Thunderstorms • Convection causes a cumulus cloud to grow into a cumulonimbus cloud
ThunderstormsThree conditions are required for their formation: • Lots of moisture in lower levels of atmosphere As moisture condenses, it releases latent heat, which keeps a cloud warmer than the air around it, so that it continues its upward motion 2. A mechanism is needed to lift the air 3. The atmosphere must be unstable (the air must continue to cool with increasing altitude so that the growing cloud will stay warmer than its surroundings).
How air is lifted • Air-mass thunderstorms form as the Earth’s surface heats up during the afternoon and air rises • Mountain thunderstorms form as a result of orographic lifting • Sea-breeze thunderstorms form from convection cells that arise from differences in temperature between ocean and land • Frontal thunderstorms form as advancing cold fronts push under warm fronts. These can persist long into the night
Cumulus stage: warm air rises vertically upwards, condenses and releases latent heat. • Mature stage: Precipitation falls from condensed droplets. As it falls, it cools the air around it. The newly cooled air then sinks to the ground via a downdraft. Updrafts and downdrafts thus exist side by side. • Dissipation stage: Once the warm air on the ground has been completely cooled, there is no updraft and the precipitation can no longer form.
Severe thunderstorms • Cold fronts may bring in upper level, low-pressure systems marked by pools of cold air. • This cold, high air increases the temperature difference between the upper and lower parts of the storm, causing more instability in the air. • The strength of the updrafts and downdrafts then intensifies, and may become supercells: characterized by updrafts as strong as 240km/h.
Lightning • Lightning forms when friction between updrafts and downdrafts separates electrons from atoms, either in the cloud or on the ground. • To relieve the electrical imbalance, an invisible channel of negatively charged air (a stepped leader) moves from the cloud toward the ground. • When the stepped leader nears the ground, a channel of positively charged ions (the return stroke), rushes up to meet it. • As the return stroke surges from the ground to the cloud, the channel is lit up with 100 million V of electricity.
Lightning heats the air to 30,000°C; 5 times hotter than the surface of the Sun!Thunder is the sound you hear as this hot air rapidly expands and contracts.
Hail • Hail forms if part of a cumulonimbus cloud is high enough that water droplets freeze • If strong updrafts and downdrafts exist side by side in the cloud, ice pellets can grow by constantly cycling up to the region where they encounter more supercooled water droplets. • Eventually, the ice pellets get too heavy and fall to the Earth as hail.
Tornadoes • Form when wind speed and direction change suddenly with height (wind shear) • This produces a horizontal rotation near Earth’s surface. • If the rotation happens close to a thunderstorm’s updraft, the rotating air can be tipped upright.
Tornadoes tend to form in the spring during the late afternoon and evening, when the temperature difference between polar air, which still has winter characteristics, and tropical air, which is becoming warmer, are the greatest. • A tornado watch means there may be a tornado. People are watching for one. • A tornado warning means one has been sighted. Take cover in an interior room or hallway on the lowest floor. Get under a sturdy piece of furniture.
Cyclonic storms • Form during the summer and fall in the tropics • Large, rotating, low-pressure storms • Called cyclones, typhoons, or hurricanes.
How cyclonic storms form: • Air rises from warm waters, storing latent heat. • Latent heat is released as water vapor condenses into clouds and rain. • Rising air creates an area of low pressure at the ocean surface. • As more warm air moves toward the low pressure center to replace rising air, the Coriolis effect causes it to rotate (counterclockwise in the northern hemisphere). • As more energy is released from condensation, air moves faster and faster- 240km/hr or more!
Cyclonic storms continue as long as atmospheric conditions allow warm air to be fed into the system at the surface and removed from the system in the upper atmosphere.