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UNIT 2 Chapter 1: Inquiring about Weather Unit 2: Weather Dynamics Chapter 1: Air Masses Science 10 Mrs. Purba Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Why do these air balloons rise? • • • Warm air rises and cool air sinks. In a hot air balloon, a heater heats the air inside the balloon. When the weight of the warm air plus the balloon is less than the weight of the cooler air outside the balloon, the balloon will rise. Air masses work on the same principles, rising and falling when they confront an obstacle, such as another air mass. Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Air Masses • An air mass is a huge body of air with similar temperature and moisture properties. • The types of air masses formed on Earth are determined by their source region. Which air mass is shown nearest Nova Scotia, and what are the characteristics of it? Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather There are four types of air 1. Continental Air - which is air over a large land mass. 2. Maritime air – which usually originates over a large water body and is moist. 3. Tropical Air - which is formed in region which are usually warm 4. Polar Air – very cold air, usually in the areas of the polar ice caps. Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Air Masses What other air masses can you see? How do they affect other parts of North America? Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather High Pressure Systems • When an air mass cools over an ocean or a cold region of land, a high pressure system forms. • As the air mass cools, the air mass becomes more dense. • When the air mass contracts, it draws in surrounding air from the upper atmosphere. How does wind form in this process? Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Low Pressure Systems • Air masses that travel over warm land or oceans may develop into low pressure systems. • When an air mass warms, it expands and rises. As it rises, it cools. • Water vapour in the air may condense, producing clouds or precipitation. What kind of weather is expected when there is a low pressure system? Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather The Coriolis Effect and Wind • The Coriolis effect describes how Earth’s rotation steers winds and surface ocean currents. • Coriolis causes freely moving objects to appear to move to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. • The objects themselves are actually moving straight, but the Earth is rotating beneath them, so they seem to bend or curve. • As wind or an ocean current moves, the Earth spins underneath it. Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Section 1.2 The Coriolis Effect and Wind • As a result, an object moving north or south along the Earth will appear to move in a curve instead of in a straight line. • Wind or water that travels toward the poles from the equator is deflected to the east, while wind or water that travels toward the equator from the poles gets bent to the west. • The Coriolis effect bends the direction of surface currents to the right in the Northern Hemisphere and left in the Southern Hemisphere. Explain in your own words why the actual path of wind is curved in the northern and southern hemispheres. UNIT 2 Chapter 1: Inquiring about Weather Spin-offs On A Rotating Sphere • The ocean and atmosphere are in constant motion. • Powered by the Sun and a rotating Earth, their interactions play a critical role in shaping weather and climate. • Natural variations in winds, currents, and ocean temperatures can temporarily affect weather patterns. Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Section 1.2 Jet Streams • A jet stream is a narrow band of fast-moving air that lies between the • • • • troposphere and the stratosphere. A jet stream can have a speed up to 300 km/h or greater at altitudes of 10 km to 12 km. Jet streams form large temperature differences between two air masses. This explains why polar jet streams are powerful, and why they have turbulent weather. Storms form along jet streams and generate large-scale weather systems. • What do the jet stream and seasons have in common? UNIT 2 Chapter 1: Inquiring about Weather Fronts • When cold air masses move south from the poles, they run into warm air masses moving north from the tropics. • The boundary between two air masses is called a front. • Air masses usually don’t mix at a front. • Each air mass has its own temperature and pressure. • The differences in temperature and pressure cause clouds and precipitation. • Types of fronts include cold, warm, occluded, and stationary fronts. • An approaching front means a change in the weather, and the extent of the change depends on the difference between conditions in the air masses. Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Section 1.2 Cold Front • • • • • • A cold front forms when a cold air mass runs into a warm air mass. The cold air mass moves faster than the warm air mass. So the cold air mass lifts the warm air mass out of its way. As the warm air rises, its water vapour condenses. Clouds form, and precipitation falls. If the warm air is very humid, precipitation can be heavy. • Temperature and pressure differences between the two air masses cause winds. • Winds may be very strong along a cold front. UNIT 2 Chapter 1: Inquiring about Weather Section 1.2 Cold Front • Cold fronts often bring sudden changes in the weather. • There may be a thin line of storms right at the front that moves as it moves. • In the spring and summer, the storms may be thunderstorms and tornadoes. • In the late fall and winter, the storms may bring snow. • A squall line is a bunch of severe thunderstorms that form near a cold front. UNIT 2 Chapter 1: Inquiring about Weather Warm Front • When a warm air mass runs into a cold air mass, it creates a warm front. • The warm air mass is moving faster than the cold air mass. • The warm air mass then flows over the cold air mass. • As the warm air rises, it cools. • This brings about clouds and sometimes light precipitation. • Warm fronts move slowly and cover a wide area. • After a warm front passes, the warm air mass behind it brings warmer temperatures. Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Occluded Front • With an occluded front, a warm air mass becomes trapped between two cold air masses. • The warm air is lifted up above the cold air. • Cloudy weather and precipitation along the front are typical. Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Section 1.2 Stationary Front • Sometimes two air masses stop moving when they meet. • These stalled air masses create a stationary front. • Such a front may bring clouds and precipitation to the same area for many days. UNIT 2 Chapter 1: Inquiring about Weather Fronts Why does an approaching front signal a change in weather? Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Section 1.2 Extreme Weather • Thunderstorms are extreme weather events that include lightning, thunder, strong winds, and hail or rain. • A tornado is a violent, funnel-shaped column of rotating air that touches the ground. • When tornados form over water, waterspouts occur. • What causes a thunderstorm? UNIT 2 Chapter 1: Inquiring about Weather Section 1.2 Extreme Weather When strong horizontal winds hit the rapidly rising air in a thunderhead, funnel clouds can result. Strong winds tilt the funnel cloud (A). The funnel cloud becomes vertical and touches the ground (B). A tornado forms as the funnel cloud travels along the ground. (C). What characteristic of a tornado makes it so dangerous? UNIT 2 Chapter 1: Inquiring about Weather Section 1.2 Extreme Weather • The tropics, the regions closest to the equator, are the ideal location for the formation of intense storms called tropical cyclones to occur. • Wind speeds of tropical cyclones may reach 240 km/h. • Tropical cyclones are also called cyclones, typhoons, or hurricanes. • Hurricane season extends from late summer to early fall. UNIT 2 Chapter 1: Inquiring about Weather Extreme Weather This is a cross-section of a hurricane (A) and a satellite image of a hurricane (B). Why do tropical cyclones originate in the tropics? Section 1.2 UNIT 2 Chapter 1: Inquiring about Weather Section 1.2 Section 1.2 Review • Earth’s shape, tilt, and orbit affect weather. • Five main air masses affect North America. The cooling and warming of air masses creates high and low pressure systems, respectively. • Fronts form where two air masses meet. • The Coriolis effect and differences in atmospheric pressure create global wind systems. • Rapidly rising warm air results in extreme weather such as tropical cyclones, thunderstorms, and tornadoes.