Forecasting Weather: Air Masses and Fronts Weather is a combination of local conditions at any one time. It's what's happening outside right now, right here. Weather includes: Temperature Humidity Air Pressure Cloud Cover Precipitation Wind Speed Wind Direction Dew Point Cloud Type Air Masses moving from areas of high pressure to areas of low pressure bring weather from one location to another. An Air Mass is a large body of air with the same temperature and moisture content. Air Masses take on the characteristics of where they form. If an air mass forms over water, it will be moist. If the air mass forms over land, it will be dry. Air masses are named depending on where they form. The first letter stands for the moisture content, the second letter tells us about temperature. Moisture: m = maritime (wet) c = continental (dry) Temperature: P = polar (cold) T = tropical (warm)
For the United States, there are four main types of air masses, but six air masses 3 warm air masses and 3 cold air masses: mp = maritime Polar (wet and cold) mt = maritime Tropical (wet and warm) cp continental Polar (dry and cold) ct = contintental Tropical (dry and warm) The boundaries between air masses are called fronts. A front is an area where 2 air masses meet. It's a zone of transition between different air masses. When 2 air masses meet, the warm air rises. As it rises, it cools and water vapor in the warm air condenses. This eventually leads to precipitation, but how much precipitation depends on the type of front.
A Cold Front is when cold air replaces warm air by moving in under warm air and pushing the warm air up. Cold fronts tend to move quickly, and bring thunderstorms, heavy rain or snow, then cooler and drier air. So, precipitation from a cold front may be more intense, but over a shorter period of time. A cold front is shown on a weather map as a line with blue triangles. Cold Front
A Warm Front is when warm air replaces cold air by moving up and over the cool air. Warm fronts move more slowly, so they bring drizzly rain that may last for several days, then clear, warm, and moister (more humid) air. A warm front is shown on a weather map as a line with red semicircles. Warm Front
A Stationary Front happens when a cold air mass meets a warm air mass, but the cold air mass doesn't have enough force to move the warm air. Stationary fronts often bring many days of cloudy, wet weather. A stationary front is shown on a weather map with a line with alternating blue triangles and red semicircles. Occluded front An Occluded Front happens when a cold front catches up to a warm front. The warmer moist air gets caught between the cool air and the colder air, and lifts over both. Occluded fronts often bring cool temperatures and large amounts of rain or snow.
Fronts show where there is a large change in temperature and wind direction. Fronts tend to move from west to east across the United States, but can also move north or south. High and Low pressure systems also tend to travel from west to east across the US because of the Prevailing Westerlies. High pressures tend to bring good weather, while low pressures are usually associated with storms. High Pressures are also called anticyclones. They are areas where there is a higher pressure than the air around it. High pressures tend to form where air masses are moving away from each other. The air in a high pressure sink, move outwards in a clockwise rotation and the air warms. High pressures are usually associated with dry, clear weather. Because the air moves clockwise, cooler air is brought south in front of the High pressure. Warmer air is brought north behind the High pressure.
Low Pressures are also called cyclones, and are areas where there is a lower pressure than the air around it. Low pressures tend to form where air masses are coming together. The air in a low pressure moves inward with a counterclockwise rotation and rises. The air cools. Warmer air comes from the south in front of the low pressure. Cooler air comes from the north behind the low pressure. Low pressures are usually associated with clouds, rain, and stormy weather. Low pressures tend to be found along the boundary between air masses with contrasting temperatures and/or humidity. A warm front will often lead the way for a Low Pressure, with a cold front following the Low Pressure system.
Meteorologists (weather forecasters) use all this information to explain current weather and put together a weather forecast.