Why Hot Air Rises
The pressure altimeter measures the weight of the air above some point. The air weight is measured using the ocean surface as the base. The weight of air is weighed using the weight of mercury as a comparison since a column of mercury one-inch high equals a similar of air one thousand feet high. Atmosphere is standardized using this scale to say that 29.92 inches of mercury is the average weight of the entire atmosphere. This is 1013.25 in millibars.

Weather is produced by the sun's effects on the earth in conjunction with the rotation of the earth and variations in the earth's surface and GRAVITY. The total combination is a heat distribution system based upon ever-changing highs and lows of air pressure. The 750-millibar chart illustrates the 10,000-foot level of air pressure.

Aircraft weigh the air using a pressure altimeter. We correct this weight for the time and place using a dial up number called the Kollsman window. This converts the atmospheric pressure into feet of altitude. Just as we normally set clocks and watches for the area, so does the altimeter do the same for all locally situated aircraft. Only by using the local altimeter setting can aircraft be safely separated in altitude.

Un-named Weather
Weather exists for which we have no terminology. Convective Available Potential energy (CAPE) is a situation where the heat of condensation will create a thunderstorm. When reading the weather look for SBCAPCE OF 2000-3000-j/kg or higher as indicative of conditions creating thunderstorms.

Some storm systems do not move. They are called steady state storms. they continue to exist by drawing warm, msoisst air into itself. TAFs will have existence of CBs.

Convection Checklist:
1. Check air temp and dew point every l00 miles
2. Get specialist to give convective outlook
3. If temp and dew point are below 50 degrees things will be o.k.
4. If temp and dew point are 60 or higher in the morning expect thunderstorms.
5. When enroute check on WSTs (Convective sigments.
6. If temp and dew point are rising…leave the area. Read the outlook
7. Day or night temp dew point over 60 is too dangerous.for flying
8. Don't try to out run the problem…get on the ground.

 Making Turbulence
Permanent high pressure exists at the poles. Permanent low pressure exists at the equator. The sun heats the air in the tropics and it settles at the poles. Or it would if it were not for the rotation of the earth. Rotation causes what should be one big circulation cell to divide into three. Where most of the people live and fly, one of these cells have winds that go to the right. The winds at the surface are from the southwest while winds aloft are from the west and northwest. The 30 to 60 degrees of latitude are the battleground of the dry polar winds and the moist winds from the tropics.

If it were not for the sun all airflow would be smooth and orderly. The sun disrupts the flow of air. Turbulence is simply the effort of the air to return to the smooth and orderly flow that would exist were it not for the sun effects.

Turbulence:
The student/passenger introduction to turbulence and weather should be, likewise, gradual both as to duration and severity. The better a pilot's knowledge of wind/weather patterns the better able he will be to select desirable conditions. The stress of turbulence is harder on the pilot and passengers than on the aircraft. Turbulence makes flying unpleasant. It is the unpredictability of turbulence that causes this stress. The dislike of turbulence is inherently related to our instinctive fear of falling. Exposure and contemplation directly affect perception of turbulence. Passengers and students detect the demeanor of the pilot/instructor in turbulence and infer the amount of concern required. Gradual exposure is the best way to overcome turbulence anxiety. You don't need to like it to overcome its effects on your flying. Turbulence is harder on people than it is on airplanes. Reduced climb, heading changes, and altitude excursions affect the airplane.

Turbulence is not limited to inside thunderstorms and other vertical clouds; it can be found in areas below and around them. Virga is an indicator of microburst activity. Strong winds over mountainous terrain produces mechanical turbulence. High level turbulence is related to the jet stream. Temperature differences, over 6 degrees Celsius, associated with frontal movements produces turbulence and wind shear. Anything stronger than light turbulence is capable of making aircraft control impossible and causing structural damage. Severe turbulence can tear an aircraft apart.

Turbulence accidents most often occur during takeoff and landing. 40% of turbulence accidents are 40% caused by control problems. If you suspect that there may be a problem the decision not to depart is wise; the corresponding landing choice may not be available. In strong winds minimize the use of flaps and use higher approach speeds. You can even request the use of a taxiway going into the wind. Once on the ground you may be in conditions that do not allow a safe taxi. Get ground help. A friend of mine once found that any attempt to taxi caused him to become airborne.

Va, maneuvering speed is commonly thought of as having to do with control movement. Va is the turbulent air penetration speed. Va is based on weight, the heavier the weight the higher the Va. I find that thinking of it as driving over country railroad tracks. The lighter the car the higher the bounce. Flight into turbulence at below Va makes the plane stall before it breaks. Know your Va. Once way to determine the approximate Va at below gross weights is to change the Va by half the percentage of weight reduction. Find the actual weight reduction below gross as a percentage of gross. Increase the published Va by half of the weight reduction percentage. A 30% reduction of weight would result in a 15% increase in Va. If you are flying light, go slower than the published Va.

There is another speed used in turbulence that is different than Va; it is called structural cruise speed or Vno. Unlike Va this is shown on the airspeed indicator as the meeting point of the orange and green. This is a speed below cruise that is recommended for rough air penetration. Vno does not offer the structural assurances offered by Va. The further below Va you are in turbulence the less likelihood there is of structural damage.

Don't turn in turbulence. Any bank greatly increases the potential to overstress the airframe. The enemy when flying in turbulence is losing control. Fly attitude, stay level, and take whatever altitude nature lets you have. Managed turbulence will not cause structural failure.

Turbulence can cause up/down drafts that exceed the capabilities of a G.A. Aircraft. It is best to maintain level attitude at reduced airspeeds and power settings and not to try to contain altitude. ATC will give you an altitude "range" under such conditions.

Turbulence is caused by rising air. Air rises because of movement across rising terrain or because it is warm. Likewise, air temperature gets colder at higher elevations. Why, then, doesn't the cooler air stop rising? Air pressure decreases as it rises. As air rises into decreased pressure it expands. The expansion causes a decrease in temperature. The lapse rate or change in temperature as at 2 degrees Celsius or about 3.5 degrees Fahrenheit.

Whenever the warmer rising air contains moisture a new factor affects the lapse rate. Warmer air can contain more moisture than cooler air. As warm air, containing moisture, rises, it expands due to decreased pressure and cools. As the air expands and cools it releases heat into the surrounding atmosphere. This process called condensation causes the moisture to become visible at the dew point. The dew point is where air, at a given temperature, contains sufficient moisture for it to become visible. When this process begins to cycle, to repeat itself over and over, we get rising thermals of air and turbulence.

Thermal activity can cause initial warm air to rise, expand, cool, condense, warm, rise, expand etc. at a rate that easily exceeds the climb rate of an aircraft. We often see this as initial scattered puffy clouds, which, as the sun rises, become cumulus clouds. Any one or all of the cumulus can become a towering cumulus thunderstorm. A given thunderstorm can have winds and turbulence exceeding the structural capacity of any aircraft. For this reason, any flight planned into areas subject to this type of weather should be on the ground by mid morning.

A special form of thermal is called a dust devil. Dust devils do not occur nearly as often as thermals and they are often quite visible but not if in a paved area. Dust devils are spinning thermals that move with the prevailing winds. Fly upwind of dust devils. Do not takeoff or land in the vicinity of a dust devil.

All thermals can be avoided by flying at altitudes above their cap level. This means that flying above the clouds will be relatively smooth. The hotter it is the higher you must fly. Maintaining level flight in thermal conditions requires that the pitch attitude of the aircraft be varied to counter the up and down drafts of the thermal area. Slow down if it gets rough.

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