What Exactly Is The Dew Point?

The dew point is a temperature at which the air becomes saturated. But as most weather definitions go, this is too simple of a definition.

Prepare yourself, what we are about to delve into actually takes some thinking. Why? Because the concept of moisture in our atmosphere, how we measure it, and how it changes, is pretty complex. Sorry about that.

However, you just may learn something that will be on Jeopardy, so let's dig in.

(This is a sling psychrometer (sigh-crom-a-ter) which is used to measure important atmospheric data, which then can be used to calculate the dew point)

First of all, it is important to realize that the dew point temperature is just that, a temperature. But the air temperature and dew point temperature are two very different things. The dew point temperature is the point at which the air must be cooled in order for the the air mass to become saturated (assuming there is no drastic change in pressure or moisture content).

So, when the air temperature and dew point are equal, does that mean it rains or snows? Not necessarily, because the process of forming rain is more complex. Also, keep in mind that rain falls from above. So, while the surface air may be saturated, that doesn't mean the air above you is saturated. In fact, before rain develops, there needs to be a cloud, and the process of cloud formation depends upon more than just saturated air. Such factors could be the type and size of aerosols which water vapor can condense upon. These aerosols are called cloud condensation nuclei (CCN). You see, it is not as simple as saying "if you have saturation, then you have a cloud or rain drop." However, it is the first step, and that's why dew point and saturation play a close role in forecasting rain.

The dew point is used a lot by TV meteorologists and seen on most current weather conditions because it's a great indicator of the moisture content of the air, or humidity. The higher the dew point temperature, the higher the humidity. Okay, this is where some thinking comes in, because the relative humidity and dew point both describe the moisture content of the air, yet they are different at the same time.

About Relative Humidity:
One thing you'll notice is that when the air temperature is close to the dew point, the relative humidity is high (often 80% or greater). But the relative humidity is "relative"or dependent. Relative humidity is dependent upon the temperature. So, if the temperature changes, the relative humidity will change. Dew point is a better "absolute" measure of the air's moisture content. Here's a good example of why:

Temperature Dew Point Relative Humidity
90 80 = 67%
100 80 = 47%
110 80 = 33%

From this table above, look at the relative humidity of 33%. Looks like a comfortable relative humidity, right?

Probably not. Sticky, sultry and oppressive are appropriate adjectives for 110 degree heat with an 80 degree dew point But why is the relative humidity so low? Because relative to that hot temperature of 110 degrees, the air is no where near saturation. Remember, we would have to cool the air down to 80 degrees (with no pressure change) in order to saturate that air mass (remember that's the definition of dew point). That's a 30 degree cooling! If our air temperature is 90 degrees, now the air is 67% saturated and we only have to cool the air 10 degrees to reach saturation. What if the air temperature was 80 degrees and the dew point was 80 degrees? The air is saturated, it 100% relative humidity and it is likely that you would be experiencing either rain, snow, mist or fog.

So, relative humidity is just that...relative, meaning it depends. It depends on the temperature of the air. Dew point temperature is a better "absolute" measure of moisture in the air.

In general, I consider dew points above 70 degrees F very uncomfortable. Most people consider dew points near 55 degrees F and higher to be necessary for severe weather. That's just a guideline though.

Why does cooling the air to the dew point cause saturation?

The cooler the air, the less energy it has. It takes energy for molecules to break free of the liquid bonds (or solid in the case of ice). So if you cool the air, you're reducing the energy the molecules need to escape the liquid bonds and become a gas. In turn, this encourages condensation. If you were to heat the air, you're adding energy and encouraging evaporation.

This doesn't quite explain why cooling to the dew point will cause saturation. In order to understand saturation, first understand that water in our atmosphere is constantly evaporating and condensing. H2O molecules are constantly coursing back and forth from a liquid to a gas and vice versa. What is important to the growth or decay of a water droplet is which process is dominant. For example, if you had 10,000 molecules evaporating and 1,000 molecule condensing, then evaporation is dominating and a rain drop or ice crystal is not likely to form. But when you cool the air, you reduce the energy molecules need to evaporate. At some point 5,000 molecules are condensing and 5,000 molecules are evaporating. The temperature at which this occurs, is called the dew point. It's the point at which condensation and evaporation are equal. Or you could consider it the temperature at which condensation has a chance to dominate over evaporation. And the reason that's important is because when condensation dominates over evaporation, it's finally possible to form a rain drop or ice crystal.

