HUMIDITY
AND
DEW POINT
DEW POINT
The dewpoint is the temperature of air which
is needed for condensation or dew (at that
particular temperature).
If you take a glass of ice water and it develops
condensation on the glass surface, the air on
the glass has condensed to its dewpoint and
created dew.
Dewpoint actually measures how much water
vapor is in the air.
RELATIVE HUMIDITY
Relative humidity is a measure of how much water
vapor the air actually could "hold" at a certain
temperature.
The relative humidity represents how close the air is
to saturation.
Saturated air will have an RH of 100 %. You need the
RH of 100% to have rain form in clouds.
How do we measure
Humidity??????
To measure relative humidity use a sling
psychrometer (hygrometer)
Dry bulb thermometer and a wet bulb
thermometer mounted together
Dry bulb tells actual temperature
Wet bulb shows how much water can be
evaporated – temperature lowers as water is
evaporated
The difference in temperature on the 2
thermometers is an indication of the amount of
water vapor in the air.
Wet bulb.
Dry Bulb
How relative humidity (RH) is
measured?
Wet- and Dry Bulb Psychrometer
Wet- and Dry Bulb Psychrometer
Psychrometric Chart.
If the difference
between wet and dry
bulb is 6º F and the
temperature is 72º F
(dry bulb), then the RH
is 54%.
Dry air: the water will evaporate
quickly and cause a large drop in the
wet-bulb temperature.
This makes the difference in readings
on the 2 thermometers greater.
Moist air: little water will evaporate from the
wet-bulb and the temperature decrease will be
small.
The difference between the wet bulb and dry
bulb will be small.
When the Wet bulb temperature = the dry bulb
temperature………
100% HUMIDITY!!!
The "wetter or damper" you feel,,
the higher is the relative
humidity.
If you feel the air is dry around
you, the relative humidity is low.
Glass of water demo
Assignment
•Blk text pg 505 6-9
CLOUDS AND PRECIPITATIONS
V. Clouds & Precipitation
A. Cloud Formation (condensation)
1. All clouds require 3 things:
a. water vapor
b. condensation nuclei- something for water to bond with
c. cooling- air must be cooled below dew point . Expansion, or when
air rises.
When air in the atmosphere sinks it compresses causing heating.
Cloud families
•High altitude
•Middle altitude
•Low altitude
•Vertical
development
C. Cloud Types
1. Cirrus- wispy, feathery high altitude clouds.
a. Made of ice crystals
2. Stratus- flat, spread out layers.
Low to middle altitude.
a. form through horizontal air
movement
3. Cumulus- fluffy, rounded, medium altitude clouds.
a. form through vertical air movement
b. cumulus- “piled”
4. Cumulonimbus- large rain/storm
cloud
a. nimbo – “rain”
b. cumulo- “column like”
Cirrus
Cumulunimbus
stratus
cumulus
G. Forms of Condensation
1. Frost- when the temp of the air is below 0°C, then the water vapor
condenses as a solid.
2. Fog- cloud formation near Earth’s surface.
a. Ground/radiation fog- during nighttime with sky clears, the ground
loses heat rapidly by radiation.
1.) breeze mixes air at ground level
2.) air is cooled below the dew point and fog forms.
b. advection fog- when warm, moist air blows over a cool
surface.
Topic Questions 511 blk text
• Complete the following questions.
• 10, 11, 12a, 14a,b, 15a.
• Read pg 512-514 topics 16-17. then draw diagram 27.12 in your notes and
describe in your own words how rain drops forms and the types of
precipitation.