Why does the wind seem stronger during the day than at night?

 Continuing with our Answering Viewer Questions series, Susie asks why it seems like the wind is stronger during the day than at night. Outside of the influence of storms, our winds are typically a bit stronger during the day compared to at night. This is because stronger winds found just above the surface can be mixed down to the ground a little easier during the day.

This can be visualized using an atmospheric sounding, or a vertical profile of a column of air above one single point. The red line plotted below is the temperature, while the green line is a dew point. The bottom of the plot the surface, while the top of the chart is typically miles up in the sky.

A typical daytime sounding has strong "lapse rates" in the lower levels. Simply put, the air cools quicker over short distances. This steeper slope allows the air above to easier mix down toward the ground unimpeded. And often stronger winds -- commonly referred to as the "low-level jet" -- are found just above the ground. These winds are also mixed down to the ground, producing stronger winds during the day.

Compare this to a nighttime example. The only change from the last sounding to this one is the temperature in the lower levels near the surface. Since we cool off quickly at night with the loss of solar radiation, the temperature at the surface becomes cooler than that above. This is called an "inversion" where there is no longer cooling with height. This layer of relatively warmer air prevents that mixing mentioned above, which typically results in lower wind speeds at night compared to the day.

To summarize: stronger winds during the day are a result of deeper mixing, where air above the surface is easier transported down to the ground. While the loss of daytime heating prevents that mixing effect.

Summer-like weekend ahead, cooler and active pattern returns next week

 Hurricane Isaias is just hours away from making landfall near Pensacola, Florida as a strong hurricane. Sustained winds as of 6PM Friday are near 115 mph as the center of the storm is moving North at 18 mph. Storm surge, heavy rain, and tornadoes will accompany the storm across the gulf coast as it makes landfall Friday evening, with impacts carrying over into Saturday.

Locally here in Northern Illinois, we will see a very summer-like weekend with limited influence from the tropical system to the South. A weak blocking pattern aloft will keep us dry, as the rain shield will stay off to the Southeast. A few upper level clouds will spread over the area Saturday, but we will remain rain-free with highs near or above 80 degrees. Chances for showers and even a few storms will begin to move in later in the week, with the initial cold front arriving Monday night.

Showers will be higher in coverage toward the middle of the week, as the upper-level pattern brings several waves our direction. With varying degrees of moisture, we may not see widespread and heavy rain potential locally. But a cooler pattern with more frequent chances for rain will take over the second half of next week.

Isaias strengthens, but a warm and dry weekend remains on track

Showers Friday:

It's been a beautiful stretch of weather across the area so far this week. Sunshine has been plentiful, and our temperatures have consistently climbed into the 70s, ranging from the low 70s early in the week to the upper 70s more recently. 

Combined with low humidity and dry conditions, it's made for a fantastic first full week of October. But as the saying goes, all good things must come to an end. Fortunately, our interruption from the sunshine will be brief as our weather pattern aims to dry back out for the upcoming weekend.  

Today's rain showers are tied to a weak disturbance and accompanying warm front lifting east across the Central Plains. Over the past 24 hours, this system has sparked several severe thunderstorm warnings and flash flood warnings across Iowa. As it continues its eastward trek, it will bring an opportunity for a few showers, possibly a rumble of thunder around or just after the midday hours. Amounts should be light for most locales, and once this system moves through, clouds will decrease into the night. 

Latest on Isaias:

Since our last update, Isaias has continued to gain strength as it spins over the central Gulf, reaching category 2 status with maximum sustained winds of 105 mph. At the same time, its central pressure has fallen to 972 millibars. Landfall remains expected sometime late Friday evening along the Gulf coast, possibly still as a category 2 hurricane.  

Weekend Outlook:

The core of Isaias is still expected to remain well to our southeast after landfall, keeping the heaviest rain and strongest winds away from northern Illinois. That said, we may see some of the system's influence in the form of high-level clouds late Saturday into Saturday night as tropical moisture spread north into the Great Lakes. Even with those clouds, a ridge of high pressure positioned to our north will help keep our weather dry and summer-like with afternoon highs landing in the upper 70s and low 80s. Saturday looks to be the warmest day of the stretch with highs topping out around 81 degrees. This warmth aims to linger into the start of next week.   
 

What is a diurnal temperature range?

 

The first week of October did not disappoint with comfortable temperatures and sun-filled skies! For the last six days afternoon highs have warmed into the low to mid 70s, with a few spots even tagging 80 degrees! And we are likely to see those 70s, and even 80s, continue into at least the weekend if not early next week.

This warmth has been aided by some dry air thanks to high pressure. This has allowed the diurnal temperature range - difference from morning low to afternoon high - to range from roughly 20-30 degrees! That's a very typical early fall afternoon.

The reason for this high range is simply due to the dry air we've had in place. Dew point temperatures over the last several days have remained in the 30s and 40s. Dry air allows our air mass to heat up more efficiently than an air mass that has moisture in it. Think of those humid days in late spring and summer. It may feel hot, but the range in temperature from the morning low to afternoon high isn't as great. But when there is dry air (like what we've had this past week) morning lows in the 40s turn into afternoon highs in the 70s! This can also make it a little more difficult to figure out what to wear throughout the day.

