What is a water year and what is it used for?

Posted on October 5, 2026 by WeatherGuys Editor

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.

The water year is widely used to model and track the movement of water through a landscape. The water year is also used in water resource management applications, such as drought forecasting and computer simulations of watershed processes. The water year specifies the amount of water that flows into the system in that year. The water year is used to compare precipitation from one water year to another. When compared with a predetermined normal water year, the amount of precipitation defines a water year as very dry, dry, wet, or very wet.

The concept of the water year period is based on knowledge that precipitation falling in the latter part of a calendar year may be frozen and not drain out of a watershed until the next spring.

Additionally, Oct. 1 is generally the time of year when a river’s baseflow is at its lowest point. The baseflow is the result of shallow groundwater discharging directly into the river in places where the river channel lies below the elevation of the water table. The water table tends to be at its highest in the spring and begins declining throughout the summer and early fall.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Seasons

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Why is the reference, or normal, climate period a 30-year average?

Posted on September 28, 2026 by WeatherGuys Editor

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.

Climate data for meteorological fall (September, October, November) from Wisconsin State Climatology Office. (Map credit: Purdue’s Midwestern Regional Climate Center)

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.

The WMO has recommended that each member country recompute their 30-year climate normals every 10 years to get a better estimate for the current baseline climate.

The 10-year window allows us to see climate change in action, as the decadal updates monitor the trend of what is considered “normal.” Climate is not stationary: We are seeing trends in our normal climate conditions. Recomputing the 30-year normal every 10 years provides information for decision-makers in climate-sensitive sectors and industries such as agriculture, water management and energy.

Climatologists also compare global average temperature with the pre-industrial conditions of 1850-1900. The global average temperature in 2024 was 2.62 degrees Fahrenheit (1.35 degree Celsius) above the pre-industrial average of 56.7 degrees Fahrenheit, making it the warmest global temperature on record.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, History, Seasons

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How did our summer look climatologically?

Posted on September 21, 2026 by WeatherGuys Editor

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.

Monthly statewide average precipitation anomalies in inches for Wisconsin between September 2025 and August 2026 compared to the 1991 to 2020 normal. Precipitation anomalies are from NOAA’s National Centers for Environmental Information.(Image credit: Wisconsin State Climatology Office)

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.

Based on average dew point temperatures, this was a muggy summer, with 2026 ranking as the 10th-most-humid summer since 1979.

As far as precipitation goes, the summer was a dry one. Only a few areas in western and southern Wisconsin saw above-normal precipitation. Much of the state experienced some level of drought, with conditions ranging from moderate to extreme drought.

Twenty tornadoes occurred between June 1 and Aug. 31, two more than the normal summer period. Wisconsin’s total tornado count for 2026 is currently at 48, making this year our state’s second-most tornadic year since 1950. Wildfires in Ontario and northern Minnesota resulted in heavy smoke throughout Wisconsin in mid-July. During the third week of July, Wisconsin became shrouded in wildfire smoke. Blazing wildfires in parts of Ontario and northern Minnesota created dense plumes of smoke that were spread across the Midwest and the East Coast by northeasterly winds. The Air Quality Index reached the “hazardous” category across much of northern Wisconsin on July 15, a result of the thick smoke, and Wisconsin’s worst wildfire smoke event occurred on July 16.

For more detailed analysis and maps, visit the State Climatology Office website at https://climatology.nelson.wisc.edu/august-summer-2026-climatesummary.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, Seasons

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What is considered “heavy rain”?

Posted on September 14, 2026 by WeatherGuys Editor

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.

Rain gauges catch falling precipitation and record its amount and intensity. There are three major types of rain gauges: the standard rain gauge, the tipping bucket rain gauge and the weighing rain gauge.

The standard rain gauge is a cylinder with a funnel leading into a narrow measuring tube. It collects rainwater and a person manually reads and records the water level in the tube daily.

A tipping bucket rain gauge has a funnel that channels water into one of two small, balanced side-by-side buckets. When a specific amount of rain fills a bucket, it tips over, empties and triggers an electronic pulse to record the amount and timing. This is ideal for automated weather stations and real-time rainfall tracking.

A weighing rain gauge has a collecting bucket that sits on a mechanical scale and continuously weighs the accumulated water and records the total mass over time on a chart or digital logger.

Weather radars measure rain rate by sending out radio waves and calculating how much energy bounces back from raindrops. Colors on a weather radar image show the deduced intensity of precipitation. Green indicates light rain, mist, or drizzle; yellow indicates moderate rain (0.11 to 0.3 inches per hour), orange areas indicate heavy rain, and red are downpours of more than 0.3 inches per hour. Purple or magenta colors indicate extreme rainfall, severe storms or large hail.

Steve Ackerman and Jonathan Martin, professors in the UWMadison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Severe Weather

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Two Questions about Clouds

Posted on September 8, 2026 by WeatherGuys Editor

One of our readers awoke recently to some beautiful clouds in the summer sky and two excellent questions popped into her mind. First, what holds the clouds up in the air. And then what makes some clouds appear to be fluffy on top but flat on the bottom?

The fluffy appearance of the tops of some clouds are evidence of convection — that is, the air parcels within the cloud are buoyant and literally bubble to the top.
(Photo credit: Alex Buzurnyuk)

Clouds are composed of tiny liquid water droplets, whose diameters are about the width of a human hair, and tiny shards of ice in a variety of shapes, or habits. Whether a cloud is mostly liquid water droplets or ice particles depends, as you might guess, on the ambient temperature of the air in the cloud.

Tiny cloud liquid water droplets can remain in the liquid state to temperatures as low as about 14 degrees Fahrenheit, and when they do they are known as supercooled liquid water droplets. These droplets feel the downward directed force of gravity just like a baseball or a watermelon would. Because the droplets are so small, and therefore have small masses, the gravitational force can easily be balanced by an upward directed friction force resulting from the interaction of the droplets with the air molecules around them and so the droplets remain suspended.

This is what holds clouds up in the air.

When these droplets grow, by a variety of interesting processes, they gain water mass, and eventually the gravitational force overwhelms the friction force and the now larger droplets fall to the surface.

The fluffy appearance of the tops of some clouds is evidence of convection — that is, the air parcels within the cloud are buoyant and literally bubble to the top. As the air rises, it cools by expansion as it encounters environments with lower and lower pressure the higher it goes. This cooling increases the relative humidity of the air, and once the humidity gets to 100%, condensation of the invisible water vapor begins to produce the cloud liquid water droplets.

The bottom of clouds often appears flat because the first level at which rising air parcels begin to condense is usually rather uniform over a given region. This level is known as the lifted condensation level — that is, the level at which lifted air parcels first begin to experience condensation.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on Wisconsin Public Radio at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena

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