Dry Bulb Temperature & Heat Sensor

01  The Instrument — What Dry Bulb Temperature Measures

Dry bulb temperature — shortened to dry bulb or Tdb — is the standard ambient air temperature measured by a thermometer or electronic sensor shielded from radiation and moisture. It is the temperature reading that appears in every weather forecast, news broadcast, and NWS observation: the actual temperature of the air, measured in the shade, at a defined height above the ground, without the cooling effect of evaporation. In thermodynamic terms, dry bulb temperature measures sensible heat — the energy content of air that changes its temperature without altering its moisture content, as distinct from latent heat, which is the energy bound up in water vapor.

The term ‘dry bulb’ originates from the classical psychrometer — a paired-thermometer instrument in which one sensor (the dry bulb) measures air temperature directly while the other (the wet bulb) has a moistened wick that cools by evaporation. The relationship between the two readings (typically along with atmospheric pressure) is used to determine humidity variables such as relative humidity and dewpoint. In modern electronic weather stations, the ‘dry bulb’ is simply the ambient air temperature sensor — the thermistor or RTD housed inside the station’s radiation shield — distinguished from the wet bulb by its dry, unmodified measurement.

In a cyclonePort weather surveillance station, the temperature sensor is housed inside a radiation shield that prevents direct solar heating of the sensor while maintaining airflow — the same principle that defines all WMO-standard meteorological temperature measurements. The sensor delivers ±0.2°C accuracy continuously, with data flowing through RadarOmega alongside wind, humidity, pressure, rain, lightning, and camera feeds.

02  Dry Bulb vs. Wet Bulb — Understanding the Distinction

The dry bulb / wet bulb distinction is foundational to understanding how meteorologists characterize heat stress — and why no single temperature reading fully describes how hot or cold a person actually feels.

03  Dry Globe Temperature — Capturing Radiant Heat Dry Bulb Cannot See

Dry bulb temperature measures the shaded air. Globe temperature reveals how much additional heat load is being imposed by the sun and surrounding surfaces. That distinction becomes critical on sunny days, over artificial turf, pavement, rooftops, and other environments where radiant heat can greatly increase real-world heat stress beyond what air temperature alone suggests.

What Dry Globe Temperature Is

Globe temperature (Tg) is measured inside a matte black sphere — typically 150mm in diameter — that absorbs solar and infrared radiation from the surrounding environment, just as human skin and clothing do. The globe serves as a standardized indicator of radiant heat load. Unlike a wet bulb sensor, it uses no water or wick, so its reading is not influenced by evaporative cooling. Globe temperature reflects the combined effects of radiation, ambient air temperature, and air movement, making it useful to compare with dry-bulb temperature when assessing additional heat load from the sun and hot surrounding surfaces.

The difference between globe temperature and dry bulb temperature is the diagnostic signal: it quantifies how much additional thermal load the sun and surrounding hot surfaces are imposing on anyone in that environment. A reading of dry bulb 30°C alongside dry globe 38°C reveals an 8°C radiant heat differential — conditions far more stressful than the air temperature alone suggests.

04  How Temperature Sensors Work — RTD and Thermistor Technology

RTD Sensors (Resistance Temperature Detectors)

An RTD is a temperature sensor built from a metal element — almost always platinum — whose electrical resistance increases predictably and nearly linearly with temperature. The most common standard is the Pt100: a platinum element with 100 ohms of resistance at 0°C, increasing by approximately 0.385 ohms per degree Celsius (the alpha coefficient specified by IEC 60751).

Platinum is used because it is chemically stable, has a highly linear resistance-temperature relationship across a wide range (roughly –200°C to +850°C, depending on sensor construction), and exhibits minimal drift over years of continuous operation. RTDs achieve accuracy of ±0.1°C to ±0.3°C in typical meteorological ranges and are widely used in research-grade and professional meteorological instrumentation.

Thermistors

A thermistor is a semiconductor device (typically a ceramic metal-oxide composite) whose electrical resistance changes dramatically with temperature — far more so than a platinum RTD. This high sensitivity makes thermistors excellent for detecting small temperature changes but introduces a challenge: the resistance-temperature relationship is highly non-linear, and is typically modeled with equations such as the Steinhart–Hart equation rather than the near-linear approximation often used for RTDs over limited ranges.

