![]() Drops that reach the critical size can continue to grow to become cloud drops. 0.3% supersaturation).ĭroplets below the critical size are 'haze drops' and these make up the haze you see on very humid days. The critical supersaturation is the supersaturation needed to attain the critical drop size, and is generally small (e.g. In a nutshell, there is a critical drop size below which drop size decreases for decreasing supersaturation and above which drop size increases for decreasing supersaturation. The combined interaction of these are described by Köhler theory and describe droplet growth in terms of drop size, solute and supersaturation (RH-100%). These work by forming a solute in water increasing the energy needed to break bonds and evaporate the water ( Raoult's_law) Some of these aerosols are classed as cloud condensation nuclei (CCN) and enable droplet formation at lower relative humidities. Luckily for us, our atmosphere is not pure air but has small particulates suspended in it (aerosols). Reasons for this are a bit complicated but it has to do with very small droplets being more likely to evaporate as their curvature is very large ( Kelvin effect, saturation vapor pressure is higher over curved surfaces than flat ones). In a mixture of pure dry air and water vapor, water will not condense until around 400% RH. When RH > 100% net condensation occurs, but water has its own ideas. Saturation vapor pressure is the partial pressure of vapor when evaporation and condensation rates are equal, represented by RH=100%. The amount of water vapor in the air can be expressed as relative humidity (RH) which is the ratio of water vapor pressure ($e$) and saturation water vapor pressure ($e_s$). The onset of rain is dependent on many things including humidity, but a specific value of humidity is not a sufficient condition for rain. The second map better comports to our ideas of areas that are more “humid.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 desert Southwest looks like most people would expect, but not so much everywhere else. If you only saw the first map, you would be forgiven for scratching your head in confusion. ![]() The two maps below show the 1) average annual relative humidity and 2) the average annual dew point. This is a truer reflection of the moisture regime. The average dew point in Alaska is the lowest of all 50 states. A cold airmass simply cannot hold a lot of moisture. This is a function of the low temperatures. Dew points under 30☏ feel notably dry.Īs noted earlier, when looking at relative humidity, Alaska is the most humid state. A dew point over 60☏ is where it starts to “feel humid.” Dew points under 60☏ generally feel comfortable. in the summer months, this is a common value. It’s the amount of moisture that makes you sweat even at night without any physical exertion. A dew point over 75ׄ☏ is very oppressive. There are some magical dew point numbers that represent handy guides for determining how much moisture is in the air. There are other measures of moisture not discussed here, they include: specific humidity, mixing ratio, and vapor pressure. Looking at the sponge diagram, the dew point represents the temperature if the dry part of the sponges (yellow areas) were removed. In short, the dew point is a temperature value that represents the minimum temperature an airmass can achieve given the amount of moisture in the air. While not technically a direct measure of moisture, dew point is a relatable measurement to most people. The most popular is something called the dew point. There are a number of ways to measure the moisture in the air that do not have this issue of relativity. This leads to a situation where the same city has a very high relative humidity at one time of day and a very low humidity at another time of the day – even with no change in the amount of moisture in the air. If the amount of water vapor in the air is constant throughout the day, the relative humidity changes dramatically as the temperature rises and falls. In most instances, the air is coolest in the morning and warmest in the late afternoon. In the previous example, the warmer airmass actually contains 2.7 times as much water vapor as the cooler airmass – even though they both are reporting 50% relative humidity.Īnother aspect to this is the (diurnal) trajectory of temperatures throughout the day. This demonstrates why using relative humidity is a terrible metric for surface moisture. ![]() ![]() In the graphic above, an 80☏ airmass that is 50% full of water vapor is shown as a sponge that is significantly larger than a 50☏ airmass that is also 50% saturated. ![]()
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