Download STATION MODEL The "station" what ? Since we started offering you

STATION MODEL
The "station" what ? Since we started offering you the synoptic charts from the South African
Weather Service there have been many queries regarding what all those lines and black circles
with lines sticking into them and those numbers mean.
A synoptic chart means "weather chart" and meteorologists need to plot weather data
collected from hundreds of weather "stations" on a chart - why they are called stations, I don't
know. Some kind of convention had to be adopted so that any meteorologist anywhere in the
world can read such a chart and know what it means. Hence, each weather station on a
synoptic chart is depicted by a circle and data collected at that station is plotted as shown in
the simple example below.
The example above tells us the following :
dry bulb temperature : 16C
dewpoint temperature : 14C
atmospheric pressure : 1008.9 hPa
pressure tendency in last 3 hrs : steady fall of 2.6 hPa
wind direction : NW
windspeed : 30 knots
cloud cover : 8/8 (or 100%)
current weather : rain
cloud base is at 100m
clouds : cumulus and stratocumulus
rainfall past 3 hours : 15,0 mm
visibility : 5km
Wow - all that from a simple little diagram - yes, it is that powerful. Usually, not all the data
is shown, and only temperature, current weather and pressure, cloud and wind data is shown.
Let me explain how I deciphered the station model so that you can become an expert at
interpreting synoptic charts.
Firstly let's pretend the circle is like a clock, so that I can use "the time" as the locator for
data....
Let's start at 11 o'clock - the dry bulb temperature is the actual temperature of the air and is
given in centigrade in South Africa and Fahrenheit in other countries like the USA.
At 7 o'clock we have the dewpoint temperature, which is the temperature at which water
vapor will condense (into cloud/fog) if the air is cooled to that temperature. It is a useful
indicator as to how much moisture is in the air - the closer the dewpoint is to the actual
temperature, the more humid the air is.
At 9 o'clock, we have the "current weather symbol", and there are many symbols representing
many types of weather, and in this case, the single dot means light rain. The table to the left
shows some of the more common types of weather and their symbols (if you go to
meteorology school you have to learn all of them).
Also at 9 o'clock we have the visibility shown in kilometers.
At 2 o'clock we find the air pressure given in hecto Pascals
(used to be millibars, but the metric system calls for it to be
expressed in metric units - 1mb = 1 hPa). Air pressure is
expressed using one decimal place, i.e. 1008.9 hPa and the
last three digits are used, so for example:
1008.9 = 089, 1033.5 = 335, 995.3 = 953
BUT, what happens if the pressure is 1095.3 hPa you ask
would that not mean 953 also ?....don't worry - the highest
pressure ever recorded is about 1080mb and the lowest about
880mb, and neither were recorded in SA, so we never have to
worry about not reading the correct figure from the pressure
data on the station model....to complicate things a little, over the land (non-coastal stations)
the figure at 2 o'clock actually does not represent a pressure reading, but rather the height
above sea-level that a a certain pressure level is found - for example, over the land a number
like 546 on the 850 hPa chart represents a height of 1546m where the 850 hPa pressure
(surface) is found in the atmosphere.
Wind data is represented by the "stick" that points out of the station model. Wind direction is
always given where the winds blow FROM, i.e. as though the winds blow INTO the station,
so in the example above, the wind direction is NORTH WEST (i.e. the wind is coming FROM
the NW or 315�). The speed is represented by "feathers" or "barbs" that are usually
perpendicular to the stick. Each barb is 10 knots and a knot is one nautical mile per hour
which is equal to 1.8 kilometers per hour. So in the example above the wind speed is 20 knots
or 36 km/h. Half a barb is 5 knots. At 50 knots, windspeed is indicated by a solid triangle as
shown below.
Finally, the cloud cover is shown by how much of the station model is filled. See the
examples above.
COLD FRONTS
Cold fronts are mid-latitude phenomena - meaning they are not tropical in nature and occur
mostly south of 25�S or north of 25�N. The word "front" means "boundary" and a cold
front is a boundary between cold and warm air, where cold air is under-cutting (moving
under) warmer air ahead of it. Fronts usually form far south where the westerly windbelt
meets the polar easterlies - somewhere near 60�S. A disturbance may cause the cold easterly
air to invade the warmer westerly air and since cold air is denser, will, move under the warm
air. Warm air could also invade the cooler air and ride up over it since, as you should know,
warm air is less dense and it is force to rise by the colder air beneath it, it will want to rise
further and as it does, air that rises, cools and as it cools, any water vapor in the atmosphere
will condense and form clouds. It is suggested that you familiarise youself with synoptic chart
terminology before reading this article unless you know basic meteorological terms.
