Dew Point, Air Temperature and Precipitation (2023)

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RESPONSE:

Your question was this: "In areas of precipitation, the dew point and air temperature are higher than in the surrounding areas not receiving rain. In areas of precipitation the dew point and air temperature are also the same or close to being the same. What can you say about the relationship between air temperature and the air's dew point for stations within areas of precipitation?"

Background and Definitions:

Air Temperature is defined as the measure of the average speed of atoms and molecules. The higher the temperature the faster they move. Dew point is the temperature at which water vapor saturates from an air mass into liquid or solid usually forming rain, snow, frost or dew. Dew point normally occurs when a mass of air has a relative humidity of 100 % (and air temperature and dew point are the same). If the dew point is below freezing, it is referred to as the frost point.

Precipitation occurs if there:(1) is any aqueous deposit, in liquid or solid form--that develops in a saturated atmosphere (relative humidity equals 100 %) and falls to the ground generally from clouds. Most clouds, however, do not produce precipitation. In many clouds, water droplets and ice crystals are too small to overcome natural updrafts found in the atmosphere. As a result, the tiny water droplets and ice crystals remain suspended in the atmosphere as clouds. (2) The state of being precipitated from a solution. (3) Precipitation is water in liquid, or solid phase falling through the atmosphere toward the surface of the earth the condensation and sublimation from which precipitation is derived are associated with the release of a significant amount of heat energy to the atmosphere. Causes of Precipitation-clouds and precipitation are the result of rising and adiabatic cooling of air. Precipitation Formation Processes--precipitation is usually produced in clouds that have been uplifted far enough for the air to have cooled adiabatically to well below the original dew point, thus forcing the condensation or sublimation of large quantities of precipitation collisions between smaller water droplets in clouds cause them to coalesce into larger water droplets which eventually fall to the ground (http://io.uwinnipeg.ca/~wbuhay/nov10.htm).

a. What, then, can you say about the relationship between air temperature and the dew point for stations within areas of precipitation?

Dew points indicate the amount moisture in the air: the higher the dew points, the higher the moisture content of the air at a given temperature. Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapor content -see below for unstable conditions) in order to reach saturation. A state of saturation exists when the air is holding the maximum amount of water vapor possible at the existing temperature and pressure.

When the dew point temperature and air temperature are equal, the air is said to be saturated. Dew point temperature is NEVER GREATER than the air temperature. In other words, dew point temperature is always less than air temperature (in fact, the air is said to be unsaturated when the air temperature is greater than the dew point temperature). Therefore, when the air cools (decrease in air temperature, moving closer to dew point temperature), moisture must be removed from the air and this is accomplished through condensation. This process results in the formation of tiny water droplets that can lead to the development of fog, frost, clouds, or even precipitation.
Relative Humidity can be inferred from dew point values. Humidity is a measure of moisture content of air. Relative humidity is a percentage measure of moisture in the air compared to what the air actually is capable of holding at a particular temperature. So a relative humidity of 50 percent indicates the air, at the current temperature, holds 50 percent of the moisture it is capable of holding. In very dry climates, the RH is low...and in moist climates with high precipitation, the relative humidity is high.

So, what does this imply about the relationship between temperature and dew point in areas of precipitation?

When air temperature and dew point temperatures are very close, the air has a high relative humidity (i.e., areas of high precipitation). The opposite is true when there is a large difference between air temperature and dew point temperatures, which indicates air with lower relative humidity.

Locations with air temperature and dew point temperatures very close with a high relative humidity indicate that the air is nearly saturated with moisture; clouds and precipitation are therefore quite possible. Weather conditions at locations with high dew point temperatures (65 or greater) are likely to be uncomfortably humid. The higher the relative humidity (in areas of high precipitation), the closer the temperature and dew point will be (at constant pressure and constant water vapor content). Said another way, the more humid the air, the closer the temperature and the dew point, because warm air (and thus, warm air temperature) holds more water than cold air. The difference between the dew point and current temperature can also gauge how much moisture is present in the air. The smaller the difference between current temperature and the dew point, the more moisture is present in the air. High dew points can also lead to unstable weather. Dew points also are used as an indicator for determining the likelihood of thunderstorms in the summer.

Rising air will cool (air temperature decreases) by expansion. As a volume of air cools (air temperature decreases), its capacity to hold moisture decreases (dew point temperature cools and moves closer towards the current air temperature) and it will eventually reach saturation (relative humidity of 100% with the temperature and dew point being the same). If cooling continues (air temperature decreases), water vapour will condense on tiny particles in the air to form minute liquid water droplets or solid ice crystals. This is the most common way that clouds are produced. Cloud bases are located at the elevation at which saturation (100% relative humidity, with the air temperature and dew point temperature the same) is first achieved. Further cooling and condensation causes these small water droplets or ice crystals within clouds to multiply, eventually collecting together to form larger precipitation particles, which reach the earth's surface as either snow, rain, drizzle, or hail. Saturation can also occur if water vapour is added to already moist air. Enough condensation can then take place to maintain saturation levels of water vapour (air temperature and dew point the same, with a relative humidity of 100%). This can be seen at times when cold air moves over a warmer, wet surface, causing evaporation, which often produces saturation that is called steam fog. This frequently occurs over Canada's coastal and inland water bodies in the fall when cool air flows over a still relatively warm water surface. Rain evaporating into already humid air to produce pre-frontal fog is yet another means of achieving saturation-condensation.

b. Does the dew point temperature always have to be above a certain value (i.e., same as air temperature) for precipitation (i.e., a thunderstorm)?

