Remember how 天気 is weather (and how 天 often refers to sky things?). Well, when those sky things are dry, you have dry weather.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
Because of the continuous, twenty-nine day drought, Fugu is about to dry out.
The long, cruel dry weather killed all the vegetables in my field.
The supply of bacon Koichi got for the starving workers was like a welcome rain after a drought.