I mentioned several times about cooling the "air temperature", I am using the air temperature to approximate the temperature of the water vapor in the air. Although air is made of Nitrogen, Oxygen, water vapor and other stuff, we can make the assumption that the "air temperature" is also the temperature of the water droplet. It's kind of like trying to separate the temperature of the water in tomato soup from the temperature of the entire bowl of tomato soup. For all practical purposes, the temperature of the water is probably the same as the temperature of the entire soup. I mention this because it's not the temperature of the air as much as it is the temperature of the water vapor that is important

Heavy rainfall in Rameswaram region.

For the past 3 days, it's unusually heavy rain over Rameswaram region. Recorded around 40cm in 3 days. Even a cloud burst of 19cm on 15-Dec-08.
Almost there is no such rain forecast thru GFS models and no visible evidence in Satellite images. Purely a local disturbance.
Satellite pic shows...

Still lot's of activity over the South-Arabian sea in the form of stable "97A".
And cloud activity can also be seen over the South-BAY. There's a GFS suggestion of LOW pressure formation over that area and moving in a WEST direction.
Chennai:
Night was mild around 24 degrees and Now (3:45PM) the temperature is gradually reducing. Tonight we might get up to 22 degrees. From tomorrow morning there'll be DEW.

Will there be a final showdown??

Will there be a final showdown of rain in Chennai before the end of this Season..??
As far as the latest GFS models, the answer is "NO".
Latest satellite pic shows some activity near the west of Srilanka.

These cloud formations in southern bay would not move North or North-westwards, unless the Anticyclone seen over Northern bay region moves away.(pls view following image for Anti-cyclone over north bay). That anticyclone is there for the past 15 days.

Latest "Sea Surface Temperature Chart" also shows cooling down near the tamilnadu coast as well as central bay. This phenomenon will drastically reduce the cloud formation over the Bay.

Chennai:
The low cloud formation during mornings have reduced. Today it was almost NIL.
Only good thing is the North-east air current is strong without carrying any moisture. For the past 3 nights, the minimum temperature was not low, this time of year it should be COLD around 21 degrees.

Last: "97A" in Arabian sea (remnant of 07B) is turning BACK, this is a very rare phenomenon. We'll report on it if it gathers strength and moves in a East Direction.

Chennai witnessed a classic cricket TEST match

"Well done Team India !!"
Our forecast for the whole of test match stood perfect.
Thanks to weather and Team India, Chennai witnessed yet another classic test match.
Tomorrow we'll post the HIGH LIGHTS package video download in our other site Free Download Shop

For the past two days the minimum temperature was not low, even it's increasing every night. Yesterday (14-Dec-08) night it was 26.2°C (2:14am).
Thru the day it was HOT and mildly humid.
Even the cloud formations seen in mornings for the past 2 days was not seen today.
Today there was very little cloud movements.
Going by the latest satellite pic,

There's still a Anti cyclone over the northern bay.
And we can see lot's of activities around Southern bay region just EAST of Srilanka.

Hats off to "INDIAN CRICKET TEAM"..!
The GFS suggests, that there'll be a MILD low pressure system forming in another 36 hrs and it'll move across Srilanka (again) in another 2 days. No suggestion of moving up and crossing Tamilnadu coast. This system will trace the path of "07B".

Still the remnants of "07B" can be seen over South-central Arabian Sea with some power. This system will enter the COOLER waters near Arabian peninsula and die out.
Only 30% chance of reaching the Yemen or Saudi Arabian coast.

Going to be a Thrilling Final day...

It's going to be a thrilling final day of 1st test match in Chennai (Ind v Eng).
What's in store the day and days ahead?
Going by the latest satellite pic, shows nothing for next 2 days.
That's good news for the test match.

