Transcript Relative Humidity and Dew Point

Aim: What is relative humidity?
Do Now: Answer the following in your
notebooks;
1) What do you think of when
you hear it is humid out?
2) Explain what type of
pressure is outside and how
you know.
3) Since it is windy here, what
does that tell you about the
pressure around RVC?
1) What do you think of when you hear it is humid out?
it’s wet, its hot, it’s uncomfortable
2) Explain what type of pressure is outside and how you
know.
There is low pressure outside because it is lousy
weather (wet, windy, cloudy)
3) Since it is windy here, what does that tell you about
the pressure around RVC?
The surrounding areas must have different (higher)
pressures; wind blows from High --> Low pressure
I. Vocabulary
a) Humidity – is a measure of how much water
vapor is in the air.
b) Parcel – a
section or part of
something.
A parcel of air is
a section of the
atmosphere we
are discussing.
Which temperature air can hold the most water vapor?
- Warmer air can hold more water vapor than
cooler air.
WHY?
- Warm air expands and has
more space to hold water
vapor
Cold air
Warm air
II. Relative Humidity (RH) : describes “how
full” the air is with water vapor
(expressed as a percent (%)).
- compares the size of the air to the amount
of water in the air.
What What
caused
this parcel
of airRH
to as
change
it’s size
happens
to the
the air
(expand)?
temperature
changes?
- heat causes air to expand and allows it hold
more water vapor.
- Saturation the air is 100% filled with water
vapor.
When saturated, which temperature air will
hold more water vapor?
Cold air
WHY?
Warm air
Warm air is more
spread out and can
hold more water
vapor
Which parcel of air has a
higher relative humidity?
They have the same
RH = 100%
- When the Relative humidity = 100%, the
air is Saturated and Condensation can occur
(makes clouds and rain)
Take the worksheet from
the back top bin and work
on the side the says
What is Humidity? only
III. Changing the Relative Humidity
a. Water vapor enters or leaves the
air. (evaporation or condensation)
b. The temperature changes. (changes
the size of the air)
How can you change the Relative
Humidity of the air?
(Think, what is the definition of RH?)
IV. Dew Point Temperature
- always lower than the air temperature
- indicates when the air will be saturated
with water vapor. (RH = 100%)
Example: Below the parcel of air has a dew
point temperature of 750F.
RH = 50%
Air Temp. = 870F
What will this
dew point
temperature do
to the air?
RH = 100%
Air Temp. = 750F
- As air temperature gets closer to the dew
point temperature…
– there is better chance for precipitation.
1) At what time
is the RH the
highest? 6am
2) At what time
is RH the
lowest? 3pm
3) When is the
greatest
6am chance of rain?
V. Measuring DP and RH:
- Sling Psychrometer measure the RH and DP.
- uses a dry bulb
temperature and wet
bulb temperature.
- The wet bulb
temperature is always
lower.
Why would the wet bulb thermometer always at a
lower temperature? What is the water doing?
- Calculate the difference between dry and
wet bulb temps. This is called the wet
bulb depression.
- Use ESRT p.12
Example:
A student measure the temperature of the
air using a sling psychrometer and found that
the Dry Bulb Temp. = 240C and the Wet Bulb
Temp. = 200C. What is the wet bulb
depression temperature? Relative humidity?
Dew Point?
Example:
Dry bulb = 240C
Wet Bulb = 200C
Depression = ? 40C
RH = ? 18 C
DP = ? 69%
When trying to
determine Relative
Humidity and Dew
Point, create a list of
the information you
need, and the
information you have
to make solving the
problem easier.
- Subtract the wet bulb from the dry bulb value.
- Use the ESRT page 12 charts to look up the
Dew Point Temperature and the Relative
Humidity.
Work on the
other side of
the worksheet
you already
have