Air Pressure and Moisture


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Air Pressure and Moisture

  1. 1. Air Pressure & Moisture
  2. 2.   Air Pressure Air Pressure (also known as atmospheric pressure) is a measure of the force of the air pressing down on the Earth’s surface. Air Pressure depends on the density (d = m/v) of the air. Less dense air exerts less air pressure.
  3. 3. Factors that Affect Air Pressure: 1. Temperature – places of high temperatures usually have lower air pressure than places of low temperatures.
  4. 4. 2. Water vapor – less dense air can hold more water than colder denser air. Air with a large amount of water vapor in it exerts less pressure than dryer air. (Low pressure systems)
  5. 5. 3. Elevation or altitude – as elevation increases, the air becomes thinner, or less dense. Air pressure decreases with increasing elevation. Higher elevation – less dense and lower pressure Lower elevation – more dense and higher pressure The Rockies
  6. 6. Thin air and low pressure makes it difficult for climbers to breathe (Oxygen level drops). Helicopters will not work due to the lack of air pressure.
  7. 7. Measuring Air Pressure  Air pressure changes with changes in temperature and elevation; the standard air pressure is measured at a temperature of 0 degrees Celsius at sea level.
  8. 8. Not on notes…Thinking in terms of air molecules…  As the number of air molecules on the surface increases, there is an increase in force on that surface, and as a result the pressure increases.  In contrast, a reduction in the number of air molecules above a surface will result in a decrease in pressure.
  9. 9. Atmospheric pressure is measured with an instrument called a "barometer," which is why atmospheric pressure is also referred to as barometric pressure. 2 types: a) Mercury – Evangelista Torricelli. At sea level, the weight of the atmosphere forces mercury 760 mm.  When air pressure increases, the column of mercury rises in the tube.
  10. 10.  Aneroid - In an aneroid barometer, a partially evacuated metal drum expands or contracts in response to changes in air pressure.
  11. 11. Average Air Pressure on Earth (not on notes…)  Average sea-level pressure is 1013.25 (mb) or 29.921 inches of mercury (in Hg).  Average pressure on Earth ranges between 900 mb and 1040 mb.
  12. 12. Air Pressure and Weather  Barometers help us forecast the weather.  If pressure is high, it prevents warm, moist air form rising into the upper atmosphere. (good fair weather occurs)  If the pressure is low, the air masses move apart in the upper atmosphere. This reduces the pressure on the layers of warm air below. Warm air rises, clouds form, and rainy weather may occur.  At sea level air pressure is normally around 760 mm.  Extremely strong hurricanes have air pressures between 30 and 70mm.
  13. 13. A Look at Air Pressure Air Pressure and Weather (2 minutes)
  14. 14. Moisture in the Air  Evaporation – the process by which water molecules escape into the air. (The atmosphere holds 14 million tons of moisture!)  Humidity – Moisture in the air.  Relative humidity – describes how far the air is from saturation (at a given temperature).  It is a useful term for expressing the amount of water vapor when discussing the amount and rate of evaporation.
  15. 15. Why and how?  Relative humidity is commonly stated during weather reports because it is an important indicator of the rate of moisture and heat loss by plants and animals.  Example: 1 kg of air can hold 12 g of water vapor, but is actually holding 9 g. The RH would be 9/12 x 100, or 75 %. If it were 12/12 x 100, it would be 100%.
  16. 16. Air Pressure and Air Moisture
  17. 17. Measuring Relative humidity  Meteorologists measure RH with a psychrometer.  It uses the difference in readings between two thermometers, one having a wet bulb and the other having a dry bulb, to measure the moisture content or relative humidity of air.  If humidity is low, evaporation is quick. If high, evaporation is will take place more slowly.
  18. 18. Interpretation of pyschrometer readings… Tdb Twb dry bulb temperature (°C) wet bulb temperature (°C) Let’s do some examples: Example Dry Bulb 1. 2. Wet Bulb Difference Relative Humidity 15 C 5 C ̊ ̊ 10 C ̊ 14 % 30 C 28 C ̊ ̊ 2C ̊ 86 %
  19. 19. Measuring Relative Humidity Water Vapor 2 min
  20. 20. Dry Bulb Wet Bulb Relative Humidity 40 38 Difference is 2 RH = 83% 48 34 Difference is 14 RH =12% 56 50 Difference is 6, RH = 65% 60 46 Difference is 14, RH = 30% 62 56 Difference is 6, RH = 69% 66 64 Difference is 2, RH = 90% 68 52 Difference is 16, RH =31%
  21. 21. Mini Lab: Determining Relative Humidity 1. Define humidity: The amount of water vapor in air. 2. Define relative humidity: Tells you how close air is from total saturation. The ratio of water vapor in the air in volume or air to how much Water vapor or air is capable of holding. 1. Relative humidity changes due to: Temperature. 1. Warm air is capable of holding more or less moisture than cool air? More. There are less molecules in warm air. 5. Relative humidity is expressed as a: Percentage (%)
  22. 22. Mini Lab: Determining Relative Humidity 1. You will determine the average relative humidity of the classroom. 2. Dampen “wet” bulb. 3. Spin for one minute. 4. Record dry and wet bulb readings. 5. Determine relative humidity (use chart). 6. Repeat 2 more times. 7. Obtain average for all readings.