It has nothing to do with the launcher, go to the google now screen that comes up in the stock launcher when you swipe up the home button.
first find the weather card and tap the button in the top right of the card(one with 3 vertical dots) and that will take you to the card set up questions. First question will ask you if this card is helpful right now. Select yes.the second One will ask you which units you prefer for the temperature. Select Fahrenheit.