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Weather Fronts CScope Unit 9 Did you know? Today’s weather is influenced by air from thousands of kilometers away, perhaps from Canada or the Caribbean Sea? What is an air mass? An air mass is a huge body of air that has similar temperature, humidity, and air pressure. Once the air mass enters the U.S., they are blown from west to east (known as the jet stream). What is a front? As air masses travel across land and oceans, two masses may bump into each other. If they have different temperatures and densities, they form a front. Because different densities do not mix (like oil and water!), this collision of fronts may cause storms and change the weather. A front may be 15-200 km wide and reach up to 10 km in the troposphere! There are 4 fronts. . .create a grid to write the descriptions. We will also draw a diagram for the warm and cold fronts. Cold Front Stationary Front Warm Front Occluded Front Cold Front A cold front forms when denser cold air moves under warm air. The warm air is pushed upward, condenses and forms clouds or precipitation. Cold fronts move quickly so they can cause violent storms at times. After a cold front, the air will be cool and dry. Warm Front A warm front forms when warm air moves over a slow moving cold air mass. If the air is humid, light showers, rain, or snow in the winter will fall. If the air is dry, clouds may form. Warm fronts move slowly so it can be rainy, foggy, snowy, or humid for several days! Stationary Fronts Sometimes a cold and warm air mass will meet, but neither has enough force to move the other so they face each other in a “standoff”! Water vapor in the warm air condenses into precipitation and if the stalled area remains, it may bring many days of clouds and precipitation. Occluded Fronts At an occluded front, a warm air mass is caught between two cooler air masses. The denser cool air masses move underneath the less dense warm air mass and push it upward. The cooler masses mix and the ground becomes cooler so the warm air mass is occluded, or cut off, cools, condenses, and the weather turns cloudy and rainy or snowy. The Highs and Lows Atmospheric pressure at the Earth’s surface is one of the keys to the weather. Weather maps feature H’s for high pressure and L’s for low pressure. High Pressure High pressure occurs when air pressure is higher than the surrounding air pressure. The air is slowly descending. As it lowers, it warms so clouds do not form. This is why this pressure is associated with fair weather. The sinking air spirals outward, or clockwise, and is steered west to east by upper level winds. Low Pressure Air rises near low pressure areas. When a weather forecaster says a low pressure area or storm is moving toward your region, this usually means cloudy weather and precipitation are on the way because as air rises, it condenses and forms into clouds. Counterclockwise circulation of air surrounds this area with the system being steered from west to east by upper level winds. Reading A Weather Map, Page 526 Homework Project Copy the Lows, Highs, Warm and Cold Fronts for each day on the U.S. map. Shade in precipitation. Record the weather facts for each day. You can use weather.com to find the information. We will summarize the information on Thursday.