High-quality glass-encapsulated thermistors achieve ±0.1–0.2°C accuracy and can maintain this stability over years — comparable to RTDs — with proper calibration. However, cheaper thermistors degrade more rapidly, exhibit more temperature hysteresis, and have a narrower linear range. For professional weather station applications requiring reliable long-term accuracy, platinum RTDs are generally preferred; thermistors are common in consumer-grade weather instruments where cost is the primary constraint.

Signal Conditioning and Temperature Compensation

Raw sensor resistance output requires conversion to temperature using calibration curves (IEC 60751 for RTDs, Steinhart-Hart for thermistors) implemented in the station’s onboard electronics. Professional stations include temperature compensation for the electronics themselves — ensuring that changes in the circuit board temperature due to ambient conditions don’t introduce measurement errors into the sensor reading. cyclonePort’s temperature measurement system includes onboard signal conditioning, factory calibration coefficients stored in non-volatile memory, and firmware-level thermal compensation across the full operating range.

04  The Radiation Shield — Why Housing Matters as Much as the Sensor

The most accurate temperature sensor in the world will produce meaningless data if it is installed in inadequate housing. This is the most frequently overlooked aspect of outdoor temperature measurement — and the most common source of systematic error in field deployments.

The fundamental problem: any temperature sensor installed outdoors will absorb solar radiation, which heats the sensor above the true air temperature. The error depends on solar angle, cloud cover, wind speed, and the thermal characteristics of the housing — but it is always positive (too warm), and it is frequently large. Research has documented solar radiation errors of up to 1.8°C in passively ventilated shields under low-wind, high-radiation conditions. Without wind to ventilate the housing, the trapped air inside heats up rather than representing the ambient air outside.

Types of Radiation Shields

cyclonePort temperature sensors are housed in a radiation-shielded enclosure designed for continuous outdoor deployment. The shield design achieves ±0.2°C accuracy across the full operating temperature range through a combination of thermal mass management, passive ventilation geometry optimized for typical wind conditions at facility sites, and white UV-stable construction that minimizes solar absorption.

05  Heat Index — What It Is, How It Is Calculated, and Its Limits

The heat index — also called apparent temperature or the ‘feels like’ temperature — is a calculated value that expresses how hot the air feels to a person based on the combined effect of dry bulb temperature and relative humidity. The scientific foundation of the heat index was developed by Robert Steadman in 1979 and has been the primary NWS heat safety metric for public communication in the United States ever since.

The Physics of Heat Index — Why Humidity Changes the Feel

The human body cools itself primarily through sweating. When sweat evaporates from skin, it carries heat away from the body surface. The effectiveness of this cooling depends strongly on how readily water vapor can move from the skin into the surrounding air, so it decreases as relative humidity rises. In dry air, sweat evaporates rapidly and cooling is more effective. In humid air, especially near saturation, sweat evaporates more slowly, reducing cooling and increasing the risk of overheating during the same level of exertion.

Heat index captures this effect by expressing the temperature that a person in the shade would need to experience in 40% relative humidity to feel the same level of thermal discomfort as they actually feel at the current temperature and humidity. At 90°F and 65% relative humidity, the heat index is approximately 105°F — the air feels 15°F hotter than the thermometer reads because the humidity is suppressing evaporative cooling.

The NWS Heat Index Formula

The NWS heat index is computed using the Rothfusz regression equation, itself derived from Steadman’s original biometeorological tables. The equation is a multivariate polynomial in temperature (T, in °F) and relative humidity (RH, in percent):

Critical Limitations of the Heat Index

Heat index is useful as a public communication metric, but has important limitations that make it less suitable for safety-critical heat-stress assessment:

• Shade only: The NWS heat index is calculated for shaded areas. According to the NWS, direct sunlight can increase the apparent temperature by up to about 15°F (8°C) beyond the calculated heat index. A person working in full sun on a 90°F day with a calculated heat index of 100°F may experience conditions up to 115°F.
• Light-wind assumption: The heat index formula is based on light-wind conditions. Lower wind speeds produce higher effective heat stress than the heat index predicts; higher wind speeds in humid conditions may make conditions feel less extreme than the heat index indicates.
• No physical exertion factor: Heat index is calculated for a person at rest or in light activity. Workers performing heavy physical labor generate metabolic heat that adds substantially to the environmental heat load — a factor not captured by the heat index.
• WBGT is more accurate for active outdoor populations: For occupational safety and athletic heat stress management, WBGT (Wet Bulb Globe Temperature) — which includes solar radiation (black globe) and more accurately represents the conditions experienced by a physically active person in direct sun — is generally the preferred metric. cyclonePort stations with WBGT provide a more complete heat stress picture.

