COLUMBUS (WCMH) — As the seasons change, it’s a great opportunity to review some commonly used weather terms to better understand your forecast and how to take action.
These words, phrases and terms are commonly used by Storm Team 4 meteorologists to describe both day-to-day weather, and potentially damaging storms. Being familiar with them will let you quickly make decisions to keep you and your family safe.
Watches and warnings
Ahead of weather that could harm life or property, a watch will be issued. A watch means that the atmosphere is favorable for that extreme, and potentially damaging weather. When this turns into a warning, it means that the weather event is imminent or actively occurring, so the time to take action and shelter if needed is now.
A thunderstorm capable of producing a tornado, winds reaching at leas 58 mph (50 kt) and/or hail at least 1″ in diameter (the size of a quarter).
The rate at which wind velocity changes from point to point in a given direction (as, vertically). One example of shear can be speed shear, where speed changes between the two points, but not direction. Another is direction shear, where direction changes between the two points, but not speed. A combination of the two is also possible.
Breezy conditions mean that wind speeds have picked up to 15-25 mph.
Windy conditions are stronger than breezy. The National Weather Service defines winds at 20-30 mph as windy.
Atmospheric (barometric) pressure
Pressure is the exertion of force upon a surface by a fluid (e.g., the atmosphere) in contact with it. The pressure of the atmosphere usually expressed in terms of the height of a column of mercury. Pressure is measured by electronic sensors called barometers.
High pressure systems form when downward pressure creates a clockwise air rotation in the Northern Hemisphere, and a counterclockwise circulation in the southern. High pressure is also referred to an anti-cyclone. Weather attributed to high pressure usually involves light wind and clearing clouds.
Low pressure systems generate counter-clockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. Unlike high pressure, areas of low pressure are often associated with unsettled weather like increasing clouds, wind and rain or thunderstorms.
An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The trough (or trof) is opposite of ridge.
An elongated area of relatively high atmospheric pressure. A ridge is the opposite of trough.
(abbrev. LTNG) A visible electrical discharge produced by a thunderstorm. The discharge may occur within or between clouds, between the cloud and air, between a cloud and the ground or between the ground and a cloud. Because of this electrical charge, it is not safe to be outdoors. Remember, “when thunder roars, go indoors” and “When you see a flash, dash inside.”
Negative Lightning strike
Lightning strikes are categorized as positive or negative based on the charge from cloud to ground. Negative lightning strikes usually originate from the lower-level clouds in a thunderstorm, which have a negative charge. The bolts often strike directly under the thunderstorm where the ground is positively charged. These are the most common types of lightning strikes, nearly 95% of lightning strikes carry a negative charge.
Positive Lightning strike
Lightning strikes are categorized as positive or negative based on the charge from cloud to ground. Lightning that strikes from the anvil (top) of the thunderstorm tends to carry a positive charge because most of the particles in the anvil are positively charged. These are the strikes that can travel far from the parent thunderstorm. Positive lightning strikes are way more powerful than negative lightning strikes. They can carry as many as 1 billion volts, which is ten times more powerful than a negative lightning strike and can strike more than 20 miles away from the storm. Positive lightning makes up less than 5% of all strikes.
High-altitude winds that steer weather systems and air masses near the surface that affect large regions. Jet stream dynamics play an important role in storm development (wind shear) and moisture transport (low-level jet stream).
Red Flag Warning
A National Weather Service forecast to alert the public and firefighters that conditions are favorable for wildfires due to low humidity, gusty winds and dry vegetation.
A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). When the dew point is reached, we often see small water droplets or dew form. A higher dew point indicates more moisture present in the air, and the air will often feel more humid.
A ratio, expressed as a percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. Therefore, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. Note that while relative humidity and dew point are related, they are not interchangeable.
The heat index is the apparent temperatures, or a measurement of how hot it really feels outside when the relative humidity is added or factored in to the actual air temperature. Note that the more humid the air is, the harder it is for evaporation to occur, which can slow down a person’s natural ability to cool themselves off. Intern, this is why it feels hotter when it is more humid outside.
