Lab 3: Moisture in the Atmosphere
Why do we care about water in the atmosphere?
What terms would you use to describe how much
water there is in the atmosphere?
Water in the Atmosphere
Amount of Water in Atmosphere is Regulated by:
sun, ocean currents, aerosols, rain, land use, etc.
Describing water in the Dragonfly with Dew
• Mixing ratio (kg water
vapor / kg dry air)
• Specific humidity (kg
water vapor / kg air)
• Vapor pressure
• Dewpoint temperature
• Relative Humidity From:
Relative Humidity Equations
water vapor in the atmosphere x100
water vapor capacity
mixing ratio x100
saturation mixing ratio
actual vapor pressure x100
saturation vapor pressure
You MUST have units in your
calculations for them to be
From: http://www.lebell.ca/2009/07/22/spider-web-dew-drops/ correct!
Measures dry bulb and wet
• Dry bulb is the
• Wet bulb is the temp
after the water
IF it is below freezing outside
this method has problems.
How to use it:
1. Wet cotton wick with water.
2. Shade with body while measuring to
avoid direct solar heating.
3. Read initial wet temp then swing for
10 sec. Swinging helps liquid
4. Read wet temp again and then
swing again for 10 sec.
5. Repeat until the wet bulb temp stays
constant. Note: the wick must
remain wet during this time.
6. When the wet bulb temp is constant
record it, then turn over the
thermometer and read the dry temp
and record that as well.
• Water requires heat to
evaporate (ex. boiling water)
• Evaporated water takes away
heat and the surroundings
• Difference between wet bulb
and dry bulb temperatures show
the potential for evaporative
From: http://www.sustainablesushi.net/tag/overfishing/ cooling.
Dry Air Wet Air
Dry air has “room” in the Wet air doesn’t have “room” for
air for more water to much water.
Saturated air is “full” of water
and can’t take any more air in
without pushing liquid water out
(no evaporation). like sponge
Dry air: Large difference between the bulb temps because
there is evaporation and the wet bulb temperature decreases.
Wet air: Small difference between the bulb temps because
there is less evaporation and the wet bulb temperature
Standard Deviation (σ - sigma):
1.σ is a measure of the variability of a data set
2.Low σ means that the data points tend to be very close
to the mean.
3.High σ means that the data are spread out.
4.One σ is ~68% of the data. Two σ are ~95% of the data.
(images from wikipedia)
Things to consider about measurement error:
• Errors accumulate
• Error of measurement (thermometer, human) vs. true
variability (actual temperature changes)
Instruments can have internal error of two kinds:
Accuracy vs. Precision
High accuracy, low precision High precision, low accuracy
Time to make measurements!
Online: Temp, Dew Point
(Convert from F to C)
Outside: Psychrometer (only if above
freezing) and Cloud observation