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Air Temperature

Air Temperature. Chapter 3. Air Temperature. Last Chapter - Heating and heat transfer mechanisms conduction, convection, radiation This Chapter - How temperature varies Daily, monthly, yearly Controls of temperature Organization of temperature data Measuring air temperature

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Air Temperature

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  1. Air Temperature Chapter 3

  2. Air Temperature • Last Chapter - Heating and heat transfer mechanisms • conduction, convection, radiation • This Chapter - • How temperature varies • Daily, monthly, yearly • Controls of temperature • Organization of temperature data • Measuring air temperature • instruments

  3. Daily (Diurnal) Temperature Variations • Daily cycle of temperature is very much like a seasonal cycle • Warming begins in the morning (spring) • Hot in the afternoon (summer) • Cooling in the evening (fall) • Coolest late evening/early morning (winter) • Just like seasons - due to differential between incoming (solar) and outgoing (earth) radiation

  4. Daytime Warming • We know from chapter 2 that solar radiation heats the earth’s surface which in turn heats the air by conduction • How this heat is distributed depends greatly on the wind • Calm wind - hot air remains near the surface • Thermals are small and don’t bring much hot air up • Temperatures 5-6 feet above ground can be 30º cooler! • This is why temperature measurements are not taken at the surface (also in the shade and over grass)

  5. Daytime Warming

  6. Daytime Warming • On a windy day, temperature does not change as much with height • The wind creates turbulent “eddies” which transfer heat from the near the surface upward and cooler air from above downward

  7. Effect of Wind on Temperature

  8. Maximum Daily Temperature • The sun is highest at noon. So why does the maximum daily temperature usually occur around 4 PM or so??? • Has to do with absorption and emission • When do we have warming? • Absorption > Emission • This is usually the case up until about 4 PM (average)

  9. Maximum Daily Temperature • Incoming solar radiation is maximum at noon • But incoming radiation is greater than outgoing radiation until late afternoon • So warming continues until outgoing becomes greater than incoming

  10. Maximum Temperature • 4 PM is the average time the maximum temperature occurs but…. • It can be at other times due to several factors • ???? • Clouds - keep temperature lower and causes temp max to occur earlier • radiation is reflected • Bodies of water - air over water is generally cooler than air over land during summer • air can move ashore during the day and drop the temperature • seabreezes are a good example

  11. Maximum Temperature • 4 PM is the average time the maximum temperature occurs but…. • It can be at other times due to several factors • ???? • Large storms and their fronts - During winter, the passage of warm fronts can cause highest temp to be late at night or early in the morning

  12. Factors Influencing Max Temperature • Soil Type • Some types absorb more energy due to color, texture, etc. • Soil Moisture • Wet soil heats up slower • Some energy goes into evaporation • Vegetation • Evaporation again • Clouds, Humidity, Haze • Reflect radiation

  13. Factors Influencing Max Temperature • So ideally, to get the hottest daytime temp: • Soil which is a good absorber (sand for one) • Dry soil • No vegetation • No moisture in the air • What type of area does the above describe? • Desert areas

  14. Atlanta vs Phoenix • Both at same latitude - same amount of solar radiation coming in • AtlantaPhoenix humid dry cloudy clear lots of trees less trees 87º July avg. 105º July avg.

  15. Nighttime Cooling • Around 4PM the earth starts losing more IR energy than it gains in solar energy • Both the air and earth cool by radiating IR energy - radiational cooling • The earth radiates at a greater rate so it cools faster • Just as air near the surface warms by conduction during the day, it cools by conduction at night • By late evening/early morning, air at the surface is cooler than the air above

  16. Nighttime Cooling • Defn. - Radiational Inversion - an increase of T with height due to radiational cooling at the surface of the earth • Also called a nocturnal inversion

  17. Factors Influencing Min Temperature • Wind • Light wind - colder temps • Strong wind - warmer temps • Wind causes coldest air near surface to mix with warmer air above • Less chance of an inversion • Length of night • Longer nights - more time for radiational cooling

