, zero evaporation would have occurred, and both thermometers would read identical temperatures.
Exercise 2: Evaluating Barometer and Wind Vane Clues. An amateur weather watcher notes two instrument readings at 8:00 AM: the barometer reads 1,026 millibars (high pressure) and is rising steadily, while the wind vane points due north. Predict the general weather conditions for the day, identify the direction from which the wind is blowing, and explain your reasoning.
Solution 2: The weather watcher can predict fair, pleasant, clear, and sunny weather for the day. A barometric reading of 1,026 millibars is significantly higher than standard sea-level pressure (1,013 mb), and a rising barometer indicates that an anticyclone High Pressure system is dominating the area. High-pressure systems feature sinking, dense air that warms and evaporates clouds, ensuring clear blue skies. The wind vane pointing due north indicates that the wind is blowing from the north (a north wind), which will likely bring cooler, crisp Canadian continental polar air into the region.