Download RH = 100% - Montville.net

Cloud Formation: Lifting Processes
Atmospheric Lifting
• In order for air to form clouds, the air must be
lifted and rise in altitude
• There are 4 types of Atmospheric Lifting
– Orographic
– Convective
– Convergent
– Frontal
Orographic Lifting
• Air is forced over a barrier
such as a mountain
– Windward Side: wet and cold
– Leeward side: dry and hot
• Causes a “rain shadow”
– dry area on the leeward side
of a mountainous area (away
from the wind). The
mountains block the passage
of rain-producing weather
systems and cast a "shadow"
of dryness behind them.
• i.e. Sierra Nevada, Rocky
Mountains, Cascades
Convective Lifting
• Air lifting due to local
surface heating
– The surface is heated
from the Sun’s radiation
causing the air to warm
and rise
– Causes rain and
thunderstorms
• i.e.- Hadley Cell along
the equator
Convergent Lifting
• Air flows towards areas
of low pressure, forces
air to rise
• Causes thunderstorms
• i.e.- Florida
Frontal Lifting
• Air lifting occurs along
boundaries of different
air masses
– Cold Fronts
– Warm Fronts
• Cool air is denser than
warm air
• cool air sinks down and
warm air rises
– Create different types of
weather
Stable vs. Unstable
AIR
• Air stability is the measure of its
temperature at given heights or
altitudes.
• Cool air is denser so it wants to
sink and warm air wants to rise
to create stable conditions
• Stable air does not rise
• Unstable air will rise freely to
create stability
• Meteorologists focus on unstable
air
– produce the weather systems due
to movement of air and water
vapor.
• Unstable air is associated with
various ways of lifting air as
discussed in the previous slide.
Stable vs. Unstable AIR
Humidity and Saturation
• Humidity is a general term for
the amount of water vapor in
air.
• Relative humidity (RH) =
water vapor in air as a ratio
expressed as %
– Example: 32% or 100%
• RH = 100% = clouds form
= condensation level
(dew point)
• Air is saturated when it
contains the maximum
quantity of water vapor that it
can hold at any given
temperature and pressure.
• When saturated, warm air
contains more water vapor
than cold saturated air.
ELR – Environmental Lapse Rate
• Lapse Rate = speed of temperature
decrease with altitude through the
atmosphere (in this case the troposphere)
Adiabatic Processes
• process where NO HEAT is added or removed to
the system
• As altitude increases in the troposphere,
temperature naturally decreases
• Air is denser near surface. As the air rises, it
cools as it expands
• The rate at which it cools is called the Adiabatic
Lapse Rate
– Two Types
Dry Adiabatic Cooling
Saturated (Wet) Adiabatic Cooling
Adiabatic Processes
Dry Adiabatic Cooling:
• Does not contain water
vapor
• Air rises and expands as
temperature decreases in
troposphere
Saturated (Wet) Adiabatic
Cooling:
- Occurs when Relative
Humidity = 100%
- Occurs when water vapor
forms a cloud
LCL & Dew Point
• LCL = the lifted condensation level is the height at which the
relative humidity (RH) of an air parcel will reach 100% when it is
cooled by dry adiabatic lifting.
• Condensation and cloud formation occur at the LCL
• Air rises and contains water vapor
• Through adiabatic cooling, the temperature and pressure decrease.
This causes the air parcel to expand as it cools as it rises.
• At a certain altitude in the troposphere the rising air condenses.
This is called the ‘lifting condensation level’.
• The Dew Point is when the temperature of water vapor condenses
into water droplets and forms a cloud
• This is why a cloud appears to be flat at the base