Outline Ch. 5-Weather
Lesson 1-Describing Weather
I.
II.
What is weather?
a. Weather- atmospheric conditions along with short-term changes, of a certain place at a
certain time.
Weather Variables
a. Variable-a quantity that can change.
b. Air temperature, rainfall, air pressure, wind speed and direction, humidity, cloud
coverage and precipitation aka (rainfall).
c. Meteorologists- scientists who study and predict weather.
d. Air Temperature- Measure of the average kinetic energy of molecules in the air.
i. Kinetic energy- energy an object has due to its motion
ii. If temp. is high molecules have a high kinetic energy, so warm air molecules
move faster than molecules in cold air.
iii. Air temperature varies with time of day, season, location and altitude (height)
e. Air Pressure- column of air exerts on the air or a surface below
i. Air pressure ↑ as height/altitude ↓
ii. Air Pressure ↓ as height/altitude ↑
iii. Measured using a barometer, measured in mb (millibars)
iv. Barometric pressure= air pressure
f. Wind- wind is created from movement of air from high pressure to low pressure
i. Wind speed is measured by an anemometer
ii. Wind direction measured by a wind vane.
g. Humidity- amount of water vapor in the air
i. Measured in grams of water per cubic meter of air g/m^3
ii. High humidity= higher content of water vapor in the air
iii. Ex. High humidity makes sweaty sticky day
h. Relative Humidity-amount of water vapor present in the air compared to the max.
amount of water vapor the air could contain at that temperature.
i. Saturated air is full of water vapor and can’t hold any more
ii. The warmer the air the more water vapor it can contain, than cold/cooler air
iii. Measured using a psychrometer and given as a percentage
iv. Ex. 100% relative humidity=air can’t hold any more moisture (water vapor) it
will rain! 50% half moisture
i. Dew point- temperature at which air is fully saturated because of decreasing
temperatures while holding the amount of moisture constant. Ground level- 0 degrees
Celsius- dew forms below 0 degrees Celsius ice crystals form. 0 degrees C=32 degrees F
30 degrees Celsius= 86 degrees Fahrenheit, higher in the atmosphere clouds form
j. Clouds & Fog- Clouds are water droplets or ice crystals suspended in the atmosphere
i. Clouds can have different shapes and be present at different altitudes
1. Stratus—flat, white, layered, appears gray if heavy alt. up to 2000 m
2. Cumulus- fluffy, heaped or piled up (cotton ball appearance) 20006000m
ii.
1. Cirrus clouds- wispy feather like above 6000m ice crystals
2. Warm air containing water vapor cools as it rises in the atmosphere,
when the cooling air reaches its dew point, water vapor condenses on
small particles in the air and forms droplets.
iii. Fog- suspension of water droplets or ice crystals to/or at Earth’s surface
k. Precipitation- water as liquid or solid form, that falls from atmosphere, rain, snow, sleet
or hail (what determines the precipitation type is temperature)
i. Snow is frozen crystals of water reaching surface
ii. Sleet starts as snow and melts as falling by warm air and refreezes when it
passes below freezing air
iii. Hail- large pellets of ice, starts as small piece and lifted and dropped by an
updraft within a cloud, layer of ice is added with each lifting, when it’s too heavy
to updraft to lift it falls to earth
l. Water Cycle- series of natural processes in which water continually moves among
oceans, land and atmosphere
Lesson 2- Weather Patterns
I.
II.
Pressure Systemsa. High Pressure System-large body of circulating air with high pressure at its center and
lower pressure outside the system.
i. Dense air sinks, it moves away from the center to areas of low pressure.
ii. High pressure systems brings sunny skies and fair weather.
b. Low Pressure System- Large body of circulating air with low pressure at its center and
higher pressure outside the system.
i. Causes air inside the low pressure system to rise, rising air cools and the water
vapor condenses, forming clouds and sometimes precipitation-rain or snow.
Air Masses- large bodies of air that have uniform temperature, humidity, and pressure.
a. Forms when a large high pressure systems lingers over an area for several days, the air
mass takes on the temperature and moisture characteristics of the surface below it.
b. Some air masses can extend for a thousand km or more
c. Classified by their temperature and moisture characteristics.
d. Air masses that form over land are called continental air masses –drier, clear skies, high
temperatures if tropical, polar, fast moving and cold in weather and cool in summer
e. Maritime masses form over Atlantic and Pacific Oceans ( Polar-cool and humid, cloudy,
rainy weather) Tropical- hot, humid air during summer and bring heavy snowfall during
winter.
f. Warm air masses- form in the equatorial regions are called tropical.
g. Cold air masses- polar
h. Near Poles- coldest air masses called artic and Antarctic air masses.
III.
Fronts- boundary between two air masses
a. Drastic weather changes often occur at fronts
b. Changes in temperature, humidity, clouds, wind and precipitation are common at
fronts.
c. Cold fronts- Colder air mass moves toward a warmer air mass, cold front forms
d. Warm fronts- Warm air front forms when lighter warmer air moves toward colder
heavier air.
e. Stationary Front- When the boundary between two air masses stalls.
f.
IV.
Occluded front-Fast moving cold front catches up with a slow-moving warm front, an
occluded or blocked front forms.
Severe Weather
a. Thunderstormsb. Tornadoesi. Formation
ii. Tornado Alley
iii. Classifying Tornadoes
c. Hurricanes
d. Winter Storms
e. Severe Weather Safety