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 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.

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 Earth’s surface due to local conditions. Examples: Sea breeze or onshore breeze Land breeze or offshore breeze

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

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 aszones of convergence. Convergent zones are moist (wet).

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).

13. What are Jet Streams? Jet streamsare 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.

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. Nimbostratus=low rain cloud A cloud resting on the Earth’s 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.

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. Dryadiabaticlapse rate Moistadiabaticlapse rate