A mass of warm air will be cooled from below by the ground; it will therefore be more stable since the cooler air will resist the tendency to move vertically. Humidity will remain stationary, resulting in layered clouds and poor visibility. Stratiform clouds will blanket large areas with fog, rain, or snow. An approaching warm front will gradually slide over the cooler air below for a distance of hundreds of miles. High altitude cirrus clouds (hook-shaped wisps) will sometimes be visible days in advance. As the front gets closer (or as the pilot flies toward it), thickening and lowering clouds can be expected which can eventually become dense enough to form precipitation. If a warm front is coming through, you may want to try to beat it because the weather will likely be bad for several days.