Download Ch7 Changes in Weather Pt2

What Causes Changes in the
Weather ?
• Air Masses / Air Movement
• Fronts
• Pressure Systems
Air Movement
Winds are large horizontal movements of
air near the Earth’s surface.
Breezes are smaller, local horizontal air
movements.
Currents are vertical air movements.
Winds are named for the direction from which they
come. Ex: a southwesterly wind is from the south
west, a sea breeze is blowing onshore from the sea.
Cause of Winds
The primary cause of winds are
differences in air temperature, which
cause differences in air pressure.
Air always moves from high pressure to
low pressure.
The rate of change of air pressure
between two locations is called the
pressure gradient.
Local Breezes
Local breezes are generated by the
unequal heating of the Earth’s surface
due to local conditions.
Examples:
Sea breeze or onshore breeze
Land breeze or offshore breeze
Planetary Convection Cells
Unequal heating of the surface result in
density differences in the atmosphere
which cause vertical air movements
called convection cells.
See diagram
Divergence Zones
In high pressure regions air is
descending and as it hits the surface it
spreads out.
These areas are known as zones of
divergence.
Divergent zones are dry.
Convergence Zones
Air from divergent zones comes together,
or converges, in the regions of low
pressure, where it rises.
These areas are known as zones of
convergence.
Convergent zones are moist (wet).
Planetary Winds
The rotation of the Earth and the unequal
heating of the surface result in a series
of wind belts known as the planetary
wind system.
Note: without rotation the winds would blow
from the poles (high pressure) directly to the
equator (low pressure).
What are Jet Streams?
Jet streams are high-speed, high altitude winds
that have a controlling influence over the
direction traveled by air masses.
These flow in a wavelike motion, instead of a
straight line, and are part of the planetary
convection cells.
The midlatitude jet stream is called the polar jet
stream and has a major role in the weather of
the midlatitudes.
Clouds
Condensation is the process by which
gaseous water vapor changes to liquid
water.
In order for condensation to take place a
surface must be available, however small.
This surface is called the condensation
nuclei.
Water vapor changing directly to a solid is
called sublimation. (occurs below 0oC)
Dew and Frost
When water vapor condenses directly
onto a cold surface, dew is formed.
If water vapor comes in contact with a
freezing surface, the vapor will
sublimate, and frost will form.
How Do Clouds Form?
• Clouds form as humid air is cooled to the
dew point.
• Condensing water vapor form tiny drops
around dust particles in the atmosphere;
these tiny drops are called droplets – they
are so small that the slightest air movement
keeps them from falling to the surface of the
Earth.
• Millions of droplets together form a cloud.
• As droplets move the bump and collide into each other
combining to form bigger droplets. Once the droplets
reach between 0.05mm and 0.5mm they fall in the form of
precipitation.
Classifying Clouds
Clouds are divided into four basic groups:
High - Cirro
Middle - Alto
Low - Strato
Vertical
The word nimbus means “rain”, so clouds that
produce rain usually have the term nimbus in
their name.
Ex. Nimbostratus=low rain cloud
A cloud resting on the Earth’s surface is called
fog.
Types of Clouds
• Cirrus Clouds (high) = very high, white,
thin, whispy clouds that contain ice.
• Cumulus Clouds (heap) = large puffy,
individual clouds that look like balls of
cotton, usually fair weather clouds.
• Stratus Clouds (low) = near the ground,
gray, layers or sheet clouds that usually
bring precipitation.
What is Smog?
• Smog is a combination of aerosols and
water vapor in stagnant air.
• Smog usually occurs during a temperature
inversion when warmer, less dense air is
above a layer of colder, denser air.
Precipitation
Precipitation occurs when water droplets
or ice crystals in a cloud grow large
enough to fall.
Rain is large water drops that fall.
Drizzle is fine water drops that float down
gently.
Sleet is rain that freezes as it falls.
Snow is ice crystals that fall.
Hail is layered ice that falls.
Moisture and Energy Transfer
An adiabatic temperature change is any
change in the temperature of a system
without any heat being added or removed.
This usually involves changes in pressure.
For example as pressure increases and air
is compressed its temperature increases.
Dry adiabatic lapse rate
Moist adiabatic lapse rate