The weather instrument here is used to determine dewpoint and relative humidity.

Based on the temperatures shown, the approximate

dewpoint temperature and relative humidity are

(1) –19°C and 4%

(2) –5°C and 25%

(3) 8°C and 40%

(4) 12°C and 53%

22oC

14oC

Why do clouds form?

Because the actual

temperature equals the dewpoint!

If temperature drops to dewpoint, air is 100% humid and clouds may form.

LessHumid More

Humid

LessHumid

MoreHumid

LessHumid

100%!

This happens when air expands!

Air expands when it rises in the

atmosphere to lower pressures.

If temperature drops to dewpoint, air is 100% humid and clouds may form.

• Air molecules are always moving with their heat energy.

• As they spread apart at lower pressures they spread their heat energy. (“they do work on the surrounding space”)

• The air gets colder!

Temperature drops about 10oC every

1000 m.

As air rises and cools it

approaches the DEWPOINT.

As air approaches the dewpoint

humidity increases.

Condensation (cloud formation)

may occur.

Water droplets can only condense ON something (like ON

your bathroom mirror).

Dust in atmosphere act as the surface

(condensation nuclei)

Precipitation

Air rises and expands

Air cools to DEWPOINT

Condensation nuclei present

3 STEPS TO CLOUD FORMATION

Why do clouds often form near the tops of

mountains?

1) Air rises and warms up until evaporation occurs

2) Air rises and cools down until evaporation occurs

3) Air rises and warms up until condensation occurs

4) Air rises and cools down until condensation occurs

It reaches the dewpoint!

Why does air rise?

Air expands, cools, and reaches the dewpoint when it rises. So…

THREE BASIC REASONS:

1. It is “naturally” less dense (hot, wet).

2. It is forced up by an obstacle.

3. It is forced up by more dense air.

Air may rise on its own when it becomes low pressure.

Low Pressure: warm, humid...

1.

This can lead to afternoon thunderstorms.

Which sequence of events affecting moist air within Earth’s atmosphere causes cloud

formation?

(1) rising → expanding → cooling → condensation

(2) rising → contracting → warming → evaporation

(3) sinking → expanding → warming → condensation

(4) sinking → contracting → cooling → evaporation

2. Air may be forced to rise if wind strikes a large obstacle like a

mountain.

This can even happen at very tall buildings!

Burj Dubai Tower, Dubai, UAE

Taipei 101, Taipei, Taiwan

Cool & Wet

Warm & Dry

The side facing the wind is

WINDWARD. Rising,

expanding, cooling air is

wet.

The other side is LEEWARD.

Sinking, compressing, warming air is

dry.

The result is a “rain shadow” caused by mountain ranges.

The result is a “rain shadow” caused by mountain ranges.

The Atacama desert of Chile in South

America is the driest place on Earth.

The cross section below shows the direction of air flowing over a mountain. Points A and B are at the same elevation on opposite sides of the mountain.

Compared to the air temperature and humidity at point A, the air temperature and humidity at point B are usually

(1) cooler and drier (3) warmer and

drier

(2) cooler and wetter (4) warmer and

wetter

Less dense air may be forced to rise if it “bumps” into more dense

air.

3.

This occurs at a WEATHER FRONT.

more dense less dense

Clouds are usually associated with weather fronts.

Which weather condition is shown along the cold front?

(1)fog

(2) Haze

(3) snow

(4) thunderstorms

The symbol indicates

1) Rain

2) Cold Front

3) Warm Front

4) Snow

The storm symbols probably result from

(1) Decreasing atmospheric moisture

(2) Increasing humidity

(3) Higher atmospheric pressure