FIG.1 Formation of a cold front in the southern hemisphere.
Below is a cross-section through a cold (blue) and warm (red) front - clouds are Ns
(nimbostratus), Ac (altocumulus), As (altostratus), Cs (Cirrostratus), Ci (Cirrus).
FIG.2 Formation of a cold front in the southern hemisphere.
HOWEVER, here is the BIG catch. Most times these cold or warm air invasions into the
warmer westerlies die out and no low pressure cell forms etc...WHY ? Well, the mid-latitudes
are controlled by what is know as a "top-down" process...a "what ?" I hear you ask....
FIG.3 The "top-down" process.
Well, I think most of you have heard of the "jetstream" - this is a strong current of air (like a
river) in the upper atmosphere (10-13km altitude) that snakes around the globe usually
between above the surface westerly wind belt (remember, you name a wind FROM where it
blows, so westerlies mean the winds blow FROM west TO east). Now, these upper
winds/jetstream are FAR stronger than the surface winds...think of a river - the surface waters
flow faster than the bottom waters - why, well, friction slows the water at the bottom of the
river - the atmosphere is NO different....now the jetstream is not a constant speed airflow or
"river", rather, think of it like traffic on a highway - in some places the traffic speeds along
and at other places the traffic slows down to perhaps to a crawl. The jetstream is no different now where it really speeds along in the upper atmosphere, meteorologists refer to it as a
"divergent" area, and where it slows down, a "convergent" area. Once again, think about the
traffic analogy - when cars on a highway move away from a congested area, the cars start
getting spaced further apart as they pick up speed - i.e. the cars are diverging from one
another, and where they encounter a snarl up, they become closer together and "converge" on
one another....
Where air in the upper atmosphere diverges it really means that air is leaving a certain area
faster than it can be replaced, and somehow, it needs to be replaced ASAP - most of it is
replaced from the upper atmosphere, but some of the air is replaced from the surface, almost
like a giant vacuum cleaner in the upper atmosphere sucking up air from the surface. See
figure 3 above.
Now, lets go back to the original scenario that I explained on the surface, i.e. the cold air from
the easterlies undercutting the warm westerly air...the warm is forced upwards since it is less
dense....NOW here is the connection - IF the upper atmosphere /jetstream is DIVERGENT,
i.e. needing air to replace from the surface, it will AID the warm air at the surface (that has
been forced upwards by the cold air) to RISE to the upper levels and the stronger the upperlevel divergence the more air that is needed from the surface and the faster the air will rise
and bingo - a LOW PRESSURE cell is formed on the surface along the boundary between the
cold polar easterly air and the warmer westerly air. This connection between the upper and
lower atmosphere is called the "top-down process" - since a low pressure will not develop
sufficiently into a deep low pressure and cold front if the upper atmosphere does not aid it
with strong divergence aloft.
FIG.4 The upper and low circulations super-imposed - dashed lines are surface isobars (equal
pressure lines) and solid isobars are of the upper-level ciruclation.
Cold fronts with their low pressure cells move eastwards, since the air in the JETSTREAM
moves eastwards and the surface features are "dragged" in the same direction. However, since
the air in the Jetstream moves a lot faster than at the surface, the area of strong divergence
aloft (and which is causing the low pressure at the surface to exist) sometimes OVERTAKES
the surface low pressure and the connection between the upper level divergence and lower
atmosphere low pressure is disrupted and the low pressure at the surface will weaken and
eventually die out and the cold front weakens and dissipates. Hence, sometimes the jetstream
in autumn in the southern Atlantic is such that active cold fronts may be formed in the
southern Pacific Ocean west of Chile and move eastwards and appear strong on satellite
images in the mid Atlantic but weaken and die out or move south of the country owing to the
way the jetstream is aligned in the upper atmosphere.....So, to cut a long story short, at certain
times of the year, the jetstream is further south (in summer) and most of the cold fronts move
south of the country, and during winter, they occur further north and CT gets a lot more rain.
