The recent few days have been refreshing after a stretch of brutally high dew points. Dew point temperatures for Des Moines have yet to decline below 60 since the 3rd of July. This makes for the longest period above 60 for Des Moines since the early 1930s. The featured plot presents the yearly maximum streak of having dew point temperatures above 60 and the time period over which the streak occurred. The plot nicely shows most of these streaks happening over July and August. With August only a week old, there is plenty of time to increase our streak this year.

Voting:
Good = 21
Bad = 5

Tags:   dewpoint   2011  

Dew point temperatures once again soared into the upper 70s and lower 80s on Monday. These temperatures were remarkable for the calendar now being in August! The featured chart presents IEM calculated yearly maximum period where the dew point remains above 75 degrees and the total number of hours above 75 degrees. This year is still behind last year by those metrics, but there is still plenty of time to go in August to catch up. While not shown on the chart, many of these humid periods occurred during August.

Voting:
Good = 17
Bad = 3

Tags:   dewpoint   2011  

Our recent stretch of extremely hot and muggy weather has gotten Iowa a bit of publicity as some of our automated stations have calculated heat indexes that exceeded 130 degrees! Typically, this is blamed on the abundant transpiring corn crop in the state, but that is not the entire story. The primary source of automated weather data in the state are located at airports, but there are two classes of instruments. The federally administered ASOS sites have higher quality and sometimes different sensors than their state run AWOS counterparts. Unfortunately, their data is typically thrown together in the same pot and reported as such. While the ASOS sites did indicate gaudy heat indexes exceeding 110 at times, they were not as high as the AWOS sites which got all the headlines. The featured chart presents a comparison of an ASOS and AWOS site separated by 30 some miles and both surrounded by farm fields. The bottom plot raises the most doubt as even while visibilities were at 2-4 miles, relative humidity was just 75% at the AWOS site and near 100% at the ASOS site. This is probably an indication that the AWOS air temperature has a warm bias. The second plot shows another issue of having the AWOS dew point rise dramatically during the morning hours at a much higher rate than the ASOS site. It is hard to imagine a natural process that could be putting that much water into the atmosphere in the early morning hours. The first plot of the ASOS site shows a reasonable depiction of transpiring corn (slowly rising dew point throughout the day peaking late afternoon coinciding with plant transpiration), but the dew point temperature barely gets to 80 degrees which keeps the calculated heat index at reasonable values. The moral of the story is that not all automated sensors are alike and while heat indexes over 110 surely were felt in Iowa these past days, the high end values over 120 are questionable.

Voting:
Good = 54
Bad = 5

Tags:   awos   heatindex   dewpoint  

A push of warm and moist air has once again invaded the state making today feel more like summer than fall. The featured chart presents the climatology of near surface water vapor mixing ratio, which is a measure of the amount of moisture held in the atmosphere. The summer of 2010 has certainly seen more than its fair share of humidity as noted by the dominance of red in the lower plot. The grey filled area in the top plot represents the range of mixing ratio. It is interesting to see that the lowest values in the summer are still higher than the maxes in the winter.

Voting:
Good = 25
Bad = 8

Tags:   dewpoint   2010   climate  

The featured chart presents the number of cycles per year that the dew point observed at Des Moines goes from a value above 70 to below 50 and then back to 70+ after 1 August. For 2010, we have already experienced one such cycle and have an outside chance of completing another today with a storm system drawing warm and moist air north. The chart would indicate that returning back to the 70s after our second dip into the sub 50s would not be uncommon. The maximum value shown of 5 cycles was in 1997.

Voting:
Good = 23
Bad = 5

Tags:   dewpoint  

The featured chart presents the IEM computed number of hours per year that the Des Moines Airport ASOS reports a dew point at or above 70 degrees F. The year to date total for this year is the second largest since 1970 and already larger than the full year totals for the past four years. Thankfully our dew points today will be well below 70 making for a nice summer day. Humidities will be on the rise for next week along with thunderstorm chances.

Voting:
Good = 30
Bad = 7

Tags:   dewpoint  

The featured map displays maximum observed dewpoints on this past Wednesday. Observation sites in both the ASOS and AWOS networks reported dewpoints in the lower 80s, which is a rare event. Dewpoint values just in the lower 70s are considered dangerous! A front was able to sweep the state clean of this obscene air for now, but there is plenty of time in summer left for it to return.

Voting:
Good = 72
Bad = 12

Tags:   dewpoint   2010  

Heat and humidity returned to Iowa on Thursday with most of the state experiencing dew point temperatures in the lower 70s. The featured chart presents the number of days per year that at least two ASOS sites in the state report a 70+ dew point on the same day prior to 18 June. This year has already seen 13 days, which is the second highest total since 1973. Another uncomfortable day is in store for today.

Voting:
Good = 46
Bad = 153

Tags:   dewpoint   2010  

The featured chart presents a quick analysis of near storm environment for the archive of NWS issued Tornado and Severe Thunderstorm warnings back to 1986 by month. The box and whisker plots help to visualize the range of values common during the issuance of the warning. In general, dew points are a bit higher for tornado warnings, which is expected as higher dew points indicate more energetic environments and perhaps higher relative humidities (lower cloud heights). For April, once the dew points reach the 50s severe weather appears to be more possible. Some severe weather occurred on Sunday over far southeast Iowa, where surface moisture was available (dew points much higher than the rest of the state). Higher dew points are expected to make their way north today with a warm front and our chances of severe weather will as well.

Voting:
Good = 18
Bad = 12

Tags:   climate   nws   dewpoint  

Our recent stretch of weather has seen very comfortable dew point temperatures in the 60s. The featured graph is a climatology of dew point observations from the Des Moines Airport. The five lines shown represent the lower (0) and upper bound (100) of 1 PM observed dew points along with the 25th and 75th percentile and also the median value (50). The months of July and August are highlighted for their rather consistent daily values due in part to increased plant transpiration.

Voting:
Good = 22
Bad = 18

Tags:   dewpoint  

Dew point temperatures on Tuesday reached the upper 70s and lower 80s, which makes the air extremely uncomfortable to deal with. A powerful thunderstorm complex, fueled by this massive amount of moisture in the air, brought relief mid afternoon. Dew points and temperatures will be on the rise again today with another round of thunderstorms eventually cooling most of the state off.

Voting:
Good = 22
Bad = 7

Tags:   dewpoint  

Continuing with the series of climate features, today a plot of when the highest dew point temperature for the local day occurs. First off, let us start by saying that dew points are much less diurnal in nature than air temperature and outside of sensor bias, dew point is not a function of air temperature. The featured plot shows two obvious extremes around midnight, this is due to air mass advection bringing in more or less moist air during most of the 24 hour period (excuse brevity of explanation please). The plot also shows maxima:

• in the mid morning during the growing season due to dew evaporation before drier air is mixed down due to boundary layer growth,
• the early evening in the late summer probably due to increased corn/soy plant evapotranspiration and thunderstorm/frontal activity,
• and mid afternoon in the winter perhaps due to evaporation(?).

As with the other features, let me know what you think and how you think this is explained incorrectly. :)

Voting:
Good = 26
Bad = 6

Tags:   climate   dewpoint  

The featured chart is a histogram of the percentage of years that a 75+ degree dew point was observed per day since 1973. The frequency is roughly 1 in every 5 years during the summer with July having the most days.

Voting:
Good = 17
Bad = 11

Tags:   summer   dewpoint