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How do we know if an extreme weather event is due to our changing climate?
Weather events result from an interaction of many factors, including current conditions of the atmosphere. Weather is considered extreme when it’s far outside of normal for a particular place and time. Weather-related extreme events include heat waves, freezes, heavy downpours and tornadoes. Attributing an extreme weather event to the observed climate change is referred to as the science of extreme event attribution.
Researchers take two complementary approaches to explore the cause-and-effect relationship between climate change and extreme weather. One method analyzes the statistics of how often a type of extreme event occurs in our current climate with how often the event occurred in a climate without human-caused climate change. This approach relies on improved climate model simulations. The other approach examines how climate change has altered the characteristics of the event. As an example, heat waves have become hotter because the atmosphere has warmed. Continue reading
Category: Climate, Meteorology, Severe Weather
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What is a water year and what is it used for?
A water year is used in hydrology to describe the 12-month period for which precipitation totals of an area are determined.
The U.S. Geological Survey, or USGS, operates a nationwide system of instruments to monitor discharge in rivers and issues comprehensive reports on the use of water resources. The USGS defines a water year as the 12-month period from Oct. 1 through Sept. 30 of the following year. This definition standardizes the reporting of water statistics in their publications. The water year is designated by the calendar year in which it ends, so on Oct. 1 we entered water year 2027. Continue reading
Why is the reference, or normal, climate period a 30-year average?
Climate normals are 30-year averages for various climate observations such as temperature, precipitation and snowfall. A 30-year period smooths out weather variability while getting a good representation of our current climate. The current 30-year normal covers the period 1991-2020. This averaging provides a baseline for comparisons and assessing whether a particular day or month is warmer or cooler than normal, or if it is wetter or drier than normal.
The World Meteorological Organization (WMO) recommends 30 years as the reference for current average conditions, and member nations follow that recommendation. For the United States, NOAA’s National Center for Environmental Information (NCEI) is the official source of the country’s climate normals. The computation is nontrivial and more complicated than one might initially anticipate. Climate scientists employ complex statistical calculations to ensure the observations accurately represent the full 30-year climate period. The determination of normal must account for the occasional missing data, change in station location and instrument upgrades. Continue reading
How did our summer look climatologically?
Meteorological summer is defined as the weather from June 1 through Aug. 31. The reference, or normal, climate period is the 30-year period of 1991-2020. The State Climatology Office recently completed its analysis of our 2026 summer conditions and created a comparison with previous summers.
The statewide average temperature during the summer of 2026 was 68.5 Fahrenheit, which is 1.3 degrees above normal. This was the seventh consecutive summer during which Wisconsin experienced a warmer-than-average temperature. All three summer months were above their normal monthly temperature. The largest departures of three to four degrees were scattered throughout northern Wisconsin. Continue reading
What is considered “heavy rain”?
The National Weather Service defines rainfall rate as the intensity of liquid precipitation falling over a specific, short duration, typically expressed in inches or millimeters of accumulation per hour.
The American Meteorological Society defines heavy rain as a rainfall rate of 0.30 inches of rain per hour. Rainfall rate measures how hard rain falls at any given moment, while accumulation measures the total depth of water accumulated. Continue reading
