Weather The Big Picture – Paragliding

Structure of the Atmosphere / Density Altitude

To understand the weather it helps to know a little about the air. Air is simply a gas comprised primarily

of nitrogen and oxygen and a variable amount of water vapor. Air has mass and weight. At sea level

where the air is most dense it exerts a pressure of 14.7 pounds per square inch.

Air density is affected by altitude, temperature, humidity and pressure. Air density alters wing

performance. The higher the altitude, temperature or humidity the less dense the air will be and the faster

you will have to move to generate lift.

For every 1000ft of altitude you gain, the air density drops approximately 4%.

For every 5ºF increase in temperature, the air density will drop 1%.

 

Water Vapor

The amount of water vapor a mass of air can hold depends on its temperature. As air heats up it is able to

hold more water. Low relative humidity means that the air at its current temperature could hold a lot more

water. When the temperature of an air mass drops, its relative humidity increases and when relative

humidity reaches 100%, water condenses and a cloud is formed.

Humid air is lighter then dry air and therefore has a tendency to rise just as when air is heated.

 

Solar Heating and Circulation

Solar heating and the resulting high and low pressure systems causes all atmospheric circulation. The sun

heats the ground, which heats the air above it. Different surfaces heat up at different rates. Some examples

of terrain that heat up slowly are water, snow, green grass, and forests. Examples of terrain that heat up

more quickly are asphalt, dry fields, dark soil and dark rocks.

As air warms up it expands, becomes less dense, and tends to rise. When warm air rises, cooler air moves

in from the surrounding areas to replace it.