the accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings. speeds up to 5,000 ft/min (~60 mph) are best but it may be dangerous to move a thermometer at that speed. errors up to 15% can occur if the air movement is too slow or if there is too much radiant heat present (from sunlight, for example).