Unequivocally no. In fact, weather stations have reported thunderstorms with air temperatures of 8 degC and a dew point of 4 degC. Why? Because what is really important is that the air must be unstable, usually achieved by warming at the bottom or by cooling high up or both. Then a trigger is required to release the instability, usually heating and input of moist air (high dew point), but if the air is unstable enough just the heating will do. Other triggers are forced lifting of air over hills or forced lifting by convergence (e.g. sea breezes).

c. What is the impact of stable and unstable air masses on the relationship between dew point, temperature and precipitation?

First, to visualize what 'stable' and 'unstable' states mean in a physical sense, stand a round pencil on end on a level surface. From Newton's First Law of Motion, it will remain upright until a force is applied. Once displaced, the pencil falls over, failing to pass through its original (upright) position. This is the UNSTABLE state. Now lay the pencil on its side, at the bottom of an incline. Displace the pencil slightly up the incline, and then remove the force of displacement. The pencil will return to its original position. This is the STABLE state.

In the atmosphere, whether air that is displaced does so in an unstable or stable environment depends upon the vertical temperature profile of the air -- its lapse rate -- and upon the moisture content of the parcel. These differences are fundamental to understanding why clouds take up the form they do. Unstable Atmosphere is defined as a condition in the atmosphere where isolated air parcels have a tendency to rise. The parcels of air tend to be warmer than the air that surrounds them (http://www.geog.ouc.bc.ca/physgeog/physgeoglos/a.html#anchor254166).
In the atmosphere, when a 'parcel' of air moves vertically upwards (or downwards), it cools (upward motion), or warms (downward motion), in accordance with thermodynamic rules ... if the air is unsaturated (air temperature > dew point temperature), the cooling/warming will be at a rate of 3 degC per 1000 ft (or 10 degC per 1 km): This is known as the Dry Adiabatic Lapse Rate/DALR; If the air is/becomes saturated (air temperature=dew point temperature), this rate is roughly halved in the lower troposphere, due to the release of latent heat upon condensation. This rate is known as the Saturated Adiabatic Lapse Rate/SALR.

Such ascent/descent is said to be adiabatic, which means that the energy/heat changes are confined to that particular parcel. Provided the parcel is warmer (less dense) than the environmental air through which it is passing, it is buoyant, and rises. If the parcel is colder (denser) than ambient air, then it will descend, or try to descend. Because the rates of cooling (ascent), and warming (descent) of individual parcels are fixed, the important variable is the overall lapse rate (i.e. the rate of change of temperature with height) of the atmosphere. On average, this is 1.98 (call it 2 degC) per 1000 ft, or 6.5 degC per 1 km in the troposphere, but this average conceals a wide variety of cases that are important for weather stations.

Where the temperature falls off slowly with height, or indeed rises, e.g. in a slow moving anticyclone, or a tropical maritime air mass, then an air parcel subject to lifting/adiabatic cooling will readily find itself colder than its surroundings ... denser ... and try to return to its original position: The air is ABSOLUTELY STABLE. Where the temperature falls off quickly with height, e.g. in a cold/polar air mass over NW Europe in late winter/spring, then an air parcel subject to ascent, although cooling, may still find itself warmer/less dense than its surrounding air ... it will be buoyant, and tend to rise further: the air is ABSOLUTELY UNSTABLE.

Problems arise when, on ascent, the dew point of the air is reached, and the rate of cooling (decrease in air temperature) is therefore less - it follows the SALR figure. If, however, the parcel is still warmer/less dense, then it will continue to rise, and the condition of the air is said to be CONDITIONALLY UNSTABLE i.e. conditional upon whether the parcel is saturated or not. This is by far the most common situation in the 'real' atmosphere, accounting for some 65-70% of situations taking the troposphere as a whole.

Stable air masses generally imply the absence of 'free' vertical motion, and any ascent that does occur must be forced, i.e. frontal (dynamic) or orographic (mechanical) ascent, and the cloud structure is essentially layered. (NB: Forced ascent comes about in several ways: frontal ascent due to large-scale air motion within frontal systems, with of course adjacent descent; convergence into an area of low pressure - the converging air can't go down near the surface - it has to go up; and topographical forcing, that is, air is forced to rise over major upland ranges.)

Unstable air masses imply free vertical motion (given an initial trigger action), and the cloud structure is 'heaped' or cumuliform. If the vertical motion is vigorous and deep enough, and there is sufficient moisture, then heavy showers/thunderstorms are likely. (NB: Trigger action: method of causing air to rise initially, which in the lower troposphere include not only the 'wide-area' triggers noted above under stable conditions, but also smaller/mesoscale mechanisms such as differential heat response between land and sea, coastal convergence, etc.) For more information on these subjects, see a good textbook on meteorology, for example, Essentials of Meteorology:(Taylor and Francis/D.H.McIntosh and A.S.Thom) (http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/maps/upa/dew... ).

In other words, the relationship between the air temperature, dew point and precipitation is somewhat complex. Weather involves humidity, pressure, temperature and the likes. We cannot speak about one without drawing in the others. That said, the relationship suggests that as the relative humidity rises, the dew point and temperature also rises. The higher the relative humidity is, the higher the dew point and the air temperature. Dew point is always less than the air temperature. When the relative humidity is 100%, the dew point and air temperature are the same (with the exception of unstable air masses), and the air is said to be saturated (i.e., rains, snows, etc.). With unstable air masses, a deviation between the dew point and air temperature may exist, but precipitation (i.e., thunderstorms) may still occurs due to other atmospheric conditions.

I hope this helps and good luck with your studies.

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