Despite these limitations, heat index remains the most widely used heat comfort metric in the United States for public communication, consumer weather products, and many workplace safety applications. cyclonePort calculates and displays heat index in real time from every station’s temperature and humidity data in RadarOmega.

06  Wind Chill — The Cold-Weather Counterpart

Wind chill is the cold-weather counterpart to heat index: a calculated apparent temperature that expresses how cold the air feels when wind-driven heat loss from exposed skin is taken into account. While heat index describes how high humidity makes hot conditions feel worse, wind chill describes how wind makes cold conditions feel worse.

When wind blows across exposed skin, it disrupts the thin, insulating layer of warm air that forms against the skin surface and carries the heat away more rapidly. The faster the wind, the faster the heat loss, and the colder the skin temperature drops — even though the actual air temperature is unchanged. The current NWS/Environment Canada wind chill formula was developed jointly in 2001 by U.S. and Canadian scientists and medical experts:

Like heat index, wind chill requires both dry bulb temperature AND a second variable (wind speed) — underscoring why a complete weather surveillance station, not a single sensor, is needed for meaningful apparent temperature calculations. cyclonePort computes and displays wind chill automatically from the station’s temperature and anemometer readings whenever conditions are within the valid range (≤50°F, wind >3 mph).

07  Derived Metrics from Dry Bulb Temperature

Dry bulb temperature is the primary input for a family of calculated weather variables. Each serves a distinct operational purpose:

08  NWS Heat Index Danger Levels — The Operational Threshold Framework

The NWS categorizes heat index values into four risk levels that correspond to escalating health impacts with prolonged exposure and physical activity. These categories are widely used in public education and situational awareness, but local warning thresholds and heat-response plans vary by jurisdiction and may use additional criteria beyond heat index alone.

09  Operational Applications

Outdoor Athletic Programs — Heat and Cold Safety

Air temperature — along with the heat index and wind chill derived from it — drives the most time-sensitive safety decisions in outdoor athletics. Heat index determines work-rest protocols, cooling requirements, and practice cancellation thresholds for summer programs. Wind chill determines safe exposure limits, equipment requirements, and postponement decisions for winter programs.

The fundamental limitation of relying on a regional weather forecast or phone app for these decisions is geographic specificity: a stadium or athletic field may experience temperatures 5–10°F different from the nearest airport observation station due to surface heating from artificial turf, urban heat island effects, or local topography. A cyclonePort station on the facility measures the actual temperature at that location — not a regional average — updated continuously throughout practice.

• Summer football, soccer, and field sports: Turf surface temperatures routinely exceed air temperature by 30–50°F on sunny days. A cyclonePort air temperature reading of 90°F at the sideline with 70% relative humidity produces a heat index of approximately 105°F in the shade — and potentially 120°F in full sun on the field surface. Used alongside WBGT, these measurements help athletic staff make more informed decisions about practice modification, rest breaks, hydration, and cooling.
• Winter football and outdoor athletics: In cold weather, on-site temperature and wind data allow wind chill to be monitored in real time so athletic staff can respond before conditions become hazardous. Exact cold-weather thresholds vary by sport and governing body, but this data can help athletic directors, trainers, and coaches evaluate whether additional protection, schedule changes, or postponement should be considered.
• Marching band and outdoor performing arts: Marching band, drum corps, and other outdoor performance programs can face many of the same environmental heat risks as athletic teams because they involve prolonged outdoor exposure and sustained exertion. On-site monitoring of temperature, humidity, heat index, and — where available — WBGT supports safer scheduling and environmental awareness for both athletic and performance programs from a single station.

Emergency Management — Heat Wave and Cold Event Response

Heat is consistently among the deadliest weather hazards in the United States. Public health agencies and emergency managers use temperature, heat index, forecast trends, and other local criteria to activate heat emergency plans, open cooling centers, issue public advisories, and coordinate wellness checks for vulnerable populations.

