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Chapter 38 Weather Humidity Humidity is the measure of the water vapor in the air. It is defined as the mass of water per volume of air. What is relative humidity? Why is it called “relative”? 1. 2. a. b. c. d. Amount of water vapor air can hold Relative b/c it is compared to something else 75% humidity = air holds 75% of the moisture it could hold at that temperature 50% - if the air can normally hold 6 gallons of water at the temperature, how much would it be holding at 50% humidity? Air Saturation When does air saturation occur? 4. a. b. 5. 6. Something is saturated if it can’t hold anymore Air saturation occurs when the air can’t hold anymore water Warm air can contain more water vapor than cold air. Air cools when it expands. This causes the water in the air to move slower and condensation occurs. Clouds & Precipitation 7. How are clouds created? Air cools and moves slower Molecules condense on particles in the sky and create clouds 8. List the five forms of precipitation. Rain, sleet, snow, hail, mist 9. What causes fog? Condensation that occurs near the ground Dew Point 10. What happens at the dew point? Air cools & holds less water Saturated air causes condensation to occur on available surfaces = dew Air Pressure, temperature, & density are the key variables that control weather. 11. Air Pressure 12. 13. Define pressure. Force exerted on an area What is kinetic energy and how is it related to air pressure. Energy of movement Faster molecules = more energy = harder collisions = higher pressure Heat Exchange 14. Explain how air density effects air pressure. Denser air = more collisions = higher pressure 15. When heat is added to air its temperature, pressure or both increase. 16. 3 Things Add Heat to Air Solar radiation Moisture condensation Contact with warm surface Adiabatic Process 17. What is an adiabatic process? HEAT exchange = 0, Change in temperature due to change in pressure Air rises = Less pressure = air expands = colder temperature Air cools 10 degrees Celcius for every 1 km rise 18. Behavior of an Air Parcel •Air Pressure on the balloon (or air parcel) decreases as you move up in the atmosphere •This creates less pressure outside the balloon •Balloon expands •Air inside the balloon cools off as it has more room to move Rising Air 19. As rising air cools, its ability to hold water decreases so the relative humidity of rising air increases . If this rising air cools to its dew point a cloud forms. 20. Condensation of water releases heat, heats the air around it and slows the cooling process. Stable Air 21. How is a Chinook wind formed? Cold air moving down a mountain is compressed (moves closer together) Causes the air to warm up Created as air comes down Rocky Mountains across the Great Plains Temperature Inversions Describe a temperature inversion. Air continues to rise as long as it is warmer and less dense that the surrounding air Some conditions cause cold air to sink and warm air to rise above it The cold air remains below because it is less dense than the air above it Causes a temperature inversion Smog What causes the Los Angeles smog? Cold air from ocean trapped by hot air from Mojave Desert Air can’t escape, traps all the pollutants and causes a layer of smog CLOUDS 1) What are the three main types of clouds? What is the pre-fix for each? High = cirro, above 6000 m Medium = alto, 2000-6000 m Low = strato, below 2000 m Common Clouds Cumulus clouds: look like floating cotton Cumulonimbus: dark clouds, usually mean a storm AIR MASSES 5) Type of air mass determined by two things: i) Surface air mass forms over (lower case letters) Maritime (m) or Continental (c) ii) Source region where air mass forms (upper case letters) (a) (a) Arctic (A), Polar (P), or Tropical (T) 6) What are the 6 possible air masses? mA – cool, moist, unstable cA – cold, dry stable mP – cool, moist, unstable cP – cold, dry, stable mT – warm, moist; usually unstable cT – hot, dry, stable or unstable LIFTING 1) Different weather occurs when two different air masses meet. 2) What cause convectional lifting? Unequal heating of earth Convection currents create lifting 3) Orographic lifting is lifting upward over an obstacle Draw what happens due to orographic lifting FRONTS 10) What is a front? What type of weather do they typically bring? Contact zone between two different air masses Wind, Clouds, Rain, and Storms 11) Frontal lifting occurs due to a difference in temperature, moisture, and pressure. 12)What is the difference between a warm and cold front? Cold = cold air mass moves into an area with a nonmoving warm air mass Warm = warm air moves into cold air Less obvious, light to moderate rain/snow, air warm behind front, clouds scatter WARM FRONT COLD FRONT High cirrus clouds, drop in temperature, and drop in pressure before,thundersto rms, air cools behind front VIOLENT WEATHER a) Thunderstorms – humid air rises, cools, and condenses into single cumulus cloud Rising warm air continues to feed the cloud Rain falls, takes cold air with it Creates lightning and thunder b) Tornadoes Winds rotating over a large area speed up when you decrease the area (figure skater) Funnel shaped cloud from a cumulonimbus cloud Only called a tornado when it touches down Tornado Alley – stretches from northern Texas through Oklahoma, Kansas, and Missouri c) Hurricanes Air and water temperatures become equal Increase in moisture and temperature of air causes strong vertical wind Causes rising air to tail in and spiral Grows as more air rises