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Airport Windsock

Weather Pressure Systems

Pilots preparing for a VFR flight are expected to read and understand aviation weather maps, METARs, TAFs and interpret this information to assess if their flight can be successfully completed. We refresh some of these once learned subjects and discuss, among other things, warm and cold fronts and the effect on the flight they have in terms of visibility and cloud base.

All of this while avoiding to become a meteorological textbook, there are more than enough websites in that field, so were are not going too. In the menu on the left you will find pages with weather maps, radar and satellite images suitable for aviation weather flight planning.

It doesn't really feel like it but the huge amount of air above our heads does have some weight. This is called atmospheric pressure and varies from location to location. So this will be our subject here.

Atmospheric Pressure

Pressure Altitude Lapse RatePressure Altitude Lapse Rate

We need a good understanding of this for a couple of reasons: pressure distribution drives the winds and cloud formation to some extent, and as altitude indicators operate by detecting the atmospheric pressure changes they must be set properly to indicate correct flight altitude. Temperature has also an effect on air density and thus to indicated altitude.

Atmospheric Standards

For reference purposes an international standard atmosphere (ISA) has been created and its parameters have been agreed upon. This way aircraft and engine performance can be compared under equal circumstances. Below you will find the some of the specifications:

Gravity32,170405 ft/sec29,80665 m/sec2
Absolute zero-459,688 °F-273,16 °C
Standards at sea level (45° Lat)
Pressure29,9213 inHg760 mmHg
Pressure14,696 psi (lbf/in2)1,0332 kgf/cm2
Temperature59 °F15 °C
Abs Temp518,67 °R288,15 K
Specific wt0,076474 lb/ft31,2250 kg/m3
Density0,0023769 lb-sec2/ft40,12492 kg-sec2/m4

At sea level: the standard barometric pressure is 29,92 inHg, 1013,25 hPa or 760 mmHg with an average lapse rate of 1 inHg/1000 ft (.01 inHg/ 10 ft) which is about 4%.

Standard temperature is 15 °C, dry adiabatic lapse rate of 3,56 °F or 1,98 °C/1000 ft, saturated adiabatic lapse rate of 2,7 °F/1000 ft or 1,51 °C/1000 ft and a dew point drop of 2 °C/1000 m.

For a complete overview of the ISA standards click the next link to an image of the ISA Atmosphere.

The difference between the dry adiabatic lapse rate and the rate at which the dew point drops is around 8 °C/1000 m.

Using these standards you can calculate the local cloud base by taking the temperature and dew point reading and multiplying the difference by 125 m/°C. The actual cloud base depends, of course, on local variations, but you will get a good estimate of where clouds can be found.

Pressure systems

Synoptic Weather Chart

Around the world pressure readings are taken and synoptic maps are drawn by meteorologists. On these maps points of equal pressure are connected with a line (called isobars) and wind direction and speed are noted.
This way areas of low and high pressure are made visible. From day to day it can be seen that these systems are moving, generally from west to east. Lows may deepen and highs may build influencing weather along the way.

High Pressure areas

High pressure areas can occur when air becomes colder (winter high pressure areas can be quite strong and lasting). The air molecules become denser, heavier and sink towards the Earth, air temperature will increase adiabatic due to compression. Clouds are less likely to form and skies are usually clear.

With this temperature inversion, clouds will develop due to instability below the inversion. The pressure on the isobars will be increasing as you approach the high pressure zone toward the inside. The airflow is clockwise (northern hemisphere) and towards the low pressure area over the ground, see figure.

A ridge is an elongated area of high pressure with low pressure on either side, flying weather is usually fine to fair.

Low Pressure areas

A lower pressure develops when air becomes warmer. The air molecules become lighter and will rise. The pressure lowers towards the center of the area involved and air flow is counterclockwise (northern hemisphere) due to Coriolis. Clouds will appear due to rising (instability) of the moist warm air and the weather will usually deteriorate. This rising air will flow back to the high pressure area at higher altitudes in the atmosphere.

A through is an elongated area of low pressure with higher pressure on either side. A col is a neutral area in the middle of two low and two high pressure systems, weather can be unsettled and can go for the worse or clear up. This depends on which pressure system is getting stronger.

Effects of flight into Highs & Lows

Whenever you fly into a low pressure area you will need to correct for the wind to the left. On the contrary, flying into a high pressure area the wind correction is to the right. Just think of how wind flows parallel between the isobars at altitude, on the ground the wind offsets to the left (northern hemisphere ie., reverse that for southern). Do remember to set your altimeter setting, especially when flying from high to low (look out below, is the adage).

Written by EAI.

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