# Slide 1 - Atmospheric and Oceanic Sciences

Document Sample

```					Lab 3: Moisture in the Atmosphere

From:
Alice DuVivier
Purpose

From:
Alice DuVivier

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.
Purpose
Describing water in the                  Dragonfly with Dew

atmosphere
Absolute measures
• Mixing ratio (kg water
vapor / kg dry air)
• Specific humidity (kg
water vapor / kg air)
• Vapor pressure
• Dewpoint temperature
Non-absolute measures
• Relative Humidity        From:
http://green.yahoo.com/blog/environmentalgraffiti/48/cr
eepy-crawlies-caught-in-morning-dew.html
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!
Observing Humidity
Sling Psychrometer
Measures dry bulb and wet
bulb temperatures
• Dry bulb is the
atmospheric temp
• Wet bulb is the temp
after the water
evaporates
NOTE:
IF it is below freezing outside
this method has problems.
Observing Humidity
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
evaporate.
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.
Evaporative Cooling
Evaporative cooling:
• Water requires heat to
evaporate (ex. boiling water)

• Evaporated water takes away
heat and the surroundings
become colder.

• Difference between wet bulb
and dry bulb temperatures show
the potential for evaporative
From: http://www.sustainablesushi.net/tag/overfishing/   cooling.
Evaporative 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.
evaporate.
Saturated air is “full” of water
and can’t take any more air in
without pushing liquid water out
(no evaporation).  like sponge
Evaporative
Cooling

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
doesn’t decrease.
Statistics
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)
Statistics
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!

Inside: Psychrometer
and Pressure

Online: Temp, Dew Point
(Convert from F to C)