081 Relative humidity and precipitation

The class starts off with a demonstration of calculating the relative humidity, heat index, and risk level for activity using a wet and dry bulb thermometer. Tables based in part on the heat index calculator.
Other weather calculators.

  1. Look in the first column on the left. Find the row with the nearest dry bulb temperature to the measured dry bulb temperature.
  2. Move to the right along the row until the wet bulb depression in the top row is matched.
  3. Read the relative humidity provided by the table.
Relative humidity from dry bulb and wet bulb depression
Web bulb depression in °C: Dry bulb minus the wet bulb
Dry bulb T in °C 0.5 1.0 2.0 2.5 3.0 3.5 4.0
28 96% 93% 85% 82% 78% 75% 72%
29 96% 93% 88% 84% 80% 76% 73%
30 96% 93% 86% 82% 79% 76% 73%
  1. Look in the first row at the top. Find the column with the nearest dry bulb temperature to the measured dry bulb temperature.
  2. Move down the column until the relative humidity nearest to the one you determined in the first table is found.
  3. Read the effective temperature in °C provided by the table.
Heat index as an effective temperature in °C given a dry bulb temp. and Rel. Hum.
Dry bulb temperature in °C
Relative humidity 212427282930313235
60% 21242829.5 32 33 35 38 46
70%21252930.7 34 35 37 41 51
80%22263032.1 36 38 40 4558
90%22263133.7 39 41 43 5064
100%22273335.6 42 44 49 54 72
  1. Use the effective temperature from above to determine the risk of heat cramps, heat exhaustion, or heat stroke from the table below.
Effective temperature in °C:Risk level
Less than 32 °CLow risk: Safe to exercise with proper hydration
32 °C to 41 °CModerate risk: Heat cramps or heat exhaustion likely
Above 41 °CHigh risk: Heat stroke likely.

Discuss the implications for runners and other athletes in Micronesia


Warm air rises (convection) and cools. Cooler air holds less water than warmer air. The result is that water condenses out of the air as the air rises. This condensation is visible as clouds. Condensation requires dust, smoke, or other microscopic airborne particles for the water to condense upon. As a result of this condensation, precipitation can occur. Precipitation is produced by three different processes, although the first process is dominant in Micronesia.

  1. Collision-coalescence process occurs in warmer clouds, such as over tropical oceans. The tiny droplets of condensed water in the clouds collide and coalesce (stick together) forming larger drops of water. As the larger droplets fall they "sweep up" smaller droplets. This also occurs in colder clouds in the levels below where the ice crystals form. Cloud condensation nuclei are required. Collision-coalescence is the process that produces most of the daily rain in Micronesia.
  2. Bergeron process occurs in clouds that are cold enough to have supercooled water and ice crystals. Ice crystals form, and when they get large enough they fall to the surface. Freezing nuclei are required. The Bergeron process relies on supercooled liquid water interacting with ice nuclei to form larger particles. One by-product of this process are charge separations which lead to electrical discharges - lightning. The superheated air explodes outward generating a shock wave that dissipates as thunder. If there is lightning and thunder, then the Bergeron process is at work. In Micronesia, if there is no lightning and thunder, then the rain is being formed by the collision-coalescence process and not the Bergeron process.
  3. Orographic precipitation is precipitation from clouds formed as a result of air being forced up a slope. Orographic precipitation is rain being generated by the terrain, typically by air (wind) being forced up a mountain. The air cools, water condenses, and then collision-coalescence produces rain. The precipitation is referred to as "orographic precipitation." This is the generator of the rain on the windward side of high islands such as Oahu and Hawaii. Thus Waimanalo and Hilo are rainy, Waianae and Kona coasts (leeward) are dry.

Other notes

Types of Precipitation
Rain, snow, sleet, hail, dew (radiative cooling), or fog. Radiation fog.
Supercooled water, ice necessary to lightning, thunder. Distance versus time. 330 m/s, about three seconds per kilometer. So divide by three to get distance in kilometers. No shelter under trees! Squat. Stay in car.

Air masses:
Two. Maritime equatorial mE air mass ("doldrums" for lack of wind) most of the year. Jan-Feb ("trade wind season") maritime tropical mT air mass.

No warm and cold fronts. Pressure waves along the equator, local heating effects.
InterTropical Convergence Zone: ITCZ

Tropical storms
Areas of organized convection
Tropical depression (Get numbered!)
Requirements: SST > 28 C, low vertical shear, high pressure anti-cyclone aloft

Tropical Storm Category AWeak Tropical Storm26-43 knots (30-49 mph)33-56 knots (40-64 mph)
Tropical Storm Category BSevere Tropical Storm44-63 knots (50-73 mph)57-81 knots (65-94 mph)
Typhoon Category 1Minimal Typhoon64-82 knots (74-95 mph) 82-105 knots (95-120 mph)
Typhoon Category 2Moderate Typhoon83-96 knots (96-110 mph)106-121 knots (121-139 mph)
Typhoon Category 3Strong Typhoon97-113 knots (111-130 mph)122-144 knots (140-165 mph)
Typhoon Category 4Very Strong Typhoon114-134 knots (131-155 mph)145-171 knots (166-197 mph)
Typhoon Category 5 Devastating Typhoon135-170 knots (156-194 mph)172-216 knots (198-246 mph)

El Niño (and La Niña)
Westerly wind burst
Surface layers slide west
Convection moves west