Celsius - noun - measurement of temperature (0 degrees is freezing/100 is boiling) In the summer, the average temperature here is 20 degrees Celsius.
chilly - adjective - cold. It's a bit chilly today.
clear - adjective - when the sky is blue because no clouds are blocking the sun. On a clear night you can see a lot of stars.
cloud/cloudy - noun/adjective - water in the sky that appears as a white or grey mass. It may look cloudy in the morning, but the sun always comes out by afternoon.
cold spell - adjective + noun - a period of colder than average weather. They reckon there'll be a cold spell, so we put off our camping trip.
cool - adjective - temperature in between warm and cold. The days were boiling hot, but the nights were cool and comfortable for sleeping.