Upper Level Observations

Atmospheric phenomena are not two dimensional, and are not limited to the surface level. The atmosphere is a layer, tens of kilometers width, thus it is impossible to understand, what is happening inside of it, without measurements along with height.

Surface measurements can be compared to a doctor trying to assess one's illness only by looking at, or touching his forehead by hand. The same way, the modern medicine requires blood tests, x-ray and so on, in order to learn and understand the inner parts of the body, so is meteorology, requiring upper observations to better understand the atmosphere.

Upper level data are critical to understand the clouds formation processes – are the conditions sufficient for rain clouds to develop? Thunderstorms? Are the conditions sufficient for low clouds and fog development?

Upper level data are especially important for aviation. It is a known fact, that planes do not fly at surface level, thus the conditions for their flight can be identified by upper level balloons. The upper level data are being used to construct wind and temperature maps for the flight heights, so the aviation companies could use them in order to calculate the fuel consumption and planning the flight route as well as for mapping the areas and the heights, where the weather might dangerous for flying.

Vertical profile of the temperature is essential to identify freezing level and the levels at which icing danger might occur: levels at which the air temperature is lower than zero, though the water droplets are still liquid (super chilled). These droplets might freeze once touching the body of the plane.

Vertical profile of the wind is essential for identifying strong wind affecting aviation such as jet stream at high levels and shear winds at the lower level. Jet stream at the upper levels is an extremely strong wind that can effect considerably flight duration, thus it is important to recognize its location and strength.

(Flying in the same direction as the Jet Stream results in a much shorter flying time than flying against thus its influence can be minimized by changing flight altitude). A strong vortex exists at the low layer of the Jet Stream, also known as air pockets, which are felt by passengers during flights.

The upper level measurements are being performed by using sensors that are connected to a balloon transmitting the data wireless to the ground. That is the radiosonde.

The balloon is inflated nowadays by helium, and gets larger and larger as it rises to higher altitude, where the pressure becomes smaller and smaller. At about the altitude of 20-30 km, the balloon explodes and the sensors slowly fall to the ground using a parachute.

Along its rising and falling, the radiosonde performs measurement of the pressure, temperature and humidity at the different atmospheric levels. The position of the balloon is set by a global positioning system (GPS) and according the leeway of the balloon, it is possible to calculate the wind speed and direction at different height levels.

In order to construct a three dimensional global image of the atmosphere, meteorological services worldwide fly radiosondes simultaneously, at distinct hours.

Here, at Bet Dagan, the radiosondes are flown twice a day, at the main hours determined by the WMO –00 and 1200 UTC.

In addition, pilot balloons are being flown at 0600 and 1800 UTC. The pilot has no sensor and no GPS. Tracking the pilot is performed by using a theodolite, and the wind speed and the direction can be calculated upon knowing the balloon's location.

As the tracking of the balloon depends on the ability of the observer to see the balloon, wind measuring by the pilot is limited to several kilometers only. 

The upper level observations page allows to watch the radiosonde data – altitude, pressure, temperature, wet bulb temperature and wind, via table as well as via a thermodinamic diagram, also known as a "tephigram".

The tephigram is used to identify stability conditions of the atmosphere at different levels, and the chances for a fog or for convective clouds. It shows the temperature (on the right, solid line) and the wet bulb temperature (on the left, dotted line), all in one form where isothermal lines (vertical), isopotential temperature lines (horizontal) and isobars (oblique, rounded) are also drawn.

In addition to the updated radiosonde data, data from a 24 hours earlier fly is also drown.

The difference between the two flies is being colored, to emphasize the change in the atmosphere. Warming up of an atmospheric layer is colored in red, and cooling down in blue. Also, yellow indicates a decrease in humidity, and green - an increase in humidity.