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Measures of Humidity

Climate �feedbacks�

We talked briefly about the positive feedback processes of climate change in previous lectures. What is “feedback”?

Feedback is a concept that explains the interaction of the climate system that alters changes in climate. When the rate of climate change is amplified (either by warming or cooling), the process is called “positive feedback”. The upper figure demonstrates the basic way that these feedbacks operate.

On the other hand, when the rate of climate change is suppressed, then the process is called “negative feedback” (lower figure).

Primary Climate System Feedbacks • Radiation feedback (hotter planet radiates

more energy out to space, E=sT4)

• Snow/ice-albedo feedback

• Water Vapor feedback

• Cloud feedback (high versus low clouds)

So, climate feedbacks are a loop of cause and effect; positive (amplifier) and negative feedbacks (stabilizer). Some feedback processes are more complicated than others. Here are a few important feedbacks that affect our climate system.

Temperatureà radiation feedback Energy emitted = σT4


éradiation to space



The temperature of the Earth is increasing due to a rise in greenhouse gases in the atmosphere. Thus, how will the climate feedback system change with this temperature increase?

First, increases in temperature will alter radiation feedback because the energy emitted from a blackbody is proportionate to its temperature to the fourth (σT4). Feedback process: Increasing CO2 concentration in the atmosphere – increasing temperature – increasing associated energy radiation to space – decreasing temperature

Thus, increasing CO2 is a negative feedback process in the long term. However, this feedback process in the climate system is far more complex. This is not the only feedback loop that we know of.

Snow/sea ice albedo feedback

Melting of snow/sea ice directly affects the albedo of the Earth (less ice = decrease in albedo)

Measuring Earth’s Albedo ?id=84499

Also, we have seen how recent warming has been impacting the arctic sea ice (see the following two slides)

Polar amplification!

Global temperature departures from average during January through May 2020, compared with a 1951-1980 average. (Berkeley Earth).

Greater climate change observed near the pole responds to changes in the radiation balance (e.g. intensified greenhouse effect). This phenomenon is known as “polar amplification”.

Melting sea ice in the Arctic decreases the Earth’s albedo. Changes in albedo are likely contributing to significant temperature increases in the northern hemisphere. The increase in surface temperature is observed mainly in the higher latitude in the northern hemisphere, where most sea ice is, and where there is a greater continental distribution (more continent is located in the northern hemisphere than in the southern hemisphere. continent heat capacity is lower than the water body – ocean).

Polar Amplification

“Over the past 100 years, it is possible (33-66% confidence) that there has been polar amplification, however, over the past 50 years it is probable (66-90% confidence)” [The Arctic Climate Impacts Assessment (ACIA), 2005, p22]

Although polar amplification has been a known phenomenon for over 100 years, such amplification has been more and more prominent in the recent past.

Further reading: Polar amplification effect amplification-effect

IPCC AR5 report about polar regions ( 5_hezel_sbsta40_short.pdf)

Snow/sea ice albedo feedback


êsnow and ice


Melting of snow/sea ice directly affects the albedo of the Earth (less ice = decrease in albedo).

Feedback process: Increasing CO2 concentration –> increasing temperature –> melting snow/sea ice –> decreasing albedo –> less energy reflected to the space –> further increasing temperature

Water vapor feedback

Clausius-Clapeyron relationship

Warm air holds more water vapor!

NASA: Sea Surface Temperature vs Water Vapor urselinks/fall16/atmo336/lectures/sec1/ev ap_cond.html

Clausius-Clapayron relationship is a way of characterizing discontinuous phase transition between two phases of a matter of a single constituent. This concept explains the relationship between the temperature and water vapor, which is by far the most concerning greenhouse gas in Earth’s atmosphere. This figure shows how the water-holding capacity of the atmosphere (water vapor pressure) increases by 8% per temperature increase in Celsius. Importantly, this relationship is mainly a function of temperature, and not directly dependent on other parameters like pressure or density.

What does this figure tell us? “warm air hold more water vapor!”

