Pressure
AVM 303 Aviation Meteorology
CACS 209 General Meteorology
Introduction
• Role of Atmospheric Pressure in Weather Prediction:
• Pressure changes have historically been linked to weather changes.
• A falling barometer (glass) often signals the approach of bad weather.
• Significance in Aviation:
• Atmospheric pressure is foundational to understanding meteorological phenomena.
• Pressure patterns help predict weather systems, influencing flight planning.
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Quote from the Handbook of Aviation Meteorology
"The study of atmospheric pressure may be said to form the foundations of
the science of meteorology."
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Atmospheric Pressure
• Definition: Atmospheric pressure is the force per unit area exerted by the atmosphere on any
surface in contact with it.
• Conceptualization:
• Pressure can be visualized as the weight of a column of air above a surface.
• The higher the surface, the less weight of air is pressing down, thus lower pressure at higher altitudes.
• Pressure and Altitude: Atmospheric pressure decreases with an increase in height.
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The Weight of the Atmosphere on the
Surface of the Earth
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Units of Measurement
• Standard Unit of Force:
• Newton (N) is the standard unit of force.
• Atmospheric pressure at sea level = 101,325 N/m² or 1013.25 hPa.
• Units of Atmospheric Pressure:
• Hectopascal (hPa) = 1 millibar (mb) (Earlier system).
• Mean Sea Level Pressure (MSLP) in ISA:
• 1013.25 hPa
• 29.92 inches of mercury
• 760 mm of mercury
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Mercury Barometer
• Definition: Instrument used to measure atmospheric pressure.
• Working Principle: Atmospheric pressure is measured by the height of a column of
mercury.
• Units of Measurement:
• Height can be read in terms of:
• Inches of mercury (commonly used in the USA).
• Millimeters of mercury.
• Hectopascals (hPa) or millibars (mb).
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A Mercury Barometer
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Aneroid Barometer
• Definition: A compact device for measuring atmospheric pressure.
• Mechanism:
• Consists of partially evacuated capsules.
• Capsules expand and contract in response to pressure changes.
• Changes are transferred via a system of levers.
• A pointer moves over a scale to indicate the pressure.
• Advantages:
• More compact and portable than mercury barometers.
• Does not use liquids.
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An Aneroid Barometer
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Met Office Aneroid Barometer
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Barograph
• Definition: A barograph is a device • Purpose:
used to continuously record • Measures pressure tendency
atmospheric pressure changes. (changes in pressure over a period).
• Mechanism: • Pressure tendency is vital for
meteorological forecasting.
• A paper-covered rotating drum
replaces the scale of a standard • Application: Used by meteorologists
barometer. for weather prediction and analysis.
• Records the rise and fall of
atmospheric pressure over time.
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A Barograph
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Pressure Variation with Height
• As height increases, atmospheric pressure decreases.
• Barometric lapse rate: Rate at which pressure falls with height.
• At higher altitudes, the pressure lapse rate decreases.
• Warm air causes a slower fall in pressure with height.
• Cold air causes a faster fall in pressure with height.
• Effect of temperature: Higher pressure at a given altitude over warm air; lower
pressure over cold air.
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Diurnal Variation
• Daily pressure change, typically around 1 hPa in temperate latitudes and
up to 3 hPa in the tropics.
• Likely due to the natural oscillation of the atmosphere over a 12-hour
period, maintained by the 24-hour temperature cycle.
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Diurnal Variation
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Types of Pressure
QFE (Field Elevation Pressure):
• Atmospheric pressure measured at the aerodrome reference point.
• Altimeter reads zero feet when the aircraft is on the aerodrome.
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Types of Pressure
QNH (Sea Level Pressure):
• Barometric pressure at the airfield (QFE), adjusted to mean sea level
(MSL) using ISA temperature and pressure lapse rate.
• Does not account for temperature deviations from ISA.
• When QNH is set, the altimeter reads the elevation of the aerodrome.
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Types of Pressure
QFF (True Sea Level Pressure):
• Forecasters use QFF to calculate the true mean sea level pressure for
accurate weather analysis and forecasting.
• Accounts for actual temperature deviations.
