Introduction:
In this lab you are going to set up a simple hygrometer, make observations over a two week period, and graph the data collected. Use Numbers (here's a tutorial) on your laptop and this graph rubric.
Dew point is the temperature to which air must be cooled, at a constant pressure, in order for it to be saturated with water.
The amount of water vapor in the air is referred to as humidity.
When the humidity is high, the dew point is also high.
Water vapor can compose no more than 4% of the mass of air.
When a weather report says the "relative humidity" is 100%, that means that 4% of the air's mass is water vapor − the air can hold NO MORE water vapor. Use this website for a local weather report.
A hygrometer is a device for measuring changes in humidity.
Today's meteorologists use digital hygrometers to measure relative humidity.
Atmospheric humidity was first measured by reading a pair of thermometers known as a 'wet- and dry-bulb psychrometer'.
The dry bulb is a glass thermometer that registers the actual air temperature. The wet bulb is a similar thermometer but with the bulb enclosed in a muslin bag and secured with a string that dips into a bottle of distilled water to keep the muslin bag damp. This thermometer measures the wet-bulb temperature, which, unless the relative humidity is very close to 100%, is generally lower than the dry-bulb temperature. This is because the temperature is lowered by the evaporation of water from the muslin bag.
The difference between the dry-bulb and wet-bulb temperatures is called the wet-bulb depression. It is this, along with the dry-bulb temperature, that enabled meteorologists to calculate the dew point temperature and relative humidity.
Heat Index:
The old saying goes, "It's not the heat, it's the humidity". The way the human body reacts to warm temperatures is a function of both the actual temperature, and the moisture in the air.
Low humidity lets the body cool itself by the evaporation of perspiration. (Evaporation is a 'cooling' process.) As the humidity rises, less perspiration evaporates, and it becomes more difficult for the body to dissipate heat − possibly leading to heat illness.
The heat index is an attempt to measure the apparent temperature, or the way it feels outside. Heat index is calculated using a formula containing actual temperature and absolute humidity (grams of water per cubic meter of air). Generally, when the heat index rises above 90 degrees, people should avoid being outside for too long. When the heat index is 105 degrees or higher, it becomes dangerous. (Another type of "felt" temperature is wind chill.)
Note that the 'official' heat index is calculated using an 'in the shade' temperature, so if you are in the sun, it could actually feel warmer than the heat index indicates.
Dew Point Temperature:
Another way to measure how hot it feels outside is to use the dew point temperature. The dew point temperature is an indicator of the absolute humidity (humidity in the air).
You know that the warmer the air, the more water vapor it can hold. Therefore, the higher the humidity, the higher the dew point!.
If the air temperature drops to its dewpoint temperature, water vapor condenses out of the air onto surfaces in contact with the air in the form of dew.
No matter the actual temperature, most people begin to feel uncomfortable when the dew point approaches 70 degrees, and dew points above 70 degrees are opressive.
The table below (left) shows the "absolute humidity" in g/m3 (upper line) and the "dew point temperature" of the air in oC (lower line) for certain air temperatures as a function of relative humidity.
Example: At an air temperature of 50 oC and a relative humidity of 70%, the absolute humidity is 58.1 g/m3 and the dew point temperature is 43 oC.
Use the chart below for a 'quick' comparison of Celsius and Fahrenheit temperatures.
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