What Causes Changes in the Weather

1
What Causes Changes in the Weather ?

• Air Masses / Air Movement
• Fronts
• Pressure Systems

2
Air Movement

• Winds are large horizontal movements of air near
  the Earths 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.

3
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.

4
Local Breezes

• Local breezes are generated by the unequal
  heating of the Earths surface due to local
  conditions.
• Examples
• Sea breeze or onshore breeze
• Land breeze or offshore breeze

5
(No Transcript)
6
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

7
(No Transcript)
8
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.

9
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).

10
(No Transcript)
11
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).

12
(No Transcript)
13
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.

14
(No Transcript)
15
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)

16
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.

17
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.

18
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. Nimbostratuslow rain cloud
• A cloud resting on the Earths surface is called
  fog.

19
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.

20
(No Transcript)
21
(No Transcript)
22
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.

23
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.

24
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

25
(No Transcript)