• May differ from QNH due to temperature differences affecting the
pressure lapse rate.
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Why is the study of atmospheric pressure crucial in meteorology?
A) It controls temperature variations
B) It influences aircraft altitude
C) It predicts weather changes
D) It affects cloud formation
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Why is the study of atmospheric pressure crucial in meteorology?
A) It controls temperature variations
B) It influences aircraft altitude
C) It predicts weather changes
D) It affects cloud formation
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What is atmospheric pressure?
A) The force per unit area exerted by the atmosphere on a surface
B) The speed of wind in the atmosphere
C) The temperature change with height
D) The weight of the Earth’s atmosphere
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What is atmospheric pressure?
A) The force per unit area exerted by the atmosphere on a surface
B) The speed of wind in the atmosphere
C) The temperature change with height
D) The weight of the Earth’s atmosphere
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What happens to atmospheric pressure as altitude increases?
A) It increases
B) It remains constant
C) It fluctuates
D) It decreases
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What happens to atmospheric pressure as altitude increases?
A) It increases
B) It remains constant
C) It fluctuates
D) It decreases
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Which of the following units are equivalent to 1 hPa?
A) 1 inch of mercury
B) 1 millibar
C) 1000 Pascals
D) 1 Joule
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Which of the following units are equivalent to 1 hPa?
A) 1 inch of mercury
B) 1 millibar
C) 1000 Pascals
D) 1 Joule
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The height of the mercury column in a barometer is directly proportional to which of
the following?
A) Temperature
B) Humidity
C) Atmospheric pressure
D) Wind speed
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The height of the mercury column in a barometer is directly proportional to which of
the following?
A) Temperature
B) Humidity
C) Atmospheric pressure
D) Wind speed
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Which of the following is an advantage of an aneroid barometer over a mercury
barometer?
A) More compact and portable
B) Requires less maintenance
C) Uses liquid for more accurate measurements
D) More expensive and complex
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Which of the following is an advantage of an aneroid barometer over a mercury
barometer?
A) More compact and portable
B) Requires less maintenance
C) Uses liquid for more accurate measurements
D) More expensive and complex
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What feature distinguishes a barograph from an aneroid barometer?
A) Use of mercury
B) Digital display
C) No mechanical components
D) A rotating drum that records pressure changes
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What feature distinguishes a barograph from an aneroid barometer?
A) Use of mercury
B) Digital display
C) No mechanical components
D) A rotating drum that records pressure changes
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Which of the following is a key use of a barograph?
A) Measuring humidity
B) Forecasting wind speed
C) Recording continuous pressure changes
D) Measuring temperature variation
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Which of the following is a key use of a barograph?
A) Measuring humidity
B) Forecasting wind speed
C) Recording continuous pressure changes
D) Measuring temperature variation
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What is the relationship between height and atmospheric pressure?
A) Pressure increases with height
B) Pressure decreases with height
C) Pressure remains constant with height
D) Pressure varies randomly with height
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What is the relationship between height and atmospheric pressure?
A) Pressure increases with height
B) Pressure decreases with height
C) Pressure remains constant with height
D) Pressure varies randomly with height
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What does the altimeter read when QFE is set?
A) Mean sea level altitude
B) Zero feet when the aircraft is on the aerodrome
C) Aerodrome elevation
D) Pressure altitude
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What does the altimeter read when QFE is set?
A) Mean sea level altitude
B) Zero feet when the aircraft is on the aerodrome
C) Aerodrome elevation
D) Pressure altitude
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Atmospheric pressure plays a fundamental role in the science of meteorology.
A) True
B) False
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Atmospheric pressure plays a fundamental role in the science of meteorology.
A) True
B) False
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Atmospheric pressure increases with altitude.
A) True
B) False
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Atmospheric pressure increases with altitude.
A) True
B) False
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The pressure at a higher altitude is greater than that at a lower altitude.
A) True
B) False
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The pressure at a higher altitude is greater than that at a lower altitude.
A) True
B) False
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Atmospheric pressure is the force exerted by the weight of the air above a surface.
A) True
B) False
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Atmospheric pressure is the force exerted by the weight of the air above a surface.
