The weather instrument here is used to determine

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% 22 o. C 14 o. C

Why do clouds form? Because the actual temperature equals the dewpoint!

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

If temperature drops to dewpoint, air is 100% humid and clouds may form. This happens when air expands Air expands when it rises in the atmosphere to lower pressures. !

• 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 10 o. C 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)

3 STEPS TO CLOUD FORMATION Air rises and expands Air cools to DEWPOINT Condensation nuclei present Precipitation

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 rea ches the dewp oint!

Air expands, cools, and reaches the dewpoint when it rises. So… Why does air rise?

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.

1. Air may rise on its own when it becomes low pressure. Low Pressure: warm, humid. . .

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 The side facing the wind is WINDWARD. Rising, expanding, cooling air is wet. Warm & Dry 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

3. Less dense air may be forced to rise if it “bumps” into more dense air. This occurs at a WEATHER FRONT.

Clouds are usually associated with weather fronts. more dense less dense

The symbol indicates 1) Rain 2) Cold Front 3) Warm Front 4) Snow Which weather condition is shown along the cold front? (1) fog (2) Haze The storm symbols probably result from (3) snow (1) Decreasing atmospheric moisture (4) thunderstorms (2) Increasing humidity (3) Higher atmospheric pressure