☁️Meteorology Unit 7 – Atmospheric Pressure and Wind
Atmospheric pressure and wind are fundamental concepts in meteorology, shaping weather patterns and climate. These forces drive air movement, creating high and low-pressure systems that influence our daily weather experiences.
Understanding pressure gradients, wind formation, and measurement techniques is crucial for predicting weather patterns. From sea breezes to jet streams, various wind types play significant roles in atmospheric circulation and local weather phenomena.
Atmospheric pressure refers to the force exerted by the weight of the atmosphere on a unit area of surface
At sea level, standard atmospheric pressure is 1013.25 millibars (mb) or 29.92 inches of mercury (inHg)
Pressure decreases with increasing altitude because there is less air above to exert force
High-pressure systems have air sinking and diverging at the surface, resulting in generally clear, stable weather conditions
Low-pressure systems have air rising and converging at the surface, often associated with cloudy, unstable weather and precipitation
Pressure differences between high and low-pressure systems drive atmospheric circulation and wind patterns
Isobars are lines on a weather map connecting points of equal pressure, helping to visualize pressure gradients and wind flow
How Pressure Changes with Height
Pressure decreases exponentially with increasing altitude in the atmosphere
The rate of pressure decrease is approximately 1 mb per 10 meters in the lower atmosphere
At higher altitudes, the rate of pressure decrease slows down due to the thinning of the atmosphere
The pressure at the top of Mount Everest (8,848 m) is about one-third of the pressure at sea level
The relationship between pressure and height is described by the hypsometric equation, which takes into account temperature and humidity
In the International Standard Atmosphere (ISA) model, pressure decreases by half for every 5.5 km increase in altitude
Pressure changes with height affect the density of air, which has implications for aircraft performance and weather patterns
Aircraft require longer runways for takeoff at high-altitude airports due to the reduced air density
Measuring Atmospheric Pressure
Atmospheric pressure is measured using a barometer, which can be either a mercury barometer or an aneroid barometer
Mercury barometers measure pressure by balancing the weight of the atmosphere against a column of mercury
The height of the mercury column is proportional to the atmospheric pressure
Aneroid barometers use a sealed, flexible metal chamber that expands or contracts with changes in pressure
The movement of the chamber is translated into a pressure reading on a dial or digital display
Barometers are calibrated to display pressure in various units, such as millibars (mb), inches of mercury (inHg), or hectopascals (hPa)
Pressure readings are often adjusted to sea level pressure (SLP) to allow for comparison between locations at different elevations
Barographs are instruments that continuously record atmospheric pressure over time, creating a graph called a barogram
Altimeters in aircraft use pressure measurements to determine altitude above sea level, as pressure decreases with height
Pressure Systems and Weather Maps
High-pressure systems (anticyclones) are characterized by sinking air, clear skies, and generally stable weather conditions
Winds flow clockwise and outward around high-pressure systems in the Northern Hemisphere
Low-pressure systems (cyclones) are characterized by rising air, cloudy skies, and often unstable weather with precipitation
Winds flow counterclockwise and inward around low-pressure systems in the Northern Hemisphere
Weather maps use isobars to represent areas of equal pressure, allowing meteorologists to identify pressure systems and gradients
Closely spaced isobars indicate a strong pressure gradient and higher wind speeds, while widely spaced isobars suggest weaker winds
Troughs are elongated areas of low pressure, often associated with frontal systems and unsettled weather
Ridges are elongated areas of high pressure, typically bringing clear skies and stable conditions
The movement and interaction of pressure systems play a crucial role in determining weather patterns and the development of various weather phenomena (fronts, storms)
What Causes Wind?
Wind is the horizontal movement of air from areas of high pressure to areas of low pressure
Pressure gradients, which are the differences in pressure over a distance, are the primary driving force behind wind
The strength of the pressure gradient determines the wind speed, with stronger gradients resulting in faster winds
The Coriolis effect, caused by Earth's rotation, deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
This deflection creates the characteristic circular flow around pressure systems
Friction with the Earth's surface slows down wind speed and affects wind direction, especially in the lower atmosphere (planetary boundary layer)
Thermal differences, such as those between land and water or between different latitudes, can also generate wind patterns (sea breezes, jet streams)
Topography, including mountains and valleys, can channel or block wind flow, creating local wind patterns (mountain breezes, gap winds)
Convection, the vertical movement of air due to heating and cooling, can lead to the development of small-scale wind patterns (thermals, dust devils)
Types of Winds
Geostrophic wind is a theoretical wind that results from the balance between the pressure gradient force and the Coriolis force
It flows parallel to isobars at a constant speed and does not consider friction
Gradient wind is a more realistic approximation of wind flow that accounts for the pressure gradient force, Coriolis force, and centripetal acceleration
It flows parallel to curved isobars and varies in speed
Surface wind is the actual wind experienced near the Earth's surface, influenced by friction, topography, and local factors
Sea and land breezes are coastal wind patterns driven by temperature differences between land and water
Sea breezes blow from the cooler ocean to the warmer land during the day, while land breezes blow from the cooler land to the warmer ocean at night
Mountain and valley breezes are local wind patterns in mountainous areas, caused by temperature differences between the slopes and the valley floor
Jet streams are narrow bands of strong winds in the upper atmosphere, typically located at the boundaries between air masses of different temperatures
The polar jet stream and the subtropical jet stream are the most prominent examples
Monsoon winds are seasonal wind patterns that reverse direction between summer and winter, primarily affecting tropical and subtropical regions (South Asia, West Africa)
Wind Measurement and the Beaufort Scale
Wind speed is typically measured using an anemometer, which can be a cup anemometer, a propeller anemometer, or a sonic anemometer
Cup anemometers measure wind speed based on the rotation rate of three or four cups mounted on a vertical shaft
Propeller anemometers measure wind speed based on the rotation rate of a propeller mounted on a horizontal shaft
Wind direction is measured using a wind vane, which points in the direction from which the wind is blowing
Wind speed is usually expressed in meters per second (m/s), kilometers per hour (km/h), or knots (nautical miles per hour)
The Beaufort scale is an empirical measure that relates wind speed to observed conditions at sea or on land
It ranges from 0 (calm) to 12 (hurricane force) and describes the effects of wind on waves, trees, and structures
Wind gusts are sudden, brief increases in wind speed that can significantly exceed the average wind speed
Gusts are important for aviation, construction, and other activities sensitive to wind conditions
Wind shear refers to changes in wind speed or direction over a short distance, either horizontally or vertically
Wind shear can be hazardous to aircraft, especially during takeoff and landing
Pressure Gradients and Wind Patterns
Pressure gradients are the driving force behind wind, with wind flowing from high pressure to low pressure
The strength of the pressure gradient determines the wind speed, with stronger gradients resulting in faster winds
Closely spaced isobars on a weather map indicate a strong pressure gradient and high wind speeds
The direction of the pressure gradient force is perpendicular to the isobars, from high to low pressure
The Coriolis force, caused by Earth's rotation, deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
This deflection results in wind flowing roughly parallel to isobars, rather than directly across them
Geostrophic balance occurs when the pressure gradient force is balanced by the Coriolis force, resulting in geostrophic wind
Gradient wind balance occurs when the pressure gradient force, Coriolis force, and centripetal acceleration are in balance, resulting in gradient wind
Friction with the Earth's surface disrupts the geostrophic and gradient wind balance, causing wind to cross isobars at an angle (Ekman spiral)
The angle of deflection is greater over rough surfaces (land) than over smooth surfaces (water)
Convergence and divergence of wind can lead to vertical motion in the atmosphere, affecting weather patterns and the formation of pressure systems