Questions: Wet-Bulb Temperature and Psychrometric Process

Question 1

Air has a dry-bulb temperature of 38°C and a wet-bulb temperature of 24°C. Which statement best describes this air mass?

• A. The air is nearly saturated — evaporation is almost impossible
• B. The air is significantly below saturation; vigorous evaporation would cool a wet surface to 24°C
• C. The wet-bulb temperature is below the dew point, indicating supersaturation
• D. Relative humidity is 100% because the wet-bulb reading is valid

Answer: (B)

A 14°C wet-bulb depression (38 − 24 = 14°C) indicates dry air significantly below saturation. The wet-bulb temperature is the equilibrium temperature reached when evaporation saturates the surrounding air — the larger the depression, the drier the air and the more vigorous the evaporative cooling. Option C is wrong: the wet-bulb temperature always lies between the dry-bulb temperature and the dew point; it cannot be below the dew point. Option D is wrong: 100% RH would give zero wet-bulb depression.

Question 2

City A has a forecast of 46°C dry-bulb, 26°C wet-bulb. City B has 36°C dry-bulb, 35°C wet-bulb. Which city poses a greater physiological heat risk to outdoor workers, and why?

• A. City A, because the absolute air temperature is far higher and drives more heat into the body
• B. City B, because with wet-bulb near 35°C, evaporative cooling from sweat becomes nearly ineffective, preventing the body from shedding metabolic heat
• C. Both are equally dangerous because both exceed 35°C dry-bulb
• D. Neither is dangerous — heat stress depends only on solar radiation, not air temperature or humidity

Answer: (B)

Human thermoregulation depends on evaporative cooling: sweat evaporates from skin, removing latent heat. This process requires that the ambient air be below saturation. When wet-bulb temperature approaches ~35°C (skin temperature), the gradient driving evaporation nearly vanishes — the body cannot cool itself regardless of how much it sweats. City A's 26°C wet-bulb still allows effective sweating. City B's 35°C wet-bulb is near the physiological lethal threshold, making it far more dangerous despite the lower dry-bulb temperature.

Question 3

In mostly saturated air (100% relative humidity), the wet-bulb temperature is higher than the dry-bulb temperature because saturation stores more heat.

• T. True
• F. False

Answer: False

In saturated air, evaporation from a wet surface cannot occur because the air cannot hold any more water vapor. With no evaporation, there is no evaporative cooling, and the wet thermometer reads the same temperature as the dry thermometer. At 100% RH, wet-bulb temperature = dry-bulb temperature = dew point. They converge to the same value, not with wet-bulb above dry-bulb. The wet-bulb temperature can never exceed the dry-bulb temperature.

Question 4

The wet-bulb temperature encodes information about both air temperature and moisture content, making it a more complete thermodynamic descriptor than temperature alone.

• T. True
• F. False

Answer: True

The wet-bulb temperature is determined by the combined effect of temperature (which drives evaporation rate) and moisture content (which limits how much evaporation can occur). Two air parcels at the same dry-bulb temperature but different humidities will have different wet-bulb temperatures. This is why the wet-bulb temperature appears on thermodynamic diagrams as a conserved variable along moist-adiabatic processes, and why it is used in precipitation-type forecasting — it captures the full thermodynamic state more compactly than temperature alone.

Question 5

Why does whether precipitation reaches the surface as rain or sleet depend on the wet-bulb temperature of the sub-freezing air layer below a warm aloft layer, rather than the dry-bulb temperature?

Model answer: As melted snowflakes fall through sub-freezing air, they evaporate slightly, and this evaporation draws heat from the surrounding air — cooling it below the dry-bulb reading. Whether the drops refreeze into sleet depends on the actual temperature of the air after accounting for this evaporative cooling, which is the wet-bulb temperature. If the wet-bulb temperature of the cold layer is below 0°C, the evaporatively cooled air can refreeze the drops; if the wet-bulb is above 0°C, rain reaches the surface. Using dry-bulb alone would overestimate the air temperature in the cold layer and lead to incorrect precipitation-type forecasts.

This is an elegant example of wet-bulb temperature's practical power: it captures the thermodynamic equilibrium that falling hydrometeors approach as they evaporate, not the initial state of the air. Forecasters therefore use wet-bulb temperature — not dry-bulb — as the threshold for sleet/freezing rain versus rain.