41 votes

Why would it be drier indoors during the winter vs the summer?

The only physics you need to know is that if the temperature is higher the air† can hold more water vapour (the gaseous phase of water). Cool molecules don't have enough energy to free themselves from ...
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28 votes
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At what humidity does it start to rain?

Short answer: humidity is not a proxy for rain starting and no, it does not start raining automatically when 100% humidity is reached (haze or clouds can form though). The onset of rain is dependent ...
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28 votes
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How do I convert specific humidity to relative humidity?

Relative humidity is just $e/e_s$, the ratio of the vapor pressure to saturation vapor pressure or $w/w_s$, the ratio of mass mixing ratios of water vapor at actual and saturation values. If you have ...
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  • 13.9k
19 votes

How do I convert specific humidity to relative humidity?

Here is an implementation that I wrote in R (with documentation in the header). In addition to laboriously working through a few textbooks and online references, I have compared the results to a ...
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15 votes
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Why (actually) is the night sky so bright in the city? How far up is that happening?

What's actually happening is scattering of light, both off of aerosol particles and nitrogen and oxygen molecules. For a review of quantitative models of light pollution as a function of distance ...
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  • 5,892
14 votes

Why would it be drier indoors during the winter vs the summer?

Because cooling air tends to take out moisture, while heating air doesn't change the actual moisture levels. When air gets colder, it can't "hold" as much water vapor (lower saturation vapor ...
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13 votes

Why does warm air "hold" more moisture?

Saying that warm air "holds" more moisture is technically incorrect, but is a common colloquialism. Let's break it down to the technicalities. Let's consider a glass of water with a vacuum (no air) ...
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12 votes

How to calculate relative humidity?

No, this is not enough. Relative humidity is defined as the water vapour partial pressure relative relative to the saturation water vapour pressure: $$ \phi = {{e_w} \over {{e^*}_w}} $$ where ${e^...
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  • 11.1k
11 votes

What does the notation "kg kg-1" for units mean for specific humidity

Yes, the unit is kilogram per kilogram, but it means kilogram of water (moisture) per kilogram of air.
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  • 20.3k
11 votes

Why is q used for specific humidity?

The specific humidity q is a quotient - mass of water vapor in mass of moist air. The mass is expressed per volume, i.e. the density of water vapor $\rho_v$ and the density of dry air $\rho_d$ are ...
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  • 1,662
11 votes

Why would it be drier indoors during the winter vs the summer?

A psychrometric chart is very useful for those kind of calculations. There is a lot of information and the chart looks overwhelming at first. You can click on the charts to see a larger version. 35°...
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10 votes

How do the pros measure humidity?

The WMO report Instruments and Methods of Observation has a chapter on Measurement of humidity and a separate chapter on Measurement of upper-air pressure, temperature, humidity. The first deals with ...
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  • 772
9 votes

Is "humidity" the quantifiable metric for mugginess?

Relative humidity is just about the most over-used, relatively useless measure of moisture there is. It's just how full the air is in terms of what it can hold in moisture. Warm air holds more ...
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8 votes
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Why is q used for specific humidity?

I haven't been able to find any particular references which hold themselves out as the origin of $q$ as the symbol of choice for specific humidity, but the origin of the term "specific humidity" ...
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  • 96
8 votes

Why do anticyclones bring clear skies in summertime?

Why anticyclones are also called areas of high pressure Let's call anticyclones areas of high pressure. Areas of high pressure (anticyclones), pull air towards the earth's surface. The increase in ...
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8 votes
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How to calculate relative humidity from temperature, dew point, and pressure?

You can refer to this question for more detail on the origin of this formula (based on the Magnus approximation), but if you do some algebra to the expression there for dew point ($TD$) as function of ...
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7 votes
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How to calculate specific humidity with relative humidity, temperature, and pressure

It looks like your program is using an approximation based on $q \approx w = w_s*RH$ with an approximation of Clausius-Clapeyron to find $w_s$. Looking at a few values of RH,T and P, your ...
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7 votes

What the humidity metric is hiding?

Weather services rarely state the absolute humidity because it is not easy to determine. Instead, they state the relative humidity. Absolute humidity is the mass of water vapor divided by the mass ...
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7 votes
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Relative Humidity approximation from Dew Point and Temperature

The equation for relative humidity is $$RH=100\times \frac{e}{e_s(T)}=100\times \frac{e_s(T_d)}{e_s(T)} \tag{1}$$ Where $T_d$ is the dewpoint temperature and $T$ is the temperature, $e$ is the water ...
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6 votes
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Why doesn't it snow even when it is below freezing?

Farrenthorpe is right, a little surfing the net for snow distribution would clarify the picture for you. However, I think you are referring to certain times when it rains further north and snows ...
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6 votes

Why does warm air "hold" more moisture?

What is the deep physics or chemistry reason this happens? In one word, entropy. In a thousand words (i.e., a picture), Source: Wikipedia article on Phase Diagram The above is a phase diagram of ...
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6 votes

Why does warm air "hold" more moisture?

The water molecule is just another molecule in the atmosphere - except that it's triatomic, whereas 99.97% of the dry atmosphere is monatomic (Argon) and diatomic (Oxygen and Nitrogen). It's also ...
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  • 121
6 votes

How is relative humidity determined from a wet and dry bulb readings?

Edit: 1 May 2021 The following procedure uses the less accurate method from page 455 onward from the scanned sections of the book pictured below, from the original answer. The procedure is a ...
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  • 20.3k
6 votes

How do the pros measure humidity?

We use something like an Onset HOBO electronic sensor. They've got logging (and in some cases, networking) built in. They'll typically log both temperature and relative humidity at regular intervals ...
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  • 4,060
5 votes

What does the notation "kg kg-1" for units mean for specific humidity

The basic idea is that by mentioning $\mathrm{kg}\, \mathrm{kg}^{-1}$ you make it clear that you measure the weight. You can think of it as $$ 0.01\, \mathrm{kg}\, \mathrm{kg}^{-1} = 1\, \mathrm{wt.}\...
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  • 1,148
5 votes
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Is it hotter in different places with the same air temperature?

A temperature is a temperature but the effect of that temperature is not always the same. Some things that add to our perception include sun intensity, wind, humidity, and simply what we are used to. ...
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5 votes

Is it hotter in different places with the same air temperature?

While perceptions about the climate can have an effect on how we react to our environment, Guangdong and Minnesota are fundamentally different climates altogether. Using the Koppen-Geiger climate ...
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5 votes
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How to predict when contrails are likely to form over a specific location using public data?

As described here, condensation of cloud particles (i.e., contrail formation) will occur if the mixing between hot and moist exhaust from an aircraft engine and ambient environmental air results in ...
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