Humidity

Under what conditions do you see the above?

Humidity

• Measure of the quantity of water vapor in the air

• Overall atmosphere is 78% nitrogen, 21% oxygen, and only 1% water vapor.

Idealized representation of the atm: a “parcel” of air

Measuring relative humidity

• For a parcel of air, RH is the ratio of the amount of actual water vapor in the air (AMR) divided by the maximum amount of water vapor the air can hold at the given temp (SMR)

A plot of saturated mixing ratio (left) shows that as temperature increases, the maximum amount of water the air can hold increases. Warm air can hold more water vapor than cold air

The curve indicates where RH = 100%, or where the actual amount of water vapor in the air is at the maximum for that given temperature.

Temperature along the line is called the dew point temperature.

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Air temperature

When RH = 100%:AMR = SMR Air is saturated with water vaporAir temp = dew pointWater condensates out of airSuspended liquid water droplets form in the air

To reach RH = 100%Add more water vapor to the airCool the air down to its dew point temperatureLower the air pressure of the parcel

What are our diurnal patterns of humidity?

When are our maximum and minimum relative humidities? [Lexington weather conditions]

Heat (or comfort) index: the human element in humidity

When RH is high, air is near dew point and holds near maximum amount of water vapor

Inefficient evaporative cooling for humans

Temp experienced by humans is higher than air temp

Risk of heat exhaustion or stroke

Explain why and where in their homes people use these appliances and how they work

Humidifier Dehumidifier

Where would you expect to use an evaporative cooling system (also

known as a swamp cooler?

Measuring humidity• Automated sensors most

commonly used today, but basic principle of sling psychrometer (right) still applies

• A sling psychrometer measures difference between the dry bulb temperature and the wet bulb temperature

Dew

• High humidities during the day

• Clear night with radiative cooling

• Air temp cools to dew point

• Condensation forms on surface (dew)

Frost• Radiation frost

– Occurs on days with high humidities and clear calm nights

– Dew point temperature goes below freezing

• Advection frost– More common on cloudy, windy nights with strong cold

air advection – Dew point temps go below freezing

Clouds:Air temp cooled to dew pointLiquid water condensates out of water vaporRH = 100%

But most of the water vapor is at the surface. A mechanism is needed to get water vapor to cool, condensate and form clouds. What might that mechanism be?

Large amount of water vapor in red. The y-axis isdenotes altitude above the surface.

LIFTING: If you can get the surface air to rise it will cool, reach dew point, and form clouds

Large amount of water vapor in red. The y-axis isdenotes altitude above the surface.

Surface

Cooler aloft

Sources of lifting

The secret of weather prediction is knowing whether or not air is rising, or if it is moving downward.

An atmosphere with a propensity for rising air is unstable. This air may cool to form clouds, and potentially, rainfall.

Stable denotes an atmosphere with air that is not rising or being lifted. It is not likely to form clouds or rainfall.

Adiabatic processes – changing temperature by changing air pressure

• Adiabatic warming • Adiabatic cooling

Atmospheric stability

• Stability is a measure of the probability of cloud formation

The troposphere is the layer of the atmosphere in which most of our relevant weather processes take place.

Skew-t plot

Red line shows air temp and blue line shows dew point temp

At what pressure level are clouds likely toform ?

Adiabatic processes

• Changing the temperature of a parcel of air by changing air pressure – Adiabatic cooling: parcel is lifted, surrounding air

pressure decreases, and air parcel expands, parcel temp decreases

– Adiabatic warming: parcel descends, surrounding air pressure increases, parcel contracts, parcel temp increases

Lapse rates

A lapse rate is a rate of temperature decrease per change in altitude. Three relevant lapse rates for the basics of understanding stability:

ELR – environmental lapse rate;applies to environment surrounding parcel

DALR (dry adiabatic lapse rate) and SALR (saturated adiabatic lapse rate); apply to parcel

Lifting condensation level (LCL) – where parcel DALR switches to SALR

Stability determined by comparing lapse rates

• Lapse rate – rate of change of temperature

• The steeper the slope of a lapse rate line, the slower the rate of cooling

• Levels of atmospheric stability: stable, unstable, conditional instability

• Level of stability determined by looking at all three lapse rates.

Stable atmospheric conditions

Clear skies; any cloud development is high and lacking a strong vertical dimension. Vertical motions resisted.

A thunderstorm, a hallmark of unstable atmosphericConditions. Atmosphere promotes vertical motion.

Unstable atmospheric conditions

Conditional instability

• Conditional instability is the most common state of the atmosphere.

• A lift is needed to make a stable parcel go unstable.

• What are the sources of lifting?

Sources of lifting

Convectional surface heating

Wind convergence and surface heating (along theITCZ)

Orographic lifting

Rainshadow effect, a product of orographic lifting

Rainshadow deserts

• North American rainshadow deserts: Death Valley (Sierra Nevada), eastern Washington and eastern Oregon (Cascades)

Orographic lifting

Orographic lifting

Frontal lifting