OED sense 6 of the adjective close, relates to the weather. and is the opposite of fresh.
Why do we describe warm and slightly oppressively humid weather as "close"? It has been the case since the sixteenth century.
- Of the atmosphere or weather: Like that of a closed up room; confined, stifling, without free circulation; the opposite of fresh.
[1533 J. Heywood Play of Wether sig. Diiii Wynde rayne nor froste nor sonshyne wold she haue But fayre close wether her beautye to saue.]
1591 R. Percyvall Bibliotheca Hispanica Dict. at Bochorno A close hot weather.
1599 T. Moffett Silkewormes 48 Keepe them not in roomes too hot and close.
1748 B. Robins & R. Walter Voy. round World by Anson ii. vii. 213
We had now for several days together close and sultry weather.
1834 F. Marryat Jacob Faithful I. i. 5 The little cabin being so unpleasantly close.
1855 A. Bain Senses & Intellect i. ii. 165 The opposite of freshness is shown in the close or suffocating odours.