Pressure & weather patterns

How can the atmosphere exert a pressure?

Relate the molecular picture to the large-scale weight of the atmosphere.

Force distributed over an area

Atmospheric pressure is force per unit area exerted by the air. Molecular impacts produce pressure in all directions. In the nearly hydrostatic atmosphere, the upward pressure-gradient force balances the weight of the air, so pressure at a level reflects the weight of the column above that level. Higher pressure does not mean that air pushes only downwards.

The SI unit is the pascal, one newton per square metre. Aviation meteorology commonly uses the hectopascal: 1 hPa = 100 Pa. The older millibar has exactly the same numerical value as the hectopascal. Inches of mercury are also used in some regions; 1 inHg is approximately 33.864 hPa, so 1013.25 hPa is about 29.92 inHg. Read the reported unit and the aircraft subscale before selecting a setting.

A mercury barometer balances atmospheric pressure against the weight of a mercury column: a higher pressure supports a higher column. An aneroid barometer uses the deformation of an evacuated flexible capsule as external pressure changes. A mechanical altimeter also uses pressure-sensitive capsules, but its scale translates pressure into altitude through a standard-atmosphere relationship. A barometer measures pressure; it does not directly measure the weight or temperature of a chosen local parcel.

Why does a hectopascal span more feet at high altitude?

Use the amount and density of air in a layer to explain the non-linear pressure profile.

Remove air from above the measurement level

Climbing leaves some of the atmospheric mass beneath you, so less weight remains overhead and pressure decreases. The hydrostatic relation is dp/dz = −ρg: the pressure drop per metre depends on air density. Dense lower air contributes more mass in each metre of vertical column than thin upper air. Pressure therefore falls rapidly per unit height near the surface and more slowly per unit height aloft.

Equivalently, a fixed pressure change of 1 hPa represents a smaller height interval near sea level and a greater one high up. An average approximation near mean sea level is 30 ft, or about 9 m, per hPa. It is not a constant suitable for an arbitrary layer to FL400. The EASA examination convention permits 30 ft/hPa for its specified altimetry calculations between MSL and 700 hPa; that convention is developed with worked examples later.

In ISA, pressure is approximately half its sea-level value at 18,000 ft, around 500 hPa. Air density reaches approximately half its sea-level value nearer 22,000 ft, and a quarter near 40,000 ft. Pressure and density are different physical quantities and their fractional decreases do not occur at identical heights because temperature also changes through the atmosphere.

What do the shapes between the isobars mean?

Identify a pressure pattern from its surrounding values, not from the weather symbol alone.

Contours join equal pressure

An isobar joins points of equal pressure reduced to a common reference level on a surface chart. Read the chart's isobar interval and units. Closely spaced isobars indicate a larger horizontal pressure gradient than widely spaced ones at the same map scale. They are a clue to stronger winds, but latitude, friction and curvature are also required to relate the gradient to the actual wind.

A high has pressure greater than the surroundings and a low has pressure lower than the surroundings. The labels are relative: a 1010 hPa centre can be a high if nearby values are lower, while a 1020 hPa centre could be a low inside an even higher field. Closed contours help identify a centre, but the full surrounding pattern is what defines it.

A trough is an elongated extension of relatively low pressure, and a ridge is an elongated extension of relatively high pressure. A col is a saddle region between two highs and two lows, often with a weak gradient near its centre. A trough need not have a closed low, and a ridge need not be a separate closed high. Neither isobar geometry nor the letter H or L alone guarantees a specific cloud or visibility condition; moisture, stability and vertical motion complete the weather diagnosis.

Why can raw station pressure mislead a surface chart?

Remove the large height effect before comparing weather systems between stations.

Reduction to mean sea level

A mountain station normally measures lower pressure than a coastal station simply because it is higher, even when both sit beneath the same broad weather system. Plotting their raw values together would create an artificial low over high terrain. Surface weather analysis therefore uses pressure reduced to mean sea level. QFF is the sea-level pressure obtained from station pressure using the relevant meteorological temperature assumptions for the fictitious layer below the station.

The reduction effectively adds the weight of an estimated air column between mean sea level and the station. A cold, dense assumed column contributes more pressure per metre than a warm, less dense one. Temperature and the reduction procedure therefore influence the resulting value. At high elevations this extrapolation becomes increasingly uncertain; weather services use specified methods and may apply special treatment over high terrain.

QNH also reduces station pressure to sea level, but through the standard-atmosphere altimeter relationship so that an altimeter on that setting indicates the aerodrome elevation on the ground. QFF is intended for meteorological comparison, not as an interchangeable flight altimeter setting. QFE refers to pressure at the chosen aerodrome datum. A common datum removes a height bias; it does not make every reduction method identical.

Can a surface high become an upper low?

Examine the spacing of pressure surfaces through cold and warm columns.

Thickness depends on mean temperature

The vertical distance between two pressure surfaces is called their thickness. A warmer layer has greater thickness than a colder layer between the same two pressures. In a cross-section, equal pressure surfaces are therefore farther apart in the warm column and closer together in the cold column. This is a statement about the mean virtual temperature of the layer, not just the surface thermometer reading.

A shallow cold surface high can weaken with height and be replaced by relatively low heights of upper pressure surfaces: the cold column contracts strongly. Conversely, a warm surface low may weaken aloft or sit beneath relatively high upper pressure-surface heights because the warm column is deep. Other systems are deep through the troposphere. Temperature structure decides how the pattern changes; a surface H does not require an H directly above it on every chart.

Upper charts commonly use a constant pressure, such as 500 hPa, and plot the height of that surface. A low height means that this pressure is reached at a lower altitude than in the surrounding area. A ridge is a bulge towards higher heights, while a trough is an extension of lower heights. Compare charts on their own coordinate systems: surface isobars describe pressure at a reference level; upper contours describe height at a reference pressure.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.