Solar radiation heats up the earth. The ground is heated up and releases the heat into the
atmosphere. In a similar but less rapid manner, the sea is heated and this heat is also released into the atmosphere. In turn, the heated atmosphere then causes various weather patterns to occur. The fact that the earth warms and cools more rapidly than water will later help us understand several other weather patterns later. In a nutshell, all weather patterns derive their activity from heat via solar radiation.
Pressure and temperature diminish with height
The atmosphere that surrounds the earth has mass and so is pulled downward by gravity. The weight of air above and around us is felt as pressure on all earthly matter. This is called atmospheric pressure, and is measured in Hectopascals (hPa), or formerly in Millibars (mb).
Further standardization has been agreed upon with the establishment of one unit of atmosphere at sea level per 1013.25 hPa (or 29.92 inches of mercury). Thus atmospheric pressure is reduced by 1 hPa per 8.5 meters (28 feet) of altitude as we move upward. Air then becomes more rarified to the point that external supply of oxygen is required for paragliding above 3500 meters (11,500 feet). A
fight beyond this height without oxygen support is extremelly dangerous.
Any variations in atmospheric pressure from place to place is accompanied by a tendency for air at greater atmospheric pressure to flow into the area of lower pressure. This horizontal displacement of air is commonly known as wind.
As mentioned previously the heated earth releases heat into the atmosphere. When this happens, the warm air rises vertically and a neighboring cold air current fills the void. This vertical displacement of air is called an ascending current of air or thermal updraft.
Thus it is evident that there are two types of motion in the air:
1. The wind, which is the horizontal movement in the air.
2. The ascending or descending air current caused by thermals, which is the vertical movement in the air.
Most non - pilots cannot understand the difference between wind and a thermal. In practice, it is not always clear when to differentiate them since they can often occur simultaneously. Beginner pilots should fly under steady conditions with a wind speed at launch of 0 to 15 km/h (0 to 10 mph) and under an experienced pilot's supervision. Indeed, any seasoned pilot knows that paragliding is directly dependent on weather conditions and this calls for a good knowledge of meteorology.
Temperature is measured in degrees Celsius ( ºC ) in most countries while degrees Fahrenheit ( ºF ) are still used in the U.S.
As you know the air's temperature varies throughout the day according to the solar heating. Low temperatures usually occur in the early mornings and high temperatures two hours after the sun passes meridian level. The heating by the sun causes thermals, so thermal production usually follows the temperature pattern. Temperatures typically fall with altitude by about 0.65 ºC or 1.08 ºF, for every 100 meters (300 feet).
Humidity represents the amount of water vapor contained in the air. Water exists in three different states:
1. Gaseous in the air in the form of water vapor 2. Liquid as precipitation or water droplets 3. Solid as hail or ice
The air may contain a limited amount of water vapor in proportion to the temperature. For instance, a cubic meter of air with 1013 hPa pressure can contain up to 8 g of water at zero Celsius (0.28 oz at 32 ºF), 13 g at 10 ºC (0.46 oz at 50 ºF), 25 g at 20 ºC (0.70 oz at 68 ºF) and 40 g at 30 ºC (1.41 oz at 86 ºF). The air holding the largest possible amount of water vapor is called saturated. When air becomes saturated, the water vapor turns to liquid. This liquefaction process is achieved either by increasing the water vapor content by evaporation from the surface or when the temperature drops, as we will see later in the chapter on thermal flying.
Absolute humidity is the amount of water vapor in grams contained in a volume of atmospheric air. Relative humidity is the mass of water vapor existing in a given unit of volume of air in relation to a mass of saturated air, and is expressed in percentage terms. Dew point represents the temperature at which the atmospheric air becomes saturated as it cools. Any further cooling converts a part of the water vapor into water droplets which we see as cloud or fog.
Pressure and temperature determine the density of a gas such as air. Water vapor density is roughly five eighths (5/8) that of dry air. Therefore, it is necessary to know the relative humidity to
determine the air density. Furthermore, an increase in altitude will reduce the density by roughly one per cent per hundred meters (300 feet) of altitude increase. In contrast, density increases when there is a drop in temperature. So, it seems that there are no guidelines for raw sensory perception of density and thus must be measured scientifically.
Flying in low-density air means flying at a relatively higher airspeed compared to a situation with the same conditions but with high air density. For example, on a hot, dry day all true airspeeds will be higher. Flying at high altitude occurs inlower density air and also results in relatively higher airspeed. It is important to remember that your airspeed is lower when for example, you decide to fly in the vicinity of the sea with a high humidity breeze. Air density cannot be easily measured at the flying site, but it can be generally assessed by noting the temperature, humidity and altitude.
The density factor affects you mainly at launch and landing, and usually is not detectable in flight since the force of the relative wind is the same in all air densities.
Isobars
Isobaric chart containing indication of geostrophic and surface wind direction.
Isobars are curved lines on a weather map which connect points that have the same atmospheric pressure. In general, isobar patterns curve around large areas of low pressure, or barometric
depression, and around areas of high pressure, or anticyclone. A difference in pressure at separated points on the earth's surface results in a flow of air from high pressure to low pressure. This flow is nature's way of equalizing the pressure imbalance, which we know as wind. An isobar chart can inform us of many things. For example, in the earth's northern hemisphere:
1. The closer the isobars are to each other, the stronger the pressure gradient and thus the stronger the wind.
2. Winds invariably blow parallel to isobars because as the winds flow the earth turns below them.
The result is an apparent turning to the right in the northern hemisphere. We call this the Coriolis effect. According to the Buys Ballot's Law: In the Northern Hemisphere, if one stands with his back to the wind, the area of low pressure is to his left. In the Southern Hemisphere the reverse is true.
The explanation lies in the deflection, caused by the earth's rotation, in the movement of air from areas of high pressure to areas of lower pressure.
3. Wind rotates anti-clockwise around depressions (lows) and clockwise around anticyclones (highs).
The opposite occurs when we are in the southern hemisphere.