Chennai, was bit hot today and humid. And even the yesterday's night temperature was not that low (around 25 degrees). And in night we even had a small drizzle or two.
North-east is almost at it's end now.
Some GFS models suggest that we might have a LAST depression almost the same area as that of "07B".
Take a look at this model...
A low pressure system indicated to form around 16-Dec-08.

This system will not move north because still there is a ANTI cyclone in northern bay.
Studying the "Sea Temperature Charts (STC)" for the past 4 days suggests that the HEAT potential for the LOW pressure system in southern bay is decreasing.

"Save energy" - visuals #1

Energy Conservation DAY

Tip from "Clean Energy Ideas"
If you are interested in how to conserve energy, then this article can be of use to you in explaining the basic and advanced methods of how we can conserve energy including the use of natural resources.
Using natural energy resources doesn't necessarily mean you will conserve any energy, yet we do advise every household to research the possibility of implementing natural energy resources to influence their home power supply. This will conserve the fossil fuels we are currently consuming at an alarming rate.
Many little things can be done in an attempt to conserve energy in the home. It is knowing how to do these little energy conservation processes that many people do not know about, understand, or just don't care.
Another way relating to how we can conserve energy is to ensure whenever we have finished using appliances that may have a light on them (such as a standby light), we switch the appliance off, or unplug after use.
Little things can make a big difference in conserving energy around the home, and this is especially true for dishwashers, washing machines and dryers.
Modern dishwashers have energy saving wash cycles, we strongly advise the use of such features, washing machines can be set to lower temperature washes, when used with the right washing powder, and a dryer, well if you own a dryer, get rid of it and dry your washing naturally on a washing line.
Hybrid cars can make a difference to your carbon footprint, and can help you save energy. No, they will not eliminate carbon emissions from their exhausts, yet they will greatly reduce the amount of carbon you pump into the air each year.
The only reason you should need a 4x4 vehicle is if you live in a rough terrain area, or constantly snowy area.
When you choose your next car, be sure to check the miles per gallon to see how efficient your car will be, this not only helps the environment but it saves you money on fuel too.
When conserving energy, we can also look to our heating methods, why not reduce your heating thermostat or radiators by only 1 degree for a lengthy time, and compare your heating bill?
If you recycle more, you will be helping to conserve energy put into making new versions of what you have recycled, and this also reduces the burden on landfill sites and incinerators which are not good for the environment.

"Brown clouds" - images

"Brown clouds" threaten health in Asian cities

Taken from http://southasia.oneworld.net
Man-made atmospheric brown clouds fed by black carbon and soot are masking Asian cities and causing glacial melting and extreme weather events, says the latest UNEP report. These clouds containing toxic particles and carcinogens risk the health and livelihoods of three billion people, warn scientists.

A three-kilometre-thick “brown cloud” of man-made pollution, which stretches from the Arabian Peninsula to China to the western Pacific Ocean, is making Asian cities darker, speeding up the melting of Himalayan glaciers and impacting human health, according to a new United Nations Environment Programme (UNEP) report.

Atmospheric Brown Clouds (ABCs), resulting from the burning of fossil fuels and biomass, has resulted in the formation of particles such as black carbon and soot which absorb sunlight and heat the air, experts write in the study released today in Beijing.

The clouds also “mask” the actual warming impact of climate change by anywhere between 20 and 80 per cent because they include sulfates and other chemicals which reflect sunlight and cool the surface.

Shifting weather patterns
The artificial lowering of temperature by ABCs is leading to sharp shifts in weather patterns, causing significant drying in northern China while increasing the risk of flooding in the Asian nation’s south. Monsoon precipitation over India and South-East Asia has dropped up to seven per cent since the 1950s, with the summer monsoon both weakening and shrinking.

Meanwhile, the health and food security of three billion people in Asia are threatened by ABCs, which impacts air quality and agriculture.

Achim Steiner, UNEP’s Executive Director, voiced hope that “Atmospheric Brown Clouds: Regional Assessment Report with Focus on Asia” will serve as an early warning of the phenomenon, which he hopes will now be “firmly on the international community’s radar.”

