Temperature • Temperature is the measure of the average kinetic energy of the particles in a material. • Higher temperature means particles are moving faster. • Temperature Scales • Fahrenheit (used in United States) • Celsius (used in the rest of the world) • Kelvin (SI unit of temperature)
Air Pressure • The Earth’s atmosphere, because air has weight, exerts pressure. • Density of air decreases as altitude increases so pressure also decreases.
Pressure-Temperature-Density • Air Pressure versus Temperature • At the same density, warm air exerts more pressure than cooler air. (Faster particles=greater force) • Air Pressure and Density • At the same temperature, more dense air exerts more pressure. (more particles=greater force) • Temperature and Density • At the same pressure, warm air is less dense than cool air. (Heat=expansion=lower density) • Think of this in a balloon!
Humidity • Humidity is the amount of water vapor in the atmosphere at a given location. • Two ways to express water vapor content: • Relative Humidity • Dew Point
Relative Humidity • The amount of water in a volume of air relative to the amount of water vapor needed for that volume of air to reach saturation is called relative humidity.
Dew point • The dew point is the temperature to which air must be cooled at constant pressure to reach saturation. • When temperature falls to this level, dew begins to forms (condensation). • If the dew point is nearly the same as the air temperature, then the relative humidity is high.
So at 62% humidity, the air contains 62% of the water vapor needed for the air to be saturated
Sample Calculation • What is the relative humidity of 1 kg of air containing 20 g of water at 40 °C? • There can be up to 50 g of water in a 40 °C sample. • (20g/50g) x 100 = 40% relative humidity
Class activities • Complete the Problem-Solving Lab on page 294 and answer the Think Critically questions on the same sheet of paper.