Measures of Humidity



The same concept can be explained by “relative humidity” and “water vapor capacity”. • Vapor pressure – contribution of water vapor to total atmospheric pressure • Humidity – amount of water vapor in the air

Imagine you have a balloon that is perfectly sealed. No air or water vapor goes in or out of this balloon. In this figure, “water vapor capacity (red solid line)” indicates that your balloon is “saturated (= relatively humidity 100%)” at the temperature and the amount of water vapor that exists in the balloon. Now, your balloon is first saturated at 16 degree Celsius or 60 degree Fahrenheit with 10g of water vapor per cubic meter. How can you change the saturation status? It is easy – you just need to change its temperature! If you heat up your balloon, for instance, to 100F, your balloon will no longer be saturated (a). Instead, to saturate this warm balloon (100F to be exact), you need 4 times more water vapor (b)!

Measures of Humidity • Relative humidity – how close the air is to saturation – Saturation represents the maximum amount of water

vapor the air can hold – Saturation depends on temperature – Saturation vapor pressure

In this figure, water vapor capacity is depicted in the yellow circle. The amount of water vapor in the atmosphere does not change regardless of the temperature (blue circle). Instead, water vapor capacity increases with increasing temperature. Therefore, relative humidity decreases when you increase the temperature.

• Temperature and relative humidity are inversely related

Measures of Humidity

This relationship is rather obvious if you plot the typical hourly temperature with the relative humidity for 24 hours. Temperature increases in the morning at around 8 am. You observe the highest temperature of the day in the afternoon. The temperature decreases when the sun sets. As you can see, relative humidity is almost a mirror image of temperature. The relative humidity is highest in the early morning when the temperature is the lowest, and is at its minimum when the temperature is at the highest of the day.

• Temperature and relative humidity are inversely related

• Dew point temperature

Measures of Humidity

You must have seen dew in the grass or a windshield in the early morning, when the temperature is lowest. This is because the air becomes saturated and the excess amount of water is condensed to form moisture! When the temperature is close to freezing level, the dew turns into frost. Both are exactly the same phenomenon.

Dew point temperature—the critical air temperature at which saturation is reached.

Also, when warm air rises, the temperature decreases adiabatically. At some point, the air becomes saturated, and the excess amount of water is condensed. This is called cloud!

Water vapor feedback


ñH2O vapor


Feedback process: Increasing CO2 concentration – increasing temperature – high temperature can hold more H2O vapor (which is a greenhouse gas!) – further increasing temperature

Studies show that water vapor feedback roughly doubles the amount of warming caused by CO2! Further reading:

Cloud Feedback

Cloud feedback is the coupling between cloudiness and surface air temperature in which a change in radiative forcing perturbs the surface air temperature, leading to a change in clouds, which could then amplify or diminish the initial temperature perturbation.

Cloud feedbacks are more complicated



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Feedback process: Increasing CO2 concentration –> increasing temperature –> enhance cloud formation (due to enhanced evaporation from the ocean) –> clouds emit infrared radiation back to the Earth’s surface (positive feedback)


– cloud reflects sunlight (negative feedback)

Condensation • Conversion of vapor to

liquid water • Surface tension makes it nearly

impossible to grow pure water droplets

• Need supersaturated air • Need particles to grow droplets

around, a cloud condensation nuclei

• Liquid water can persist at temperatures colder than 0�C without a nuclei – supercooled

How big does a rain drop need to be to reach Earth without evaporating?

The drop would have to be approximately .2 mm or larger in diameter. Typical rain drops are 2 mm in diameter.

• Lifting condensation level (LCL)

Adiabatic Processes

Large masses of air can be cooled to the dew point ONLY by expanding as they rise. Because of this limitation, adiabatic cooling is the only prominent mechanism for development of clouds and production of rain.

When warm air rises, it cools down. This is called adiabatic cooling. When the air cools, it holds less moisture (capacity decreases). As a result, relative humidity increases. The altitude at which air becomes saturated (100% relative humidity) is called lifting condensation level (LCL).

Perhaps you have seen clouds like those shown in the slide – tall puffy clouds with a flat bottom. This happens because rising warm air continuously brings moisture to higher altitudes and, at a given point, air becomes saturated (LCL). Clouds will form above the LCL.

Lenticular clouds

Examples of cloud formation due to atmospheric lifting!

– Cirrus clouds – Cumulus clouds – Stratus clouds

Clouds Not all clouds precipitate, but all precipitation comes from clouds!

The Oxford English Dictionary: (Cloud is) “a visible mass of condensed watery vapor floating in the air at some considerable height above the general surface of the ground.”

At any given time, about 50 percent of Earth is covered by clouds. Clouds play an important role in the global energy budget.