However, sometimes for a few days or even weeks, the jetstream may align itself such that
cold fronts will weaken as they approach CT frustrating forecasters to no end....
THE SYNOPTIC CHART
Since we started offering you the synoptic charts from the South African Weather Service
there have been many queries regarding what all those lines and "H"s and "L"s mean.
A synoptic chart means "weather chart" and meteorologists need to plot weather data
collected from hundreds of weather "stations" on a chart - why they are called stations, I don't
know. Some kind of convention had to be adopted so that any meteorologist anywhere in the
world can read such a chart and know what it means. Hence, each weather station on a
synoptic chart is depicted by a circle and data collected at that station is plotted as shown in
the simple example below.
On the chart above, we see the following :







isobars, or lines joining areas of equal pressures, much like contours on a map
indicating equal heights
pressure values in hecto Pascals, e.g. 1024 hPa indicated by the upper blue oval on
the map above
high pressure cells, i.e. the South Atlantic High Pressure cell is seen SW of Cape
Town
low pressure cells,i.e. the low pressure cell at the bottom of the map associated with a
cold front and
coastal low pressure cell along the Namibian coastline
station model as seen at Cape Town where the wind is NW 15 knots, and the temp is
15C
Air flows from a high pressure which is the center of descending or sinking air (which
suppresses cloud development), and hence clear, skies, to areas of low pressure where air
converges or rises and where clouds usually form. However, air cannot flow across isobars
form high to low pressures, but instead blows parallel to isobars. Well, if air flows parallel to
the isobars, how does air get from the high pressure to the low pressure, surely air would just
go around these cells instead ?
The answer is that in reality, near the surface, air actually does flow across the isobars owing
to friction between the air and the surface of the earth. Hence, flow is across the isobars
towards centers of low pressures and away from centers of high pressures. In the southern
hemisphere, the flows clockwise around low pressures and counter-clockwise around high
pressure cells.
A pressure gradient exists between high pressure cells and low pressure cells. Think of a
high pressure being a mountain and a low pressure being a valley - if you drew a line from the
highest point of the mountain to the lowest point of the valley you would have one side of a
triangle of a right-angle triangle, and little trigonometry would tell you that the ratio of the
height to the horizontal distance is the gradient, the the higher the mountain and lower the
valley, the steeper the gradient.
Well, the stronger a high pressure is (i.e. the higher the pressure) and the deeper a low
pressure is, the stronger the gradient will be, or if the distance is shortened between a high
pressure and low pressure, so too, would the pressure gradient be increased, all resulting in
stronger winds . On a synoptic chart, isobars that are closely packed together indicate stronger
winds, and vice versa. On the map above, the strongest pressure gradient exists between the
South Atlantic High pressure and the deep low pressure (988 hPa) near the bottom of the map
and strong SW winds wil be found in the region between them.
The red arrows indicate the direction of the wind, and remember, wind is named where it
blows from.
TEPHIGRAMS
A "tephi" what ? If you are a pilot or a meteorologist, you are about the only people on this
planet able to decipher this very useful chart.
On the tephigram you will see myriads of criss-crossing lines - they will be explained, but all
this diagram really is telling us what the conditions are like from the surface to the upper
atmosphere. It is essentailly an energy diagram. It tells us which layers are moist and which
are dry, and what the winds are doing at certain altitudes. Data collected by a weather balloon
and radiosonde is plotted on a tephigram.
Let's look at individual features of the tephigram :







dry Bulb temperature sounding line
dewpoint temperature sounding line
significant levels
flight Levels
temperature lines (vertical)
potential temperature lines (horizontal)
pressure level lines (curving from bottom left to upper right)
The lines on the tephigram all represent a particular atmospheric variable, and so let's look at
them all individually without the sounding data plotted on it.
