The day with the least amount of daylight is also the start of the winter season. This at first may seem counter-intuitive as the sun is the energy source for heating the ground which then heats the air. We have about another month yet to go before we see the coldest average air temperatures as shown by the featured chart presenting the daily climatologies of air and soil temperatures along with daylight length. You can see that none of these curves exactly line up with their maximums and minimums shifted in time slightly.

Voting:
Good = 24
Bad = 5

Tags:   daily   climate   soil   daylight  

Temperatures yesterday warmed 37 degrees for Ames after a chilly start in the low teens. Having this large of a daily warm up is somewhat rare for this time of year as shown by the featured chart. The chart presents the frequency of 20+, 30+, and 35+ degree differences between the daily low and high temperature. The spring and fall seasons show up clearly in this chart along with a general annual signal of the 20 degree line. The dip in frequencies for the largest differences in the summer is due to shorter nighttime periods (less time to cool off) and abundance of water vapor to help moderate any temperature moves.

Voting:
Good = 16
Bad = 3

Tags:   climate   highs  

The weather these past few days has been remarkable for cool high temperatures along with mostly clear skies overhead. The featured chart attempts to look at the relationship of having cool days along with mostly clear skies or northerly winds. While the determination of these two criteria was somewhat arbitrary, the signal appears to be reasonable. Having northerly winds is common for these cool days, but least important during the summer time when the air to our north may not be that cold. Clear skies are the least common during May and June, when are surface heating rates are the largest.

Voting:
Good = 8
Bad = 5

Tags:   climate   clouds   highs  

Today is the first day of school for many, so the featured chart presents a refresher of something you should learned in middle school math. There are many ways to statistically represent a distribution of values. Three of those ways are the mean, the median, and the mode. The featured chart presents these three values for all daily temperatures on a monthly and annual basis. In general, these three values are close to each other except during the winter and springtime. The last value is for the entire year and there is a large difference between the mode and median/mean. Our summertime can be very persistent thanks to high humidities holding temperatures around 60 over night and in the low eighties during the day. This is about what our weather has been like for the past week and will be like for most of the coming week.

Voting:
Good = 15
Bad = 2

Tags:   stats   climate  

The featured chart presents the frequency of a given day being one of the seven warmest for the year. July is clearly the winner with most days having an one in ten chance of being one of the seven warmest for the year. The red bars indicate the warmest seven days so far this year and given that these days fall outside of the higher probabilities, perhaps we can expect warm days ahead in July with highs well into the 90s? The forecast has highs above 90 on Thursday and Friday.

Voting:
Good = 13
Bad = 2

Tags:   highs   climate  

High temperatures on Wednesday struggled in the 60s and were only a few degrees warmer than the coldest high temperatures on record for the date. This was thanks to a thick cloud cover and cold air brought in from our north. The featured chart looks at a measure of the amount of cloud cover on days were the high temperature was at record minimums and maximums. It makes intuitive sense that the coldest days are typically more cloudy during the spring, summer, and fall months as the sun is effective at warming the ground thanks to its high declination. Clouds are less important in the winter months as temperatures are dominated by organized air masses and the sun is less effective. For the warmest temperatures, having more sunny conditions are the most important in October it appears. Outside of a blip for warmest days in May, the plot has two clear annual signals!

Voting:
Good = 14
Bad = 5

Tags:   highs   climate   clouds  

Recently, Bartlesville Oklahoma went from a low temperature of -28 F on the 10th of February to a high temperature of 82 F on the 17th. This is a remarkable 110 degree change on the seventh day. Can any site in Iowa compete with this? The featured chart presents an IEM computation of historical climate data in Iowa and the largest value is 106 degree change on the 9th day. This happened during a remarkable period in February 1930 for Webster City after a low temperature of -34 was followed by highs in the 60s and then 72 on the 24th. Please note that the number for the 0th day is simply the difference between the high and low temperature (Tripoli 18 Jan 1996).

Voting:
Good = 13
Bad = 3

Tags:   climate   high   low   extreme  

Very cold air has settled into the state thanks to a robust area of high pressure. The altimeter reading (a measure of pressure) yesterday morning was a few hundredths of an inch below the highest value reported so far this year. The featured chart presents the yearly events of when the highest and lowest altimeter readings were observed. The first chart compares the air temperature against the day of the year for the observation. The highest pressure events are associated with colder temperatures than low pressure events as confirmed by the second chart. Low pressure events tend to be with dynamic storm systems (upward motion) that are windier than high pressure events (downward motion). The last chart shows the time of day that each event type occurs. High pressure events mostly occur during the morning hours when the atmosphere is still cooling, while low pressure events happen about any time of day.

Voting:
Good = 22
Bad = 6

Tags:   climate   pressure  

The featured chart presents the daily computed correlation between the high and low temperature. Higher values imply a closer relationship between changes in highs and lows. For example, a cold low temperature would typically imply a cold high temperature for that day. There is a clear annual signal shown, but the question is what causes it... One reason may be that the increase in moisture and warming soil temperatures cause overnight lows to moderate and not vary as much. Another potential explanation is that air masses dominate in the winter season and perhaps that causes highs and lows to be more regular. What do you think? Feel free to comment on this via facebook!

