Monday, July 27, 2015

Dew points on the rise this time of year

The 'comfort' factor for the past week or so hasn't been all that great due to high dew point temperatures.  A lot of this has been driven by the type of air mass that has been in place - very tropical and warm.  But, the increase in dew point temperature can also be attributed to what's around us.  And that's corn.  Believe it or not maturing corn can, and does, have a role in our weather.

It's a process known as 'evapotranspiration'.  And it's a combination of evaporation and transpiration.  We all know what evaporation means - the process by which water changes from a liquid to a gas or vapor.  But, we might not know what transpiration means.  Transpiration is when moisture escapes through the plant's leaves and enters the atmosphere.  Think of a plant stem as a giant wick that soaks up moisture from the soil with its roots.  As it does, some of that moisture is then released back into the atmosphere.  As the corn crop matures, or tassels, the area of the plant leave grows allowing more moisture to escape into the atmosphere.  That added moisture to the atmosphere can cause dew point temperatures to rise.  Something we typically see this time of year, especially in areas heavily surrounded by corn crops.  Once the crop has reached its peak, the evapotranspiration rate isn't as high.

A fun experiment to try would be to cover a plant with a plastic bag.  Within and hour or two, you will notice moisture on the inside of the bag.  That's transpiration.

These next couple of days, dew points will be touching close to 70 degrees, or higher.  That doesn't feel too comfortable when you step outside.  While most of it will be driven by the air mass that's in place, the higher dew point readings are also caused by evapotranspiration.

Many research projects have been conducted to try and understand this process, and to really gain an understanding of why this happens and how much of an impact it may actually have in the corn belt area.  Here is an actual research paper/project from an old colleague of my from school.   Differing types of vegetation along with soil characteristics could have an impact on evapotranspiration.


Wednesday, July 22, 2015

Knowing the difference between dew point and relative humidity

With the recent hot and humid days, you've likely heard Meteorologists talk about the dew point temperature a lot.  Dew point temperatures typically over 60 to 65 degrees makes the air feel uncomfortable, or sticky.  Dew point temperatures over 70 degrees just feel down right miserable.

I received an email the other night wondering why we don't talk about the relative humidity that much.  After all, it does relate to humidity.  And that's a good question:  why do we stress so much about the dew point temperature?

To understand why, we first need to explain what the difference is between dew point and relative humidity.  Dew point is an absolute measure of how much water vapor is in the air.  The dew point temperature will always be lower, or equal, to the air temperature.  Relative humidity is a relative measure of how humid the air is, or, how close the air is to saturation.  Relative humidity is expressed as a percentage, and is the ratio of the pressure of water vapor in a parcel of air relative to the saturation pressure of water vapor in that same parcel of air at a specific temperature.  In simpler terms, a relative humidity close to, or at, 100 percent means the atmosphere is saturated and rain or clouds are likely. 

A dew point temperature at 65 degrees or higher means the air feels sticky, or humid.  A dew point temperature less than 65 degrees may not feel as humid and people are able to tolerate it a little more.  Relative humidity can be thought of as how close the air is to saturation.  For example, in the early morning when the temperature is close to the dew point temperature, the relative humidity will be near 100%.  But it may not feel humid outside because the dew point temperature is low.  During the day, the temperature will rise with the heating of the sun.  This causes a larger spread between the dew point and temperature, which can cause the relative humidity to decrease.  In very warm and humid conditions, the dew point temperature can often times rise to 70 degrees or higher.  No matter how warm the temperature gets, a dew point temperature of 70 degrees will always represent the same amount of water vapor in the air in absolute terms, but the relative humidity will change throughout the day.  This is why we use dew point temperature more so than relative humidity during the summer time because it is a better measure of how humid it feels.

Tuesday, July 21, 2015

Cooling temperatures Tuesday night

Clear skies, light winds and high pressure overhead could cause temperatures to drop into the upper 50's by Wednesday morning.  The last time the temperature fell into the upper 50's in Rockford was back on the morning of July 15th.

So, give the A/C a break tonight and open the windows.  You may even be able to keep them open for Wednesday.

Much needed break from July's heavy downpours

We're a little over half way through the month of July, but we're already almost one inch above our monthly rainfall average.  This month, along with last month, has had not shortage of those heavy summer downpours.  But now we're entering into a stretch of dry weather that could last through the end of the week.

