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

3.2 Saturation and humidity Finally it is saturated : evaporation rate same as condensation rate Saturation pressure increases for higher temperature

3.2 Saturation and humidity  Higher temperature  saturated vapour pressure higher  water content in air higher also

3.2 Saturation and humidity Relative humidity The ratio of the actual water content in the air to the water content in saturated air at the same temperature

3.2 Saturation and humidity Relative humidity Questions? Higher relative humidity means higher water content?

3.2 Saturation and humidity Relative humidity

3.2 Saturation and humidity Dew point is the temperature required to cool a parcel of air to reach saturation dew point of the flask of air is 10  C Dew point

3.3 Atmospheric stability Adiabatic 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 stability Dry 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: 5  C per 1 km (high moisture content) to 9  C per 1 km (low moisture content).

3.3 Atmospheric stability 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 5  C per 1 km

3.3 Atmospheric stability (a) Absolute stability

3.3 Atmospheric stability (b) Absolute instability

3.3 Atmospheric stability (c) Conditional instability

3.3 Atmospheric stability Why is the air more polluted at night? Examples in daily life

3.3 Atmospheric stability Examples 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!

3.3 Atmospheric stability Examples in daily life

3.3 Atmospheric stability Examples in daily life

One evening in Hong Kong

3.3 Atmospheric stability Examples in daily life Why are there mid-afternoon rain showers in summer?

3.3 Atmospheric stability Examples in daily life 2. Mid-afternoon rain showers in summer  Some 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 0  C? 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