Unit 2 Water and Weather - Spokane Public Schools

6.1 Introduction to WeatherWeather is a term that describes the condition of the atmosphere in terms oftemperature, atmospheric pressure, wind, and water. The major energy source forweather events is the Sun. Weather events tend to happen when air masses interactor changes locations. An air mass is a large body of air (sometimes coveringthousands of square kilometers) with consistent temperature and moisture contentthroughout. In this section, you will learn how weather happens. You will also learnimportant terms used for talking about weather.Weather factorsTemperaturePressureWhat is wind?The temperature of air determines whether it rises or sinks. TheSun warms Earth’s surface. As air near the surface is warmed, itexpands and becomes less dense. The less-dense air rises.Eventually the warm, less-dense air that rose from the surfacecools. The same chain of events that made the air rise now worksin reverse and the air sinks back to the ground (Figure 6.1).When warm air rises from Earth’s surface, an area of lowatmospheric pressure is created. This lower-pressure area drawsin air from surrounding higher-pressure areas. Eventually thewarm air that rose from the surface cools and becomes denser.This dense, cool air sinks back to the surface causing an area ofhigh atmospheric pressure.Wind is the horizontal movement of air that occurs as a result of apressure difference between two air masses. The greater thedifference in pressure, the greater the speed of the air flow. Mostof these pressure differences are due to unequal heating of theatmosphere.weather - the condition of theatmosphere as it is affected bytemperature, atmosphericpressure, wind, and water.air mass - a large body of air withconsistent temperature andmoisture content throughout.wind - the horizontal movement ofair that occurs as a result ofpressure differences between twoair masses.Figure 6.1: Convection in theatmosphere.122UNIT 2 WATER AND WEATHER