• Cloud types – High clouds (over 6 km) – Middle clouds (from 2 to

6 km) – Low clouds (less than

2 km) – Clouds of vertical

development • Grow upward from

low bases to heights of over 15 km occasionally

Cloud Families

Cloud categories are largely based on altitude: • High clouds—Altocumulus clouds—found above 6 kilometers (i.e., cirrus

clouds) • Middle clouds—between about 2 and 6 kilometers (i.e., altocumulus and alto

stratus). • Low clouds—below 2 kilometers (i.e., stratocumulus and nimbostratus). • Clouds with vertical development (i.e., cumulus clouds).

Clouds – Cirrus clouds (high clouds)

Feathery appearance.

Cirrus: Detached clouds in the form of white, delicate filaments, mostly white patches or narrow bands. These clouds may have a fibrous (hair- like) and/or silky sheen appearance. Although cirrus clouds may look less dense, considering that they form in the high altitude, they are always composed of ice crystals. Since ice crystals are a blackbody that absorb and re-radiate outgoing infrared radiation, having more cirrus clouds contribute to warming (positive feedback)!

– Cumulus clouds (middle to low clouds)

Puffy white cloud that forms from rising columns of air.

– Stratus clouds (low clouds) Low clouds, usually below 6500 feet, that sometimes occur as individual clouds but more often appear as a general overcast.


Cumulus: Detached, generally dense clouds and with sharp outlines that develop vertically in the form of rising mounds, domes or towers with bulging upper parts often resembling a cauliflower. The sunlit parts of these clouds are mostly brilliant white while their bases are relatively dark and horizontal. Precipitation of showers or snow may be associated with cumulus clouds.

Stratus: A generally gray cloud layer with a uniform base which may, if thick enough, produce drizzle, ice prisms, or snow grains. When the sun is visible through this cloud, its outline is clearly discernible. Often when a layer of stratus breaks up and dissipates blue sky is seen. We also call stratus clouds as overcast.

Both cumulus and stratus clouds are middle to low clouds and can block sun light from reaching the ground. (Imagine an overcast day. You will feel cold because there is less energy from the sun on ground.) With this, having more cumulus and stratus clouds contribute to a cooling effect (negative feedback)!

Contrails: Man-made clouds

Jet contrails = condensation trails caused by the exhaust from airplanes that contain water vapor, and are not much different from natural clouds. If the air is very cold (which it often is at high altitudes), then the water vapor in the exhaust will condense out into what is essentially a cirrus cloud.

Sailors have known for some time to look specifically at the patterns and persistence of jet contrails for weather forecasting. On days where the contrails disappear quickly or don’t even form, they can expect continuing good weather. While on days where they persist, a change in the weather pattern may be expected.

Contrails: Man-made clouds

If contrails persist for a long enough period of time, they can spread out

across the sky due to the prevailing winds at the level at which they

formed. The two figures show how contrails generated on this particular

day spread out fairly quickly due to the stronger jet stream of air aloft.

Persistence of contrails is neither an indication that they contain some kind

of chemical, nor that it is some kind of spray. It is simply an atmospheric


Contrails are a concern in climate studies as increased jet traffic may result

in an increase in cloud cover (specifically, cirrus cloud coverage). It has been

estimated that in certain heavy air-traffic corridors, cloud cover has

increased by as much as 20 percent. However, the world’s goals for

reducing aircraft emissions are still unclear as strategies vary by nation.

Below, I am sharing an interesting and informative reading about the

unknowns of contrails and the complicating relationship with jet fuel


Greening the Friendly Skies By Mark Betancourt, 4 November 2020





The Coming Surge of Rocket Emissions

The launch plume from a test missile, photographed on 10 October 2013 by astronaut Luca Parmitano, diffuses into the middle and upper atmosphere during the first several minutes after launch. As the number of rocket launches increases in the future, rocket engine emissions will increase proportionally. Credit: © European Space Agency/NASA

By Martin N. Ross and Darin W. Toohey 24 September 2019, EOS

Due to the unique nature of the combustion chemistry, it turns out that rocket engines emit even larger amounts of black carbon than a modern jet engine. This means that it is more potent than contrails!

“With 114 launches in 2018, the number of launches has been growing at an average rate of about 8% per year for the past decade. Rocket emissions have also been growing.” (EOS)

A missile launch seen from space: an unexpected surprise!

Cloud feedbacks are more complicated Because cloud feedbacks are more complicated than other feedbacks, it is likely that it

causes uncertainty in climate predictions.



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