How do we measure Humidity?
Water in the Atmosphere
Water is the only substance on our planet that exists in our atmosphere in all three states: solid, liquid, and gas. Within the troposphere water is constantly switching back and forth between these states in the water Cycle.
Evaporation - liquid to gas
Transpiration - liquid to gas from a plant
Condensation - gas to liquid
Freezing - liquid to solid
Melting - solid to liquid
Each of these transformations involves energy, and the specific amount of energy involved in each type of transformation can be found on the cover of the ESRT.
We discuss that amount of moisture in the air in terms of humidity. There are several different methods of measuring and describing humidity.
SPECIFIC HUMIDITY IS A MEASURE OF THE WEIGHT OF WATER PRESENT IN ONE CUBIC METER OF AIR.
The specific humidity changes with the temperature of the air, because temperature effects the air's capacity, or the amount of moisture it can hold.
HOT AIR HAS A HIGHER CAPACITY - meaning it can hold more moisture.
COLD AIR HAS A LOWER CAPACITY - meaning it can't hold as much moisture.
Rather than measure the weight of moisture in the air, in our class we will use Relative Humidity as our way of discussing humidity.
RELATIVE HUMIDITY MEASURES HOW FULL OF WATER THE AIR IS. It compares the specific humidity to the air's capacity.
Relative humidity is measured using a PSYCHROMETER.
A psychrometer is a device made from 2 different thermometers. One of these thermometers is completely normal, and simply measures the air temperature. This one is called the DRY BULB.
The other thermometer has a damp cloth tied around it. This one is called the WET BULB.
BECAUSE EVAPORATION OCCURS ON THE WET BULB, THE WET BULB VALUE IS ALWAYS LOWER THAN THE DRY BULB
How to Find the Relative Humidity Using a Psychrometer:
- Find the dry bulb and wet bulb temperatures
- Find the difference in the dry bulb and wet bulb temperatures
- Look at the ESRT to find where DRY BULB and DIFFERENCE VALUES ALIGN