In the case examined in class, warm, moist air moved north from the Gulf of Mexico to a low pressure are over the Dakotas. A cold front to the southeast gradually moved east towards the warm air. When warm air moves over cold air, it rises, nimbostratus clouds form, and precipitation often occurs. Clouds are not visible in an infrared image, meaning that they are not developed enough. In a visible image, clouds are apparent, indicating that they must be low-level clouds.
Warm air is lifted and clouds can form if enough water vapor is present.
Warm air lifts over cold air and clouds can form if enough water vapor is present. Symbols on fronts, such as blue triangles for cold and red half circles for warm are always drawn and pointing in the direction in which the air is advancing.