surrounded by
stratocumulus On
Earth, common
weather phenomena include wind,
cloud, rain, snow,
fog and
dust storms. Some more common events include
natural disasters such as
tornadoes,
hurricanes,
typhoons and
ice storms. Almost all familiar weather phenomena occur in the troposphere (the lower part of the atmosphere). Weather occurs primarily due to air pressure, temperature and
moisture differences from one place to another. These differences can occur due to the
sun angle at any particular spot, which varies by latitude in the tropics. In other words, the farther from the tropics one lies, the lower the sun angle is, which causes those locations to be cooler due to the spread of the
sunlight over a greater surface. The strong temperature contrast between
polar and tropical air gives rise to the large scale
atmospheric circulation cells and the
jet stream. Weather
systems in the mid-latitudes, such as
extratropical cyclones, are caused by instabilities of the
jet stream flow (see
baroclinity). Weather systems in the tropics, such as
monsoons or organized
thunderstorm systems, are caused by different processes. Because the Earth's
axis is tilted relative to its orbital plane,
sunlight is incident at different angles at different times of the year. In June the Northern Hemisphere is tilted towards the
Sun, so at any given Northern Hemisphere latitude sunlight falls more directly on that spot than in December (see
Effect of sun angle on climate). This effect causes seasons. Over thousands to hundreds of thousands of years, changes in Earth's orbital parameters affect the amount and distribution of
solar energy received by the
Earth and influence long-term climate. (See
Milankovitch cycles). The uneven solar heating (the formation of zones of temperature and moisture gradients, or
frontogenesis) can also be due to the weather itself in the form of cloudiness and precipitation. Higher altitudes are typically cooler than lower altitudes, which is the result of higher surface temperature and radiational heating, which produces the
adiabatic lapse rate. In some situations, the temperature actually increases with height. This phenomenon is known as an
inversion and can cause mountaintops to be warmer than the valleys below. Inversions can lead to the formation of
fog and often act as a
cap that
suppresses thunderstorm development. On local scales, temperature differences can occur because different surfaces (such as oceans, forests,
ice sheets, or human-made objects) have differing physical characteristics such as
reflectivity, roughness, or moisture content. Surface temperature differences in turn cause pressure differences. A hot surface warms the air above it causing it to expand and lower the density and the resulting surface
air pressure. The resulting horizontal
pressure gradient moves the air from higher to lower pressure regions, creating a wind, and the Earth's rotation then causes deflection of this airflow due to the
Coriolis effect. The simple systems thus formed can then display
emergent behaviour to produce more
complex systems and thus other weather phenomena. Large scale examples include the
Hadley cell while a smaller scale example would be
coastal breezes. The
atmosphere is a
chaotic system. As a result, small changes to one part of the system can accumulate and magnify to cause large effects on the system as a whole. This atmospheric instability makes weather forecasting less predictable than tidal waves or eclipses. Although it is difficult to accurately predict weather more than a few days in advance,
weather forecasters are continually working to extend this limit through
meteorological research and refining current methodologies in weather prediction. However, it is theoretically impossible to make useful day-to-day
predictions more than about two weeks ahead, imposing an upper limit to
potential for improved prediction skill. ==Shaping the planet Earth==