Earth Science
Chapter 16 Notes
Section 16.1 – What is Weather?
Weather – State of the atmosphere at a specific time and place
Examples of Weather Conditions
1) Air Pressure
2) Wind
3) Temperature
4) Amount of Moisture in the Air
Temperature – Measure of the average amount of motion of
molecules
High Temperature – fast moving molecules
Low Temperature – slow moving molecules
Wind – Air moving in a specific direction.
Wind occurs because air moves from regions of high pressure to
regions of low pressure
Wind Vane – measures wind direction
Anemometer – measures wind speed
Humidity – the amount of water vapor in the air
More water vapor can be present when the air is warm because
the molecules are farther apart.
Condensation – Forming liquid water from water vapor
The air becomes saturated when condensation takes place.
Relative Humidity – Measure of the amount of water vapor present
compared to the amount needed for saturation at that specific
temperature
Dew Point – the temperature at which air is saturated and
condensation forms.
Dew point changes with the amount of water vapor in the air.
When temperatures are near 0℉ - frost forms
Forming Clouds – clouds form when air is forced up, expands and
cools.
Classifying Clouds
Clouds are classified by:
1) Shape
2) Height
3 Main Types of Clouds
1) Stratus
2) Cumulus
3) Cirrus
Stratus
- Form layers or smooth even sheets
- Fog is a stratus cloud that has cooled to its dew point
- Form at low altitudes
- Occur during fair weather, rain or snow
Cumulus
- White puffy clouds with flat bases
- Fair weather or thunderstorms
Cirrus
- Fibrous or curly
- High, thin, white feathery clouds
- Fair weather – can indicate approaching storms
Height Prefixes
- Cirro – High clouds – Example - cirrostratus
- Alto – Middle clouds – Example – altostratus
- Strato – low clouds
Nimbus – Dark rain cloud – Water content is so high that light can’t
pass through them
Cumulonimbus – Thunderstorm cloud
Nimbostratus – Layered clouds that bring steady rain or snow
Precipitation – Water falling out of clouds – Occurs when cloud
droplets grow large enough to fall from Earth
Size of Raindrops Depend on:
1) Updrafts in a cloud
2) Rate of evaporation
4 Main Types of Precipitation
1) Rain – drops of water falling in temps above freezing
2) Snow – drops of water falling in temps below freezing
3) Sleet – Raindrops pass through a layer of freezing air
4) Hail – forms in cumulonimbus clouds