fog/foggy -noun/adjective- thick water vapor that blocks your vision. We couldn't see the bridge because there was too much fog.
forecast -noun/verb- the expected weather for the future. According to the 5 day forecast, it's going to rain on our wedding day.
freezing cold -adjective+noun- common expression used to describe a very cold day. It was a freezing cold day yesterday.
frost -noun- ice crystals on a frozen surface. Some flowers are so strong they can withstand frost.
frostbite -noun- a skin condition caused by over exposure to the cold. I lost my hat while I was skiing, and I ended up with frostbite on my ears.