extreme events
Thresholds
A threshold is a boundary that, when reached and surpassed, results in dramatically new conditions. Sometimes, thresholds are referred to as “tipping points”.
What are examples of thresholds in weather?
Thresholds are at work all around us. For example, certain types of plants die when temperatures drop below 32° F. At 33° F, a plant might be fine, but its cells break when water in them freezes at 32° F.
Hurricanes need surface water temperatures in the ocean to be at least 80°F before they can form. They won’t form at 79° F, but will form more frequently and with greater energy when more of the ocean is above 80° F.
How is the idea of thresholds used to understand and prepare for weather?
Thresholds exist in nature, but we also use this concept to identify tipping points related to our built world and the impacts of weather. For example, we may define thresholds to help us categorize different things. Wind speed thresholds help us classify tornadoes, with 40-72 mph winds regarded as the least damaging class (F-0) and 261-318 mph winds as the most damaging class (F-5), with other classes in between. If the winds in a tornado pass the 260 mph threshold, the tornado is classified as an F-5.
Thresholds also help weather forecasters determine when to issue a storm advisory—for example, when hail reaches one inch in diameter and winds reach at least 58 mph, a severe thunder storm warning will be issued.
We can also find thresholds in things we build. For example, we know that a dam can only hold back so much water before it collapses or water flows over it. We also know that a building is designed to withstand certain strength winds.
Knowing these kinds of thresholds can help us understand what type of impacts to expect, given certain weather conditions. Understanding thresholds can help us identify actions needed to prevent the threshold from being reached (e.g., divert water away from a dam) or to respond to the threshold before it becomes a problem (e.g., evacuate people downstream from a dam).
