Condensation level

Lower limit of Sc clouds

Cumulus clouds over Kandel / Rhineland-Palatinate on a summer's day

In meteorology, the level of condensation or the cloud condensation zone denotes the height at which the air temperature equals the dew point . Thus, the air is completely saturated with water vapor . A relative humidity of 100 percent is synonymous with this . In addition to the vertical temperature profile , the height of the condensation level depends crucially on the moisture content of the rising air parcel . In meteorology, a distinction is usually made between two levels of condensation, the elevation condensation level and the convection condensation level .

In the earth's atmosphere , a condensation level represents the altitude at which clouds are formed, since the water vapor begins to condense as the altitude continues to rise and the temperature thus falls (in the standard atmosphere under standard conditions) . This area is therefore also referred to as the cloud base or cloud base , although these terms are not completely synonymous with the level of condensation. The cloud base can differ depending on the type of cloud, but it shows a correspondence with the level of condensation , especially in the case of cumulus- type clouds caused by uplift . The cloud height can be measured using a laser cloud altimeter (ceilograph) or at night with the help of a cloud spotlight and a sextant . In the synoptic weather observations carried out every hour around the world, the lower limits of the clouds are also estimated by the experienced weather observers.

In aviation, the term cloud ceiling ( English ceiling ) used. A main cloud base is only referred to as a "ceiling" if the degree of coverage is at least 5/8.

In order to be familiar with the field of cloud formation, it is therefore essential to also deal with the various types of condensation levels. In meteorology, this is where you enter the aerology department . As already roughly explained above, a condensation level describes the height or the layer at which an ascending air parcel is saturated with water vapor and condenses or resublimates in higher layers (freezes, ice clouds such as cirrus are formed) during a further rise .

Nowadays it is quite possible to determine these processes and procedures by calculation, but in many cases it is also very well possible to make specific statements for a specific area using graphical methods. For the graphical determination z. B. the cumulative cloud lower and upper limits and the lower and upper limits of stratified clouds, one therefore uses the data obtained from radiosonde ascents, which are deciphered in a simple logarithmic temperature-pressure diagram and thus form a TEMP graphic. The TEMP key represents a data telegram from the radiosonde , which in groups of five contains temperature and dew point, as well as pressure height and wind direction / speed of the high-altitude winds . The radiosondes are started four times a day, namely always at 00Z, 06Z, 12Z and 18Z. The data from the radiosondes provides meteorologists with a vertical sounding of the earth's atmosphere for a specific area.

The elevation condensation level

The lifting condensation level (HKN ) represents the height or area at which saturation first occurs due to forced uplift , mostly due to orographic effects (overflowing a mountain / mountain range) or due to lifting and sliding on a front and in the further course oversaturation with layer cloud formation occurs. Such an uplift can also occur in areas where wind shear occurs, with the same effect.

The still unsaturated air parcel rises due to the effects mentioned above, up to the point at which condensation occurs. Until it has reached this point, the package continues to cool down dry adiabatically, with its mixing ratio remaining the same until saturation is just reached. If the package is now raised even further, beyond the point of water vapor saturation, further cooling leads to condensation and thus to the formation of clouds. Depending on how high this HCN is, a distinction is made between condensation and sublimation .

If the package has condensed at a certain point, it does not stop (the causes of the rise still exist), but rather it continues to rise. However, the ascent is no longer dry adiabatic as it was at the beginning from the ground, but rather, due to the previous condensation, wet adiabatic.

Graphical determination in the temperature-pressure diagram

In the temperature-pressure diagram with the logarithmized pressure axis, the HCN forms the intersection of temperature and dew point when one moves upwards from with the dry adiabatic and from with the line of the saturation mixture ratio ( ). The HCN is always formed from the starting level, i.e. always from the height zero, which corresponds to the height of the place at which the radiosonde was started. ${\ displaystyle T}$ ${\ displaystyle T _ {\ mathrm {D}}}$ ${\ displaystyle T}$ ${\ displaystyle T _ {\ mathrm {D}}}$ ${\ displaystyle R _ {\ mathrm {w}}}$

So you have now determined an HCN and you know the height from which source or layer clouds can form. Here, however, there is still something very important to consider: In the early hours you can find very high moisture values in the area close to the ground, these decrease continuously towards the afternoon hours.

