When warm and cold air collide, warm air is pushed up and can form clouds.
Image Gallery Center for Science Education
Earth
CHILI Education
Cold weather front cloud hi-res stock photography and images - Alamy
Insider Blog: The science behind fronts - KESQ
Lesson 4 Meteorology, PDF, Cloud
Weather works: Here is how a cold front is formed
Weather systems - Weather and climate
Image Gallery Center for Science Education
Lesson 4 Meteorology, PDF, Cloud
Fronts CK-12 Foundation
MetLink - Royal Meteorological Society Weather Systems