Transcript HUMIDITY, PRECIPITATION AND CLOUDS

Meteorology Notes: Part III
Clouds
Precipitation
Weather Map and Weather Systems
Station Models
Ingredients Needed for Cloud
Formation
1. Air at saturation (100% relative humidity
or the dew point)
2. Cloud Condensation Nuclei (bits of
microscopic dust and dirt)
HUMIDITY, PRECIPITATION AND CLOUDS
Warmer air can hold MORE
water vapor
The rate of increase is
EXPONENTIAL (it increases
by a factor of 3 every 10°C)
The amount of water
vapor that can be held by
air depends on the
temperature. Condensation
occurs at 100% relative
humidity
�Points A 1-5 are all at
100% relative humidity. B3
is 80%, C3 is 33%, M is
50%.
� For air at temperature
and humidity of B3, C3 and
M to reach saturation
(100% relative humidity) it
must either
�
ADD WATER VAPOR
OR
BE COOLED
�
�
How Can We Cool Air
AIR COOLING occurs by:
1. Contact with cold surface (Forms fog – a “ground cloud”
2. Lifting of air up through the atmosphere (Forms what we
generally call “clouds”
1. Contact with Cold Surface
Ground level condensation may also occur
when stationary air is cooled by a ground
surface that cools, usually overnight. Cooling is
fastest when no cloud cover exists to reflect
outgoing long-wave terrestrial radiation
Cold surfaces are common in mountains, where snow reflects solar radiation and remains cold, cooling
air in contact with the ground.
2. Uplift - How Clouds Form
4. Cloud formation continues as high
as air rises. When air stops rising,
cloud development stops. The more
uplift, the taller and deeper the clouds
3. Relative humidity reaches 100%
(saturation) at dew point temperature.
Water vapor starts to condense on
particles above this level - cloud base
level. This level is called the lifted
condensation level, or in other words,
the cloud base.
2. Rising air expands with less pressure
at altitude. This causes cooling, which
causes relative humidity to rise. Air
continues to rise as long as it is forced
upwards, or if it has a higher
temperature than the surrounding air.
1. Air at ground level is forced to
rise.
CLOUD TOP
CLOUD
CLOUD
BASE
**Read this
slide from the
bottom to the
top**
“Parcel” of Air
Cloud Formation and Stability
The size and shape of clouds depends entirely on the
stability of the air. Stability is the ability of air to resist
rising. An unstable atmosphere produces deep, puffy
clouds. A stable atmosphere produces shallow, flat
clouds.
LIMITED
UPLIFT CAUSES
SHALLOW
CLOUDS
UPLIFT CONTINUES
CLOUD BASE at
CONDENSATION LEVEL
Here, uplift is so strong
that the cloud is very deep,
and rainfall occurs.
WHY AIR RISES (1) – Orographic Lifting
Cloud development stops when
air is no longer forced up.
Condensation
level
SNOWDONIA
Westerly winds blow
humid air onshore from
the Atlantic Ocean
Air sinks down on leeside
of high land, contracts due
to denser air, warms up,
relative humidity falls and
rainfall diminishes.
Orographic Lifting – When
air is lifted by encountering
higher ground and having
no where to go but up.
WHY AIR RISES (2) - CONVECTION
Air meets air equally warm,
stops rising, descends
Solar radiation heats up
ground surface
LCL
Air warms by contact
with warm ground
surface, becomes less
dense and rises
Rising air expands, cools,
relative humidity rises until
condensation of water
vapour occurs above
condensation level.
WHY AIR RISES (3) - FRONTAL
Rising air expands, cools and relative
humidity rises until it reaches 100%
(saturation) at condensation level above
which clouds form as water vapour
condenses on particles.
Warmer, lighter air is
forced to rise by
denser cooler air.
Condensation
level
Cooler, denser air
sinks downwards and
forces lighter, warmer
air to rise
Naming Clouds - Cloud Types
Clouds are named by their height and their shape. The four basic types of
clouds based on shape are cirrus, cumulus, stratus, and nimbus. By
adding a prefix to some of these basic cloud types you can also specify
the height.
Precipitation
 Definition: All solid and liquid forms of water that fall
from clouds
Not all clouds produce precipitaiton
• To produce precipitation, cloud
droplets need to fall
• -Coalescence occurs when cloud
droplets stick together and grow to
form larger droplets
• -Once droplets are too large to be held
in the air by gravity, they fall
producing precipitation
• -Different types of precipitation are
produced by different conditions at cloud
level and the temperature changes on the
way down.
Snow
Sleet
Sleet is
rain that
froze
before it
hit the
ground
Freezing Rain
Regular Rain occurs when
it is above freezing
Freezing
Rain
freezes on
contact and
coats
everything
Hail formation – only in T-Storms
How to Read and Understand the
Weather Map
H=High
Pressure
L=Low
Pressure
Cold Front
Warm Front
Occluded
Front
Lines are
Isobars
Fronts
• When two air masses collide there is a
narrow region separating the two masses
called a front.
• Types of Fronts
1. Cold Front
2. Warm Front
3. Occluded Front
4. Stationary Front
•
Cold
Front
Advancing cold air displaces warm air along a
steep boundary
• Characterized by a thin line of heavy to
moderate precipitaion
• Cold fronts are represented by a blue line with
triangles.
•
Warm
Front
Advancing warm air
displaces cold air in a slow
and gradual slope.
• Characterized by extensive
cloudiness and
precipitation.
– Why?
• Represented by red lines
with half circles.
Stationary
Front
• Blue line with triangles
• When two air masses meet and
neither advances into the other
territory the boundary between
the two masses is said to have
stalled.
– Characterized by small temp. and
pressure difference between the
masses.
and red half circles.
Occluded Front
• When cold air moves
rapidly and overtakes
the warm front.
– The cold air wedges
the warm air upward
and it is squeezed
upward between two
cold fronts.
• Represented by
alternating purple
triangles and half
circles.
High Pressure Systems
Low Pressure Systems
Isobars
Pressure Gradient Force