BÀI GIẢNG KHÍ TƯỢNG THỰC HÀNH CHƯƠNG 3

CHAPTER 3 Observation of Air Temperature and Dew-point Temperature

CHAPTER 3

Observation of Air Temperature and Dew-point
Temperature

The horizontal distribution of the temperature in the atmosphere causes the distribution of the
atmospheric density, as well as giving rise to flow of the air (wind), clouds and precipitation
phenomena. Therefore, the observation of the atmospheric temperature is one of the basic
elements to conduct a synoptic aspect of the meteorological analysis.
The water vapor in the atmosphere is the source of clouds and precipitation phenomena. It is
also involved in the heat transfer in terms of latent heat through evaporation and condensation.
Since the water vapor absorbs or emits infrared radiation, it also affects the variation of the
atmospheric radiation. Therefore the water vapor is playing an important role in the atmospheric
phenomena.
3.1 Definition and unit
The temperature of the atmosphere is called air temperature.
The partial pressure of the water vapor in the atmosphere is called water vapor pressure or is
simply called vapor pressure in meteorology. The maximum value of the vapor pressure in the
atmosphere at a given temperature can be determined and is called saturated vapor pressure. Its
value varies depending on whether the atmosphere faces water or ice. The ratio of the vapor
pressure to the saturated vapor pressure at a given temperature is called relative humidity. Dewpoint temperature is defined as the temperature to which the air must be cooled down at constant
vapor pressure in order to reach saturation.
Air temperature and dew-point temperature should be recorded in the unit of degree Celsius
up to the order of one-tenth.
3.2 Measuring instruments and their installing conditions
For the observation of air temperature and dew-point temperature, an aspiration (ventilated)
psychrometer, an assmann psychrometer, a sling psychrometer, a non-ventilated psychrometer,
and an electric psychrometer (combination of an electric thermometer and a dew-point
hygrometer or an electric humidity hygrometer) are used (see Fig.3.1-Fig.3.4).

By the dew-point hygrometer the dew-point temperature is measured directly.
The non-ventilated psychrometer is not recommended for use because it tends to show the
temperature of the air just near the instrument, which is not representative of the air temperature
around the observation site.
The aspiration psychrometer should be screened in an instrument shelter (see Fig.3.5).
To make accurate temperature observation, instruments should be well exposed in a fresh air
stream from the sea, which has not been in contact with or passed over the ship. They should also
be adequately shielded from spray, precipitation or the heat radiated by the sun, the sea and from
the ship itself.

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CHAPTER 3 Observation of Air Temperature and Dew-point Temperature

3.3 Dry and wet-bulb thermometer
3.3.1 Maintenance of wet-bulb thermometer
Wet-bulb temperature is observed with a wet-bulb thermometer which is a thermometer
covered by a wet gauze (or muslin wicking) over the thermometer bulb. A thermometer
without any covers is called a dry-bulb thermometer. The wet-bulb thermometer is set on the
left side of the aspiration psychrometer and on the outer projecting side of the sling
psychrometer. The gauze has to be wet with the purest water available (preferably distilled

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CHAPTER 3 Observation of Air Temperature and Dew-point Temperature

water). Any minerals or other impurities would change the evaporation characteristics of the
water. If the gauze gets dirty or absorbs sea spray, it should be replaced immediately. The gauze

should be renewed at least once a month with normal use. The thermometer bulbs should also
be kept clean. When you change the gauze, examine whether the thermometer bulbs are clean.
Fig.3.6 shows various types of wet-bulb covered by wet gauze.

3.3.2 Observation by psychrometer
About 10 minutes before observation,
you have to make wet the thermometer
bulb of the wet-bulb thermometer with
pure (preferably distilled) water by a
squirt without splashing water droplets
over other parts of the psychrometer. It
takes about 5 minutes to get a stabilized
value of the wet-bulb temperature when
the air temperature is more than O°C. But
it sometimes takes 10-20 minutes when
the wet-bulb is frozen.
You should hold the thermometer in
front of you facing the wind such that the
dry-bulb is not downwind of the wetbulb
thermometer. You should also shade the
psychrometer from the sun. Read the drybulb thermometer first and then the wetbulb thermometer as soon as possible to
avoid any influence of your body heat or
breaths. Read the 1st

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CHAPTER 3 Observation of Air Temperature and Dew-point Temperature

decimal place of the scale first, then read the higher places.

When reading a thermometer, care should be taken to keep your eyes at the vertical position,
otherwise there will be an error due to parallax (see Fig.3.7) for all types of liquid-in glass
thermometers.
The minimum unit of graduation for a thermometer is generally O.5°C or O.2°C. For
meteorological observation, that of 0.2°C is recommended and the reading should be given by
estimation to the nearest tenth of a degree.
3.3.3 How to get dew-point temperature
Unless you are using a dew-point hygrometer, you need to get the dew-point temperature as
follows by making use of a table "Table for Finding the Dew-point Temperature" published separately
by the JMA or table for saturated vapor pressure (see Appendix 2).
(1) Method by using "Table for Finding the Dew-point Temperature"
The table shows the dew-point temperature under the given values of the difference between the
dry-bulb temperature and wet-bulb temperature (depression of wet-bulb temperature: t-t') and the wetbulb temperature (t'). But the table gives you the value in the unit of O.5°C. You are requested to
calculate it in the unit of O.l°C through interpolation as shown in the following examples:
Example A)
dry-bulb temperature (t) = 20.8°C
wet-bulb temperature (t') = 19.4°C tt' = 1.4
In the table you can find:
dew-point temperature = 19.0 for t-t' = 1.0 and t' = 19.5 (the nearest value for 19.4), and
dew-point temperature = 18.8 for t-t' = 1.5 and t' = 19.5.
Then make interpolation to get the dew-point temperature for t-t' = 1.4 and t' = 19.5:
19.0 + (18.8-19.0)x(1.4-1.0)j(1.5-1.0) = 18.8
Finally make interpolation to get the dew-point temperature for t-t' = 1.4 and t' = 19.4, assuming
the variance of t' is approximately the same with that of the dew-point temperature (19.4 -19.5 = 0.1):
18.8 - 0.1 = 18.7°C

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CHAPTER 3 Observation of Air Temperature and Dew-point Temperature

3.4.2 Dew-point hygrometer
The sensor of a dew-point hygrometer, which is coated with lithium chloride, uses
characteristics of the lithium chloride which tend to absorb water vapor in the air. When the
vapor pressure of the liquor of lithium chloride is saturated, the liquor's temperature is closely
related to dew-point temperature of the air. This sensor is usually enclosed in a cylinder other
than that for the thermometer of air temperature, because the sensor generates heat. Comparison
check with the dry and wet-bulb psychrometer is needed to this instrument. If a difference of
dew-point temperature between them is more than 1°C, lithium chloride should be recoated.
3.4.3 Electric humidity hygrometer
The hygrometer has a capacitance sensor of a non-conductive high polymer film. The
sensor uses characteristics of hygroscopicity in the film to measure relative humidity as a
function of capacitance value of the sensor. This sensor is usually enclosed in a same cylinder of
that for air temperature. Adequate wind speed is about 4 m/ s. Comparison check with the dry
and wet-bulb psychrometer is also needed to this instrument.

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