On Controlling Humidity

[Drax]

I have heard people state in the past that "humidity is lower in the winter," or that, for those using humidors/pumidors, you should lower your humidity in the winter-time.

I had thought this was an odd statement, if not because it still rains in the winter (or at least, where I am, it does), and when it rains, the relative humidity is certainly 100%.

This led me to ask a pretty easy question -- what's the humidity look like in my area during the year? It turns out this is a very easy question to answer. For Virginia Beach, I give you the graph below:



Apologies on the size. For reference:

Green: Max humidity recorded that day
Red: Min humidity recorded that day
Blue: Average humidity
(Purple: a rolling of the average to make it a little smoother)

This confirmed a couple of things for me, basically that it's actually pretty humid here. But also -- it gets really humid here frequently.

The number of days the max humidity equals or exceeds 90%: 250
The number of days the max humidity equals or exceeds 80%: 309

The number of days the avg humidity equals or exceeds 90%: 39
The number of days the avg humidity equals or exceeds 80%: 134

I think the most noticeable thing is the swing that happens every day. The average swing in this area is 37 points (as high as 70 points, as low as 5).

Okay, so who cares? Well, first of all, it shows how much natural humidity varies daily (as opposed to the semi-constant environment of the humidor -- mine oscillates around 5 points).

Second, I realize that these measurements are outdoor, and our homes are quite different.

But this has interesting implications for those of us using humidors -- especially when comparing to those massive warehouses... speaking of which...

I decided to look at Hong Kong:



The number of days the max humidity equals or exceeds 90%: 91
The number of days the max humidity equals or exceeds 80%: 245

The number of days the avg humidity equals or exceeds 90%: 6
The number of days the avg humidity equals or exceeds 80%: 80

The average swing in this area is 28 points (range of usually 10 to 50).

So people tend to talk about Hong Kong has being very humid, but, given the stuff I looked at, I would say I'm in a much more humid environment.

Anyway, just food for thought. Anybody have any other places they'd like me to look at, just let me know...

[Maitre_Tea]

Very interesting, and it would be a nice (albeit time-consuming) experiment for someone to do this for RH inside the home. I think that humidity shouldn't be at a constant high all the time, and it should at least mimic the seasons, so a part of the year will be more humid, while another part would be drier.

So maybe if you can keep the pumidor more humid in the winter, but drier in the spring/summer....I don't think your puerh is really going to know that it's not in its "native" home.

[teop]

Neat data set!

An additional view that may provide more insight is to factor in local temperature to examine the absolute humidity (or quantity of water in the air) throughout the year and for each of these two locations. For example, could winter days in Calgary are ofter near 100% humidity but, due to the very cold temperatures, are still very dry.

I'd suspect that with regards to Pu Erh aging we are more interested in the absolute amount of moisture that the tea is exposed to. So the 98% humidity winter day in Calgary may not be as beneficial (or harmful!) as a 60% humidity summer day in Hong Kong, since the amount of moisture is much less.

Although I'm not completely sure...

[betta]

So people tend to talk about Hong Kong has being very humid, but, given the stuff I looked at, I would say I'm in a much more humid environment.

Anyway, just food for thought. Anybody have any other places they'd like me to look at, just let me know...

Thanks for sharing us. Honestly last time after checking the RH and Temperature history of Kunming area, I realized something bothers me with these RH data. I haven't checked what would be the reference condition when the humidity measurement was carried out. I have a semi-professional humidity measurement device for wine cellar. The device is good enough to even able to detect the variation of RH in the ambient air at various altitude in my living room. But clearly I've just now measured the current ambient humidity is only 39-45%, however the AOL weather forcast measured it at 93% at 30°C dew point, which is of course questionable.
The measurement method might be correct, but due to the different reference condition they measured, I can't take the data as my ambient condition.

For example, could winter days in Calgary are ofter near 100% humidity but, due to the very cold temperatures, are still very dry.

I'm afraid it's not possible to get 100% humidity in the winter, especially in Calgary. Humidity increases as the temperature decreases, however during winter, the ambient temperature is very low and water vapour condenses, leaving very dry air and thus low RH.

