Quick Tip: Wet-Bulb Temperature — Why 95F Houston Is More Dangerous Than 110F Phoenix

Dry-bulb temperature (what the thermometer reads) does not tell you how dangerous the heat is. Wet-bulb temperature does — because it captures both heat and humidity in one number.

Wet-bulb vs dry-bulb

Dry-bulb: air temperature measured by a standard thermometer.

Wet-bulb: temperature measured by a thermometer wrapped
          in a wet cloth with air flowing over it.

If the air is dry → evaporation cools the cloth → WB < DB
If the air is saturated → no evaporation → WB = DB

The gap between dry-bulb and wet-bulb tells you how much evaporative cooling is possible.

Why wet-bulb matters for humans

Your body cools itself by sweating. Sweat evaporates and removes heat. The rate of evaporation depends on the wet-bulb temperature — not the dry-bulb.

Phoenix: 110F dry-bulb, 15% RH
  Wet-bulb: 71F
  Sweat evaporates rapidly → body cools effectively
  → Uncomfortable but survivable

Houston: 95F dry-bulb, 75% RH
  Wet-bulb: 89F
  Sweat barely evaporates → body cannot cool
  → Heat stroke risk, even at "lower" temperature

The hotter city (Phoenix) is safer because the air is dry enough for sweat to work.

NIOSH/OSHA heat stress thresholds

Wet-Bulb     Risk Level          Action
──────────────────────────────────────────────────
< 77F        Safe                Normal work
77-82F       Caution             Rest breaks needed
82-86F       Danger              Limit heavy work
86-90F       Extreme danger      Only light work, frequent rest
> 95F        Fatal threshold     Body cannot cool — death without shelter

At wet-bulb 95F, even a healthy person in the shade with unlimited water will die from heat because evaporation stops completely.

Why wet-bulb matters for cooling towers

Cooling towers reject heat through evaporation — the same mechanism as sweat. The tower can cool water down to within 5-7F of the wet-bulb temperature (this gap is called "approach").

City          Dry-Bulb   Wet-Bulb   Tower Leaving Water
──────────────────────────────────────────────────────────
Phoenix       110F       71F        76-78F (excellent)
Denver         95F       62F        67-69F (excellent)
Atlanta        95F       78F        83-85F (adequate)
Houston        95F       82F        87-89F (marginal)
Miami          92F       80F        85-87F (marginal)
Singapore      90F       82F        87-89F (marginal)

Same tower. Same design. Completely different leaving water temperatures — driven entirely by wet-bulb.

Finding wet-bulb from dry-bulb and RH

Approximate formula (Stull, 2011):
WB ≈ DB × atan(0.151977 × (RH + 8.313659)^0.5)
     + atan(DB + RH) - atan(RH - 1.676331)
     + 0.00391838 × RH^1.5 × atan(0.023101 × RH)
     - 4.686035

Or: read it from a psychrometric chart.
Or: use a calculator.

The rule

When evaluating heat danger (for workers) or cooling tower performance (for systems), use wet-bulb — not dry-bulb. A 95F day at 30% RH and a 95F day at 80% RH are completely different conditions. Dry-bulb treats them the same. Wet-bulb does not.