28

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 on many things including humidity, but a specific value of humidity is not a sufficient condition for rain. Water vapor is a gas and invisible. The amount of ...


23

Relative humidity is just $e/e_s$, the ratio of the vapor pressure to saturation vapor pressure or or $w/w_s$, the ratio of mass mixing ratios of water vapor at actual and saturation values. If you have specific humidity, which is the mass mixing ratio of water vapor in air, defined as: $$ q \equiv \dfrac{m_v}{m_v + m_d} = \dfrac{w}{w+1} \approx w.$$ ...


17

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 psychrometric chart. As you have likely noticed, it is a challenge to keep these terms and units straight - I wish I had @casey`s answer then! ##' Convert ...


14

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 from cities see Light Pollution Modeling . Some consider height as well as distance. To get a rough estimate of what size of lights-out event would be needed ...


12

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 ${e^*}_w$ is the equilibrium vapour pressure (which may be relative to either liquid water, or to ice), and $e_w$ is the partial pressure of water. ${e^*}_w$ can ...


12

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) above it. What will happen? The molecules that are at the top most layer of the water will evaporate. At what rate will the water evaporate? Better yet, what is ...


11

Yes, the unit is kilogram per kilogram, but it means kilogram of water (moisture) per kilogram of air.


11

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 used for the definition of the quotient $q$ known as specific humidity: $$ q = \dfrac{\rho_v}{\rho_v+\rho_d} $$


9

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 instruments used for surface measurements. The second deals with instruments used for measuring vertical profiles, which have to deal with a larger range of ...


8

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" itself appears to have been in an 1884 article by Dr. W. von Bezold, translated into English and published in the Smithsonian Miscellaneous Collections (Vol. 51) in ...


8

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 the number of air particles near the earth's surface causes a higher pressure for things on the earth, hence the name. Impact of high pressure areas on ...


7

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 of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain. Absolute humidity is expressed as grams of moisture ...


7

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 vapor pressure, and $e_s$ is the saturation vapor pressure, which is also known as the Clausius Clapeyeron equation. While the previous link has a decent ...


6

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 further south. There are several reasons for local anomalies, of which the most important is the dynamics of the polar jet streams. These are strong high-altitude ...


6

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 approximation is pretty close (+/- 5%) to an analytic answer. Based on the output you quoted it looks like you are providing incorrect values of RH. Note in the comments ...


6

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 temperature ($T$) and relative humidity ($RH$), you get $RH=100 \, e^{\Large \left(\frac{c\, b (TD-T)}{(c+T) (c+TD)}\right)}$ With $b=17.625$ and $c=243.04$. ...


6

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 specified by the user. Humidity sensors have a tendency to wander off calibration, so you will need to take that into account when designing your monitoring ...


5

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 $$ 0.01\, \mathrm{kg}\, \mathrm{kg}^{-1} = 1\, \mathrm{wt.}\%. $$ This is to avoid confusion with $\mathrm{vol.}\,\%$.


5

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. The Humidex as mentioned by Trevor, or some other forms of heat indexes are attempts to measure how the temperature feels as compared to what it actually is, ...


5

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 classification system, Minnesota is mostly a Temperate Continental (Dfb/Dwb) climate, meaning that is has warm summers and long cold winters, while Guangdong is a ...


5

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 water. The curve of interest is that between vapor (tan area) and solid or liquid (blue and green areas). For a given temperature (a vertical line in the diagram)...


5

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 slightly electrically polarized, and because of this it has a low energy state when in the presence of similar molecules (liquid or solid form), and at a higher ...


5

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 the mixed air exceeding the ice saturation vapor pressure as its temperature changes. A conceptual diagram of this mixing process is shown here, for exhaust (...


5

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 multistage process ideally suited to either a spreadsheet or programming code. The equations use SI units. 1. Calculate the saturated vapor pressure at the dry bulb ...


4

By way of reference, "humidity depends on water vaporization and condensation, which, in turn, mainly depends on temperature". From the information you have supplied in your comments. There are waters in Bucharest and forests in the suburbs, but no waters or forests where the country house is located. From your information, Bucharest has a number of ...


4

Humidity strongly varies as a function of time and space, which is why standard atmospheres like CIRA may choose to omit it at all. It simply doesn't make sense to include an “average” humidity profile, because such an average is not meaningful. However, there are other sources for relative humidity profiles. You can obtain them from reanalysis data. If ...


4

In coastal New Jersey, relative humidity is maximal in the early fall when the ocean temperature is warmest, and minimal in early spring when the ocean is coldest. However, the monthly variation is rather small: Atlantic City, New Jersey: 70 January 69 February 67 March 66 April 71 May 73 June 74 July 76 August 76 September 75 October 73 November 71 ...


4

Let's do this with accurate equations. We know how to calculate the saturated water vapour pressure for temperature $T$: $$e_s(T)=e_{s0}e^{\frac{h_i}{R_w}(\frac{1}{T_{s0}}-\frac{1}{T})}=610\text{ Pa} \times e^{5423(\frac{1}{273\text{ K}}-\frac{1}{T})}$$ $e_s$ is saturated water vapour pressure as function of $T$ in Pascals $T$ is the temperature in Kelvins ...


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