1. Weather Maps & Fronts
Refers to the state of the atmosphere at a specific time and place.
Influenced by pressure systems (high and low) and fronts.
Describes conditions such as air pressure, wind, temperature,
and the amount of moisture in the air.
Is caused by heat energy from the sun.

2. Changes in weather result from movement of air masses.
Weather Patterns
Air mass
Large body of air that has the properties similar to the part of Earth’s surface over which it develops.
Changes in weather result from movement of air masses.

4. Low Pressure System When air is heated, it becomes less dense
Therefore, it rises, leaving behind an area where there is less air.
Called a low pressure area
Rising air carries moisture with it. As a result, low pressure is usually associated with clouds and rain.
Indicated by a L on a weather map.
Troughs of low pressure are important because they sometimes lead to the development of a low pressure system. Troughs are indicated by lines (sometimes purple, black or green) with no triangles or semicircles.

5. High Pressure System As air cools, it becomes more dense
It falls, creating an area where there is more air
Called a high pressure area
Very little moisture in this air, so usually associated with clear, sunny days.
Indicated by a H on a weather map.

6. Isobars Isotherms (iso – same, therm – heat)
Lines that connect points of equal atmospheric pressure.
Isobars close together mean high wind.
Isobars far apart mean light wind.
Isotherms (iso – same, therm – heat)
Lines that connect locations of equal temperature.

7. Front
A boundary between two air masses of different density, moisture or temperature
Cloudiness, precipitation and storms may occur at frontal boundaries
Four types of fronts: cold, warm, occluded and stationary

8. Cold fronts Cold front – shown on a map as a blue line with triangles
Occurs when colder air advances toward warm air.
Cold air wedges under the warm air
This lifts the warm air, cools it and clouds form.
If the temperature difference is large, thunderstorms and tornadoes may form

10. Warm fronts
Warm fronts form when lighter, warmer air advances over heavier, colder air.
Shown as a red line with red semicircles.

11. Occluded fronts
Involves three air masses of different temperatures – cold, cool and warm
Shown on maps as purple lines with triangles and semicircles.
Colder air forces the warm air upward, closing off the warm air from the surface.

12. Occluded Front

13. Stationary Front
Occurs when a boundary between air masses stops advancing.
May remain in the same place for several days, producing light wind and precipitation.
Shown on map as alternating red and blue line. Red semicircles point toward the cold air and blue triangles point toward the warm air.

14. Stationary Front

15. Severe Weather
Sinking rain - cooled air and strong updrafts of warmer air cause the strong winds associated with thunderstorms.
Hail also may form as ice crystals alternately fall to warmer layers and are lifted into colder layers
May cause flooding, flash floods, strong winds

16. Thunderstorms – occur in warm moist air masses and along fronts.
Winds stronger than 89km/hour – severe thunderstorm.

17. Severe Weather
Lightning - Inside a cloud, liquid and ice particles collide causing the buildup of an electrical charge. When the charge becomes large enough, it creates a “spark” that allows the electrons to move. The current flows between regions of opposite electrical charge. Lightning can occur within a cloud, between clouds or between the cloud and the ground.

19. Thunder
Results from the rapid heating of air around a bolt of lightning.
The extreme heat of lightning causes the air around the lightning to expand rapidly. Then it cools quickly and contracts.
The rapid movement of the molecules form sound waves

20. Tornado
A violent, whirling wind that moves in a narrow path over land.
Wind moving in different directions (shearing) creates a rotating column
Ranked on the Fujita scale according to how much damage occurs F0, F1, F2, F3, F4, F5

22. Severe Weather Hurricane – the most powerful storm
Large swirling low pressure system that forms over the warm Atlantic ocean
Must have winds of at least 119 km/hour
Called typhoons in the Pacific and cyclones in the Indian Ocean.
Lose power when they reach land.
The Saffir-Simpson Hurricane Wind Scale is a category 1 to 5 rating based on wind speed.

23. Hurricane
Hurricane

24. These conditions must persist for more than 3 hours.
Severe Weather
Blizzards – winter storm with winds over 56 km/h, low temperature, visibility less than 400m, with falling or blowing snow.
These conditions must persist for more than 3 hours.
10 min

25. Severe Weather Safety
Weather Advisories - are sort of in between a WATCH and WARNING. The expected weather condition has a pretty good chance of occurring, even a likely chance of occurring, but typically an advisory is used for “less” severe type of weather conditions.
Severe weather watch – when conditions are favorable for severe weather.
Severe weather warning – severe weather already exists.