lab manual_v2
Lab
manual
for
GEOG1102
15
Lab 3 Moisture and lapse rates
Objective: To learn how to measure moisture in the atmosphere and its impact on lapse
ates. To understand the difference between the various lapse rates [environmental lapse
ate (ELR); dry adiabatic lapse rates (DALR); moist adiabatic lapse rates (MALR)]
occu
ing in the atmosphere and their interaction, as well as their significance for daily
weather processes.
1. We want to gather basic data on how warm the air is and how much water is held in
the air - we can use this information to calculate how close we are to our saturation
humidity. We will gather this data using a sling psychrometer. Take your psychrometer
and notice that there are two bulbs - one is covered in fa
ic (the `wet' bulb) and one is
not (the `dry' bulb). Both thermometers should be giving you the same reading right
now, and it should be equal to the air temperature in the room.
Keep the following points in mind when using sling psychrometers:
• Make sure that you have enough space around you that you will not hit anything
(or anyone) as you swing the psychrometer.
• Start by thoroughly wetting the wick of the wet bulb. Avoid touching the wick,
as dirt and oil from your hands can interfere with the evaporation process.
• Swing the psychrometer for a full 90 seconds. Quickly check the temperature of
the wet-bulb thermometer – remember the value. Swing the psychrometer for
another 30 seconds and quickly check the wet-bulb temperature again. Repeat
the swinging
eading sequence until the temperature of the wet-bulb
thermometer has stabilized and is no longer decreasing. Record the lowest
temperature observed as your final Tw. Record the dry-bulb temperature (Ta) at
the same time, and calculate their difference.
Dry-bulb temperature (Ta) is simply air temperature! Wet-bulb (Tw) is not dewpoint,
ut a measure of the moisture content of the air. If the air is very dry, more moisture will
evaporate from the ’sock" into the air, dropping the wet-bulb temperature due to the
latent heat of evaporation. The opposite is true for a nearly saturated airmass.
1a. Use the psychometric tables to determine the humidity of the air (Table 1), and then
the dew-point temperature (Tdew) of the air (Table 2). The wet-bulb depression is
calculated by subtracting Tw from Ta. (5)
Ta = _______________________
Tw = _______________________
Ta-Tw = _______________________
Lab
manual
for
GEOG1102
16
RH = _______________________
Tdew = _______________________
1b. Use the psychrometer to measure the atmospheric humidity outside. (5)
Ta = _______________________
Tw = _______________________
Ta-Tw = _______________________
RH = _______________________
Tdew = _______________________
1c. Go outside and observe the clouds in the sky. What kind of clouds are these? At
approximately what elevation are they occu
ing (low, mid, or high)? Are these clouds
likely to be associated with a stable or an unstable atmosphere? (3)
1d. We are now going to make some predictions about atmospheric stability based on
today's data for outside data only. Assuming the air at ground level is not saturated, your
parcel of air begins to rise at the Dry Adiabatic Lapse Rate. Calculate the elevation at
which it will become saturated. To do this, use the equation for lapse rate. Show your
work. (4)
The dry adiabatic lapse rate is given by Γ! =
!!
!"
= !!!!!
!!!!!
… Equation (i)
where Γd is the dry adiabatic lapse rate, dT is change in temperature, dz is
change in elevation, z1 is elevation 1, z2 is elevation 2 (the elevation at
which the air becomes saturated, in this case), T1 is temperature at
elevation 1, and T2 is temperature at elevation 2.
1e. The elevation in 1d. is called the lifting condensation level (LCL). The LCL, for
unsaturated air, is the height to which air must be lifted to become just saturated (i.e.,
cloudy). It is cloud-base altitude for cumulus and other convective clouds. If there was
uoyancy driven convection on a given day, you would expect to see cumulus clouds at
approximately this altitude. Discuss, based on the types of clouds and your observations
in Question 1c., whether it is likely that there is buoyancy driven convection today. (2)
Lab
manual
for
GEOG1102
17
2. Saturation commonly occurs when air is cooled to its dew point temperature, as
occurs when it rises from the surface. As the air parcel rises and does not mix with the
su
ounding air, atmospheric pressure on the parcel decreases, allowing it to expand.
Since the parcel has the same number of molecules but occupies more volume, its average
internal energy (temperature) decreases.
As an unsaturated air parcel rises, its temperature will decrease at the Dry Adiabatic
Lapse Rate (DALR) of approximately 1oC per 100 m. If the air parcel is cooled to its dew
point temperature and continues to rise above the height at which this occu
ed, it will
cool at a slower rate (MALR – Moist Adiabatic Lapse Rate). This rate ranges between
0.5 and 0.9oC per 100 m. This rate of cooling is slower than the DALR due to the latent
heat released when water vapour condenses, partially offsetting adiabatic cooling. The
MALR varies because the amount of condensation depends both on the amount of water
vapour in the parcel as well as on atmospheric pressure.
2a. Consider an air parcel at 26oC that is forced to rise from sea level to 5 kilometres. The
parcel reaches its dew point temperature at 1.5 km. Assume that it cools at an average
MALR of 0.5oC per 100 m once above this height. Construct a table indicating the
parcel’s changing temperatures from the surface every 500 m above the surface to a
height of 5 km. (2)
2b. In a second column of your table indicate the temperature changes in a parcel of air
that started at 12oC at the surface, reached its dew point temperature at the same height,
ut cooled at a MALR of 0.7oC per 100 m afterward. (2)
2c. Why would the warmer parcel of air cool at a slower rate between 1.5 and 5 km? (1)
3. An air parcel has a surface temperature of 21oC and a dew point temperature of 12oC. It
is rising up the side of a mountain to a summit of 2400m. DALR is (approximately)
1oC/100m, MALR is 0.6oC/100 m. Show all calculations.
3a. If condensation occurs when the dew point temperature is reached, at what height
will condensation occur? (1)
3b.What is the temperature at the top of the mountain? (2)
3c. What is the temperature of the air parcel at 400 m on the lee side of the mountain?
Think of air that moves from the Pacific toward the Okanagan near Kelowna. (2)
3d. Would you expect to see clouds and precipitation on the lee side of the mountain?
Explain. (1)
Lab
manual
for
GEOG1102
18
Table 3 Psychometric table for calculating relative humidity as a percent.
Lab
manual
for
GEOG1102
19
Table 4 Psychometric table for calculating dew point temperatures in °C.