These words are comparable in that they are all used to describe a condition of the weather. Humid , which means containing water or vapour, is the most specific term in the group because it refers to a measurable physical phenomenon called relative humidity. Relative humidity is the ratio, expressed as a percentage, of the amount of water vapour actually in the air at a specific time to the total amount which could be present at the same temperature. Humid , therefore, alone or with a qualifying adverb can designate any state of atmospheric dampness: It was not humid yesterday, but it is very humid today and the weather bureau predicts that it will be even more humid tomorrow.

The other words in this group are less precise in meaning but distinctly more colourful in tone. Close can refer to the weather or to a place. When one thinks of a close day it is in terms of being choked or suffocated by weather that is heavy, hot and airless. A room described as close has probably not been ventilated for some time; its unmoving air has grown stale and thick so that it induces an uncomfortable feeling of confinement. Oppressive is the most general of these words; it can apply to any kind of harsh weather that causes depression or discomfort. A close day with a thunderstorm lurking and no breeze stirring is certainly oppressive . But so, too, is a day on which the wind never stops blowing, whether it be hot or cold, rainy or dry. It is true that oppressive is mostly used in reference to hot, sultry weather, but any climatic condition that produces a state of oppression can be characterized by this word: The unusually mild autumn was followed abruptly by two weeks of oppressive , freezing weather that kept all but the hardiest souls indoors.

In this context, sticky refers to sweatiness caused by an oppressive and humid warmth that does not permit evaporation; this may result from weather, an unventilated atmosphere, or from exertion. An implication pertaining to the adhesiveness between layers, as of cloth to sweaty skin, may also be present: a sticky day; an overcrowded room that soon became damp and sticky ; He stripped off his sticky gym clothes.