What causes the Santa Ana winds?

Posted on January 21, 2025 by WeatherGuys Editor

Santa Ana winds are dry, warm, and gusty winds that blow from the interior of southern California toward the coast and offshore. They are a type of downslope wind, which is a wind directed down a slope produced by processes larger in scale than the slope.

Santa Ana winds can occur when the pressure gradient caused by a high-pressure region over the Rockies, in combination with friction, forces air from the mountainous West down the San Gabriel Mountains in southern California.

1-minute GOES-18 Shortwave Infrared (3.9 µm) images (left) and Red Visible (0.64 µm) + Fire Mask derived product (right), with 15-minute METAR surface reports plotted in yellow, from 1801 UTC on 7th January to 0000 UTC on 8th January; Interstate highways are plotted in red. (Image credit: CIMSS Satellite Blog)

Atmospheric pressure always increases as one gets closer to Earth’s surface. An adiabatic process is one in which no heat energy is gained or lost by the system in question – in this case, a descending parcel of air. So, as an air parcel descends, it is compressed, which results in an adiabatic warming. The parcel warms at a rate of 10° C per kilometer, or about 29°F per mile of descent. This adiabatic warming also results in a lower relative humidity.

Santa Ana winds cause the temperature to increase and the relative humidity to plummet because of adiabatic warming.  The wind speed also increases as the air squeezes through mountain passes and canyons, like a slow-moving river that suddenly narrows and turns into rapids. The strong winds become warmer as they descend as well as drier in terms of relative humidity. Santa Ana winds can turn large geographic areas into bone-dry tinderboxes. As in recent news, infernos from wind–related fires can rage throughout the affected area. 

There are other types of downslope winds around the world that similarly bring hot, dry conditions to regions downwind of mountains and deserts. Some of these are the berg wind of South Africa, the leveche of Spain, and the sirocco of the Mediterranean Sea.  

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena, Severe Weather

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Beyond the headlines, what else has been happening in the weather lately?

Posted on January 13, 2025 by WeatherGuys Editor

There has certainly been a lot of interesting and, in many cases, devastating weather around the country in the past couple of weeks. The heavy snow in parts of the country that don’t often see it along with the California wildfires have caught the attention of lots of us in the first days of the new year.

Wisconsin’s 2024 average temperature departure from the 1991-2020 Normals in degrees Fahrenheit. (Image credit: Midwest Regional Climate Center, Purdue University)

But in the background is a rather remarkable one-week stretch that occurred in the last week of December.

As you have heard a few times before, each day we calculate the areal extent of air colder than minus 5 degrees Celsius (23 degrees Fehrenheit) at 850 mb (about 1 mile above sea level) over the entire Northern Hemisphere.

This calculation can be used to assess the severity of the winter in a given year compared with other years stretching back to 1948. Each day from Dec. 24 through Dec. 30 of this season set the all-time minimum areal extent for that calendar day, meaning that the hemisphere was warmer over each day in that last week of December than it had been on those same days in any of the previous 77 years.

This is both remarkable and sobering as it indicates that, despite the local weather being quite wintry in some locations that don’t often see such extremity, the entire Northern Hemisphere is still clearly demonstrating the fact that the planet is warming.

What this means for the rest of the winter, both over the hemisphere and more locally in southern Wisconsin, is hard to say. However, the lack of snowfall to date in Madison will surely moderate the extreme cold that appears to be looming for midweek next week.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, History

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Why are clouds relatively flat on the bottom?

Posted on January 7, 2025 by WeatherGuys Editor

Most clouds, especially those with flat bottoms, form in rising air. The air can be forced to rise due to convection, frontal lifting, or when air near the surface flows together from different directions.  As a volume, or parcel, of air rises, it expands and cools. In addition, the relative humidity of the rising air increases. As the parcel approaches the point of saturation, water vapor condenses to form tiny water droplets or ice particles, creating a cloud. Saturation occurs at a distinct altitude, which varies depending on the temperature and humidity structure of the atmosphere. Below this condensation level clouds do not form.