[shogun89]

Lets not forget though, that all these humidity calculations are taken outside, outside humidity is higher than indoors, because of our heaters and air conditioners. So these numbers are by no means accurate to what your storing at unless you have your house all opened up.

[TomVerlain]

Also - HK is consistently higher - in terms of your red line. VA Beach has a big dynamic range, but HK seems to be a tighter band, leading me to believe there is more time each day with higher humidity. Basically, I bet the mode is higher, and that 100% humidity might not be the best for tea. Of course, it is the outdoor values. I live in FL, which is a high humidity state, but my tea is in A/C - so it is generally 40-60% indoors year in year out.

[teop]

Anyway, just food for thought. Anybody have any other places they'd like me to look at, just let me know...

Oh no! Now you've got me grinding numbers on of my own. I decided to do similar analysis to Calgary to find out just how doomed my pu erh collection really is. Here is a plot of the relative humidity in Calgary for 2009.



This definitely looks different than either the Virginia Beach (VB) or Hong Kong (HK) plots. I looked at the summary statistics you listed for VB and HK.

The number of days the max humidity equals or exceeds 90%: 105

The number of days the max humidity equals or exceeds 90%: 208

The number of days the avg humidity equals or exceeds 90%: 15

The number of days the avg humidity equals or exceeds 80%: 55

The average swing in this Calgary is 40 percent (as high as 95 percent and as low as 4 percent).

At first glance the main factor faced by Calgary would be the lower number of days that the average humidity is above 80%. Only 55 compared to 80 for HK. I'd guess that this is partially reflected in the large average swing of 40 percent observed in Calgary, compared to the 28 percent observed in HK.

I talked to a friend who studies boundary layer meteorology. She is interested in how water moves from the atmosphere the surface of things. Too bad she is not obsessed with tea, eh? She is critical of my earlier hypothesis that the absolute moisture content, where we factor temperature in to relative humidity, plays an important role in pu erh aging.

If I understood correctly, she noted that moisture transfer only depends on the vapor pressure differences between the air and the tea. Relative humidity already describes that vapor pressure for moisture in the air. We would need to know how the vapor pressure of tea leaf / bing varies with temperature changes relative to air to understand moisture transfer taking place. Her guess is that the tea needed a high relative humidity, so that the bing does not dry out, and a high enough temperature that biological processes that help drive aging can proceed at a reasonable rate.

For example, could winter days in Calgary are ofter near 100% humidity but, due to the very cold temperatures, are still very dry.

I'm afraid it's not possible to get 100% humidity in the winter, especially in Calgary. Humidity increases as the temperature decreases, however during winter, the ambient temperature is far lower than dew point and water vapour condenses, leaving very dry air and thus low RH.

These terms are tricky and perhaps they caught you up. You are correct that cold air can't hold as much water (typically measured as grams of water per kilogram of air) as warm air, but moisture can still be there for a wide range of temperatures. Even though water "freezes" at 0 C, it's not until around -42 C that water experiences the spontaneous nucleation required to really evict all moisture content from the air. Fortunately for us in Calgary, the winters are rarely that cold! The trick is that in Winter, with temperatures ranging from -20 C to 0 C, a tiny bit of moisture is going to cause the relative humidity to go quite high. At times it will even drive it up to 100%.

This high relative humidity can be seen in the Calgary data set. In the plot there is a noticeable increase in the relative humidity through December of 2009. Of the twelve days that a maximum humidity of 100% was recorded in Calgary in 2009, five of these days fall in between Jan. 1 and March 31.

[betta]

These terms are tricky and perhaps they caught you up. You are correct that cold air can't hold as much water (typically measured as grams of water per kilogram of air) as warm air, but moisture can still be there for a wide range of temperatures. Even though water "freezes" at 0 C, it's not until around -42 C that water experiences the spontaneous nucleation required to really evict all moisture content from the air. Fortunately for us in Calgary, the winters are rarely that cold! The trick is that in Winter, with temperatures ranging from -20 C to 0 C, a tiny bit of moisture is going to cause the relative humidity to go quite high. At times it will even drive it up to 100%.