Take in these nice fall days because as the old saying goes, "All good things must come to an end". And while we probably won't see a drastic temperature change, rain chances may increase within the next week.

  

Tracking Isaias: Rain chances remain limited locally

Sun-Filled October:

Sunshine has been the headline for October so far. Through the first week of the month, six days have come with in 30% cloud cover or less. Thanks to an expansive high pressure system and lack of moisture, that bright and pleasant stretch has helped make for a really nice start to October. 

 

Continued Sunshine:

That trend isn't going anywhere just yet. Today looks to deliver another healthy dose of sunshine with only a few passing clouds, making it another great day to get outside or to visit some of the local orchards and pumpkin patches. Winds will take a slight turn to the west, pushing highs near 70 degrees. 

 

Latest on Isaias:

While a weak system coming in Friday could squeeze out a shower or two, all eyes remain on the 9th named storm of the Atlantic season, Isaias. Overnight, this system has strengthened into the a category 1 storm with winds up to 75 mph. According to the National Hurricane Center, it is expected to strengthen further as it makes its turn towards the Gulf Coast, possibly into a major hurricane (Cat. 3+).   

Weekend Outlook:

We'll need to keep an eye on Isaias as it moves inland, as a small amount of tropical moisture could try to work its way north. For now, forecast models keep the core of the storm and its heaviest rainfall well to our south and east, which should leave the Stateline dry through the weekend. We could see some high clouds drift in late Saturday as the remnants move into the Tennessee Valley. Sunshine should make a return for Sunday as the system pushes further east. 

Limited rain chances through the weekend

 

After a cloudy and gloomy end to September, the beginning of October has featured an abundance of sunshine and some much-needed warmth! The sun-filled skies continued Wednesday with afternoon temperatures warming into the upper 70s and low 80s! For reference, the average high should be right around 67 degrees.

Skies will remain mostly clear Wednesday evening as temperatures fall back through the 60s, eventually down into the upper 40s overnight. With some stronger winds in the jet stream to the north, we may see a few clouds from time to time leaving skies partly cloudy at the start of Thursday. Winds will remain from the northwest during the afternoon but won't be quite as breezy thanks to high pressure sliding a little closer to Stateline. Afternoon temperatures will warm into the upper 60s and low 70s.

Clouds will be on the increase late Thursday night into Friday as both the return of warmer air and moisture move in from the southwest. This will help hold temperatures close to 50 degrees Friday morning. But with the increase in moisture will also come the increasing chance for a few showers during the first half of the day Friday. Drier air ahead of the rain in the morning will help eat away at some of the moisture for the afternoon leaving rainfall totals under a quarter of an inch for most.

By Friday evening, skies should be drying out just in time for Friday night football games. Temperatures on Friday should warm back into the low 70s. With the warm front to our north Friday evening southwest winds will start to increase, keeping temperatures in the mid-50s Friday night and helping highs Saturday warm into the upper 70s and low 80s. The warmth on Saturday will somewhat be determined by just how much, and thick, cloud cover we see Saturday afternoon. We'll likely see a gradual increase in higher level clouds from the south starting Saturday morning as the remnant moisture from what is currently Tropical Storm Isaias (soon to become Hurricane Isaias Thursday morning) move up from the Gulf and into the Ohio Valley this weekend. The extent of the warmth on Saturday will depend on a couple things; just how fast the cloud cover thickens up during the day and how much dry air from above is able to mix down near the surface. Either way, Saturday is shaping up to be a very nice afternoon.

Latest model runs show most of the moisture from Isaias shifting to our east, falling more over northeast and east-central Illinois and Indiana Sunday. But a shift to the west remains possible and the current forecast calls for a slight chance for showers (right now around 20%) Sunday. Temperatures will still remain above average in the mid and upper 70s. We may see a slightly wetter pattern return by the middle of next week.  

Tropical Storm Isaias forms; Gulf Coast faces hurricane threat

Isaias Forms in the Gulf:

The Atlantic hurricane season has been a dud so far in terms of hurricanes. However, new, this morning, we now have our 9th named storm of the season, tropical storm Isaias. While hurricane hunters are en route to investigate the system, satellite imagery this morning alone shows a healthy and well-organized tropical storm in the southwest Gulf. 

Over the next few days, attention will be focused on its path towards the Gulf Coast, where landfall is expected later this week. As of this morning, the National Hurricane Center indicates the system could reach the coast as a category 2 hurricane. However, with very warm gulf waters in its path, we may see further strengthening occur, giving Isaias a chance to become a major hurricane. 

 

Severe Potential:

The Storm Prediction Center is also monitoring this newly named tropical storm, highlighting portions of the Gulf Coast under a 15% severe weather risk. This is equivalent to a level 2 of 5 Slight Risk. The area at risk includes the western Florida Panhandle and parts of Alabama, where the threat for tornadoes and strong damaging winds is expected to increase as the tropical system makes landfall and pushes inland.