Download Relative humidity

S c i e n c e s
Weather, Atmosphere and Meteorology
Key words: Atmosphere, Ozone, Water vapor, solar radiation, Condensation, Evaporation, Humidity,
Dew-Point Temperature, Cirrus Clouds, Stratus Clouds, Cumulus Clouds,
Composition of the Atmosphere
10000 km
700 km
80 km
50 km
Ozone layer
8-18 km
100°C
50°C
20°C
0°C
-90°C
T
Pressure (atm)
Water Vapor (g/m3)
Moisture in the Atmosphere
80 km
50 km
100
10-1
10-2
10-3
10-4
8-18 km
Ozone layer
0
20
40
Moisture in the Atmosphere
Condensation is the change of the
physical state of matter from gaseous
phase into liquid phase.
Evaporation is the process by which
water molecules in liquid water
escape into the air as water vapor.
Moisture in the Atmosphere
Humidity of the air is the amount of water in it.
Warmer air contains more water than colder one. When the air reaches the maximum capacity, it
is saturated.
Relative humidity measures the percentage of water (%) in the
air.
Relative Humidity (%)
Air Temperature
°C
43
42
41
40
39
38
37
36
35
34
33
32
31
30
29
28
40 45 50 55 60 65 70 75 80 85 90 95 100
Extreme Danger: Exertional Heatstroke
highly likely
Danger: heat cramps or heat exhaustion
likely, heatstroke possible
Extreme caution: Heat cramps or heat
exhaustion possible
Exercise caution; Dehydration likely if
people do not drink adequate fluids
Moisture in the Atmosphere
Relative humidity measures the percentage of water (%) in the
air
How to calculate the Relative humidity
Psychrometer method
The Psychrometer is an instrument composed of two thermometers
•
•
one thermometer measures the air temperature (the Dry-bulb).
The other thermometer has a wet wick on the bottom of the bulb (Wetbulb).
Water evaporating from the wet-bulb removes energy and cools the
thermometer. If the air is humid, less water evaporates from the bulb and
the two thermometer read the same temperature.
Dry-bulb
Wet-bulb
The more humid is the air, the closer is the reading between the two
thermometers. If the temperature are the same, the relative humidity
is 100%
Moisture in the Atmosphere
How to calculate the Relative humidity
Psychrometer method
Measure the dry bulb temperature.
Measure the wet bulb temperature.
Calculate the difference between the
dry bulb and wet bulb temperatures.
Find the difference between the wet
and dry bulb temperatures on the
table.
The relative humidity will be at the
intersection of the row and column.
Example
TDry-bulb = 24°C
TWet-bulb = 30°C
The difference between Dry-bulb and Wet-bulb temperatures is ∆T=6°C.
So the corresponding Relative humidity would be RH=55%
Moisture in the Atmosphere
The Dew-Point (DpT) is the temperature at which water vapor
condenses into liquid water.
The condensed water is called dew. If the Dew-Point Temperature is below
0°C, the air frosts instead of condensing and it is called Frost point.
The DpT is found in a similar manner to that of relative humidity.
Moisture in the Atmosphere
How to calculate the Dew Point Temperature
Psychrometer method
Measure the dry bulb temperature.
Measure the wet bulb temperature.
Calculate the difference between the
dry bulb and wet bulb temperatures.
Find the difference between the wet
and dry bulb temperatures on the
table.
The DpT will be at the intersection of
the row and column.
Example
TDry-bulb = 24°C
TWet-bulb = 30°C
RH = 55%
The difference between Dry-bulb and Wet-bulb temperatures is ∆T=6°C.
The corresponding Dew Point Temperature would be DpT =14°C
Moisture in the Atmosphere
Why is it important to calculate the Dew Point Temperature?
The dew point is associated with relative humidity.
Relative humidity of 100% indicates the dew point is equal to the current temperature
and the air is almost saturated with water.
One of the phenomena associated to the DpT is the formation of fog and clouds.
Fog is the condensation and accumulation of water droplets suspended in the air near the Earth's
surface. Fog starts to form when the difference between temperature and dew point is generally
less than 2.5 °C and it can create a visibility hazard.
Clouds
Clouds form when the air is cooled down to its DpT.
The air cools down as it rises up from the Earth surface.
if DpT > 0°C
The cloud is composed of
water drops
if DpT < 0°C
The cloud is composed of
ice crystals
Clouds
Warm air is lighter and less dense than the surrounding air, so it rises and takes on the shape of a round
"parcel."
Several things happen as the air rises up through the atmosphere:
1 - The atmospheric pressure drops down and the parcel increases in size (PV=nRT).
2 - As soon as the air expands it gets cooler. The parcel cools at a rate of 10° C per 1000 mt of rise.
This is called the dry adiabatic lapse rate.
3 - Now the cooling parcel is no longer able to hold the water vapor. The excess water vapor begins to
condense into liquid droplets (moisture), this means the air parcel is saturated and has reached its Dewpoint temperature. The higher the parcel rises, the more it expands, the cooler it gets, and the greater
the amount of moisture that condenses out of it.
Clouds
The air parcel
expands and cools
The atmospheric
pressure applied
on the parcel
decreases
Warmed air rises with
shape of a round parcel
The Sun heats the
ground air
The air reaches the
DpT and the vapor
starts to condense
DpT Level
Clouds
Clouds form wen the parcel rises up and cools to its DpT
There are four situation that make the air to rise:
1 - Convection
2 - Topography
3 - Air convergence
4 - Weather front’s Lifting
Clouds
Clouds formations fall into three categories
Cirrus clouds are very high and
are made of ice crystals. They are
thin and feathery clouds
Stratus clouds are layered
and they are found at lower
altitudes
Cumulus clouds are puffy and
cotton-like and develop vertically
Precipitation
In order for the water vapor to condense and precipitate, the air
must:
1.Cool down till the DPT.
2.Find a condensation nucleus (particle). These nuclei can be dust,
salt, bacteria or pollutants (acid rains).
In meteorology, precipitation is a product of the condensation of atmospheric water vapor
that falls under gravity when the droplet becomes too heavy to be maintained on the air.
There are several forms of precipitation as rain, snow and hail.
The type of precipitation that forms depends on the air
temperature.
If it is above 0°C, rain forms;
if it is under 0°C, snow forms.
Precipitation
RAIN
Inside the cloud the air moves the drops of comndensing water up and down
constantly. As the droplets collide against each other, they aggregate and get larger.
When the drops become too heavy to stay up in the cloud, they fall down to the Earth.
The stronger is the updraft the bigger would be the forming drop and the resultant
precipitation.
Raindrops have sizes ranging from 0.1 mm to 9 mm, above which they tend to break up.
Precipitation
SNOW
Snowflakes are conglomerations of frozen ice crystals which fall through the Earth's
atmosphere. They begin as snow crystals which develop when droplets freeze.
Snowflakes come in a variety of sizes and shapes.
Precipitation
HAIL
Like other precipitation, hail forms in clouds when water freezes on contact with condensation nuclei.
1. Hail forms in a tall cloud with strong updrafts.
2. The frozen droplet moves through the cloud collecting other droplets at the lower part of the
cloud.
3. As the hail-stone rises up in the cloud, the outer layer freezes and increases its size.
4. This process continues till the hail-stone falls into the ground.
The cross section of a hail-stone shows the history of the formation.
Hailstones can grow to 15 cm and weigh more than 5 kg
Accretion
strata
Condensation
nucleus