Wind velocity (speed) is greatest when there is a greater pressure gradient. This means winds are faster when a large difference in air pressure exists over a short distance.
For example, the air flows out of the bicycle tire faster when the tire is inflated completely (much higher air pressure within the tube compared to outside the tube). Compare this to when the tube is only slightly inflated and the air flows out of the tube slowly (smaller difference in air pressure between the tube and outside).
There is an easy way to find an area on a map that has the greatest pressure gradient and therefore is likely to have the greatest wind speeds. Where the isobars are closest together is where the pressure gradient is greatest and where the wind speeds will be greatest.
On the map below, compare the spacing of the isobars at New York and Texas. New York is likely to have high winds compared to Texas' more calm winds.
The rotation (spinning on it's axis) of the Earth causes free flowing objects near the Earth’s surface to curve to the right of their path in the Northern Hemisphere. This greatly effects the direction of winds & oceans currents. This is called the
How does Earth's rotation affect wind?
How can differences in the Earth's surface cause small-scale winds called breezes?
Wind is the major factor in moving oceans. As the wind blows over the ocean, it transfers energy to the water and sort of "drags" the water in the same direction. Both prevailing (constant) winds and ocean currents are affected by the Coriolis effect. See ESRT page 4 for more information on ocean currents.
How are ocean currents caused?
- Convection cells – Convection is the movement of a gas or liquid caused by differences in density. Large-scale convection cells occur in the atmosphere, which cause the prevailing winds.
- Zones of diverging and converging air result from the convection cells in the atmosphere, which cause dry or wet areas.
Condensation is the process by which a gas turns into a liquid. For condensation to occur, two things must be present or happen:
Cold glass of soda: As air near the glass cools and the air temperature approaches the dew point temperature. When the air temperature equals or drops below the dew point temperature, the water vapor in the air turns to liquid (the process of condensation) on the glass (glass acts as condensation nuclei).
- The temperature of the air mass must reach it’s dew point temperature, AND
- A surface must be present for the water vapor to condense upon. This surface can be dust, pollution, or volcanic ash in the atmosphere. On the surface of Earth it can be a car, lawn or a flower. All of these are called condensation nuclei.
Clean air is free of any condensation nuclei, so condensation cannot take place.
When condensation occurs, energy is released into the atmosphere. Exactly 540 calories per gram of H2O is released into the atmosphere and is the “fuel” for many severe weather events, like Hurricanes (See ESRT front page). Energy is released during condensation because gas is the highest energy phase and as the molecules become liquid, the "excess" is released into the atmosphere.
Cloud Formation - A cloud is a mass of air that has suspended droplets of water or ice in it. In order for a cloud to form, a few events must occur.
Precipitation – Any falling liquid or solid water from clouds is considered precipitation. Once a cloud forms, precipitation can take place.
- Air must rise.
- The air will expand due to less air pressure aloft, which causes the air temperature to decrease.
- Condensation will occur if the temperature decreases to the dew point temperature and there is condensation nuclei available.
- Finally, a cloud is born!
- Coalescing – This is the process by which tiny water droplets in clouds “knock” into one another and grow into larger water droplets. The water droplets will fall to ground when they become too large to be suspended.