Pressure
High pressure has a clockwise down spiraling cold air mass giving good flying weather. This is generally true about highs moving in from the Pacific Ocean but not true if they come down from Canada. Where it comes from is important. The Bermuda High brings days of poor visibility to the Eastern U. S.
Low-pressure areas are counterclockwise upward spiraling systems that bring bad weather. The Low draws in moist air and forms spiraling cloud formations of low cumulo or stratocumulus rain clouds. The cold front formed by a Low is {,} comma shaped.
Flying from warm air into cold air is effectively flying from high to low pressure. The effect in a standard lapse rate is 35' for every degree Celsius. Every three degrees colder with a constant altimeter setting and altitude effectively puts you 100 feet lower.
As altitude increases above sea level the pressure of the air decreases at a rate of two pounds per square inch for every 2000' below 18,000'. This pressure altitude in used to determine oxygen requirements for humans. When the temperature change (lapse rate) is greater than standard the density altitude will be higher than the pressure altitude. This affects aircraft operation by decreasing power, lift, and thrust.
The barometer is a lineal scale used for weighing the air on a square inch of the earth surface. The aneroid barometer uses a closed bellows and a circular scale to do the same thing. The barometer that uses the weight of air to measure altitude above the earth is called an altimeter. The standard setter for all forms of barometers is the mercury barometer, which reads in Hg on a scale in which one inch of mercury movement in a tube is equivalent weight of one thousand feet of air if in the same tube. This change is weight is known as the lapse rate.
At sea level the weight of the air column is 14.7 pounds per square inch or the air pressure required to raise the mercury column to 29.92 inches or 1013.2 millibars. For aeronautical purposes measures are varied through the use of a kollsman window which computes weight from mean sea level (msl) without regard to terrain. As daily air pressures vary the altimeter is re-set to read changes due to the adjusted lapse rate.
By FAR you must have your altimeter set from a figure given within 100 n.m. in a worse case situation. Fact is, settings should be obtained from the closest source available. All radar facilities are required to give this setting at least once during any continuous contact with an aircraft. This setting can be confirmed + 300 feet just be having the pilot read back his altitude.
From a weather viewpoint changes in air pressure are always significant. The pressure trend that is to lower pressure means that air is being drawn in along with moisture that at will at some point condense into visible moisture. Rising air pressure reverses this process. These pressure changes in the U.S. mean that the rise and fall will cause fronts to move across the country. On average, one front crosses a given point once a week in summer and two or three such passages in the winter. Warm or cold in the passage of the fronts is always relative to the front preceding. Cold fronts move faster than warm fronts and tend to wrap-up on them when they touch. The wrapped area is known as an occluded front, which usually has the worst aspects of both the warm and cold fronts. It would not be nature's way to have the best qualities survive as offspring.
Moisture and Weather
The movement of moisture is reminiscent of a multiple juggling act where the three forms of ice take turns going through six different stages of transformation. Vapor can condense into liquid and sublimate into ice while liquid can evaporate back into vapor and the ice can sublimate into vapor. The ice that sublimated from vapor can melt into liquid and be created by freezing liquid. This entire process can be diagramed as the three forms in a line with a large circle connecting vapor and ice on the outside while liquid in the center is connected by two smaller circles one to vapor and the other to ice. The required words along the arcs of the circles are only sublimation twice hot and cold from vapor to ice and ice to vapor. Liquid and vapor are connected by the hot condensation arc and the cold evaporation arc. Liquid and ice are connected by the hot freezing and the cold melting. With a little effort the diagram can be made.
In the diagram we have the three phases of moisture gas, liquid and solid. All are forms of water. The forms are determined by the speeds at which the molecules move. The molecular speeds are determined by temperatures. Air is saturated when for every molecule evaporated another is condenses. The relative humidity is 100%. When dew point rises above 60 degrees we have a condition of high humidity. The dewpoint is the temperature where dew forms. When the air temperature and dew point are close together contaminants in the air facilitate the formation of liquid molecules which form visual obscuration.
The form changes of water while being caused by temperature changes, in turn cause temperature changes.
Each form change there is an exchange of heat. Every storm is a heat engine during which the change in water form either releases or collects heat.
Wind Indicators
Wind often is very stressful to the pilot. Takeoffs and landing techniques require greater skill under the influence of wind. Even the light and variable wind adds complexities that are difficult to plan for. Having specific wind information can either add to or subtract from a pilot's stress.
Runway selection is primarily based on wind direction except where noise abatement rules prevail. With experience a pilot learns to read winds from water, dust, DME, GPS, the windsock and weather forecasts. Regarding the latter, one thing you can be certain of is that the winds will NOT be as forecast. It is not a good idea to read the wind from a tetrahedron. Most tetrahedrons have locks that allow them to be positioned for the preferred runway without regard to wind direction. The windsock is the best wind indicator at an airport and should be noted on downwind and especially on final. What you see on downwind will enable you to make a wind adjusted pattern. A disproportionate number of landing accidents are caused by a pilot's failure to adjust the pattern for wind direction and velocity.