Due to these two causes, an HCN determined in the morning with the 00UTC or 06UTC TEMP will probably be significantly too low in the course of the day, due to the high humidity close to the ground . In order to eliminate this source of error, it is possible to determine the HCN ₁ , which forms the intersection of the and curves ( along the dry adiabates, along the saturation mixture ratio line) of the lower 30 hPa. So one averages a range from the initial level up to 30 hPa in height and thus excludes part of the air humidity contained near the ground . ${\ displaystyle T}$ ${\ displaystyle T _ {\ mathrm {D}}}$ ${\ displaystyle T}$ ${\ displaystyle T _ {\ mathrm {D}}}$

However, there is also the possibility of determining an HKN for any height in order to e.g. B. to be able to predict whether there can be condensation and cloud formation at this altitude . This HKN is then referred to as HKN ₂ .

Do you want z. B. For the level (the pressure area) 500 hPa know whether cloud formation is possible, the point of intersection of the dry adiabats with the line with the same saturation mixing ratio of the 500 hPa area is to be designated as HKN ₂ .

Convection condensation level

Convection condensation level (KKN or Cumulus condensation level) describes the height or the level at which an ascending air parcel is first saturated with water vapor and condenses as it ascends further . One important difference to the HKN should be noted: The KKN is not based on a forced uplift as with the HKN, but here it is a question of uplift due to thermal influences on the ground, the thermal . This creates a cloud caused by thermal influences. The rising air condenses at the level of the KKN, and more and more "fresh" air is fed in from below, like in a kind of hose, which allows the cumulative cloud to grow in its vertical thickness. In gliding, this "updraft vent" is also called "thermal hose".

With a suitable temperature stratification (gradient greater than moisture adiabats) and without disruptive influences on cloud formation, such as B. Wind shear in the initial stage or entrainment (drying out of the resulting cloud, Cu fractus) the cumulus would continue to grow upwards until it becomes a thundercloud (cumulonimbus, Cb) and, with the development and expansion of a downdraft zone, in the cold air flows below and thus destroys the updraft area that the cloud needs to grow and live.

During the course of the day (assuming an average summer day), the floor is heated more and more, up to a point at which the layer of air close to the floor is so heated that individual "air packages" (thermal bubbles) dissolve and rise. This point is also known as the trigger temperature . This is a very important parameter, especially for paraglider and glider pilots , the determination of which can influence the entire daily planning. ${\ displaystyle T _ {\ mathrm {A}}}$

Cumulative cloud over the fields of the southern Palatinate , recorded between Kandel and Steinweiler, due to thermal effects

The thermal bubble initially rises dry adiabatically while maintaining its saturation mixing ratio . At some point it reaches water vapor saturation and condenses on further ascent. A cumulus cloud (Cumulus humilis, Cumulus mediocris) arises.

Just like the HKN, the KKN can be graphically determined very easily: Take the intersection of the line with the same saturation mixing ratio with the temperature curve of the TEMPS and the KKN has already been formed. The starting point is again the height 0, which corresponds to the starting level of the balloon ascent. However, even with a morning graphical determination of the KKN, the air humidity close to the ground again plays a very large, falsifying role. As with the other graphical evaluations, this error can be avoided by averaging the lower 30 hPa. The point at which the mean saturation mixing ratio of 30 hPa intersects with the temperature state curve is then referred to as KKN ₁ .

Once the KKN has been determined, the trigger temperature, which is very interesting for meteorologists, can easily be determined. In the graph, you simply go from the KKN determined dry adiabatically back to the starting level and read off the temperature at the point at which you arrive . This then represents the. In order to avoid the falsifying high air humidity, it is possible to determine the from KKN1 by dry adiabatic decrease to the starting level ; due to the exclusion of moisture and the fact that the KKN1 is higher than the KKN, this should also result in a slightly higher trigger temperature. ${\ displaystyle T _ {\ mathrm {A}}}$ ${\ displaystyle T _ {\ mathrm {A}}}$ ${\ displaystyle T _ {\ mathrm {A1}}}$

If you want to determine the KKN in summer in radiant weather, it may be that the KKN1 is still too low and thus also the . This is due to the difference between temperature and dew point, called spread, or dew point difference, which increases during the day on sunny days . This condition can be countered by forming a KKN ₂ . To do this, the entire area between the initial level and KKN _{1 is} averaged . This gives an averaged saturation mixture ratio for the area close to the ground. If you now go along from this mean saturation mixture ratio to the point of intersection with the temperature state curve, then you have formed the KKN ₂ . On the basis of this, it is now also possible to determine which radiation influences in the course of the day and the high humidity values in the morning hours are excluded as sources of error. ${\ displaystyle T _ {\ mathrm {A1}}}$ ${\ displaystyle T _ {\ mathrm {A2}}}$

The KKN can also be approximately determined using a rule of thumb according to Fritz Henning , which includes the current spread:

Spread · 400 = cloud base in feet
Spread 125 = cloud base in m

example

The temperature is 30 ° C and the dew point is 10 ° C. The result is a spread of 20 ° C, because the spread is the difference between temperature and dew point. If you multiply the spread of 20 ° C by 400, you get a calculated lower limit of the cumulus cloud of 8000 ft, or, multiplied by 125, a lower limit of 2500 m.

