Learning to forecast the weather can help you to avoid getting caught in suddenly changing conditions that are potentially dangerous. The things you should be most concerned about are temperature, precipitation and wind, because these are the factors that do the most damage or create the most discomfort.
The earth is surrounded by air masses that differ from one another in temperature. Areas where the cold and warm air bump up against each other are called frontal boundaries. When a warm air mass follows cold air, it is an approaching warm front; a cold front is just the opposite. Frontal boundaries are where most of the exciting weather develops, as warm and cold air mix, often creating violent storms with high winds and precipitation.
Warm air can hold more moisture than cold air. When warm, moist air chills, the moisture that was previously invisible condenses and forms clouds. However, it is not only the formation of clouds that is important, but the sequence in which the clouds approach. The sequence tells whether a cold front or a warm front is coming and that is important for two reasons: One is because the fronts approach at different speeds and the other is because their arrival brings storms of different intensities.
When a cold front approaches, the colder air, which is much heavier than the warm air ahead of it, forces its way under the warm air mass. The result is an abrupt jump from clear skies to mountainous thunderheads and heavy rain squalls as the warm, moist air is rapidly forced to move into the cold upper atmosphere.
Cold-front storms come on rapidly and violently but pass more quickly than warm-front storms. The sequence that indicates an advancing cold front begins with thunderheads, lightning and thunder and violent squalls. As the worst of it passes, the sky will lighten, the air will be colder and the storm will give way to lighter rain and eventually high cirrus clouds and then clear skies.
Warm fronts bring warm, moist air that rides gradually up over the top of the cooler air ahead. Because of this gradual approach, storms associated with the actual frontal boundary may be several hundred miles behind the wispy cirrus clouds, which are the first indication that the front is approaching. The cloud sequence that indicates an approaching warm front begins with very high cirrus and proceeds to lower and lower cloud decks that become increasingly dark, finally bringing rain from low nimbostratus clouds.
So, with this basic understanding of how weather works, how do we know what’s going to happen next? First, keep your eye on the sky. Violent weather has a way of being very localized. Watch cloud formation and movement (both direction and speed). Monitor temperature, humidity and barometric pressure.
If humidity is high and the temperature is falling, you’re heading for the dew point, which means fog, heavy dew or other forms of precipitation are likely to follow. If the barometer is rising, more stable weather is coming. A falling barometer, conversely, indicates the approach of more variable weather.
A falling barometer in conjunction with rising humidity is the kind of condition that accompanies tropical storms and hurricanes. The tables on the previous page show what to expect as warm fronts and cold fronts approach and pass.