The FAA wind cone (sock) can be of several colors but usually orange and either 8 or 12 feet long. Windsocks are fully extended in 15-knot winds. I make a practice of having student's practice reading windsocks by comparing ATIS or tower wind directions and velocities with their guesses from the windsock. At one time Concord, CA had five windsocks. It was not unusual to have them all showing different direction and velocity.
A weather related accident is most likely to be blamed on the wind. Wind does have a lot to do with your flying. The unequal heating of this planet's mixed surfaces of land and water causes wind. The time of the year and the weather is related to wind direction and velocity. The world is not heated evenly as it moves around the sun, changes distance, leans, and turns. The uneven heating causes the movement of air masses seeking equilibrium. This uneven heating also causes pressure differences and pressure gradient forces. This causes wind. The greater the pressure differences in you altimeter setting the stronger will be the winds. The warmth of the earth causes vertical movement of air.
Local small-scale winds are caused by the flow of air to replace the air that is rising. Water instead of absorbing heat reflects most of it. Land absorbs instead of reflecting. The hotter land causes rising air thermals. When the sun goes down the land gives back to the atmosphere much of the heat acquired during the day. Land makes this change at five times the efficiency of water. At night the water is warmer than the land and the flow of the wind reverses.
Where the pressure isobars bend the effect of centrifugal force is greatest. This is the force the wind has upon itself due to differing pressure gradients. The rotation of the earth also creates a force against the air surrounding the earth. This coriolis effect causes the winds in our hemisphere to be deflected right. This coriolis effect deflects anything projected into space. The winds of the southern hemisphere are deflected left. This effect is weakest at the equator, which explains the absence of extreme highs and lows there.
Gustave Coriolis mathematically described the effect of the earth's rotation on moving objects in the atmosphere. The effect begins at the equator a zero and increases toward the poles. The effect also increases as the wind speed increases.
Winds should flow parallel to isobars but frictional force affects the flow of air below a couple thousand feet. Wind strength increases above this level. This frictional force makes the wind turn across the isobars into the low at a 30-degree angle. It is only above the frictional layer that the winds parallel the isobars.
When conditions indicate strong wind conditions take an early look at the Va for your aircraft and its present weight. The higher winds are the slower you should go. This is usually done beginning with the Va for gross weight and find the percentage you are under that weight. Easy to do on the E6B or on paper by making the following equation.. (One way to do it.) Make a fraction with your 'actual weight as the numerator over the gross weight as the denominator equals N over 100. Multiply the diagonal numbers actual weight by 100 and divide by gross weight. This is the percentage of actual weight to the 100% of gross weight. Subtract the two percentages to find the difference. Divide this difference by three and using the answer as knots use it as the amount to reduce your Va speed. Example: If you find that you are 10% below gross then you would lower your Va at gross speed by three knots. Not at complicated as it seems once you do it several times.
A mountain wave can be dry and cloudless with as much turbulence as if a visible cloud exists. Winds of 25-knots perpendicular to mountain ridges with increasing velocity at higher altitudes will contain such waves. Wide areas of weather instability are the best souses of mountain waves. The same wave system can last for a couple of days. Waves can occur over low hills if the winds are strong enough.
A Foehn wind (Chinook) are warm dry winds descending off mountains that can melt two feet of snow in a day. They are as dangerous as mountain waves. Any encounter with a downdraft demands that the pilot be prepared to do a 180 or have sufficient excess altitude to allow him to 'dive out' of the situation. This can be considered mountain drainage winds, which have thunderstorm like gust fronts.
These winds can be microscale weather involving less than one hour and a very few miles. Pilots flying in such conditions need to know the warning signs of markedly different barometric readings of neighboring mountain airports. Rotor clouds and unsteady lenticulars in a line serve as a warning of such conditions. Virga usually indicates extreme turbulence. These conditions may be of very short duration about mid-morning. The cloud type tells the kind of turbulence to expect.
Flying in turbulence is best done with a very light yoke and lots of rudder. Bring up a dropped wing with rudder not yoke. Rudder use otherwise must be coordinated and in choppy conditions do a rudder dance. If turbulence causes pitching of the nose, make major correction with power changes. I have found it better to put in the initial power change I feel necessary and then take half of the change off immediately the correction begins to occur. Use elevator to adjust for altitude excursions. When the aircraft acts like a bronco or a canoe counter primarily with rudder input and gentle yoke pressures.
NTSB reports that up to 40% of all weather related light aircraft accidents occur where the pilot cannot cope with the winds near the surface. The best way of avoiding adverse winds is just to stay on the ground. If airborne, barring engine failure, landings are optional. If the winds contribute to a bad approach make the easy choice of throwing in your hand and going around. A given airplane accident is the final result of decisions made by pilots. When an accident occurs that can be attributed to poor decision-making it is difficult to find why an otherwise cautious and competent pilot would do such a thing.
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