S7 Vertical Structure 2
What can tephigrams tell us about vertical structure of the
atmosphere?
S7.1 An operational tephigram: the basics
The tephigram is a plot of temperature T versus specific entropy, cp ln , as discussed
in theory lecture T3. A typical operational tephigram is that used by the UK
Meteorological Office. A sample will be given out in the lecture. Keep this with your
notes. It can come in handy for all kinds of calculations and estimates, both in the
practicals and in the problems.
Dry variables on the tephigram:

Isobars are the roughly horizontal lines; a thicker isobar is drawn every 10 kPa
(100 mb) and the fine isobars are every 1 kPa (10 mb). They are labelled with the
pressure in units of mb. If you look carefully with a ruler, you will find that
isobars are neither quite straight nor evenly spaced.

Isotherms (constant T) are the straight, solid lines going diagonally across the
chart from lower left to upper right. A ruler reveals that these are indeed straight
lines, evenly spaced. They are labelled with the temperature (in units of C) along
the p = 100 kPa isobar. Thick isotherms are drawn every 10 K and thinner
isotherms every 1 K. The important 0C isotherm (the “freezing line”) is shown as
a heavy dashed line.

Isentropes (constant specific entropy) are the thinner straight lines which run
diagonally from lower right to upper left. The lines are labelled with the
corresponding values of potential temperature , and drawn every 10 K, but since
the primary variable is s = cp ln , the isentropes, although straight, are not evenly
spaced.
S7.2 Introduction to moist quantities on the tephigram
The other lines on the tephigram pertain to the moisture content of the air. Two
further curves are important. The theory of these curves will be discussed in lecture
T4.

A set of dashed lines. Each of these joins together points where the saturated
specific humidity has some constant value. The contours are irregularly spaced
and the values are given along the bottom of the tephigram, in units of g kg-1. Note
that for air at a given pressure, and with a given particular specific humidity, these
lines meet the isobar at the dewpoint temperature.

The final, and rather important set of contours, are the curved lines, called
saturated adiabats. Suppose a parcel of saturated air is lifted. Immediately, water
vapour is condensed, and its latent heat of condensation goes to warm the air. As a
result, the parcel does not follow an isentrope as it would were it dry or
unsaturated. Instead, the parcel takes a path to the right (or warmer side) of the
isentrope. These saturated adiabats are labelled with the value of temperature on
them at a pressure of 100 kPa. This value is called the wet bulb potential
temperature.
A diagram illustrates these different curves.
w = constant
r s = constant
 = constant
T = constant
p = constant
Moist variables on the tephigram.
S7.3 Examples of what different types of airmass look like on a tephigram
Moist airmasses have the dewpoint temperature being almost equal to the temperature
for a significant part of the ascent. A thin layer where this is the case could indicate
cloud.
Drier airmasses have a larger difference between temperature and dewpoint
temperature. Often different parts of the ascent show different moisture characteristics
i.e. there might be dry and wet layers overlying each other, this can be a sign of fronts
and changes in airmasses. Also if an airmass has been over the ocean for a
considerable length of time it is often wet at low levels. With night time ascents it
should be possible to see the stable nocturnal boundary layer.
If the temperature increases with height for a small distance in the atmosphere, this is
called an “inversion” and can be associated with frontal surfaces or cloud. At higher
heights in the atmosphere you may see the temperature rising steadily as the height
increases – this is an indication that the radiosonde is sampling the stratosphere. This
part should also be dry.
S7.4 How to estimate heights and thicknesses on a tephigram
The tephigram can be used to estimate the height of pressure surfaces and the
thickness of layers of the atmosphere. Two methods are available:


The first is Very Crude and cannot be recommended for most purposes. The
numbers along the left hand side of the tephigram represent “standard heights” of
the different pressure levels above mean sea level. These standard heights are
based on a particular profile of temperature with height called the “ICAO
Standard Atmosphere”, which can be thought of as a schematic global mean
variation of temperature with height. If the actual temperature differs significantly
from the ICAO atmosphere, these heights will be in error, although they do serve
to give an order of magnitude estimate.
Much better, one can use the numbers plotted along the 95, 85, 75, 65 kPa, etc.,
isobars. These are the thickness in dm of the 10 kPa slab of atmosphere centred on
that level when its mean temperature takes the value at that point. For example,
suppose that the mean temperature of the 70 to 60 kPa slab were 0C; then we can
read off the thickness as 1233 m. You could use the hydrostatic relation to verify this
result. Using these numbers, the thickness of a number of slabs can be added up to
give the thickness of a deep layer of the atmosphere for any observed temperature
profile. If the 100 kPa height is known (and it can be calculated easily from MSL
pressure), then the height of any given pressure surface can be estimated.
S7.5 How to find out about clouds using a tephigram.
Tephigrams provide us with an elegant way to estimate where cloud is likely to be in
the atmosphere. The theory behind these methods will be discussed in T4 next week,
however we can use them before then.
Cloud Base: we assume that the cloud base forms at the level at which an air parcel
becomes saturated (its Lifting Condensation Level). Once saturated, any further lifting
of the parcel causes water vapour to condense and the parcel to become cloudy.
[Cloud will only form here if there is some method (i.e. convection or lifting over
mountains etc) that will lift the air parcel to this level in the atmosphere.
We can find cloud base if we know any 2 of the dewpoint, temperature and wetbulb
temperature at the ground (or whichever level we care about). Then we use Normands
Construction to find the Lifting Condensation Level.
“The line of constant rs passing through the dewpoint temperature, the line of
constant w passing through the wet bulb temperature and the line of constant
passing through the dry bulb temperature all meet at the lifting condensation
level.”

Cloud top: once the parcel has reached it’s lifting condensation level and become
cloudy, if it is lifted further it will follow a saturated adiabat until the adiabat crosses
the temperature curve. Once the parcel is colder than the surrounding air it will no
longer be able to rise and we use this level as an indication of cloud top.