What is the wet-bulb temperature?

Wet-bulb temperature is the lowest temperature air can reach by evaporating water into it — the reading a thermometer wrapped in a wet wick shows in moving air. It always sits between the dew point and the dry-bulb (ordinary air) temperature, and it equals the air temperature only when the air is fully saturated at 100% relative humidity.

How is wet bulb calculated from temperature and humidity?

This tool uses the Stull (2011) approximation, which estimates wet-bulb temperature directly from dry-bulb temperature and relative humidity at roughly sea-level pressure. For example, 25°C at 50% RH gives a wet bulb of about 18.0°C, and 30°C at 60% RH gives about 24.0°C. It is accurate to about ±0.3°C across normal conditions and drifts outside RH 5–99% or at high altitude.

What is evaporative cooling potential?

It is the gap between the dry-bulb and wet-bulb temperatures — the wet-bulb depression. That gap is the maximum cooling an evaporative (swamp) cooler or greenhouse wet wall can deliver: the drier the air, the larger the gap and the more cooling you get. When the gap is small (humid air), evaporative cooling barely helps and you need mechanical cooling instead.

Why does wet bulb matter for HVAC, greenhouses, and heat stress?

Evaporative and cooling-tower systems are sized against the wet bulb, not the dry-bulb temperature, because that is the floor they can reach. Greenhouse pad-and-fan cooling only works when the wet-bulb depression is large. And for people and livestock, wet bulb is the true danger signal in heat: once it approaches skin temperature, sweat cannot evaporate and the body cannot cool, which is why heat-safety limits are set on wet bulb.

How do I track wet bulb continuously instead of spot-checking?

Wet bulb changes with every shift in temperature and humidity through the day. Tracking it means measuring both continuously and computing it in real time. LoopString does this at the edge on a Raspberry Pi — deriving wet bulb and the evaporative-cooling potential from your sensors and alerting when conditions cross the limits you set for cooling capacity or heat safety.

Wet Bulb Calculator

Enter air temperature and relative humidity to get the wet-bulb temperature — the coldest evaporation can reach — plus how much cooling a swamp cooler or wet wall can actually deliver.

Your Air

Air Temperature (dry bulb)90°F

Measured dry-bulb air temperature.

Relative Humidity45%

Measured relative humidity (Stull is valid 5–99%).

Key Takeaways

Wet-bulb temperature is the coldest air can get by evaporating water into it — it sits between the dew point and the dry-bulb temperature.
The wet-bulb depression (dry-bulb minus wet-bulb) is your evaporative-cooling potential: dry air = large gap = strong swamp-cooler / wet-wall cooling.
Cooling towers and evaporative systems are sized against wet bulb, not dry-bulb temperature, because that is the floor they can reach.
For people and livestock, wet bulb is the real heat-danger signal: near skin temperature, sweat cannot evaporate and the body cannot cool.

Wet bulb, dry bulb, and dew point

Dry bulb is the ordinary air temperature. Dew point is where condensation starts. Wet bulb sits between them: it is what a thermometer reads when its bulb is kept wet and air blows over it, as evaporation pulls the reading down. The drier the air, the more evaporation, and the further wet bulb falls below dry bulb. At 100% humidity there is no evaporation left to give, so all three temperatures converge.

Why evaporative cooling lives or dies on wet bulb

A swamp cooler or a greenhouse pad-and-fan wall can only cool air toward its wet-bulb temperature — never below it. On a dry day the wet-bulb depression is large and these systems shine; on a muggy day the depression collapses and they barely move the needle, which is when you need mechanical cooling. Sizing and scheduling that equipment against dry-bulb temperature alone is how cooling gets undersized for the days that matter. In HVAC work, wet bulb is the number that tells you what is actually achievable.

Wet bulb as a safety limit

Human and animal cooling depends on sweat evaporating. As wet bulb climbs toward skin temperature (about 35°C / 95°F), evaporation stops working and body heat has nowhere to go — which is why heat stress in barns, greenhouses, and work spaces is judged on wet bulb, not the thermometer on the wall. Watching it continuously, and acting before it crosses your limit, is exactly the loop LoopString runs at the edge.

Frequently Asked Questions

What is the wet-bulb temperature?: Wet-bulb temperature is the lowest temperature air can reach by evaporating water into it — the reading a thermometer wrapped in a wet wick shows in moving air. It always sits between the dew point and the dry-bulb (ordinary air) temperature, and it equals the air temperature only when the air is fully saturated at 100% relative humidity.
How is wet bulb calculated from temperature and humidity?: This tool uses the Stull (2011) approximation, which estimates wet-bulb temperature directly from dry-bulb temperature and relative humidity at roughly sea-level pressure. For example, 25°C at 50% RH gives a wet bulb of about 18.0°C, and 30°C at 60% RH gives about 24.0°C. It is accurate to about ±0.3°C across normal conditions and drifts outside RH 5–99% or at high altitude.
What is evaporative cooling potential?: It is the gap between the dry-bulb and wet-bulb temperatures — the wet-bulb depression. That gap is the maximum cooling an evaporative (swamp) cooler or greenhouse wet wall can deliver: the drier the air, the larger the gap and the more cooling you get. When the gap is small (humid air), evaporative cooling barely helps and you need mechanical cooling instead.
Why does wet bulb matter for HVAC, greenhouses, and heat stress?: Evaporative and cooling-tower systems are sized against the wet bulb, not the dry-bulb temperature, because that is the floor they can reach. Greenhouse pad-and-fan cooling only works when the wet-bulb depression is large. And for people and livestock, wet bulb is the true danger signal in heat: once it approaches skin temperature, sweat cannot evaporate and the body cannot cool, which is why heat-safety limits are set on wet bulb.
How do I track wet bulb continuously instead of spot-checking?: Wet bulb changes with every shift in temperature and humidity through the day. Tracking it means measuring both continuously and computing it in real time. LoopString does this at the edge on a Raspberry Pi — deriving wet bulb and the evaporative-cooling potential from your sensors and alerting when conditions cross the limits you set for cooling capacity or heat safety.