please complete the worksheet carefully
you can use the lecture pdf to help you complete the worksheet
Lecture 12b – Quantifying
atmospheric moisture
Learning Outcomes
• Humidity on Earth. Be able to:
– explain why we care so much about atmospheric moisture
– describe/explain sensible heat flux vs latent heat flux and carry out
simple calculations related to specific heat
– explain/read graphs/carry out calculations of how we quantify
atmospheric water vapor (dew point temperature and relative
humidity)
• Cloudiness on Earth. Be able to:
– describe/explain/carry out simple calculations related to the
processes involved in cloud formation
– analyze feedback loops to explain the effects of clouds on climate
– explain why clouds don’t always lead to precipitation
Evaporation and Condensation
Vapor pressure: pressure of a gas above a liquid
Equilibrium: rate of evaporation equals rate of condensation
Saturation: maximum amount of water in gaseous phase
Saturation Vapor Pressure: maximum vapor pressure at a given temp
How we quantify atmospheric moisture
Indices of water vapor content:
1.
Vapor pressure
2.
Relative humidity
Indices of water vapor content
1.
Vapor pressure
What happens if
temperature increases?
= Undersaturated (more
evaporation can take
place if water available)
What happens if
temperature decreases?
= Supersaturated
(condensation will take
place until reach
saturation)
What happens if you add
more water vapor?
= Supersaturated
(condensation will take
place until reach
saturation)
Indices of water vapor content
1.
Vapor pressure
At what temperature
would this air become
saturated?
a) -10 oC
b) 10 oC
c) 25 oC
d) 40 oC
Temp of saturation for a parcel of air = dew point temperature
How we quantify atmospheric moisture
Indices of water vapor content:
1.
Vapor pressure
2.
Relative humidity – amount of water in air expressed as a percentage of
the maximum amount possible
– Relative humidity = actual vapor pressure x 100%
saturation vapor pressure
Indices of water vapor content
2. Relative humidity
Relative humidity = actual vapor pressure x 100 % = 10 mb x 100 % = 50 %
saturation vapor pressure
20 mb
Saturation vapor pressure
Actual vapor pressure
iClicker Question
The parcel of air is at 30 oC and vapor pressure is 10mb. What is the
relative humidity?
a)
b)
c)
d)
5%
25 %
33 %
50 %
Why does high humidity feel uncomfortable?
• Evaporation of sweat requires energy so absorbs energy from
your skin, cooling you down (latent heat flux)
• At high relative humidity, evaporation is very slow so you
cannot cool down so quickly
• At the same temperature, places with low humidity (e.g. Irvine)
feel nicer!
Clouds ≠ Precipitation
Why do some clouds cause precipitation and some don’t?
Because cloud droplets must grow big enough to fall
(Dust, sea salt,
sulfur and nitrogen
oxide aerosols, etc.)
2020 ESS 1 Discussion 5: Atmospheric pressure maps
This assignment is designed to help you understand and interpret pressure (isobar) maps because this is
a topic which many students struggle with each year.
The map below shows the forecast for Thursday 7th May 2020. The isobars are plotted in mb.
a) On the map above examine the isobars (labeled in mb) and identify whether locations X, Y and Z are
centers of high or low pressure.
X=
Y=
Z=
b) How can you tell where the greatest pressure gradient is on the map and so where it will be windiest?
c) What is the pressure gradient between X and Y given that the distance between them is 1600km?
Pressure gradient = high pressure – low pressure
Distance
d) How would wind move around Z if it were up high in the atmosphere (i.e. parallel to isobars or
spiraling inwards or outwards, in a clockwise or counterclockwise direction)?
e) How would wind move around Y if it were at Earth’s surface (i.e. parallel to isobars or spiraling
inwards or outwards, in a clockwise or counterclockwise direction)?
f) What sort of weather conditions would people who lived at X be experiencing? Explain in as much
detail as possible why.
g) Use the vapor pressure graph from Lecture 12b to help you answer this question. The air temperature
is 20 oC and the relative humidity is 10 %. Calculate
– the actual vapor pressure
– the dew point temperature