ASK TOM WHY: What is the distinction between humidity and dew point?

Posted on: May 19th, 2013 5:33 PM by CWC Staff No Comments

 

Dear Tom,

 
What is the distinction between humidity and dew point?  Thank you.

 
Thanks,

 

Arlene Steinhoff

 

Dear Arlene,

 
The relative humidity is determined by a relationship between the air temperature and the dew point. Only a certain maximum amount of water vapor can be present in the air at a given temperature and any surplus will condense out as dew, frost, clouds or fog. The dew point is that saturation or condensation temperature. Relative humidity is the ratio, expressed as a percentage, of the water vapor present to that maximum possible amount. When the air temperature equals the dew point, the relative humidity is 100 percent. The warmer the air, the greater its capacity for water vapor. Thus, the relative humidity increases as the temperature falls toward the dew point and decreases as it rises. That's why humidity usually peaks around sunrise and reaches its lowest value in the late afternoon.

Area to turn warm, thundery as tropical air arrives

Posted on: May 18th, 2013 9:23 PM by Paul Merzlock

 

 

Thunderstorm clusters roaming the Plains states Saturday evening have re-activated a dormant severe weather season, spawning at least a half dozen tornadoes across Kansas and Nebraska. Storms may reach the Chicago area as early as Sunday night. Southerly winds will bring a surge in dew point temperature. Dew points in the 60s possess sufficient moisture to support severe thunderstorms. These readings are forecast to approach muggy, 70-degree levels on Monday. Along with elevated moisture levels, temperatures are forecast to reach the mid and upper 80s. These conditions, combined with strong winds aloft may bring the area its first organized

severe weather episode. The Storm Prediction Center suggests there is a slight risk of severe thunderstorms across the Chicago on Monday.

Portions of Plains, Midwest may be primed for severe weather

Posted on: May 18th, 2013 9:15 PM by Paul Merzlock

 

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ASK TOM: Always very windy, except at night?

Posted on: May 18th, 2013 7:21 PM by CWC Staff

 

Dear Tom,

 

I live in the country, and it’s always very windy here, except at night. The wind usually quits at dusk, and by night it’s completely still. Why does that happen?
— Ashley Leutloff, Chebanse, Ill.

 

Dear Ashley,

 

Sun-induced heating during the day disappears at dusk, and, if the weather is not stormy, winds fade to calm or near calm around sunset. Nighttime cooling sets up a temperature inversion: Cooler, denser air collects near the ground, and milder temperatures prevail above. Such inversions move large-scale organized winds (that reach the ground during the day) away from the surface. Those winds continue to blow through the night several hundred feet aloft. When sunlight returns in the morning, the inversion dissipates after a few hours of heating. Large-scale organized winds return to the surface.

Southeasterly lake breezes to limit shoreline warmth while inland temps reach the warmest weekend levels in 37 weeks

Posted on: May 17th, 2013 11:01 PM by Tom Skilling No Comments

 

Lakeshore cooling persists a second day Saturday. Friday produced a temperature spread of up to 30-degrees between Chicago area beaches and the warmest inland locations.

 

 

While Kenosha and Highland Park, on the receiving end of easterly winds coming off chilly lake waters, ended up on the low end of the thermal ledger Friday with highs of 56 and 57-degrees, both Kankakee and DeKalb were warm, posting 85 and 82-degree peak readings respectively.

 

 

Saturday highs will repeat that trend with shoreline and beach temperatures holding to the low 60s while inland locations, head toward their warmest weekend temperatures since last September (37 weeks ago). Highs there will reach the mid 70s while near 80-degree readings are likely at the warmest south suburban observation sites.

 

 

80s return to the city Sunday, Monday and Tuesday after 2 day absence

 

 

Strengthening south to southeast winds return much of the city and inland areas to the 80s Sunday—the beginning of a fresh three-day string of 80s likely to run through Tuesday. It's conceivable a fourth 80-degree high may occur Wednesday.

 

 

80-degree or warmer daytime highs Sunday, Monday and Tuesday would mark the 6th, 7th and 8th time this year that temperatures have moved into the 80s.

 

 

Isolated thunderstorms unleash Friday’s heaviest downpours south of the city; spotty storm development ahead Saturday and Sunday

 

 

Isolated thunderstorms produced by towering cumulonimbus clouds (i.e. thunderheads), which reached heights up to 46,000 ft. Friday according to Doppler radar scans, are likely to re-develop in scattered fashion both Saturday and Sunday.

 

 

Though only a fraction of the full Chicago area may see these storms, the moist environment in which they’re developing could produce brief downpours.

 

 

Predicted warmth comes as meteorological spring 2013—currently the coolest in 17 years—enters its final two weeks

 

 

The predicted weekend warmth comes as the coolest meteorological spring season of the past 17 years moves into its final two weeks.

