Moisture in the atmosphere, HUMIDITY, TEMPERATURE AND DEWPOINT

1. Moisture in the atmosphere, HUMIDITY, TEMPERATURE AND DEWPOINT RRB Pg 117

2. Moisture in the atmosphere • Saturation: When the air contains as much moisture as it can hold • The higher the temperature, the more moisture air can hold • If air temperature of moist air is lowered enough, the air will become saturated • If the temp falls below the dew point, water vapor usually turns into liquid water

4. Moisture in the form of water vapor enters the atmosphere by evaporation, sublimation to a gas, and transpiration • Evapotranspiration: Evaporation and Transpiration together • Large amounts of energy are needed to change liquid water into water vapor during evaporation and transpiration, most of which comes from insolation • EUREKA!! • Transpiration- a flashback to biology

6. Measuring moisture in the atmosphere • Sling psychrometer: has two thermometers mounted so they can be slung through the air. One thermometer records air temp (dry bulb temp) and the other has a wet cloth which measures the wet bulb temp • Evaporation causes cooling • The drier the air, the greater the evaporational cooling, the greater the difference between the dry-bulb and wet-bulb temps • Use a dew point temperature table to determine the dew-point. ESRT pg 12

8. Dew point • Dew point is expressed using a unit of temperature (Celsius or Fahrenheit) • Dew point: the temp to which air must be cooled to become saturated • The only way to change the dew point is by adding or removing moisture from the air

9. Steps for Determining Dew POint • Locate the dry-bulb (air temperature) reading on the left hand side of the chart • Subtract the wet-bulb reading (measure of how dry or saturated the air is) from the dry-bulb reading • Locate the difference between the wet-bulb and dry-bulb readings across the top of the chart • Follow the horizontal row for the dry-bulb reading to the right until it meets the vertical column running down from the difference between the wet-bulb and dry-bulb readings

10. Sample Questions • If the dry-bulb temperature is 8 degrees Celsius and the wet-bulb temperature is 2 degrees Celsius, find the dew point • Note that the wet-bulb temp will always be the same or colder than the dry-bulb

11. Sample questions • If the dry-bulb temperature is 26 degrees Celsius and the wet-bulb temperature is 25 degrees Celsius, find the dew point

13. Finding relative humidity • Relative humidity: the ratio of the amount of water vapor in the air (absolute humidify) to the maximum amount it can hold (moisture capacity) • Relative humidity is expressed as a percent of saturation • Air that is saturated = 100% • To determine RH, you need a sling psychrometer and a RH table- ESRT 12

14. Finally… • If the RH is 50%, the air could contain twice as much water vapor • As the temperature of the air approaches the dew point, the relative humidity approaches 100%

15. Steps For determining RH • Locate the dry-bulb reading on the left-hand side of the Relative Humidity chart • Subtract the wet-bulb reading from the dry-bulb reading • Locate the difference between the wet-bulb and dry-bulb readings across the top of the chart • Follow the horizontal row for the dry-bulb reading to the right until it meets the vertical column running down from the top

16. Sample Problem • Find the RH when the dry-bulb temperature is 18 degrees Celsius and the wet-bulb temperature is 13 degrees Celsius • Find the RH when the dry-bulb temperature is 10 degrees Celsius and the wet-bulb temperature is 9 degrees Celsius