Air - (Easton's Bible Dictionary)
The atmosphere, as opposed to the higher regions of the sky (1 Thess. 4:17; Rev. 9:2; 16:17). This word occurs once as the rendering of the Hebrew ruah (Job 41:16); elsewhere it is the rendering of shamaiyim, usually translated "heavens."
The expression "to speak into the air" (1 Cor. 14:9) is a proverb denoting to speak in vain, as to "beat the air" (1 Cor. 9:26) denotes to labour in vain.