Another important element of forecasting is being aware of approaching fronts. This is especially true when you are on an extended offshore cruise, and the forecast is set for a larger, generalized area. When fronts pass through, the result is a change of weather, often for the worse. A cold front can be indicated in warm weather when clouds lower and cumulus clouds begin to tower above, due to warm air being displaced by approaching cold air. Also, altocumulus clouds, shown on the bottom picture of the next page, which look like fuzzy bubbles in long rows, are usually an early warning sign that a cold front is coming. If it's summertime or you're in the tropics with warm, humid air, these clouds may turn into thunderstorms as the cold front approaches. Barometric pressure will also decrease, often rapidly, and a squall line will usually accompany the front.