Ceiling
The heights above the earth's surface of the lowest layer of clouds or obscuring phenomena that is reported as 'broken,' 'overcast,' or 'obscuration,' and not classified as 'thin,' or 'partial.'

Atmospheric Properties
The atmosphere has certain properties that can be measured (or calculated). Namely temperature, pressure, and density. These properties change with altitude.

Up to the tropopause, temperature, for example, tends to decrease with altitude. This is because the sun heats the surface and the surface heats the mass of air closest to the surface. The increase in temperature causes the air to expand, which means it becomes less dense. Less dense means more buoyant. The warmed air tends to float upward. As it floats upward, it moves to an area where pressure is lower, so it expands further. As a gas expands, its temperature drops. Eventually it cools to the termperature of the surrounding airmass and rises no further.

The ("actual") lapse rate is the rate temperature decreases with altitude. When the lapse rate is low, the warm surface air will tend to be less bouyant and the airmass is said to be stable. When the rate is high, the airmass is going to tend to shoot up, possibly overshooting it's area of neutral boyancy. This is said to be an unstable airmass.

Sometimes you get an airmass where the temperature is warmer at higher altitude. This is called an "inversion".

So, the atmosphere has properties that change with altitude, but those properties also change with weather conditions. You can be in a high pressure area or you can be in a low pressure area. Wind is caused by pressure differences. Air is pushed from high pressure to low pressure, but because the earth is rotating, the air gets deflected to the side ("Coriolis force") and winds up traveling parallel to the lines of pressure rather than perpendicular as one would expect.

Because of all this variability of the properties of the atmosphere, the International Standards Organization got together and *defined* a standard atmosphere. The standard atmosphere has a temperature of 15C and pressure 29.92"hg at sea level. The lapse rate is is about 2 degrees C and 1"hg per thousand feet.

We use pressure to give us a proxy for altitude. The altimeter (set to 29.92"hg) is a glorified barometer, but it's calibrated to read "altitude". This "altitude" is what corresponds to a given pressure in the table the ISO people use for the Standard Atmosphere. I.E. your pressure altitude.

Above 18000' in the U.S. (other countries are different), this works out ok because the pressure differences aren't so great that a plane would risk hitting terrain trusting the altimeter, and everyone is using the same setting.

At lower altitudes, it's a potential problem, so we have an altimeter setting. You dial in the number that causes the altimeter to read the real altitude (above sea level, which is also an idealized value) at the surface.

Say you're in an area of low pressure. Your altimeter will read a higher altitude than what you're really at. Let's say it reads 1000' above your real altitude. What you do is dial in 28.92 and shift the entire scale downward. The altimeter setting is what the sea level pressure would to be to shift the scale such that it reads the local altitude at the pressure of the weather station (got that?).

Of course, the decrease in pressure with altitude won't necessarily match what the calibration of the altimeter says, but it's good enough for our purposes. Most accurate at the ground, with decreasing accuracy as altitude increases, but this is okay since everyone is using the same setting and it gives us a better value for obstacle clearance. In IFR flight around non-mountainous areas, the minimum enroute altitude is 1000' above the surface. This is in part to cover the altimeter error (and also to allow the pilot slight deviations).

Aircraft performance depends on the density of the air. That's (effectively) number of molecules per unit of volume. This depends on temperature and pressure. Instead of reading density directly, we use the concept of "density altitude" which is similar to "pressure altitude": it's the altitude you'd have to be at in a standard atmosphere to have the prevailing density.

Given an air pressure (read as a pressure altitude) and a temperature, you can compute the density altitude. It's not a trivial formula, so I just use my E6B to crank out the numbers as needed.

To solve a typical performance problem, you need the temperature, pressure and altitude of the station. Let's say pressure is 28.82"hg and the altitude is 250' above sea level. Without correcting the altimeter, you'd have an altitude of 1000' (pressure altitude).

In a standard atmosphere, the pressure should be 29.67 (29.92 at sea level - 0.25 for the elevation), but if you dial in 29.07 (28.82 current pressure + 0.25 for the 250' elevation) you get a reading of 250' at the current elevation. 29.07 would be the projected sea level.

Now that you know the pressure altitude, you can use that with the temperature to compute density altitude for performance. Not that some performance charts use a table with rows for pressure altitude and columns for temperature. This effectively gives you your density altitude without having to calculate it.
Morris Bernstein

ATC Radar Facilities and Weather
--Do not rely on ATC RADAR for weather information
--The pilot is responsible for aircraft safety and weather avoidance
--If there is a conflict in what you see and know and ATC's information, get on the ground.
--ATC radar is unreliable when it comes to presenting weather.
--An untrained controller may fly you into trouble

Weather Service Radar
--New WSR-88D doppler weather surveillance radar is excellent and detection weather and estimating rainfall
--Doppler detects speed and direction even spin motion differences
--160 WSR-88D radars will form an integrated network covering U.S. and island territories

Weather and Radar Processors (WARP)
--ATC to reroute traffic to avoid weather
--Real-time weather in color w/precipitation ate three different altitudes.
--ATC gets accurate localized precipitation and weather effects.

Internet AWOS
AWOS Weather Advisor internet sites:
40N in PA
http://

Chester NY
http://

Bay Area Weather
---Satellite and computer technology have given us new knowledge about why our weather is as it is.
     ---What and why what is happening now is the weather and every valley is its own microclimate
         ---Every valley modifies its weather to the terrain of the area
             ---As a pilot you should become weather sensitive to your immediate airport conditions
                 ---Experience enables a pilot to more accurately predict 'his' weather from 'the' weather

Fog of a Different Kind
---When the earth is wet the sun causes radiation fog
---When the earth is hot it ‘draws' the winds from the sea to cause advection fog
---Both kinds of fog provide Bay Area air conditioning of different kinds.
---It is not common knowledge that it is the salt in the air that triggers the formation of fog.

Geographic Factors
---The geography consists of mountain range deviators, barriers and leaks
---The Bay area is a contact point between the climatic forces of sea and land
---The coastal range is a series of channels for movements of winds and associated weather.
---Winds carry moisture through the Golden Gate spreading coastal weather along the shorelines.
---The rainfall on one valley makes the valley over the hill that much dryer

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