This formula is used in everyday weather, especially on summer days, because the cumulus clouds are often estimated a little too low. In addition to looking at the TEMP increases, the observer has a clue for the cloud base.

The formula should only be used to determine the lower limits of convective cloud cover , as it makes use of the characteristics of the KKN. Other cloud formation processes, e.g. B. are responsible for the formation of a stratus or stratocumulus layer (sliding processes on a warm front) are not taken into account here.

If one compares HKN and KKN, the statement can be made that the HKN is usually lower than the KKN. However, if the atmosphere is stratified in a dry adiabatic manner at the time of the graphical determination and the balloon ascent up to the HKN, the KKN and HKN can be at the same height.

Free convection level

The level of free convection (NFK) represents the pressure surface from which an air package that was previously forced (or dynamically) raised with the use of energy (e.g. by sliding on a mountain or on a front), now automatically (through the release of latent energies during the condensation of water vapor) continues to rise, i.e. without the necessary supply of external energies (external force). Since the air package above the NFK is sustained warmer than the ambient air - and therefore lighter - the buoyancy continues even without an external, forced lift. Above the NFK, the thermally induced lift, which is increased by the release of latent energies, only collapses when the thermodynamic conditions of the air package (temperature, water content, pressure, density) have adjusted to those of the surroundings. This happens at the level of neutral buoyancy (LNB), also called equilibrium level . The air parcel only comes to a (relative) standstill (equilibrium state) at greater heights, due to the inertia . With this overshoot, the air parcel experiences a negative lift and is thus returned to the level of neutral lift in the theoretical model . The boundary surfaces of the cloud surfaces often have a fractal structure. If the equilibrium is at tropopause level , the saturated air flows out to the side in a visible anvil shape .

Here are some reference values for weather phenomena based on the cloud upper limit temperatures (WO in ° C) and the thickness of the cumulus clouds from the zero degree limit (WD):

Temperature range	Zero degree limit
−10 to −15 ° C	WD 5000-7000 ft	Cu med, con, light rain showers , light snow showers
−15 to −20 ° C	WD 7000-9000 ft	Cu con, moderate rain showers / snow showers, heavy snow showers
−20 to −25 ° C	WD 9000-12000 ft	Cu con, Cb cal, heavy rain showers, heavy snow showers
−25 to −35 ° C	WD 12000-17000 ft	Cb cal, Cb cap, heavy rain showers, light hailstorms.
−35 to −45 ° C	WD 17000-22000 ft	Cb cap, heavy rain showers, moderate hail / sleet thunderstorms .
−45 to −55 ° C	WD 22,000-27,000 ft	Cb cap, inc, severe thunderstorms , hail showers.
Below −55 ° C	WD greater than 27,000 ft	Cb cap, inc, severe weather criteria are met.

Cu ( cumulus ) and Cb ( cumulonimbus ) are the common abbreviations for these types of clouds . “Cal”, “inc” and “cap” denote the subspecies, with “cal” standing for calvus, which describes the shape of the thundercloud as smooth, uniform. "Cap" stands for capillatus and describes a fibrous structure of the cloud upper limit, which is created by icing. “Inc” means incus, the thundercloud resembles an anvil , the image of a typical thundercloud with a heavily iced upper part is given.

Thundercloud / Cb cap, inc over the fields of the southern Palatinate

This table is particularly useful when interpreting satellite images . The table is also well suited for the approximate determination of the expected weather phenomena when evaluating the TEMPS .

The instructions for the graphic determination of the levels apply here specifically to the temperature-pressure diagram (simply logarithmic paper in which the logarithmic axis is used as the pressure axis), not to the Stueve or other diagrams.

A great deal of important data is extracted from the aerological data of the radiosondes in everyday weather forecasting . Much is already calculated by computer , but it is important to understand the relationship between the level of condensation and cloud formation. Various calculations or estimates can also be carried out quickly and easily with the aid of the diagram papers.

Web links

Andreas Kalt: “Adiabatic Processes” learning module. Water in the atmosphere. In: WEBGEO basics / climatology. Institute for Physical Geography (IPG) at the University of Freiburg , April 1, 2003, accessed on December 14, 2010 .
Radiosonde diagrams from around the world
Soundings for central Europe