Short answers, pulled from the story.
Humidity is the concentration of water vapor present in the air. The amount of water vapor needed to achieve saturation increases as temperature rises, so the same quantity of water vapor produces higher relative humidity in cool air than in warm air. For example, air near saturation holds about 8 grams of water per cubic metre at 8 degrees Celsius but up to 28 grams per cubic metre at 30 degrees Celsius.
Absolute humidity is the mass of water vapor per volume of air, expressed in grams per cubic metre. Relative humidity is the ratio of the current water vapor partial pressure to the equilibrium vapor pressure of water at the same temperature, expressed as a percentage. Specific humidity is the ratio of water vapor mass to the total mass of the air parcel, and is commonly used in HVAC system design.
High humidity slows the rate at which sweat evaporates from the skin, reducing the body's ability to dissipate heat. Blood diverted to the skin's surface is blood taken away from active muscles, the brain, and internal organs, causing earlier fatigue and reduced alertness. According to the heat index, a relative humidity of 75% at 80 degrees Fahrenheit makes conditions feel like approximately 83.6 degrees Fahrenheit.
High humidity requires pilots to use longer runways for takeoff and reduces climb performance. Cold, humid air promotes ice formation on wing surfaces, altering the wing profile and adding weight. Aviation weather reports include dew point readings specifically to help pilots account for these humidity-related risks.
Water vapor is the most abundant of all greenhouse gases. It absorbs infrared radiation emitted upward by the Earth's surface, which is why humid regions experience little nocturnal cooling while dry desert regions cool significantly at night. Water vapor compensates for roughly 70% of the average net radiative warming at the Earth's surface through latent heat released during evaporation and transpiration.
For air-conditioned buildings, the generally recommended indoor relative humidity range is 30-60%. Electronic devices operate optimally between 30% and 65% relative humidity. Below 20% relative humidity, static electricity buildup can cause spontaneous computer shutdowns and dielectric breakdown in solid-state devices.