The emagram was invented in the year 1884 by H. Hertz. It describes the temperature and the humidity of the air as a function of the height above the ground and/or the sea level. However the humidity is not represented as relative or absolute humidity, but as dew point temperature , i.e. as that temperature, at which the relative humidity would be equal to 100 %. Therefore the dew point temperature is never larger than the real temperature . Thus one can differentiate these two main lines always surely with respect to the emagram, even if they do not (as here) carry different colors.
In the meantime many further such diagrams were invented, for example 1923 the Tephigramm of N. Shaw, 1927 the Stueve diagram of G. Stueve, 1935 the Aerogram of A. Refsdal, 1945 the Pastagramm of J.C. Bellamy and 1947 the SkewT/LogP diagram of N. Herlofson. The latter dominates today and is often called emagram too. Its correct name describes where it differs from the emagram:
LogP means that the ordinate is not the height but the logarithm LogP of the air pressure P. SkewT means that the abscissa is not the temperature T but a linear combination of LogP and T. As a consequence of this the isotherms , i.e. the places of same temperature , are not vertical, but inclined straight lines. This complication is the price for the fact that same energy of air corresponds to same surface in the diagram. This facilitates the interpretation of the diagram, together with the following 5 auxiliary lines, which, contrary to the main lines, do not depend on the current data:
1. Isobars: horizontal straight lines as places of constant air pressure, often marked in hPa.
2. Isotherms: to the left inclined and straight lines as places of constant temperature, often marked in degrees Celsius.
3. Isohumes: more than the isotherms to the left (or even to the right) inclined and straight lines as places of constant absolute air humidity, often marked in g water per kg of air, whereby the relative air humidity is also constant, i.e. 100%.
4. Dry adiabat curves: to the right inclined curves which describe the condition of air bubbles which rise or descend without mixing themselves with the ambient air, whereby their relative humidity remains always smaller than 100%. Adiabats are often marked with the temperature in degrees Celsius or Kelvin which the corresponding bubble has at the air pressure 1000 hPa.
5. Wet adiabat curves: more strongly than the dry adiabat curves to the right inclined curves, which describe the condition of bubbles which rise without mixing themselves with the ambient air, whereby their relative humidity remains always equal to 100%, because surplus humidity condenses at once. Adiabats are often marked with the temperature in degrees Celsius or Kelvin which the corresponding bubble has at the air pressure 1000 hPa.
A bubble rises or sinks as long as its temperature is higher or lower than that of the ambient air. The base of the eventually developing cumulus cloud is at the height where the isohume of the bubble and its dry adiabat cross. From there it rises according to the wet adiabat. A warm air bubble mostly originates from the proximity of the ground. Then its isohume or its dry adiabat starts with the air pressure of the ground and with the dew point temperature or real temperature there.
Roughly said: We may expect the better thermics, the more the sun heats the ground (and thus the near-surface air layer) and the less (compared with the adiabats) the main line of the temperature is inclined to the right. Outside of the cumuli the dry adiabats applie and inside the wet adiabats. (But be careful: Hang gliding in the devils kitchen of a cumulus is too dangerous and therefore forbidden)
Physics theory question: If a bubble has exactly the same temperature compared with its ambient air, but a higher dew point temperature, does it rise then or does it sink? Answer: It rises, because the molecular weight of water is smaller than that of nitrogen and oxygen.
Emagrams are often supplemented at their right with wind arrows. These are represented without head, but with feathers at the tail. The speed of the air is not represented by the length of the arrow, but by the number of the feather lines at the tail. The wind arrow applies to the place where its head is.
The example above is a SkewT/LogP diagram as prognosis from the weather model AVN111 of NOAA, valid on 13 June 2002 for 09h00 local time in London and thus 11h00 local time in Zurich (with the geographical latitude 47.5 degrees and the geographical longitude 8.6 degrees), extrapolated over 15 hours. This means that measurements, which were entered into the model later than 18h00 local time in London on 12 June 2002, did not influence this diagram any more. The wind turns from southwest at the ground to northwest in the height. Colors in the text correspond to those in this diagram.