This high relative humidity can be seen in the Calgary data set. In the plot there is a noticeable increase in the relative humidity through December of 2009. Of the twelve days that a maximum humidity of 100% was recorded in Calgary in 2009, five of these days fall in between Jan. 1 and March 31.

Sorry I gave a wrong info, the dew point should be lower than ambient temperature even in winter.
To my understanding, the RH is product of vapour liquid equilibrium with the dew point as the reference. So the water vapor in the air is obtained from water evaporation from earth surface or any surface covered by water. This is true especially when the ambient temperature is above 0C. When the temperature is subzero, the surface water has very low vapor pressure and it might be in a solid form (ice/snow) with no vapor pressure. So increasing ambient humidity at subzero temperature is.... difficult to achieve
As a result I need lip balm and apply moisturizer on skin even though the forecast says the RH is above 70% during winter.
Moreover at -10°C during winter, when the ambient temperature achieve the dew point, we won't get rain, instead the condensed vapor transformed directly to solid form/snow. And as long as the ice or snow does not melt, we won't get increase of humidity.

That's the reason why I am wondering how can the RH measured by weather forcast is way higher than I measured at my garden. I suspect they measure it directly at on the coast at sea level and the sea might be far warmer than continent in winter. If it does so... then the RH measured is not applicable for those who live far away from the measurement point. But again, there's no sea nearby my city...

If you're talking about small volume container, like the puerh storage room, we can of course use humidifyer or sort of to "create" the vapor and increase the humidity artifically and easily. But it's not applicable if we're talking about increasing outdoor humidity.

[zeusmta]

Drax and teop,

This is great data you two posted. Where did you get the data? Is the raw data for various cities available on the Web?

I'm going to be doing some of my own humidity experiments over the coming weeks and months and will post some results to the board. Having data sets for my local area also would be helpful. Thanks.

[Drax]

I went to Weather Underground (www.wunderground.com).

You can enter your city/zip and then you can also get historical information. It's easily exportable into Excel, too.

Good luck w/ the experiments!

[zeusmta]

Using Relative Humidty (RH) and Temperature (T), one can calculate the actual vapor density in the air, i.e. the amount of water that is actually in the air per unit volume. In trying to understand how wet the air is, RH doesn't tell the whole story. This is because hotter air can hold more moisture than cooler air. So even with a high RH, if the temperature is cool, the air can still be pretty dry. For example, 40% RH at 100 degress F gives an actual vapor density of about 18 grams per cubic meter (g/m^3). 90% RH at a 60 degrees F, gives only about 12 g/m^3. So there is 50% more water vapor in the air at 40% RH than at 90% in this scenario.

There is an interesting website about this, including a calculator, at http://hyperphysics.phy-astr.gsu.edu/hb ... elhum.html.
I used the formulas from this site to calculate the actual vapor density in some cities of interest for the year 2009. I used average T and average RH for each day, which makes the results a rough estimate. But still it may still be useful for comparison.



Blue: Hong Kong
Black: Beijing
Purple: Kunming
Green: San Ramon, CA (where I live)

It is clear from the plot that Hong Kong is a stand out in terms of highest year-round vapor density, going up to about 25 g/cubic m. Winter in Kunming seems comparable to San Ramon (NorCal), but has significantly higher vapor density in the summer, up to around 15 g/cubic m. Beijing, which is mentioned as a climate not suitable for pu erh storage by Chan Kam Pong in his book "First Step to Chinese Pu Erh Tea," is quite high in the summer but also very low in the winter.

Recommended RH and T numbers from Chan are 20-30 deg C and 70-80% RH. That corresponds to a vapor density range of about 12 g/cubic m to 24 g/cubic m.

Not drawing any conclusions here, just some food for thought that is hopefully interesting to others here. It is clear from this data, however, that simply looking at RH alone does not tell the whole story about water vapor in the air.

Will post more later with some actual measurments I've been taking inside and outside my house here in Northern California.