A) True
B) False
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1 hPa is equivalent to 1 millibar.
A) True
B) False
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1 hPa is equivalent to 1 millibar.
A) True
B) False
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The mercury barometer measures atmospheric pressure by the height of a mercury
column.
A) True
B) False
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The mercury barometer measures atmospheric pressure by the height of a mercury
column.
A) True
B) False
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An aneroid barometer contains a column of mercury to measure pressure.
A) True
B) False
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An aneroid barometer contains a column of mercury to measure pressure.
A) True
B) False
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The aneroid barometer works by the expansion and contraction of partially
evacuated capsules.
A) True
B) False
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The aneroid barometer works by the expansion and contraction of partially
evacuated capsules.
A) True
B) False
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A barograph is used to measure wind speed.
A) True
B) False
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A barograph is used to measure wind speed.
A) True
B) False
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A barograph records atmospheric pressure changes continuously.
A) True
B) False
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A barograph records atmospheric pressure changes continuously.
A) True
B) False
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Why is the study of atmospheric pressure
important in meteorology, particularly for
aviation?
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Why is the study of atmospheric pressure
important in meteorology, particularly for
aviation?
The study of atmospheric pressure is critical in meteorology because it helps
predict weather patterns. For aviation, understanding pressure variations is crucial
for flight safety, planning, and navigating weather hazards, as changes in pressure
often indicate the approach of different weather systems that can affect flight
conditions.
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Explain the connection between atmospheric
pressure changes and weather prediction.
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Explain the connection between atmospheric
pressure changes and weather prediction.
Atmospheric pressure changes are directly related to weather patterns. A
decrease in pressure, often signaled by a falling barometer, is usually associated
with the approach of low-pressure systems, which can bring bad weather, such as
storms and precipitation. Conversely, rising pressure often indicates improving
weather conditions.
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Describe atmospheric pressure and how it
changes with altitude.
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Describe atmospheric pressure and how it
changes with altitude.
Atmospheric pressure is the force exerted by the atmosphere on a surface,
measured as the weight of a column of air above that surface. As altitude
increases, there is less air above, resulting in lower atmospheric pressure. This
means that pressure decreases with height.
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How does a mercury barometer work?
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How does a mercury barometer work?
A mercury barometer measures atmospheric pressure by the height of a column of
mercury in a glass tube. The atmospheric pressure pushes on a reservoir of
mercury, causing the mercury to rise or fall in the tube. The height of the mercury
column represents the atmospheric pressure and can be measured in inches or
millimeters of mercury, hectopascals (hPa), or millibars (mb).
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Why is mercury used in barometers for
measuring atmospheric pressure?
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Why is mercury used in barometers for
measuring atmospheric pressure?
Mercury is used in barometers because it is dense, which allows for a manageable
column height to measure atmospheric pressure. Its high density means the height
of the mercury column can precisely reflect pressure changes without requiring an
excessively tall instrument. Additionally, mercury does not evaporate easily, making
it suitable for long-term use in measuring atmospheric pressure.
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Why is pressure tendency an important
forecasting tool for meteorologists?
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Why is pressure tendency an important
forecasting tool for meteorologists?
Pressure tendency is crucial for meteorologists because it helps them predict
weather changes by tracking how atmospheric pressure rises or falls over time. A
falling pressure often indicates approaching bad weather, while rising pressure
suggests improving conditions.
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Explain the concept of diurnal variation in
atmospheric pressure.
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Explain the concept of diurnal variation in
atmospheric pressure.
Diurnal variation refers to the small daily change in atmospheric pressure, typically
about 1 hPa in temperate regions and up to 3 hPa in the tropics. This variation is
believed to be caused by a natural oscillation of the atmosphere, which has a 12-
hour cycle and is influenced by the 24-hour temperature variation. It is important for
understanding pressure tendency in weather forecasting.
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What does QFE represent, and how is it used
in aviation?
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What does QFE represent, and how is it used
in aviation?
QFE is the atmospheric pressure measured at the aerodrome reference point.
When QFE is set on the altimeter, the instrument reads zero feet when the aircraft is
on the aerodrome, representing the height above the airfield rather than mean sea
level.
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