 

 

Since March 1—the start of meteorological spring—Chicago’s temperature has averaged 43.7-degrees.  That’s a reading 1.6-degrees below the long term average and 10.4-degrees cooler than the same period a year ago.

 

 

"Trying" pollen season produces a 13th consecutive report of “high” tree pollen values Friday

 

 

This pollen season has been a miserable one for far too many area allergy sufferers.  Pollen counts are generated Monday through Friday. Friday marked the 13th consecutive report in which the tree pollen value taken by Dr. Joseph Leija and distributed by Loyola-Gottlieb Memorial Hospital was categorized as “high”.

 

 

Most humid weather so far this year to build in during the days ahead; surging moisture levels to fuel t-storm clusters

 

 

The average of a suite of 7-day computer model rainfall estimates through next Friday and centered on Chicago has jumped to 1.80”. Such an average is the product of more than 40 individual computer-generated predictions. Those forecasts included rainfall estimates ranging from as little as 0.93” to as much as 4”.

 

 

Mid-U.S severe weather potential on the rise; Chicago area at risk, particularly Monday and Tuesday

 

 

Since 2000, May has ranked as the top tornado-producing month across the greater Chicago area—a region which includes 17 counties from southern Wisconsin to northern Illinois and northern Indiana.

 

 

Of the 78 twisters which have been logged since the start of the 21st century, 26 of them—33% have occurred in March.  That’s why it’s little surprise the area may be headed for some challenging thunderstorm clusters capable of severe weather production in the days ahead.  This is particularly true of thunderstorms predicted to “bubble-up” into a vigorous overhead spring jet stream Monday and Tuesday next week.

 

 

Waves of severe weather are predicted to sweep the Plains Saturday then extend eastward into the Midwest Sunday.

 

 

Chicago is close enough to the area of potential severe weather that any storms which develop later Sunday into Sunday night will have to be monitored.  But, it’s the Monday, Tuesday and possibly Wednesday periods which appear likely to warrant the greatest attention.

Increasing humidities and resurging 80s to turn spotty weekend storms into more numerous storm clusters next week–Plains severe storm risk threatens to shift into Chicago Monday and Tuesday

Posted on: May 17th, 2013 11:49 PM by Jennifer Kohnke No Comments

 

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Cool start to weekend, then tropical weather arrives

Posted on: May 17th, 2013 10:10 PM by Paul Merzlock No Comments

 

Temperatures reached the mid and upper 80s across portions of the Midwest, along and south of a frontal boundary draped from South Dakota to Tennessee.

 
High humidity levels were pooled along this boundary, giving rise to thunderstorm clusters scattered from Texas to the Canadian border. As the front to our south lifts north of Chicago Sunday and Sunday night, similar conditions can be expected to overspread the area. Dew point temperatures which relate to the amount of water vapor in the atmosphere, are forecast to jump into the 60s on Sunday, and approach 70 degrees on Monday. This means that thunderstorm coverage and frequency can be expected to increase Sunday night, and continue into midweek. Rainfall amounts of 1 to 2 inches are forecast during this period.

ASK TOM WHY: Did some people think April 21, 1967 was the end of the world?

Posted on: May 17th, 2013 10:21 PM by CWC Staff No Comments

 

Dear Tom,

I recall a weather event in either the late 1960s or early '70s that had some people thinking it was the end of the world. It got as dark outside as if it were midnight. Can you help?

— Richard Murphy, Chicago

 
Dear Richard,

 

While it's hard to pinpoint the exact event you remember, Chicago climatologist Frank Wachowski used your exact words when he described the eerily dark conditions before the deadly F4 Oak Lawn tornado struck about 5:20 p.m. April 21, 1967, producing extensive damage and killing 33 people. The cumulonimbus clouds that produce severe weather often tower 11 or 12 miles into the atmosphere and are very effective at blocking sunlight. Virtually no light can penetrate such a deep cloud layer, giving approaching severe storms their uniquely dark and ominous appearance.

“Don’t you want to go, back to the same old place, Sweet Home Chicago.”

Posted on: May 17th, 2013 6:02 PM by CWC Staff No Comments

 

Taken from the Shedd Aquarium, Chicago's beautiful skyline and Lake Michigan are captured on a clear, sunny day. Thanks Jason Picchiotti for the great picture!

SHEDDSKYLINE

 

 

Hawk sunbathing in Deerfield.

Posted on: May 17th, 2013 6:21 PM by CWC Staff No Comments

 

A red tailed hawk sits by a window, soaking in the sun. What a beautiful picture captured by Lillie Blair Miller from Deerfield, IL. Thank you for sharing this picture!

HAWK