When you say "It is a bit nippy today." to Describe the temperature 5 degrees below freezing, you are using -

An understatement is a figure of speech employed by writers or speakers to intentionally make a situation seem less important than it really is.

Some Example of understatement
• “Deserts are sometimes hot, dry, and sandy.” – Describing deserts of the world.
• “He is not too thin.” – Describing an obese person.
• “It rained a bit more than usual.” – Describing an area being flooded by heavy rainfall.
• “It was O.K.” – Said by the student who got the highest score on the test.
• “It is a bit nippy today.” – Describing the temperature, which is 5 degrees below freezing.

Source: literarydevices.net