High pressure overhead will keep the majority of storm systems bypassing the Stateline as the heat builds across the central and southern Plains.  Surface winds will shift around to the southwest Wednesday, pulling in a little more moisture.  However, the air mass at the surface and a few thousand feet above remains dry.  So, even though a weak upper level disturbance moves in late Wednesday night, most of us should stay dry.

The next chance for measurable rainfall will come as early as Friday night, but may hold off until early Saturday morning.  A developing storm complex along a cold front Friday night could move close enough by Saturday morning to once again bring heavy rain producing thunderstorms.  Something we'll watch closely as we head near the weekend.

Monday, July 20, 2015

Rainy Start, Relief on the Way.

It was a hot, humid, and stormy weekend across the Stateline with severe thunderstorms Friday and Saturday and heat index values just shy of 110.° Thankfully we will get a break from the humidity and the rain after we get through Monday. Humidity is increasing as dew points rise into the upper 60's and low 70's ahead of our next cold front. This is giving us another humid day with some heavy downpours possible in any showers/storms we see. Showers and storms will fire up out ahead of an approaching cold front as early as the early afternoon, however better timing looks to be with the passage of the cold front later in the afternoon. Temperatures today will top out near 85° under thickening cloud cover, and with dew points in the 60's and 70's this will bring some heavier downpours in any showers or storms we see.

The good news is, high pressure builds in on Tuesday and winds shift around to the northwest. This will bring in cooler and drier air, as well as more sunshine. You don't have to worry about the rain for the majority of the week, next chance looks be to late Friday evening, however better timing arrives Saturday. Temperatures will stay in the low 80's this week but warm back into the  upper 80's with an increase in humidity as we go into the weekend.  The heaviest rain stays to the south and west of us this week. The southern plains look to get a good 2-3 inches of rain along a stalled boundary, and southern California is setting record rainfall amounts with the remnants of Tropical Storm Dolores bringing heavy rain to typically southern California's driest month.

Sunday, July 19, 2015

EF-1 tornado confirmed in southern Wisconsin

The Milwaukee National Weather Service was out Sunday conducting damage surveys from severe storms that moved through southern Wisconsin early Saturday afternoon.

While there were numerous reports of straight-lined wind damage, it was found that a tornado did cause some of the damage reported in east-central Rock and west-central Walworth counties between 2:40pm and 2:50pm.  Meteorologists confirmed than an EF-1 tornado, with winds up to 100 mph, touched down between Janesville and Elkhorn.  The width of the tornado was about 50 yards, with a path length of 5 miles.

Click here for the full report, including damage pictures and radar images, from Saturday afternoon.

Saturday, July 18, 2015

Saturday Severe Weather

 5:33 UPDATE: Tornado Watch cancelled for the rest of northern Illinois.

4:27pm UPDATE: Tornado Watch Cancelled for Walworth county in southern Wisconsin.

 3:54 UPDATE: The Tornado Watch for portions of the Stateline have been cancelled including: Jo Daviess, Whiteside, Stephenson, Carroll, Green, and Rock counties.

 Tornado Watch continues for Walworth, Winnebago, Boone, McHenry, Ogle, Lee, and DeKalb counties until 10pm.

UPDATE: Storms that moved through southern Wisconsin earlier have produced an outflow boundary, we could see additional storms fire up around this, this afternoon.
All tornado warnings have now expired. 

Tornado Warning for N Walworth smf East central Rock Counties in Wisconsin until 315pm. Storm moving NE at 40mph

N Brooklyn [Green Co, WI] public reports TSTM WND GST of E55 MPH at 02:00 PM CDT -- 2 to 2.24 inches of rain in 20 minutes

Continues for Green, Rock, and Walworth counties until 3:15pm

TORNADO WARNING: for Mchenry county until 3:00pm for storm located over Harvard, IL.

(2:30:31 PM) nwsbot: LOT continues Tornado Warning [tornado: RADAR INDICATED, hail: 1.00 IN] for McHenry [IL] till 3:00 PM CDT ...AT 230 PM CDT...A SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO WAS LOCATED NEAR HARVARD...MOVING EAST AT 30 MPH.