How’s the weather?

Meteorology notes.

The past couple of weeks we have been talking about the weather, the aviation regulations, radio navigation and started with the flight procedures. Electromagnetism has been killing me. Not literally though. But I have started drinking coffee again.

The weather is of course an interesting phenomenon, because it touches us all. The harder it rains, the easier it is to feel that touch. From the aviation point of view, weather is strongly related to safety. The possible hazards that hurricanes and severe turbulence can cause are obvious, but weather is linked to flying in many ways.

The altimeter on an aircraft indicates altitude by measuring the pressure difference of the air surrounding the airplane to a reference pressure level. The air pressure decreases when altitude increases. That means that the higher up in the air you are, the less air pressure there is. The altimeter measures the change in pressure as you go up or down, and is therefore able to tell your altitude. The altimeter is calibrated to show the correct true altitude when airborne only in standard conditions where e.g. temperature is +15 C and air pressure is 1013,25 hPa. Of course air pressure and temperature changes, everyone knows that. So what happens then with the altimeter’s indication?

To get the right reading on the altimeter, the current air pressure must be taken into account. This is done by selecting the correct reference pressure by turning a control knob on the altimeter. This reference pressure, QNH, is given to pilots by the air traffic control or by ATIS (Automated Terminal Information System). When QNH is set, the altimeter should read the field elevation on the ground within the allowed instrument error.

“From high to low, look out below!” If flying from high pressure to low pressure without adjusting the QNH on the altimeter, the aircraft’s true altitude is less than the altimeter’s reading. This of course would pose a more serious problem when flying IFR in bad weather without visual contact to the ground.

The same problem occurs when flying from warm air to cold air. Cold air is more dense than warm air. In cold weather the aircraft flies lower than what the altimeter says. To ensure obstacle clearance, a temperature correction must be made to the altimeter reading. A 4% correction is made to the altitude for every 10 C deviation from the stardard temperature. For example temperature being -3 C and altimeter showing 4000 ft, the true altitude is 3840 ft.

Clouds can affect visibility, but there are also other factors to be considered when flying in cloud. First of all, some clouds are not as friendly as others. The puffy white cumulus clouds look like cotton, but are convective clouds formed in unstable air. This means they will often have turbulence beneath and  within the cloud. The higher the cumulus (towering cumulus) the stronger the convection and the turbulence. The cumulus cloud can develop further into a cumulonimbus, in which the thunderstorms occur. Cumulonimbus always means a high risk of heavy windshear, severe turbulence and icing, hail and lightning. It is not advisable to fly into a thunderstorm. Check out the hail damage on the nose of this SAS MD-81: Airliners.net – SAS MD-81 hail damage.

Rolling thunderstorm (Cumulonimbus arcus) Fran...

Rolling thunderstorm (Cumulonimbus arcus) (Photo credit: Wikipedia)

We have also gone through different weather systems, air masses and fronts, anticyclones, depressions, precipitation, winds, local winds, turbulence, windshear, jet streams, a little bit of thermal dynamics, clouds and visibility and in-flight hazards. An interesting subject for me and a great teacher once again. Our meteorology teacher is a Finnair/Flybe first officer that graduated from the school about 5 years ago.

Psychology lessons were cancelled the previous week. So I’m looking forward to that still. Next week we’re learning morse code. We will also continue with the flight procedures, which we only started this week.

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2 responses to “How’s the weather?

  1. Hi Niina!

    This part of your blog is very intresting.
    It is nice to know how for example the weather, airpressure, temperature influences affects flying.

    Dad

  2. Nina,

    Great explanation on the weather. I am sure your instructors are teaching you that not only do you have to account for the weather, but consider the capabilities of both you and the airplane when making weather decisions.

    But the day will come when the real meaning of weather will change. It will no longer be whether or not your going. It will simply be information on what it will be like when you get there. Because your airline will dispatch you into legal weather that is so bad the birds don’t want to fly.

    Have fun in psychology.

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