Dry bulb temperature is the “normal” air temperature measured by an ordinary thermometer, while wet bulb temperature is the lowest temperature that same air can reach if water is allowed to evaporate from a wet surface into it.

Quick Scoop: Core Idea

  • Dry bulb temperature :
    • The standard air temperature you see in weather reports or on a basic thermometer.
* Measured with the bulb of the thermometer exposed directly to the air, completely dry.

* It does not directly account for how much moisture (humidity) is in the air.
  • Wet bulb temperature :
    • The lowest temperature air can reach by evaporating water into it under current conditions.
* Measured by wrapping the thermometer bulb in a wet cloth (a “sock”) and letting air flow over it; as water evaporates, it cools the bulb.

* Always less than or equal to the dry bulb temperature; they become equal when the air is fully saturated at 100% relative humidity.

Why They’re Different

  • When air is dry, water evaporates more easily from the wet cloth, causing strong evaporative cooling, so wet bulb is much lower than dry bulb.
  • When air is very humid, evaporation slows down, there’s little cooling, and wet bulb gets close to the dry bulb temperature.

How They Are Measured

  • Dry bulb thermometer :
    • Just a regular thermometer exposed to air.
  • Wet bulb thermometer :
    • Same type of thermometer, but its bulb is wrapped in a moist wick and ventilated by moving air (often spun on a sling psychrometer).
* As water evaporates from the wick, the thermometer cools until evaporation and heat gain balance; that stable reading is the wet bulb temperature.

A sling psychrometer usually has two thermometers side by side: one dry, one wet; by comparing them, you can determine humidity from charts or formulas.

What They Tell You About the Air

  • Dry bulb temperature tells you :
    • How hot or cold the air is in the usual sense.
* Used in almost every temperature-related calculation in weather, HVAC, and comfort design.
  • Wet bulb temperature tells you :
    • How much cooling is possible by evaporation (like sweating, cooling towers, or swamp coolers).
* An indirect measure of moisture content and relative humidity when combined with dry bulb.

* A better indicator of heat stress on humans and animals than dry bulb alone, especially during extreme heat and humidity.

Side‑by‑Side View (HTML Table)

Here is an HTML table comparing the two, as requested:

[10][4] [3][5][7] [4] [1][7] [7][10] [5][7] [4] [1][7] [6] [3][5][7] [6][4] [8][5][7][1] [9][5] [9][5][7]
Aspect Dry Bulb Temperature Wet Bulb Temperature
Basic meaning Actual air temperature measured by a standard, dry thermometer. Lowest temperature air can reach via evaporation of water into it.
Accounts for humidity? No, it does not directly include moisture content. Yes, strongly affected by how much moisture the air can still take (relative humidity).
Measurement method Exposed, dry thermometer bulb in air. Thermometer bulb wrapped in wet cloth with airflow over it.
Typical value Equal to the reported ambient air temperature. Always ≤ dry bulb; equal only at 100% relative humidity.
Physical process behind it Just sensible heat of the air (no phase change considered). Evaporative cooling as water changes from liquid to vapor and absorbs heat.
Key use cases Weather reports, basic comfort, HVAC load calculations. Estimating humidity, heat stress, evaporative cooling potential, design of cooling towers and some AC systems.
Human comfort and safety Shows how hot it feels in a dry sense but can underestimate heat danger in humid climates. High values indicate dangerous conditions where sweating no longer cools effectively (extreme wet bulb heat events).

A Simple Story‑Style Example

Imagine you’re in a desert at 40 °C, but the air is very dry. If you wrap a wet cloth around a thermometer bulb and wave it in the air, the water evaporates quickly and cools the bulb to perhaps 24–26 °C; that cooler reading is the wet bulb temperature.

Now picture a tropical coastal city at 32 °C with very high humidity. The same experiment barely cools the thermometer, maybe to 29–30 °C, because the air is already almost saturated and cannot evaporate much more water.

In both places the dry bulb is your “headline” temperature, but the wet bulb tells you whether your sweat can actually do its job.

Why This Topic Is Trending Lately

In the last few years, wet bulb temperature has started appearing more in climate and heatwave discussions, because it captures when heat and humidity together reach levels that can become dangerous for people working or exercising outdoors.

Researchers and weather agencies use it to flag periods when normal coping strategies like drinking water and staying in the shade might not be enough, as the body simply cannot shed heat fast enough through sweating.

TL;DR:

  • Dry bulb = ordinary air temperature.
  • Wet bulb = how cool the air can get by evaporation under current humidity.
  • The bigger the gap between them, the drier the air and the more powerful evaporative cooling can be.

Information gathered from public forums or data available on the internet and portrayed here.