Transcript Humidity & RF, edited.2 - GE-sec2i-2014

Humidity
Humidity is the state of the atmosphere with respect to its
water vapour content/It is the amount of water vapour in
the air.
the water vapour comes from evaporation from water
bodies and transpiration of vegetation
amount of water vapour the air can hold depends on the
temperature. Warm air holds more water vapour than cold
air
Air becomes saturated when it contains all the water
vapour it is able to hold at a particular temperature
The temperature at which the saturation of air occurs is
called dew point
Relative Humidity
Ratio of the actual amount of water
vapour in the air to the maximum
amount of water vapour that the air can
hold at a particular temperature.
A RH of 90% and more shows that the
air is holding a lot of moisture at that
particular temperature
RH of saturated air is 100%
Hygrometer is used to measure relative
humidity
Hygrometer
dry bulb thermometer
muslin
wet bulb thermometer
wick
distilled water
Hygrometer
the dry bulb thermometer measures the
temperature of the surrounding air
the wet bulb thermometer has its bulb wrapped
with a muslin which is kept moist by a wick dipped
into a container of distilled water
if air is not saturated, evaporation from the wet
muslin occurs
Hygrometer
as latent heat is used up during evaporation,
cooling occurs which causes the wet bulb
thermometer to register a lower reading
the difference between the dry bulb and wet bulb
temperature readings is called the wet bulb
depression
Refer to the chart to get the RH value
RH chart
if the wet bulb depression is large, the RH is low
if the wet bulb depression is low, the RH is high
Formation of Clouds
Types of Clouds
As warm air cools, its ability to
hold moisture decreases.
It becomes saturated at dew
point
further cooling after saturation
causes water vapour to
condense into water droplets
which join together to form
clouds
the base of the clouds indicates
the level at which
condensation starts
Types of clouds
1. Cirrus clouds (7 - 13 km above sea level)
highest clouds, detached, thin and wispy
associated with fine weather, bringing little or no
rain
2. Stratus clouds (below 2 km above sea level)
grey, featureless sheets of clouds that can cover the
whole sky
often bring drizzle that may last for several hours
Types of Clouds
3. Cumulus clouds (2- 7 km
above sea level)
cauliflower-shaped clouds
flat base is grey but the
parts the parts
illuminated by the sun are
brilliant
small and white cumulus clouds indicates fine weather
larger cumulus clouds may develop into cumulonimbus
clouds brining heavy showers
Types of Clouds
4. Cumulonimbus clouds (3 - 11
km above sea level)
dark grey cauliflower-shaped
clouds spread out at the top to
form anvil-looking tops
associated with heavy rain,
lightning, thunder and strong
winds
On hot mornings in tropical
areas, they grow in extent and
by afternoon they bring heavy
rain, accompanied by
lightning and thunder
Rainfall
Rainfall refers to the amount of rain that an area
receives over a period of time
It is measured in millimetres (mm) or centimetres
(cm)
The rain gauge is an instrument used to measure
rainfall
It consists of a funnel that collects and channels
rainwater into a container
It should be placed in an open area where there are
no obstructions to block the rain(grass area, not
concrete)
Standard Rain Gauge
The main features of the
gauge are:
the sharp inner edge of the
funnel which allows it to have
an accurate 5" internal
diameter;
http://www.metoffice.gov.uk/education/secondary/students/observing_rainfall.html
Standard Rain Gauge
the whole gauge is set into the ground to keep it secure
and upright with the rim 30 cm above the surrounding
short grass or gravel, this height being chosen so that no
rain splashes from the surroundings into the funnel;
the inner can is provided so that the glass jar can be
lowered gently into the gauge and can also hold the water
if the jar overflows or cracks in cold weather;
the funnel has a narrow spout so that there is little
exposure of the water in the jar to the air, to reduce
evaporation.
http://www.metoffice.gov.uk/education/secondary/students/observing_rainfall.html
Calculating Rainfall
Daily rainfall refers to the amount of rain that falls
over 24 hours
Monthly rainfall refers to the total amount of
rainwater collected throughout the month
Annual rainfall is the total amount of rainwater
collected throughout the year
Calculating Rainfall
A rainfall map shows
the distribution of
annual
rainfall in a place for
a period of time
It makes use of
isohyets which are
lines joining points of
equal rainfall to show
the distribution of
rainfall in a place
Types of Rainfall: Convectional Rain
Convectional Rainfall – Formation
1. Convectional rain (most common in Singapore)
•
Commonly occurring in warmed/heated regions
(tropical regions/continental interior in summer).
•
Air layer near the ground/sea surface is heated up by
conduction because of intense insolation.
•
Warm air expands and becomes unstable and rises –
convection uplift.
•
Rising air cools to its dew point at its level of
condensation.
Convectional Rainfall – Formation
2. Air that has been lifted cools.
3. Saturation takes place at dew point.
4. Air is cooled further as it continues to rise - water
vapour condenses into water droplets around
atmospheric particles (like dust).
5. Water droplet coalesce to form clouds.
6. Rain falls when water droplets in the air becomes
too heavy to be suspended.
Convectional Rainfall – Formation
1. Convectional rain
•
Warm moist air becomes saturated at dew point
temperature.
•
Relative humidity reaches 100%.
•
Condensation takes place upon further cooling to form
cumulonimbus clouds.
•
When water vapour in the air becomes too heavy,
convectional rain falls.
Convectional Rainfall – Formation
1. Convectional rain
•
Convectional rain is usually associated with tropical
depressions (typhoons) and thunderstorms,
sometimes with lightning.
•
Almost a daily occurrence in tropical areas.
•
Less frequent but still occur in tropical desert regions
and continental interiors in summer.
•
Usually in the afternoon.
Convectional Rainfall – Formation
1. Convectional rain
•
It brings intense, but short-lived showers and is
localised.
•
Rain drops are usually large; heavy downpour
Types of Rainfall: Relief Rain
(Source: geographyfieldwork.com/ReliefRain3.gif)
Formation of Relief Rain
•
As moisture bearing wind from the sea reaches
large obstacles such as hills and mountains, the air is
forced to rise over the top.
•
As it rises, it cools, condenses and forms clouds.
•
When the water droplets are large and heavy enough,
rain falls
•
This side which receives rain is called the windward
side
•
As the air moves over the high ground, it may warm
up and descend over the hills. Moisture may evaporate
to give generally dry weather conditions on the other
side of the mountain. This dry side is called the
leeward side (rain shadow)
References
Lau, L. (1998) Physical Geography for Distinction, Hong
Kong: Greenwood Press.