Humidity — Page 82, Lesson 71

I want to walk you through how we actually measure humidity in the atmosphere, because it’s not something you can just look at — you need instruments. Let’s start with the measurement of humidity. Atmospheric humidity is measured using a dry bulb and wet bulb hygrometer — also called a psychrometer — or an electrical hygrometer. I’ll focus on the psychrometer first, because it’s the classic tool and you’ll see it in practical use. The dry bulb and wet bulb hygrometer consists of two thermometers. The dry bulb thermometer simply gives you the ambient temperature — the actual air temperature around you. The wet bulb thermometer has, around its bulb, a muslin cloth that has its other end dipped into a reservoir of distilled water. The water rises up the muslin by capillary action and evaporates. As it evaporates, it draws heat from the bulb, which reduces the temperature of that thermometer. So the wet bulb thermometer gives you the lowest temperature to which the air can be cooled by the evaporation of water. Now, the rate at which the water evaporates depends on the relative humidity. If the relative humidity is high, the air already holds a lot of water vapour, so evaporation is slow — the wet bulb temperature stays relatively high, close to the dry bulb. Conversely, if the air is dry, evaporation is rapid, and the wet bulb temperature becomes much lower than the dry bulb temperature. So the difference between the two readings tells you how humid the air is. Let me give you the key bullet points from the book for this instrument: - If air is dry, water will evaporate from the muslin covering the wet bulb, and latent heat will lower the temperature. - If air is saturated, no evaporation will occur, and both thermometers will read the same. - To get dew point, relative humidity, and HMR — that’s humidity mixing ratio — you read them from tables or a slide rule by entering with the two temperatures you obtained. There’s a diagram for this — — which shows the dry bulb and wet bulb hygrometer. Now let’s talk about dew point temperature. Dew point, abbreviated DP, is the temperature to which air must be cooled at constant pressure for saturation to occur. That’s the exact definition. Important: the dew point temperature is not the same as the wet bulb temperature — except when the air is saturated, at which point they are equal. The dew point has a lapse rate of 0.5°C per 1000 feet. That means as you go up, the dew point temperature decreases at that rate. And here’s a quick relationship: Wet bulb = dry bulb (= dew point) at 100% relative humidity — that’s saturation. Now, let’s look at the diurnal variation of humidity — how humidity changes through the day. By day, as the temperature increases, relative humidity decreases because the maximum amount of water vapour air can hold increases as temperature rises. After 1500 hours, the temperature starts to fall, and the maximum amount of water vapour the air can hold falls, so relative humidity increases. This higher relative humidity at night is the reason for the formation of mist and fog after dark in autumn and winter. The book shows a graph of this — — and it tells us that relative humidity is maximum approximately 30 minutes after sunrise, when the temperature is at its minimum. There’s also a graph from RAF Waddington over a number of years — Figure 6.6 — which confirms this pattern with a sinusoidal curve. Finally, by definition: - Saturated air: relative humidity = 100% - Dry air: relative humidity less than 100% - For example, even 99.9% relative humidity is still considered dry air — because it’s not fully saturated. That covers the measurement of humidity, the instruments, the dew point, and how humidity varies through the day.