The Weather Wire
July 2007 Volume 15 Number 07
Avg High 85.3
Avg Low 52.3
Snow - 0.0"
Season Snow - 72.6"
Precipitation - 0.52"
Avg High 88.0
Avg Low 58.7
Avg Snow - 0"
Avg Precip - 2.16"
I’m sure most of you have heard the term inversion, but how many really know what an inversion is? Inversions occur in the atmosphere all the time and are responsible for a variety of weather. An inversion is just that, an inversion of the normal temperature profile going up in the atmosphere. Normally the air cools at 5 degrees (f) for every 1000 feet you go up in the atmosphere; hence it is colder as you go up in elevation. A good example of this was the other day when Denver hit 97 degrees, while up in Conifer a few thousand feet higher it was only 83 degrees. If there had been an inversion present the temperature would have been lower in Denver than areas in the foothills. So, an inversion layer is the warm layer of air above the cooler lower levels. This has important aspect here along the Front Range both in the winter and in the summer.
First, some basic atmospheric thermodynamics need to be explained. If you take a stable layer of air and heat it, it will expand and become lighter than the surrounding air. As a result it will begin to rise. As long as it continues to be warmer than the surrounding air it will continue to rise higher and higher in the atmosphere. Now if there is an inversion over this layer as the air tries to rise higher and it enters this warmer layer, it is no longer warmer that the surrounding air (it is now cooler) so it now begins to sink. The inversion acts as a cap on the lower cooler layer trapping that air beneath it. On most days the sun will heat the lower layer to the temperature of the inversion and the two different layers mix and the inversion is no longer there. But we see many days in Colorado where the cap is never mixed out or broken.
Looking at a winter scenario, a cold arctic air mass has just moved into Denver via a Canadian cold front. The cold air is dense and shallow (only a few thousand feet thick) and undercuts warmer air in place creating the inversion in temperature. This is a likely scenario: Denver is a rather cold, 18 degrees today while Evergreen in the foothills is a toasty 42. We have an inversion at about 2500 feet above the ground level. During the winter months the main concern is air pollution that is being trapped in the cold layer of air at the surface. Until the cold surface layer warms to 42 or above the surface air will not mix with the warmer layers above it and we wind up with a brown cloud over Denver until winds or warming temperatures break the inversion.
During the summer months we are not so much concerned about pollution as we are about thunderstorms. Thunderstorms depend on strong rising current of air to build and develop. As long as these rising currents of air are warmer than the surrounding air they will continue to rise and feed the thunderstorms. However, there are many days during the summer when warmer air usually moving in from the desert southwest will move in over a slightly cooler layer at the surface. When this happens we say that the atmosphere is capped. In other words as air rises it meets this layer of warmer air and is forced back down. This effectively stops any thunderstorms from developing. We’ve had 2-3 days in the past week of July where we have had a cap over the Front Range and thunderstorms just could not get going.
I hope this helps you the next time someone talks about an inversion or a cap and why the pollution is particularly bad that day or when you have a clear sunny summer day without seeing any thunderstorms.
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The western slope was dry during the month of June with worsening drought conditions and increased fire danger. The Front Range experienced good rainfall during the spring and slightly below normal precipitation in June resulting in drought free conditions up to this point. If the monsoon season does not kick into gear soon the drought conditions will be worsening all areas of the state in the near future.
The map below shows forecasted temperature deviances for July 2007. As can be seen, normal or slightly above average temperatures are expected for Eastern Colorado for July 2007, with Central and Western portions of Colorado expected to have above normal temperatures.
The map below shows forecasted precipitation deviances for July 2007. Normal precipitation is expected for the majority of Colorado for July 2007, though extreme northwestern Colorado may see below normal precipitation and the southeastern areas of the state may see above average precipitation.
As can be seen in the below map, little overall change is anticipated for much of Colorado through September, though some worsening of drought conditions possible in Western Colorado.
June 2007 finished 1.2 degrees above normal in the temperature department. Temperatures ranged from a record setting 100 degree reading on the 24th down to a record low temperature of 31 degrees on the 8th. The hundred degree reading was the first of the year and the first triple digit temperature since June 14th 2006. On the other hand, the 31 degree low temperature record became the latest below freezing temperature in Denver weather history. On the 17th, a record high temperature was established breaking the old record of 94 last set in 1940. On the 20th we tied the record high 97 last set in 1968. On the 21st we set a new record high of 99 degrees broke the old record of 98 last set in 1922. There were 13 days where the mercury soared in to the 90s.
Precipitation at DIA was below normal with 0.52” recorded. This was 1.04” below the normal of 1.56” and most of that rain fell on the 12th when 0.46” fell. Two other days recorded measurable moisture. Two other days recorded a trace. There were 5 thunderstorm days that were rain free.
Denver’s weather is notorious for being changeable. However this is not the case during July. Most July mornings are sunny with clouds developing during the late morning and early afternoon. By mid-afternoon, thunderstorms develop over the foothills and drift across the Denver metro area and then onto the eastern plains. Some of these storms continue to develop and reach severe status. Severe thunderstorms containing large hail, strong gusty winds and heavy downpours are not uncommon during July for the Denver metro area.
The worst hailstorm on record in Denver history moved across western sections of the city on July 11th 1990. This ferocious storm dropped baseball to softball sized hail and caused millions of dollars worth of property damage. July is the stormiest month of the year with thunderstorms occurring about every third day or usually 11 days per month. Most of the rainfall during the month is the result of slow moving thunderstorms which can cause precipitation to vary tremendously at different locations across the metro area. It is not unusual for some areas of the city to receive two or three times as much precipitation as other areas. Flask flooding is also quite possible during July.
July is also the month that Denver has the most 90 degree consecutive days. There have been 14 times where consecutive streaks of 10 or more 90 degree days have either been entirely in July or at least started in June and finished in July or started in Jusy and finished in August. The highest consecutive streak of 90 degree days is 18 and occurred twice. Once in 1901 from July 6th through July 23rd 1901 and again in 1847 from July 1st to July 18th. More recently we had 17 consecutive 90 degree days in 2000 from June 29th through July 15th.
Sunrise/Sunset (Denver area)
May 2007 to Sept 2007