1. WEATHER
PSC 1515
Chapter 10

2. Weather
Defined as the state of the atmosphere at a particular time and place.
Climate is very different – it refers to world wide and long term weather
Made up of six elements of the atmosphere
Atmospheric pressure
Temperature
Wind
Precipitation
Cloudiness
Humidity

3. Atmosphere That sphere of the planet that lies above the ground/water.
Contains water droplets and small particulates like pollen, dust (aerosols)
Currently composed of 78% nitrogen, 21% oxygen, 0.9% argon
Carbon dioxide, ozone, methane and other gases comprise about 0.1% total
This composition has changed since the beginning of the planet

4. Benefits of an Atmosphere
Carry the gases we need to live
Oxygen and carbon dioxide
Nitrogen
Protects organisms from sun’s dangerous rays
Especially ozone
Provides insulation that traps heat and keeps temperatures in habitable range

5. Air Pressure Atmosphere nearly 100 miles thick
One atmosphere = the force of a one square inch column of air from sea level to the upper edge of the atmosphere
Force per unit area – pounds per square inch is PSI
Other units – mm Hg, millibars, torrs, pascals, atmospheres (atm)
Decreases as one increases in altitude
Makes boiling points lower at higher altitudes

6. Structure of the Atmosphere
From bottom to top
Troposphere
Stratosphere
Mesosphere
Thermosphere
Ionosphere

7. Each Layer Troposphere Stratosphere
Closest to surface, about 8 miles thick
Contains almost all the water and suspended particulates – form clouds
Layer where weather occurs
Temperature decreases about 60C per kilometer
Stratosphere
From about 12 to 50 km up
Thicker than troposphere but less dense
Ozone layer – temperature rises in this layer

8. Layers continued Mesosphere Thermosphere Ionosphere
Above stratosphere to about 80 km (30 km thick)
Gets colder as altitude decreases
Thermosphere
Air that is there absorbs UV radiation
Temperature increases with altitude
Ionosphere
Not really a layer
Contains ions caused by sun’s radiation
Layer of aurora

9. Temperature vs Altitude

10. Solar Radiation and the Greenhouse Effect
The Earth is heated by radiation from the sun.
Visible light readily passes through the atmosphere and warms the Earth
When the Earth gets warm it gives off heat but in the infrared region
The infrared heat cannot pass through the atmosphere because of certain gases and particulates
This causes the atmosphere to warm even more

11. Solar Radiation in a Greenhouse

12. Temperature as a function of latitude
Latitude – zero at the equator and 90 at the poles
Tropic of Cancer – highest point the sun gets
Tropic of Capricorn – the lowest point the sun gets
Major zones
Tropical – about 300 above and below equator
Temperate - from 30 to 600 latitude
Polar – from 600 to the poles

13. Latitude and Temperature

14. Graph Temp vs Latitude

15. Temperature vs Latitude

16. Tilt affects where sun hits

17. The seasons

18. The Seasons
Terms –
Solstices (2) and Equinoxes (2)
Tropic of Cancer and Tropic of Capricorn
Solstices – highest and lowest points on planet where sun’s rays hit directly
Occur in June, Tropic of Cancer – longest day
and December, Tropic of Capricorn – shortest day
Equinoxes (2) when sun crosses the equator
Occur in March and September – equal day and night

19. Winds
Air flowing horizontally from an area of high pressure to one of low pressure
Named for the direction that they came from not where they’re going.
Can be local – ex land and sea breezes
or Global – blow consistently from same direction

20. Local Winds
Caused by differences in geography and thus temperatures. – ex land/water or mountain/valley
Sea Breeze – during day land warms faster than water. Warm air rises creating low pressure. Air from over water blows in from ocean to land
Land Breeze – reverse – Land cools faster, causes high pressure, blows out to water

21. Global Winds Caused by unequal heating of the Earth by the sun.
Alternating patterns of easterly and westerly blowing winds (name from direction from which they come)
Divided into six zones
Trade winds (north east) and (south east)
Westerlies (northern and southern)
Polar Easterlies

22. Earth’s Global Winds

23. Coriolis Effect
Air flow deflected to west because the planet rotates west to east. Looks opposite in southern and northern hemispheres but it’s really the same.
Responsible for fact that water drains counter clockwise in the northern hemisphere and clockwise in the southern

24. Water in the Atmosphere
Humidity
Amount of water vapor in the air
Amount that the air can hold increases with increasing temperature
Relative humidity – amount of water vapor present divided by the maximum capacity at that temperature
Relative humidity changes very much with temp.
Dew Point – temperature at which water in air will condense – 100% humidity

25. Dew, frost, fog Dew – water vapor that condenses on plants, cars, etc.
Frost – frozen dew or when water vapor condenses below the freezing point
Fog – when a large mass of air cools to below its dew point close to the ground – like clouds only close to the ground.

26. Clouds
Water vapor condenses on particles in the upper atmosphere to form clouds
Kinds of clouds
By shape
Cirrus (curl), cumulus (piled up), stratus (spread out)
By altitude
High clouds – prefix cirro –
Middle clouds – alto –
Low clouds – strato -

27. Clouds

28. Summary Clouds

29. Precipitation
Rain – when condensed water vapor gets too heavy and it falls
Hail – forms when ice crystals start to fall and are then pushed back up where they gather more ice and then they fall and are pushed back up multiple times
Sleet – Frozen rain – slush – mixture of water and ice crystals
Snow – 6 sided crystal of water.

30. Changing Weather
Air Masses – large pools of air that has the same temperature and humidity profile throughout. Take on the characteristics of the area over which they form and are moved by the winds
Fronts –
(warm air moves away from equator toward poles, cold air does the opposite)
When these opposite air masses collide they form a front

31. Fronts Cold – cold air moves into warm air
Pushes warm air up, it becomes saturated, forms high clouds and rain
Warm – warm air moves over cold air
Warm air sits on top of cold (less dense)
After front passes, air is warmer
Stationary – neither air mass is moving
Occluded – warm caught between two cold air masses

32. Cyclones
Any of a series of low pressure systems associated with counter clockwise motion of the air around a low pressure center.
High pressure – clockwise
Low pressure – counterclockwise
H with C
L with CC

33. Cyclones
Air masses with a low pressure center and winds that move counter clockwise around the center.
Hurricanes in Atlantic – Cyclones in Pacific
Tornadoes
Discussed more in next chapter

34. Anti-cyclones
High Pressure Areas