Ice pellets that are formed when rain or melted snow freezes into ice on its way from the sky down to the ground. This is different from freezing rain because it is frozen precipitation that freezes and becomes a solid before hitting the ground.
Rain that falls as a liquid but freezes into a glaze of ice upon contact with the ground, or any other surface that is below freezing.
Partly cloudy/partly sunny
These terms can often be used interchangeably. The National Weather Service defines both as having 3/8 – 5/8 of the sky is covered by clouds. The term “Partly Sunny” is used only during daylight hours.
One half of the El Nino-Southern Oscillation (ENSO) phenomenon is La Niña. La Niña (The Girl) is a global weather pattern that describes a dramatic cooling of ocean temperatures in the Western Hemisphere. La Niña is known for its disruptive impact on weather, specifically heavy rainfall and an increase in low-pressure systems.
The opposing warm half of ENSO is called El Niño. El Niño (The Boy), which occurs irregularly every two to seven years and is often followed closely by a La Niña pattern. It warms the oceans and creates the opposite effect in terms of not just ocean temperatures, but atmospheric pressure. It, too, is associated with irregular and sometimes severe weather patterns
A period of abnormally and uncomfortably hot and unusually humid weather. Typically a heat wave lasts two or more days and will consist of temperatures that are outside the region’s historical average.
Frost is the frozen version of dew. Frost occurs when cold, moisture-soaked air deposits water that freezes and leaves an icy film on things like plants and car windows. Frost can be a nescience to people who park their cars outside and destructive to plants left out out without being covered.
A flood, or flooding, is anytime that water overflows onto normally dry land.
A rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level. The actual time threshold may vary in different parts of the country, but usually refers to heavy rainfall or an overflow of water at a rapid rate in less than 6 hours.
A deficiency of moisture that results in adverse impacts on people, animals, or vegetation over a sizeable area. Droughts are among the most destructive forces in nature—only hurricanes are more economically damaging to the United States.
A microburst is a localized column of sinking air (downdraft) within a thunderstorm and is usually less than or equal to 2.5 miles in diameter. Microbursts can cause extensive damage at the surface, and in some instances, can be life-threatening. They start as updrafts, or columns of rapidly rising air. Once the updraft weakens, a downdraft comes crashing to the ground, bursting in all directions, producing tornado-like wind speeds, pressure and destruction.
Hail forms when powerful updrafts inside a thunderstorm force water droplets above the freezing level. The droplets freeze and form hailstones that grow as more water droplets freeze on the surface of growing ball of ice. Hailstones are suspended by updrafts until becoming heavy enough to fall to the ground. If hailstones are 1 inch or larger in diameter, the storm is classified as severe
A line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms. They develop along or ahead of cold fronts and are know to bring heavy rain, dangerous lightning, straight line wind damage, hail and even tornados and waterspouts.
Before a weather event graduates into a tropical storm, it’s a tropical depression. The initial stage of a hurricane, a tropical depression is a tropical cyclone with maximum sustained surface winds of 38 mph (34 knots) or less.
Once a tropical system has maximum sustained surface winds of 39-74 mph, it is a tropical storm. At this point, the storm is named using a list that is put together by the World Meteorological Society.
When wind speeds on a tropical storm increase to 74 mph, the storm will be upgraded to a hurricane. Category 1 hurricanes have 74-95 mph wind speeds. 96-110 mph wind speeds are upgraded to a category 2, Category 3 means that wind speeds have picked up to 111 to 129 mph, category 4 winds are 130-156 mph, and wind speeds 157 mph or greater will upgrade the storm the Category 5.
An organized band of cumuliform clouds that immediately surrounds the center (eye) of a hurricane. The fiercest winds, most intense rainfall and thunderstorms typically occur near the eye wall.
If you are interested in reviewing more terms, you can reference the National Weather Service Glossary.