The cyclonePort network provides emergency managers with site-specific temperature readings from across a monitored jurisdiction in real time — not interpolated from a single airport observation station potentially miles away. During a heat wave, temperatures can vary significantly within the same city or county depending on surface cover, vegetation, and proximity to water. Site-specific readings identify where cooling center activation is most urgent and where vulnerable populations face the greatest risk.

For cold events — including winter storms, arctic outbreaks, and dangerous wind chill episodes — the same station network provides the wind chill data that can support decision-making for school closures, outdoor worker safety protocols, and utility crew safety policies for working in extreme cold.

Construction, Industry, and Outdoor Workers

OSHA’s proposed federal heat safety standard — the first federal occupational heat regulation in U.S. history — uses heat index as the primary operational trigger metric for mandatory worker protections:

• Initial heat trigger (heat index ≥80°F): Employers must provide water access, rest opportunities, and shade access.
• High heat trigger (heat index ≥90°F): Mandatory rest breaks every two hours, buddy system, heat illness monitoring, and supervisor observation. All outdoor workers must be provided cool water and shade.

Employers who deploy a cyclonePort station at the worksite gain two simultaneous advantages: the real-time heat index data needed to implement these protocols correctly, and the documented, timestamped temperature and heat index record that demonstrates compliance with OSHA requirements and reduces liability exposure in heat-related incident investigations.

The need for defensible on-site monitoring is growing as regulators, insurers, and employers place greater emphasis on occupational heat risk. Organizations with documented local weather monitoring and formal heat-safety procedures are generally better positioned than those relying only on regional forecast data or informal supervisor judgment.

Accurate temperature data also enables proactive operational planning that reduces heat exposure risk before it reaches critical thresholds:

• Scheduling adjustments: Moving outdoor work or practice to early morning hours — before the peak heat index window of 10 a.m.–4 p.m. — can cut cumulative heat exposure by 50% or more on hot summer days. cyclonePort’s historical temperature data quantifies the actual temperature difference between morning and afternoon at the specific worksite, making the scheduling benefit concrete rather than estimated.
• Shift staggering: Industrial sites can rotate crews in and out of heat-exposed tasks based on cumulative exposure tracked through logged temperature and heat index readings — ensuring no individual accumulates dangerous heat load even when overall site heat index stays below the high-heat trigger threshold.
• Infrastructure investment targeting: Sustained high dry globe vs. dry bulb differentials from cyclonePort stations identify exactly which areas of a facility — specific equipment rows, loading docks, south-facing walls — radiate the most heat. This data directs shade structure, misting system, or surface treatment investments to where they will provide the greatest worker protection benefit per dollar spent.

In cold weather, on-site temperature and wind measurements also support continuous calculation of wind chill, an important screening metric in OSHA cold-stress guidance and other occupational safety programs. cyclonePort stations provide the combined temperature and wind speed data needed to calculate wind chill continuously at the specific worksite.

Agriculture — Evapotranspiration, Irrigation, and Livestock

Air temperature is one of the primary environmental variables used to estimate evapotranspiration (ET) — the combination of water evaporation from soil and transpiration from plants. ET determines crop water demand, irrigation scheduling, and drought stress monitoring. Accurate on-site temperature measurement is critical for ET models because regional temperature data from distant stations misrepresents the microclimate conditions at the field level.

For livestock operations, temperature and heat index monitoring supports heat stress management for animals — particularly dairy cattle, poultry, and swine, which have specific temperature-humidity thresholds for reduced productivity and mortality risk. cyclonePort weather data can support these monitoring and management protocols with continuous local observations.

Utilities, Industrial, and Infrastructure

Temperature drives equipment performance in several operationally critical ways for utilities and industrial operations. Electrical transmission lines experience increased resistance and reduced capacity in high heat, and critical protection systems that depend on thermal calculations require accurate ambient temperature data. Natural gas pipelines have temperature-dependent flow characteristics. Substation equipment has temperature-based loading limits that are directly affected by ambient conditions.

On cold winter days, equipment de-icing, pipeline freeze protection, and substation heating requirements all depend on accurate temperature monitoring. A cyclonePort station at a substation or pipeline segment provides the ambient temperature data that drives these operational decisions from the actual location — not from a distant weather station that may not represent local conditions.

10  Instrument Selection Guide

Temperature sensors for professional weather surveillance span a wide performance range. These are the specifications that determine whether a system provides operationally reliable data or misleading readings that undermine safety decisions.

