Essential Weather Instruments

Why This Matters

Weather instruments aren't just gadgets—they're the foundation of everything meteorologists do. On exams, you're being tested on more than instrument names; you need to understand what atmospheric property each tool measures, how that measurement works, and why that data matters for forecasting. The instruments in this guide connect directly to core concepts like atmospheric pressure gradients, humidity and phase changes, vertical atmospheric structure, and remote sensing principles.

Think of these instruments as falling into two categories: those that measure conditions where you're standing (in-situ instruments) and those that gather data from a distance (remote sensing). This distinction matters for understanding data limitations and applications. Don't just memorize what each instrument does—know what physical principle it exploits and how its data feeds into weather prediction models.

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In-Situ Surface Instruments

These instruments measure atmospheric conditions directly at ground level. They form the backbone of surface weather stations and provide the baseline data that all forecasting depends on.

Thermometer

• Measures air temperature using thermal expansion of liquids or electronic resistance changes—the most fundamental atmospheric variable
• Common types include mercury, alcohol, and digital (thermistor)—each with different response times and temperature ranges
• Temperature data drives stability calculations and is essential for determining lapse rates, frost forecasts, and heat index values

Barometer

• Measures atmospheric pressure by balancing air pressure against mercury column height or a sealed aneroid cell
• Pressure changes indicate approaching weather systems—falling pressure signals low-pressure systems and potential storms; rising pressure suggests high-pressure and clearing skies
• Pressure readings must be corrected to sea level for meaningful comparison between stations at different elevations

Hygrometer

• Measures relative humidity or dew point using hair tension, wet-bulb depression, or electronic capacitance sensors
• Psychrometers use wet-bulb/dry-bulb temperature difference—the evaporative cooling principle reveals moisture content
• Humidity data is critical for precipitation forecasting and calculating apparent temperature, fog potential, and atmospheric stability

Anemometer

• Measures wind speed using rotating cups, propellers, or ultrasonic pulse timing
• Cup anemometers are most common—rotation rate is proportional to wind speed, typically reported in knots, mph, or m/s
• Wind speed data reveals pressure gradient strength and is essential for advection calculations, storm intensity assessment, and aviation safety

Wind Vane

• Indicates wind direction by aligning with airflow—always points into the wind (toward the source)
• Wind direction is reported as the direction FROM which wind blows—a "north wind" comes from the north
• Direction changes signal frontal passages—a sudden shift from south to northwest often indicates a cold front has passed

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Precipitation Measurement

Quantifying water reaching the surface is essential for hydrology, flood forecasting, and climate records.

Rain Gauge

• Measures liquid precipitation depth collected over a specific time period, typically in millimeters or inches
• Standard gauges use a funnel and graduated cylinder—tipping bucket and weighing gauges provide automated continuous records
• Precipitation data feeds watershed models and is critical for drought monitoring, flood warnings, and agricultural planning

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Upper-Atmosphere Profiling

Surface measurements only tell part of the story. These instruments sample the vertical structure of the atmosphere, revealing conditions that drive weather development.

Radiosonde

• Measures temperature, humidity, and pressure at altitude using sensors carried aloft by balloon
• Transmits data via radio signal as it ascends through the troposphere and into the stratosphere—the name combines "radio" and "sonde" (probe)
• Launched globally at 00Z and 12Z (coordinated times) to provide synchronized data for numerical weather prediction models

Weather Balloon

• Carries radiosondes to altitudes exceeding 30 km before bursting from pressure differential
• Ascent rate (~300 m/min) is controlled by inflation level—providing vertical profiles of atmospheric conditions
• Data reveals critical features like temperature inversions, jet stream position, and tropopause height that surface instruments cannot detect

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Remote Sensing Systems

These instruments gather atmospheric data without physically contacting the air mass—essential for monitoring large areas and detecting phenomena at distance.

Radar

• Uses microwave pulses to detect precipitation by measuring energy reflected back from raindrops, ice, and hail
• Doppler radar measures velocity by detecting frequency shifts—revealing rotation in thunderstorms that may indicate tornado development
• Dual-polarization technology distinguishes precipitation types—differentiating rain, snow, hail, and even debris in tornadoes

Satellite

• Provides synoptic-scale imagery of cloud patterns, storm systems, and atmospheric moisture from geostationary (~36,000 km) or polar orbits
• Geostationary satellites monitor the same region continuously—ideal for tracking storm development; polar orbiters provide global coverage with higher resolution
• Multiple spectral channels reveal different information—visible light shows cloud structure, infrared indicates cloud-top temperature, and water vapor channels track mid-level moisture

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Quick Reference Table

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Self-Check Questions

1. Which two instruments both measure atmospheric pressure, but at different altitudes? What advantage does the airborne version provide?

2. Compare and contrast radar and satellite observations: What can radar detect that satellites cannot, and vice versa?

3. A weather station reports "winds from 270° at 15 knots." Which two instruments provided this information, and from which compass direction is the wind blowing?

4. If an FRQ asks you to explain how meteorologists gather data about the jet stream, which instrument would you discuss and why can't surface instruments detect this feature?

5. Both hygrometers and psychrometers measure humidity. What physical principle does a psychrometer exploit, and how does evaporative cooling reveal moisture content?