This it the tephigram plot with all the lines on it, but without any data plotted on it. There are
essentially 5 lines on the tephigram :





horizontal lines = dry adiabats (lines of equal potential temp in Kelvin)
vertical lines = isotherms (lines of equal temperature measured in °C)
bottom left to upper right curving lines = isobars (lines of equal pressure in hPa)
top left to bottom right = wet adiabats (lines of equal equivalent potential temp in
Kelvin)
dashed diagonal lines = saturated mixing ratio (lines of equal water vapour content in
g/kg)
Wow, what does all that mean you wonder...let's look at all individually and explain what we
mean.
Firstly, an adiabatic process is one where no transfer of heat takes place. For instance, if if
you took a parcel of air and made it rise into the atmosphere, it will cool as it expands (since
pressure decreases with height), but no heat is gained or lost, sure the temperature drops, but
that is a result of the air molecules being further apart, and not owing to heat loss. Heat is only
lost during diabatic processes such as evaporation and condensation.
Bergwinds are the best examples of adiabatic processes ! It is the reverse of the process of a
parcel of air rising, expanding and cooling - a parcel of air brought to the surface from higher
in the atmosphere, will compress and the temperature will increase at 10°C per 1000m as it
descends. So how does this help me on the tephigram. Well, the process just described with
bergwinds, is usefull - air at a high altitude if brought to 1000m will have a certain
temperature, called the potential temperature which is an indication as to the amount of
energy in the atmosphere. How would you calculate potential temperature - well, we will
explain at the end...
These lines represent the temperature of the atmosphere.
These lines represent the pressure of the atmosphere.
We have already described what an adiabatic process is, and the explanations above assumed
a DRY atmosphere. A wet adiabatic process is a pseudo-adiabatic process in reality, since
there is a slight heat transfer, but we will explain this now. What if a parcel of air that rises
and cools at 10°C per 1000m has moisture in it (like in the real world). You should know that
as air cools, its capacity to hold moisture decreases and at a certain temperature (called the
dewpoint temperature) the water vapour will undergo a phase change, i.e. it will condense into
the liquid form and form cloud or fog droplets. If this parcel of air continues to rise, it cools
less quickly, since condensation has released heat into the environent, i.e. air around the
parcel, so it only cools at about 6.5«C per 1000m until all the moisture is condensed out of the
parcel. When that happens, it will be dry and continue cooling at the dry adiabatic rate,
called a lapse rate of 10°C per 1000m if it is still rising.
However, let's say that we want to know how much energy there is in the atmosphere (which
is always moist) and let's assume that we choose a parcel of air in a cloud and bring it to
1000m. Well, it will warm at 6.5°C per 1000m, and not at 10°C per 1000m since as it warms
the water will evaporate, a process that needs heat to occur, so 3.5°C per 1000m goes into
evaporating the water back into vapour. When that parcel finally reaches 1000m the
temperature it now has is known as the equivalent potential temperature and is a better
measure of the energy in the real atmosphere, than the potential temperature descirbed before.
Finally, the last of the lines - the mixing ratio lines. Mixing ratio merely refers to the amount
of water vapour (grams) in the atmosphere per kilogram of air and is a great measure of the
direct amount of water vapour in the atmosphere.
So, we release a weather balloon with a radiosonde that collects the following data every 10
seconds or less as it rises :




temperature
pressure
humidity
wind direction and speed
So at almost every point in the atmosphere, we know the temperature, pressure, water vapor
content and what the winds are doing.
We plot the temperature measured by the radiosonde against the pressure it was measured at,
and soon we will have the temperature line you saw on the first figure. We also then plot the
dewpoint line which is calculated from the humidity measurements. Finally the wind data is
plotted at certain significant levels (i.e. 850 hPa, 700 hPa, 500 hPa, 400 hPa, 300 hPa, 200
hPa and 100 hPa) or at predetermined flight levels and these winds are plotted in the same
manor that station model winds are plotted.
Many, many different atmospheric variables essential to forecasting can be calculated from
the tephigram. These inlude stability indices, thunderstorm likelihoods, inversions, how
quickly thunderstorms are likely to develop in the day, how strong the SE wind will blow in
Cape Town and many other vairables. The tephigram in the first figure, shows a moist layer
and a dry layer - where the two dark lines are closer together, the more moist the atmosphere,
the further apart, the drier the atmosphere. Needless to say, to try and explain exactly how
these variables are calculated, would only be of interest to pilots and meteorologists, and are
difficult to explain in the context of this article.