11  Installation & Maintenance

Siting — The WMO Standard for Temperature Measurement

The WMO specifies temperature measurement at 1.25–2 meters (approximately 4–6.5 feet) above the ground surface, over a grass or representative natural surface, away from artificial surfaces that absorb or reflect radiation differently from natural surroundings. The sensor must be in the shade — shielded from direct solar radiation — while exposed to free airflow from all directions.

• Away from artificial heat sources: Paved surfaces, building facades, HVAC equipment, and parked vehicles all create local thermal anomalies that bias temperature readings away from true ambient conditions. Maintain at least 5–10 meters of separation from major heat sources.
• Away from water bodies and vegetation: Evaporative cooling from irrigation, water features, or dense vegetation creates local cool zones that don’t represent conditions across the broader facility. Mount the sensor in a representative location — typically a grassy or natural surface away from these features.
• Correct height: Mount the temperature sensor at 1.25–2 meters above the ground surface. Sensors mounted too high (above 3 meters) measure above the breathing zone and are increasingly decoupled from ground-level heat; sensors mounted on the ground surface measure soil-influenced temperatures, not air temperature.
• Avoid asphalt and concrete: These surfaces absorb solar radiation during the day and re-radiate it as heat in the late afternoon and evening, creating ‘urban heat island’ temperature readings that are significantly higher than true ambient air temperature. Mount the sensor over grass or natural ground where possible.

Maintenance

• Annual sensor verification: Compare the temperature sensor reading against a calibrated reference thermometer or nearby NWS/ASOS station during stable weather (early morning, no wind, overcast) when radiation errors are minimal. A persistent discrepancy of more than ±0.5°C warrants sensor inspection or recalibration.
• Shield cleaning: Clean the radiation shield interior annually to remove dust, pollen, insect debris, and biological material that accumulate on the shield surfaces and can alter its thermal and airflow properties. Use a soft brush and mild soap; rinse thoroughly and allow to dry completely before reinstalling the sensor.
• Aspiration verification (fan-aspirated shields only): Verify the aspirating fan operates at the specified flow rate. A degraded fan reduces airflow through the shield and increases solar radiation error. Replace fan motors at manufacturer-specified intervals or when reduced airflow is detected.
• Sensor element inspection: Inspect the sensor element for corrosion, moisture intrusion, or physical damage. Even small cracks in the sensor housing can introduce moisture that affects resistance readings and accuracy.

12  cyclonePort Temperature Monitoring System — Platform, Integration & Deployment

cyclonePort weather surveillance stations include a high-accuracy dry bulb temperature sensor with radiation shield as a core component of every standard configuration, delivering continuous air temperature data through RadarOmega alongside all other station sensor streams.

Technical Specifications

Specifications may vary by model. Contact cyclonePort for current engineering documentation.

What the System Delivers

• Continuous real-time dry bulb temperature — updated every second, displayed in °F and °C in RadarOmega
• Real-time heat index — NWS Rothfusz formula with NWS adjustments, displayed with risk category (Caution / Extreme Caution / Danger / Extreme Danger)
• Real-time wind chill — NWS 2001 formula, displayed with frostbite risk category when conditions are within valid range
• Dew point and apparent temperature — continuously calculated and displayed alongside dry bulb
• Temperature trend charts — 1-hour, 6-hour, 24-hour, and 7-day temperature history in RadarOmega
• Automated threshold alerts — SMS and email when temperature, heat index, or wind chill crosses user-defined thresholds
• OSHA heat index tier alerts — configurable alerts at the 80°F initial trigger and 90°F high-heat trigger thresholds
• Multi-station temperature comparison — view current temperature and heat index across all monitored facilities simultaneously
• Historical temperature archive — full record for compliance documentation, post-event review, and trend analysis
• Full weather picture — temperature alongside humidity, wind, pressure, rain, lightning, WBGT, and camera from the same station

Who Deploys cyclonePort Temperature Monitoring

Deploy Temperature Monitoring at Your Facility

cyclonePort temperature sensors are available as part of complete weather surveillance stations or as standalone deployments. Contact our team to configure accuracy specifications, shield type, alert thresholds, and derived metric outputs for your specific application. info@cycloneport.com  ·  844-737-9328  ·  cycloneport.com/contact

13  Frequently Asked Questions