Voting:
Good = 20
Bad = 7

Tags:   climate   high   low  

The featured chart presents the average temperature departure from average after a day that has either a high temperature two standard deviations (two sigma) warmer than average or a low temperature two sigma colder than average. You could think of this plot as examining how persistent really warm or cold days are. The plot is divided up by seasons showing how warm temperatures in the summer time tend to stick around longer than in the other seasons. Warm weather in the winter time is not as fleeting as one might have expected. Cold weather tends to stick around slightly more in the winter time than other seasons. Very cold air is set to arrive this weekend and stick around for most of next week.

Voting:
Good = 20
Bad = 7

Tags:   climate   high   low   extreme  

Based on some preliminary data and IEM estimates, 2010 will go down as the four year in a row wetter than a recent trend of 30 years of Iowa data. The featured chart presents the yearly areal averaged precipitation along with a trailing 30 year moving average. The flood year of 1993 and drought year of 1988 stick out in this chart. The trend line has increased in this chart by roughly 4 inches in the past 50 years (~12%).

Voting:
Good = 20
Bad = 5

Tags:   climate   precip  

A passing storm system brought rains to primarily eastern Iowa on Monday and with temperatures only in the 40s, it was certainly a cold rain. The featured chart presents the temperature at which the Des Moines ASOS sensor reported an hourly precipitation greater than 5 hundredths of an inch (think of it as the temperature during which the precip fell). The top plot shows that during the summer time, these rain events rarely occur below 60 degrees whereas in late November they occur at 55 degrees and less. Getting stuck outside during a rain is certainly more pleasant in July! The bottom plot shows a comparison between the temperature and reported one hour precipitation total. The bottom plot shows an interesting feature whereby nearly all the heavy one hour events occur at temperatures above 60 degrees. Surface temperatures can be thought of as a proxy to the amount of water capable of being held in a column of atmosphere. To get heavy precipitation rates, one needs a warm column capable of holding more water vapour to yield precipitation.

Voting:
Good = 31
Bad = 5

Tags:   climate   precip  

The first snow flakes of the season where reported yesterday afternoon over extreme northwest Iowa thanks to our large storm system to the north. The featured chart presents the first fall occurence of snow as reported by the sensors at the larger Iowa airports and the human COOP observers. The average first occurence is around the first of November, so yesterday's snow is nothing exceptional. The winds are finally expected to die down some this evening with the coldest night of the season on the way for tonight.

Voting:
Good = 44
Bad = 4

Tags:   climate   snow   2010  

A final feature on the diurnal temperature cycle. This plot presents a crude analysis of when the low and high temperature occurs each day based on minute interval data. Some adjustment was done to the plot to remove cases of having the low or high temperature set at midnight. The plot nicely shows the annual cycle of when the low temperature occurs, which is typically just before sunrise. The high temperature is more constant and typically occurs around 4 PM CDT (or 3 PM during the winter time). Many people think the highest temperature occurs at noon when the sun is the most direct (highest in the sky), but since the net downward radiation is still positive well into the afternoon, we continue to warm.

Voting:
Good = 22
Bad = 3

Tags:   climate   temp  

Continuing along looking at the diurnal cycle of temperatures, the featured graph presents the monthly average for six months out of the year. The weakest signal is shown in January and the strongest in September. It is interesting to contrast the patterns as September has the most rapid warming, but also the most rapid cooling. This is a result of still having significant sunshine, but also dry air allowing the rapid changes. It is curious to note how similiar July and May are. This analysis is based on minute interval data dating back to only 2000.

Voting:
Good = 22
Bad = 1

Tags:   climate   temp  

The featured chart presents the average daily diurnal temperature cycle. The values are the departure from the daily mean temperature. The chart nicely depicts the profile of when the warmest and coolest temperatures are each day. The magnitude of the values during the winter season are less due to the decreased power of the sun and dominance of warm and cold air advection not forced by the sun. This time of year and April see the strongest diurnal signal thanks to decreased amounts of water vapor in the air (warms and cools more quickly).

Voting:
Good = 36
Bad = 9

Tags:   climate   temp  

The colorful feature image presents the hourly temperature climatology by hour and day of the year for Des Moines based on 60 years of data. The plot shows our warmest time of the year during the mid afternoon in July and our coldest time of the year during the early morning hours in January. For mid October, you can see our warm afternoon temperatures are fleeting with cooler weather increasing with each passing day. Our actual weather looks to continue the pleasant weather streak with highs in the 60s and low 70s.

Voting:
Good = 35
Bad = 9

Tags:   climate   temp  

The featured chart presents the hourly frequency of cloudy or mostly cloudy skies by day of the year based on observation data at the Des Moines Airport. The plot has a number of interesting features about the diurnal and annual cycles in Iowa. Firstly, a good number of our clouds are a result of day time heating by the sun. The late summer and fall months show up nicely for having fewer clouds during the night time. The forecast for the next 5-7 days continues to keep most of the clouds away with very pleasant fall weather!

Voting:
Good = 68
Bad = 14

Tags:   climate   clouds  

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  

Numerous rounds of thunderstorms dumped hail and heavy rainfall on primarily southern Iowa this past weekend. The featured chart presents a summary of the number of automated weather station reports of thunderstorms from the Des Moines airport sensor. The 2010 total surpasses each of the previous years since 1993. The bottom sub chart shows the hourly normalized frequency. This year has seen relatively more thunderstorms during the night time hours than previous years. This has made for a number of nights of difficult sleeping weather. More thunderstorms are in the forecast for this week.

Voting:
Good = 17
Bad = 6

Tags:   climate   lightning  

The featured chart presents the number of years since the record high or low temperature was set for Ames. Ties were not considered. