Chapter 3 Water in the atmosphere. 3.1 Introduction Water: only 0 to 4% by volume No water no...

Chapter 3

Water in the atmosphere

3.1 Introduction

Water: only 0 to 4% by volume No water no rainbow No water no thunderstorm No water no life

3.2 Humidity and saturation 3 quantities to measure humidity

Absolute humidity Mixing ratio Relative humidity (discuss this only)

We shall discuss Saturation Relative humidity Dew point

3.2 Saturation and humidity

Saturation

At first, evaporation rate is faster than condensation rate


Finally it is saturated: evaporation rate same as condensation rate

Saturation pressure increases for higher temperature


Higher temperature saturated vapour pressure higher water content in air higher also

3.2 Saturation and humidityRelative humidity

The ratio of the actual water content in the air to the water content in saturated air at the same temperature


Questions?

Higher relative humidity means higher water content?


Dew point is the temperature required to cool a parcel of air to reach saturation

dew point of the flask of air is 10C

Dew point

3.3 Atmospheric stabilityAdiabatic temperature change

at higher altitude, atmospheric pressure lower, the parcel of air is expanded and causes lower its temperature.

temperature of the environment is lower at higher altitude also.

3.3 Atmospheric stability If rising parcel cooler than its

surrounding sink becomes stable; If rising parcel hotter than its

surrounding continue to rise becomes unstable.

3.3 Atmospheric stabilityDry adiabatic rate, wet adiabatic rate, and

lifting condensation level

3.3 Atmospheric stability

As water vapor condenses energy released (called latent heat) air heated gently, so wet adiabatic rate is smaller than dry adiabatic rate.

wet adiabatic rate:5C per 1 km (high moisture content) to 9C per 1 km (low moisture content).


Environmental lapse rate and stability

Stability of air depends on: adiabatic rate; environmental lapse rate

Adiabatic rate: characteristic of parcel

Environmental lapse rate: temperature of environment at various altitudes; about 5C per 1 km

3.3 Atmospheric stability(a) Absolute stability

3.3 Atmospheric stability(b) Absolute instability

3.3 Atmospheric stability(c) Conditional instability


Why is the air more polluted at night?

Examples in daily life

3.3 Atmospheric stabilityExamples in daily life

1. Temperature inversion

Usually occurs at night, if temperature increases with altitude, the so environment lapse rate is negative.

Hence, it is smaller than both wet and dry adiabatic rates.

Result: absolute stable air! The air is trapped!

One evening in Hong Kong


Why are there mid-afternoon rain showers in summer?


2. Mid-afternoon rain showers in summerSome place is hotter, e.g. Million Road

Air above lighter than surroundings

Air rises

Water condenses at lifting condensation level

Clouds form shower

3.4 Orographic lifting and rainshadow deserts

Formation of a desert on the leeward side of a mountain. Why?

Cloud formation

A kind of weather modification Spread silver iodide Supercooled droplet condenses on

silver iodide Big water drops form Raining

Cloud seeding

Cloud Forming Apparatus

Cloud seeding

Supercooled liquid

Must pure water be in the form of ice below 0C?

Answer: NO! Supercooled water Touch solid surface condense,

heat released

Example of supercooled liquid : Heat pack

A pack of colored liquid with a metal button inside.

If the button is bent and release.

The liquid condenses and releases heat.

Example of superheating

不可在微波爐中把水單獨加熱

superheating.mpg

Toy drinking bird

Chapter 3 Water in the atmosphere. 3.1 Introduction Water: only 0 to 4% by volume No water no...

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Transcript of Chapter 3 Water in the atmosphere. 3.1 Introduction Water: only 0 to 4% by volume No water no...