Cumulus clouds are formed where there is a deeper layer in which heating/lifting and moisture are sufficient to promote cloud development. (Photo credit: NWS Key West)

Often low clouds, like stratus and cumulus, appear to have flat bases. These clouds form as air near the ground is rising. As the air rises, it expands as pressure decreases with altitude. This expansion results in a cooling, which causes the relative humidity in the rising parcel to increase. The temperature of the rising air approaches the dew point temperature. When it reaches the height where those two temperatures are equal, the relative humidity is 100% and a cloud forms. Meteorologists call this altitude the lifting condensation level.

The temperature and dew point temperature near the ground are quite uniform and on the size scale of a cloud or a collection of clouds. So, as a layer of air near the ground rises, the height at which condensation occurs does not vary much and the cloud base height appears uniform. From the ground, these bottoms appear very flat but if you are in a plane and fly through these bases during take-off or landing, you’ll be close enough to notice that the cloud bases are not a sharp boundary.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) 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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Was 2024 an interesting weather year for Wisconsin?

Posted on December 30, 2024 by WeatherGuys Editor

Yes, 2024 was a very interesting year. The statewide average temperature was 31.4 degrees Fahrenheit, which is 12.2 degrees warmer than the 1991-2020 normal. December 2023 to February 2024 was our warmest winter since record-keeping began in 1895. The statewide average temperature for the winter was 28.3 degrees, surpassing the previous record by 2 degrees.

Average winter temperature in Wisconsin from 1895 to 2024 with the 1895-2024 mean winter temperature overlain (Image credit: NCEI Climate at a Glance).

Wisconsin’s average temperature during November 2024 was 38.7 degrees, which is 7.1 degrees above the November average temperature. Seventy out of Wisconsin’s 72 counties recorded a November 2024 average temperature much above average. Overall, 2024 is on track to be Wisconsin’s warmest year on record.

The warmth is not limited to the surface. Atmospheric scientists track the areal extent of air colder than minus 5 centigrade at about 1 mile above the surface of Earth.

This fall (September through November) we recorded the smallest average areal extent of this cold air since at least 1948. That means that 2024 had the warmest Northern Hemisphere autumn in at least the past 77 years.

This year was a tornado year. For the first time, tornadoes were observed in Wisconsin during the month of February: There were 2 of them, an EF1 and an EF2. Through November, more than 1,700 tornadoes were reported nationwide. The final count will come next year, as confirming a tornado takes time.

Three of the four wettest years on record have occurred in this century (2004, 2013 and 2024), and the state’s seasonal precipitation has increased over the long term by 1.5 inches (20% increase) since records began in 1895. May was Wisconsin’s 10th-wettest May. The statewide average precipitation during May was 5.52 inches, which exceeded the 1991-2020 normal by 1.59 inches. June 2024 ranked as Wisconsin’s sixth-wettest June on record.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) 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 the difference between sleet and freezing rain?

Posted on December 23, 2024 by WeatherGuys Editor

Rain, snow, freezing rain, and sleet all generate hazardous traffic conditions.  Freezing rain, and the less intense freezing drizzle, can create the very treacherous road condition referred to as “black ice.” A freezing fog may similarly coat objects in ice while also reducing visibility.  Black ice is so named because the affected roadway appears dark, just like wet pavement. Black ice creates nearly zero friction conditions with vehicle tires so that correcting a skid in such conditions can be nearly impossible. 

A diagram illustrating the difference between sleet, freezing rain, and snow. (Image credit: NOAA/NWS-Northern Indiana)

Sleet consists of translucent balls of ice that are frozen raindrops. It occurs when a layer of subfreezing air at the surface is deep enough for raindrops (usually freshly melted snowflakes) to freeze as they travel through the layer. Freezing rain forms when a very shallow layer of cold air is at the surface, causing raindrops to freeze on contact with exposed objects on the ground, objects whose temperature is below freezing. Thus, both freezing rain and sleet form when there is a temperature inversion near the surface – that is, when the air temperature increases with increasing altitude. Perhaps because of this underlying similarity, the difference between sleet formation and freezing rain formation is quite small, although the two precipitation types do not look alike at all. When sleet hits the surface, it bounces and covers flat surfaces such as roads and driveways with millions of icy ball bearings as opposed to the sheet of ice left in the wake of a freezing rain event.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Seasons

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