Wet Bulb Temperature: What is it & Why do you need it?

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December 11, 2023
min read
WBT is vital for weather forecasting, climate studies, and heat stress assessments, playing a significant role in agriculture by estimating the impacts of temperature, humidity, wind speed (wind chill), and visible and infrared radiation on crops.

What is Wet Bulb Temperature?

Wet-bulb temperature (WBT) is a crucial thermodynamic variable in agriculture, indicating the minimum temperature achievable through water evaporation into the air under constant pressure. It is measured by encircling the thermometer's bulb with a wet wick, with the resulting temperature reflecting the wet bulb temperature. In contrast, the dry bulb temperature signifies the current ambient temperature. Analysing the difference between these temperatures provides insights into air humidity, with greater disparities indicating lower humidity1. WBT is vital for weather forecasting, climate studies, and heat stress assessments, playing a significant role in agriculture by estimating the impacts of temperature, humidity, wind speed (wind chill), and visible and infrared radiation on crops2, 3, 4.  

How Does Wet Bulb Temperature Shape Crop Success?

Heat Stress Assessment: Heat stress harms plants by diminishing photosynthesis, reducing water use efficiency, inhibiting cell growth and division, compromising floral production, decreasing pollen viability, impairing spikelet fertility, lowering grain yield, and compromising overall plant quality. High WBT indicates environmental stress, suggesting elevated humidity levels that impede the plant's cooling through evapotranspiration5.

Impact on Photosynthesis: The disparity between dry and wet bulb temperatures limits photosynthesis. Elevated wet bulb temperatures can hinder photosynthetic efficiency, reducing the production of essential carbohydrates for plant development6.

Frost Prevention: When the WBT drops to freezing or below, frost protection is crucial for safeguarding crops, especially frost-sensitive fruit crops like strawberries and blueberries. Frost damage occurs when freezing water in plant cells expands, leading to cell rupture7. The critical temperature for frost damage varies, approximately ranging from -12°C to -2°C (10°F-28°F) based on the plant's growth stage8.

Disease Management: WBT is crucial in evaluating plant susceptibility to diseases as elevated humidity fosters favourable conditions for fungal and bacterial pathogens, triggering plant diseases. Moreover, it is a key parameter in calculating the Disease Severity Value (DSV), offering a measurable gauge for the likelihood of plant disease development9.  

Irrigation Planning: Understanding WBT helps fine-tune irrigation schedules10, preventing both under-watering and over-watering.

How do you Measure Wet Bulb Temperature?

Digital Sensors: In recent years, digital sensors have rapidly advanced and become widely adopted in various fields. They offer an efficient and user-friendly method for measuring WBT. Placed within the plant canopy or cultivation environment, these sensors provide precise, real-time data, allowing farmers to monitor WBT fluctuations consistently. Their accuracy is invaluable for optimising greenhouse management. Automated systems can use this data to adjust environmental conditions for optimal plant growth. Additionally, these sensors often include data logging capabilities, enabling farmers to analyse historical trends and make informed decisions for upcoming crop cycles11, 12.

Psychrometers: For those interested in a hands-on approach or who want to understand the fundamentals, calculating WBT manually provides valuable insight into the process. In the past, the psychrometer served as a practical instrument for gauging humidity levels within a given environment. To determine the WBT using a psychrometer, measure both the dry-bulb temperature (Tdb) and the wet-bulb temperature (Twb). The disparity between these temperatures is employed to calculate the relative humidity of the air. The wet-bulb temperature can be calculated using the following formula13:

Twb = T × arctangent  [0.151977 (RH% + 8.313659)1/2] + arctangent (T + RH%) - arctangent (RH% - 1.676331) + 0.00391838 (RH%) 3/2 arctangent (0.023101RH%) - 4.686035

T = Air temperature, in °C
RH% = Relative humidity, expressed as a percentage
Twb = Wet-bulb temperature, in °C  

If you are interested in calculating this for yourself send us an email at info@hexafarms.com and we will send you our API so you can calculate it for yourself!

What is the ideal Wet Bulb Temperature?

The psychrometric chart serves as a valuable tool for assessing moist air properties with just two input parameters, (dry-bulb) temperature and relative humidity. It allows for the determination of various air properties through simple readings or interpolation14. Key features include:

What is wet bulb temperature
  • Dry-bulb temperature (Tdb, °C): Measured using a standard thermometer and can be read from the horizontal axis on the chart.
  • Wet-bulb temperature (Twb, °C): Measured when air is cooled to saturation and can be read from the horizontal axis along the line of constant enthalpy.
  • Wet bulb depression: The difference between dry- and wet-bulb temperature.
  • Dewpoint temperature (Td, °C): The temperature at which condensation occurs when moist air is cooled which can be read from the horizontal axis after extending a line of constant humidity ratio to the saturation line.
  • Relative humidity (RH, %): Represents the air's saturation level with water vapour. Curved lines on the chart depict constant relative humidity.
  • Humidity ratio (kg kg⁻¹): Indicates the mass of water vapour per unit mass of dry air and can be read from the vertical axis.
  • Enthalpy (kJ kg⁻¹): Represents the energy content of dry air, including water vapor. Lines of constant enthalpy are typically presented on the chart.
  • Specific volume (m³ kg⁻¹): Indicates the volume of dry air per unit mass. Lines of constant specific volume are shown on the chart.

The ideal WBT values depend on crop type, growth stage, and environmental conditions. Recognising optimal values is crucial for effective agriculture, impacting planting schedules, irrigation plans, and overall crop management. The ideal WBT for crops can be determined using the formula:  Twb = Tdb - ((100 - RH)/5)

Crop-Specific Considerations

  • Heat-sensitive Crops: Such as lettuce or spinach, planting should be prioritised during periods with lower wet bulb temperatures to avoid heat stress. Optimal WBT around 12.8°C to 18.3°C (55°F to 65°F).
  • Moderate-temperature Crops: like tomatoes, peppers, and beans may tolerate slightly higher wet bulb temperatures, but monitor for any signs of stress. Mid-range WBT, around 15.6°C to 21.1°C (60°F to 70°F).
  • Heat-Tolerant Crops: Such as corn may perform well in higher WBT, around 18.3°C to 21.1°C (65°F to 70°F).

Greenhouse Environments: Utilise ventilation, shading, chalk, and other techniques to maintain optimal conditions for specific crops.

As we explore the realm of precision agriculture, the significance of understanding and monitoring WBT becomes increasingly apparent. This crucial metric, with its far-reaching influence on aspects from stress assessment to irrigation planning, acts as a guiding compass for farmers. With the accessibility of modern technologies, such as digital sensors, optimising environmental conditions for sustainable and fruitful agriculture has never been easier. In this dynamic landscape, WBT stands tall as a vital companion, shaping informed decision-making for a resilient agricultural future.

Written by:
Dr. Mohanna Mollavali

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