  18. Factors Influencing Min Temperature • Moisture and clouds • Absorb IR radiation and emit radiation back toward earth  warming • If air is moist we get dew • Condensation - release of latent heat  warming • Terrain • Cold air is more dense (heavy) than warm air • So it moves down slopes into valleys • Called cold air drainage

  19. Crop Protection • Add heat - smudge pots • Cover plants • Traps the earth’s outgoing IR radiation • Make wind • Mixes warm air downward • Add moisture to soil • Spray water on crops • Ice protects!! Keeps temps around freezing and liquid to ice phase change releases latent heat

  20. Crop Protection

  21. Controls of Temperature • These are the main factors that cause variations in T from one place to another • 1) Latitude - determines amount of solar radiation at surface • More near tropics, less toward the poles • 2) Elevation - already know that T decreases with height in the troposphere

  22. Controls of Temperature • 3) Land and water distribution - have different heating properties • Defn. - specific heat - the amount of energy needed to raise the temperature of one gram of a substance by one degree C • Water has a higher specific heat than land • Takes more energy to raise its temperature • So in summer, land gets hotter than water. In winter, land gets colder than water

  23. Controls of Temperature • 4) Ocean Currents - • East coasts of continents - warm currents flowing to the north • West coasts of continents - cold currents flowing to the south • So coastal cities on the West Coast will likely have lower summertime temps • EX. San Francisco - pretty cool (sometimes cold) in the summer due to cool Pacific water

  24. Controls of Temperature(January) • Large change in T from tropics to poles (large temperature gradient) • Lines of equal temperature are called “isotherms” • Colder over land • Warmer over water

  25. Controls of Temperature(July) • Less change in T from tropics to poles (small temperature gradient) • Warmer over land • Cooler over water

  26. Organization of Temperature Data • We arrange air temperature data in 2 main ways: • 1) By averages • 2) By ranges • Ex. If the low temp is 70º and the high is 90º on a particular day, then: • The average (or mean) daily temperature is 80º • The daily range of temperature is 20º • Which one of these tells you more about the temperature in any given area?

  27. Organization of Temperature Data Desert town vs Coastal town Daily mean 80º 80º Low temp 55º 70º High temp 105º 90º Same mean daily temperature but very different weather

  28. Organization of Temperature Data • At any location, the difference in temperatures between the coldest and warmest months in called the annual range of temperature • Largest annual range of temperature occurs?? • Over land • In areas with large annual changes in solar radiation • Near arctic circle 112ºF • Smallest annual range of temperature?? • Over water • In the tropics • Islands near equator only 1-2ºF from warmest to coldest months

  29. Organization of Temperature Data • The average (mean) annual temperature doesn’t tell us much without the annual range of temperature • Richmond and San Francisco have the same mean annual temp • But their climates are very different • Water moderates temps in San Francisco

  30. Air Temps and Human Comfort • Defn. - wind chill factor - how cold we feel due to temperature and wind • Why do we feel colder when the wind blows? • Conduction warms air near the skin • When the wind blows, the warm layer of air is moved away and more heat from the body is used to warm the colder air • Fast wind = faster loss of body heat • Look at tables 3.2 and 3.3

  31. Air Temps and Human Comfort • What about cold and moisture? • We feel colder when its also wet outside. Why?? • Water conducts heat better than air • Can cause hypothermia • big problem for ill-prepared hikers in the mountains

  32. Measuring Air Temperature • Thermometers • How do they work? • They don’t measure temperature directly but rather the effect of temperature on different substances • water, mercury, alcohol, metal, etc. • Most common thermometers are called “liquid in glass” thermometers • The liquid is usually alcohol (red) or mercury

  33. Maximum Thermometer • Uses mercury • Most important feature is the constriction in the bore - keeps liquid from going back down

  34. Minimum Thermometer • Uses alcohol (lower freezing point than mercury) • Most important feature is the index - can be forced down the bore but not back up

  35. Bimetallic Thermometer • Temperature is measured by the expansion of brass • This is what is used in most thermostats

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