DESCRIPTION OF HYDROMETEOROLOGICAL INSTRUMENTS
1. STEVENSON THERMOMETER SHELTER
Maximum thermometer
This thermometer registers the maximum temperature attained during an interval of
time. The liquid-in-glass type of maximum thermometer has a bore that is constricted
between the bulb and graduated portion of the stem. As the temperature rises, a portion
of the mercury is forced past the constriction and into the graduated section. This
mercury is retained when the temperature falls and serves to indicate the highest
temperature reached
Minimum thermometer
A thermometer that automatically registers the lowest temperature attained during an
interval of time. The alcohol-in-glass minimum thermometer contains a dumbbell-
shaped index that is kept on the bulb side of the meniscus by surface tension. The
thermometer is installed in a horizontal mounting as the temperature falls, the index is
pulled toward the bulb and remains at the minimum point as the temperature rises.
2. Aneroid barometer
The Aneroid Barometer is a robust and simple to use, wall-mounting instrument for
general purpose atmospheric pressure measurement. The instrument is of traditional
design set on a dark wood base. The 95mm diameter dial has scales graduated in both
millibars and inches of mercury, a settable pointer and an adjustment screw are also
provided. It is an ideal instrument for observing weather related pressure trends.
Dimensions: 150 diameter x 40 mm depth
3. Psychrometer
An instrument used to measure humidity. It consists of two thermometers exposed side
by side, one of which (the dry bulb) is an ordinary glass thermometer, while the other
(the wet bulb) has its bulb covered with a jacket of clean muslin that is saturated with
distilled water prior to an observation. The temperature measured by the wet-bulb
thermometer is generally lower (due to evaporation of water from the wet bulb) than that
measured by the dry bulb. The difference in the temperatures is a measure of the
humidity of the air; the lower the ambient humidity, the greater the rate of evaporation
and, consequently, the greater the depression of the wet-bulb temperature. A
psychrometer in which the ventilation is provided by a suction fan. The aspirator may be
driven by a spring or an electric motor.
, Dry Bulb, Wet Bulb and Dew Point Temperature
Dry Bulb, Wet Bulb and Dew Point temperatures are commonly used to determine the
state of humid moist air. The knowledge of only two of these values is enough to
determine the state - including the content of water vapor and the sensible and latent
energy (enthalpy).
Dry Bulb Temperature - Tdb
The Dry Bulb temperature, usually referred to as air temperature, is the air property that
is most common used. The Dry Bulb Temperature refers basically to the ambient air
temperature. It is called "Dry Bulb" because the air temperature is indicated by a
thermometer not affected by the moisture of the air.
Dry-bulb temperature - Tdb, can be measured using a normal thermometer freely
exposed to the air but shielded from radiation and moisture. The temperature is usually
given in degrees Celsius (°C) or degrees Fahrenheit (°F). The dry-bulb temperature is
an indicator of heat content and is shown along the bottom axis of the psychrometric
chart. Constant dry bulb temperatures appear as vertical lines in the psychrometric
chart.
Wet Bulb Temperature - Twb
The Wet Bulb temperature is the temperature of adiabatic saturation. This is the
temperature indicated by a moistened thermometer bulb exposed to the air flow. Wet
Bulb temperature can be measured by using a thermometer with the bulb wrapped in
wet muslin. The adiabatic evaporation of water from the thermometer and the cooling
effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in
the air. The rate of evaporation from the wet bandage on the bulb, and the temperature
1. STEVENSON THERMOMETER SHELTER
Maximum thermometer
This thermometer registers the maximum temperature attained during an interval of
time. The liquid-in-glass type of maximum thermometer has a bore that is constricted
between the bulb and graduated portion of the stem. As the temperature rises, a portion
of the mercury is forced past the constriction and into the graduated section. This
mercury is retained when the temperature falls and serves to indicate the highest
temperature reached
Minimum thermometer
A thermometer that automatically registers the lowest temperature attained during an
interval of time. The alcohol-in-glass minimum thermometer contains a dumbbell-
shaped index that is kept on the bulb side of the meniscus by surface tension. The
thermometer is installed in a horizontal mounting as the temperature falls, the index is
pulled toward the bulb and remains at the minimum point as the temperature rises.
2. Aneroid barometer
The Aneroid Barometer is a robust and simple to use, wall-mounting instrument for
general purpose atmospheric pressure measurement. The instrument is of traditional
design set on a dark wood base. The 95mm diameter dial has scales graduated in both
millibars and inches of mercury, a settable pointer and an adjustment screw are also
provided. It is an ideal instrument for observing weather related pressure trends.
Dimensions: 150 diameter x 40 mm depth
3. Psychrometer
An instrument used to measure humidity. It consists of two thermometers exposed side
by side, one of which (the dry bulb) is an ordinary glass thermometer, while the other
(the wet bulb) has its bulb covered with a jacket of clean muslin that is saturated with
distilled water prior to an observation. The temperature measured by the wet-bulb
thermometer is generally lower (due to evaporation of water from the wet bulb) than that
measured by the dry bulb. The difference in the temperatures is a measure of the
humidity of the air; the lower the ambient humidity, the greater the rate of evaporation
and, consequently, the greater the depression of the wet-bulb temperature. A
psychrometer in which the ventilation is provided by a suction fan. The aspirator may be
driven by a spring or an electric motor.
, Dry Bulb, Wet Bulb and Dew Point Temperature
Dry Bulb, Wet Bulb and Dew Point temperatures are commonly used to determine the
state of humid moist air. The knowledge of only two of these values is enough to
determine the state - including the content of water vapor and the sensible and latent
energy (enthalpy).
Dry Bulb Temperature - Tdb
The Dry Bulb temperature, usually referred to as air temperature, is the air property that
is most common used. The Dry Bulb Temperature refers basically to the ambient air
temperature. It is called "Dry Bulb" because the air temperature is indicated by a
thermometer not affected by the moisture of the air.
Dry-bulb temperature - Tdb, can be measured using a normal thermometer freely
exposed to the air but shielded from radiation and moisture. The temperature is usually
given in degrees Celsius (°C) or degrees Fahrenheit (°F). The dry-bulb temperature is
an indicator of heat content and is shown along the bottom axis of the psychrometric
chart. Constant dry bulb temperatures appear as vertical lines in the psychrometric
chart.
Wet Bulb Temperature - Twb
The Wet Bulb temperature is the temperature of adiabatic saturation. This is the
temperature indicated by a moistened thermometer bulb exposed to the air flow. Wet
Bulb temperature can be measured by using a thermometer with the bulb wrapped in
wet muslin. The adiabatic evaporation of water from the thermometer and the cooling
effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in
the air. The rate of evaporation from the wet bandage on the bulb, and the temperature
