Ch. 13 – The Nature of Storms Every thunderstorm has characteristics that are similar no matter how strong the storm is. Cumulonimbus clouds produce thunderstorms. So, How do cumulonimbus clouds from? Video A cumulus cloud may grow into a cumulonimbus cloud. 1. Moisture levels must be high enough in the lower part of the atmosphere. 2. The air must be lifted so the air can condense and release latent heat. 3. The air must continue to cool with increasing altitude for the growing cloud to stay warmer than the surrounding air.

An air-mass thunderstorm occurs when the air rose because of unequal heating of Earth’s surface within one air mass. Sea-breeze thunderstorms occur by extreme temperature differences between the air over land and the air over water. A frontal thunderstorm is produced by advancing cold air pushing warm air rapidly up the steep cold-front boundary. Thunderstorm Stages 1. Cumulus Stage – air starts to rise upward & clouds form. 2. Mature Stage – water droplets that formed at high, cool levels and sinks rapidly to the ground as precipitation. 3. Dissipation Stage – the supply of warm, moist air runs out and the updrafts slows, eventually stops and precipitation can no longer form. Video

Severe Weather What makes one thunderstorm more severe than another? One of the main factors involved is the difference in temperature between the upper and lower parts of the storm. This difference cause greater updrafts and downdrafts in the storm. Cold front and low-pressure systems are associated with this. Supercells are characterized by the intense, rotating updrafts. These are extremely powerful storms that can last for hours. Video

Lightning is produced in the same fashion as static electricity. Static electricity occurs with the friction of an object – Ex. Your feet rubbing against carpet. A lightning bolt is generated by the friction of an updraft with a downdraft of a cumulonimbus cloud. Electrons are separated from atoms because of this. Positive & Negative Ions (+ high & - low in clouds). To relieve the electrical imbalance, an invisible channel of electrons (stepped leader), moves from the cloud toward the ground. When this nears the ground a channel of positively charged ions (return stroke) rushes upward to meet it. 100 million volts of electricity. Heats the air to about 30,000 °C. The thunder is the sound produced from the superheated air rapidly expanding and contracting. Video

Hail produces almost $1 billion in damage every year. Downbursts occur when violent downdrafts are concentrated in a local area. Macrobursts can cause a path of destruction up to 5 km wide and wind over 200 km/h. Microbursts are up to 3 km wide and winds over 250 km/h. They are hard to detect and prepare for thus they produce lots of damage. Video Hail produces almost $1 billion in damage every year. Due to water droplets freezing in the cumulonimbus cloud. As they form and fall they may encounter a nearby updraft to lift them up and form more ice. Floods usually occur when wind current in the upper atmosphere are slow & weak, thus the storm moves slow and dump a lot of moisture.

Tornadoes A tornado is a violent, whirling column of air in contact with the ground. Before it hits the ground it’s a funnel cloud Usually associated with supercells. Tornadoes form when wind speed and direction change suddenly with height  wind shear. A horizontal rotation starts and if the rotation is near the storms updraft it can shift its rotation to a vertical rotation. As updrafts accelerate the rotation, air is removed from the center of the column. Video

Fujita tornado intensity scale – ranks tornadoes on their path of destruction, wind speed, and duration. Video

An average of 80 deaths & 1500 injuries due to tornadoes. Most tornadoes occur in late spring during the late afternoon and evening, when the temperature contrasts between polar air and tropical air. This occurs more commonly in the Central United States – “Tornado Alley”. An average of 80 deaths & 1500 injuries due to tornadoes. Tornado Safety  Table 13-3 (pg. 340)

Tropical Storms Tropical cyclones are large, rotating, low-pressure storms. The strongest of these occur in the U.S. and other parts of the Atlantic Ocean as hurricanes. They move counter-clockwise in the northern hemisphere due to the Coriolis effect around a low-pressure system. They need an abundant supply of very warm ocean water and some sort of disturbance to lift warm air and keep it rising. Fig. 13-10 (were they form) These tropical cyclones move according to the wind currents. They move steadily toward the west, then eventually turn poleward when they reach the far edges of the high-pressure system. Video

The first stage of the tropical cyclone is called a tropical depression. A tropical storm occurs when the winds exceed 65 km/h. It is classified as a hurricane when the winds exceed 120 km/h. An eye of the storm forms a calm center. The strongest winds occur in the area immediately surrounding the eye  eyewall. Saffir-Simpson hurricane scale classifies hurricanes according to wind speed, air pressure in the center, and potential for property damage. Table 13-4. Energy to run a hurricane is lost once it moves over land. Storm surges are produced when hurricane-force winds drive tides of ocean water toward coastal areas, where it washes over the land. Video

Wind chill – the temp the body feels with winds. Recurring Weather Droughts occurred during the 1930s in the Central U.S., which produced the Dust Bowl. Fig. 13-13 Droughts are usually due to global wind pattern that produce high-pressure systems (lack of rain) for several weeks or months. Heat waves are also associated with these patterns. The high-pressure systems blocks cooler air masses from moving into the area. Cold waves are also brought on by large, high-pressure systems. They occur however by arctic or polar air masses. Wind chill – the temp the body feels with winds. less than 15 degrees F wind chill may cause frostbite. Ex. 0 degrees F & 30 mph wind = -49 Ex. 0 degrees F & 10 mph wind = -22