He called on developed countries to help their poorer counterparts attain the technology needed to spur ‘green’ economic growth.

“In doing so, they can not only lift the threat of climate change but also turn off the soot-stream that is feeding the formation of atmospheric brown clouds in many of the world’s regions,” said Steiner.

Livelihoods at stake

The new publication points out 13 megacities as being ABC ‘hotspots’: Bangkok, Beijing, Cairo, Dhaka, Karachi, Kolkata, Lagos, Mumbai, New Delhi, Seoul, Shanghai, Shenzhen and Tehran. Soot levels in these cities comprise 10 per cent of the total mass of all man-made particles.

Since the 1970s, the Chinese city of Guangzhou, among other cities, has witnessed “dimming” – or reduction of sunlight – of more than 20 per cent, it notes.

The solar heating of the atmosphere by ABCs is “suggested to be as important as greenhouse gas warming in accounting for the anomalously large warming trend observed in the elevated regions” such as the Himalayan-Tibetan region, the study says, leading to the retreat of glaciers.

Further, the clouds contain toxic aerosols, carcinogens and other harmful particles, which could result in more people suffering from respiratory disease and cardiovascular problems.

While the effects of the clouds on food production and farmers’ livelihood could be immense, more research must be done to determine their precise role, it acknowledges, adding that the possible impact of ABCs could include elevated levels of ground-level ozone, which could result in massive crop losses of up to 40 per cent in Asia.

Scientists behind the report – produced by UNEP’s Project Atmospheric Brown Cloud – stress that brown clouds also can be found in parts of North America, Europe, Southern Africa and the Amazon Basin, and that they also require urgent and detailed research

"Brown Clouds" Add to Global Warming

Taken from Livescience.com
Pollution-filled "brown clouds" over the Indian Ocean could warm parts of the lower layers of Earth’s atmosphere as much as greenhouse gases do, a new study finds.

Aerosols, or tiny particles suspended in the air, make up clouds of pollution and are thought to have an overall cooling effect on the atmosphere as they scatter incoming light back out to space. Scientists suspect that this cooling effect could mitigate some of the warming caused by greenhouse gases.

But some aerosols, particularly soot, are very good at absorbing the sun's rays, and this absorption heats the layer of the atmosphere in which they sit.

How the heating from these aerosols compares to that from greenhouse gases has been largely unknown, but this new study, detailed in the Aug. 2 issue of the journal Nature, found that the warming from the two sources is comparable.

Brown clouds
During the six-month tropical dry season, soot and other aerosols waft over the Indian Ocean from Asia and form plumes of pollution known as "atmospheric brown clouds".

The researchers used unmanned aerial vehicles (UAVs) to measure the concentration of the aerosols over the ocean and how much heat they absorbed at different levels of the atmosphere.

The first half of the study period saw little pollution and little atmospheric heating, but during the second half, the brown clouds rolled in and atmospheric heating increased by 50 percent.

Using these results, the researchers modeled the heating effects of brown clouds in the region from 1950 to 2000 and found that it was about the same as that caused by greenhouse gases.

Himalayas and megacities

This study shows that the regional influence of aerosols, not just the global average seen in estimates by the Intergovernmental Panel of Climate Change, may be very important, said Peter Pilewskie of the University of Colorado at Boulder, who was not involved with the study.

"And, interestingly, that influence could either enhance (warm, as we've seen in this study) or mitigate (cool) the expected warming by greenhouse gases," Pilewskie said in an email interview.

The overall warming trend caused by greenhouse gases and aerosols in the area have caused the lower atmospheric layer to warm by 0.25 degrees Celsius per decade—twice the rate of warming at the surface, Pilewskie said in an analysis of the study.

This warming has had enormous implications for the Himalayas, where warming has led to a rapid glacier melt.

"The large clouds of pollution in Asia absorb enough solar energy to have climatic impacts," Pilewskie said.

Pilewskie added that other studies are investigating the possible climatic effects of pollution plumes drifting off from megacities (those with 10 million people or more), though he noted that as fewer aerosols are emitted due to the development of cleaner combustion processes, the influence of aerosols is likely to diminish in the future.