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Category Archives: Severe Weather
What is fire weather?
Wildfires need three essential elements: heat, oxygen and dry fuel. Fire weather refers to a mix of meteorological conditions that make it easy for wildfires to ignite and spread quickly. The main weather factors are low humidity, strong winds, warm temperatures and atmospheric instability.
When humidity is low, moisture is drawn out of plants, turning them into highly combustible fuel for fires. Extended dry spells or droughts remove the moisture that helps prevent ignition, making the physical environment more vulnerable. Strong winds provide fires with oxygen and carry hot embers ahead of the fire line, starting new fires. Warm temperatures heat dry vegetation, making it more likely to ignite. Unstable atmospheric conditions allow smoke plumes to rise rapidly, intensify fires, and create unpredictable shifting winds. Continue reading
Who or what triggers the decision to sound a tornado siren?
The National Weather Service is responsible for issuing forecasts, watches and warnings for a variety of weather and water hazards. A “warning” is issued when hazardous weather poses an immediate threat to life or property.
A warning can be issued for a variety of hazardous weather, including tornadoes, thunderstorms and flash floods. These threats can form quickly, last a few minutes and impact a small area. Forecasters must rely heavily on observational data from Doppler radar, satellite, and other ground-based equipment and sensors to inform them during the warning process. Continue reading
Category: Meteorology, Severe Weather, Weather Dangers
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What is the tornado scale?
A tornado is a powerful column of winds that rotate around a center of low pressure. The winds inside a tornado spiral inward and upward, often exceeding speeds of 300 mph. We classify the strength of a tornado after trained observers assess the damage it did to the area.
All tornadoes are assigned a single number from the Enhanced Fujita Scale (EF) according to the most intense damage caused by the storm. When tornado-related damage is surveyed, it is compared to a list of damage indicators and degrees of damage to help estimate the range of wind speeds the tornado likely produced. The rating is assigned based on a set of 28 damage indicators, such as barns, schools and trees; the degree of damage to each one is used to determine the EF scale of every tornado. Continue reading
Why do clouds turn green during some storms?
The visible light spectrum covers colors from violet to red. Each color corresponds to a different wavelength, with green sitting roughly in the middle. Blue and violet have shorter wavelengths. Light rays change direction when they hit particles — a process known as scattering. The sky looks blue because air molecules scatter shorter wavelengths more effectively.
Clouds are made of water drops and ice crystals that scatter light from the sun in all directions. It is the multitude of drops and crystals that make a cloud look white during the day. Sometimes only a small amount of light escapes out the bottom, and so cloud bottoms often appear grayish. Continue reading
Category: Meteorology, Phenomena, Severe Weather
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What is clear air turbulence?
Clear-air turbulence, or CAT, is the turbulent movement of air masses in the absence of any visual clues such as clouds. The atmospheric region most susceptible to CAT is at altitudes of around 23,000 to 39,000 feet. This is the region of jet streams — fast-moving, narrow bands of wind high in the atmosphere that blow from west to east.
Wind shear refers to a sudden change in wind speed or direction across a short distance. When an aircraft encounters wind shear, it may experience abrupt vertical or horizontal movements, resulting in unanticipated changes in altitude or velocity. While wind shear is commonly observed in the vertical dimension, it can also manifest horizontally. Continue reading
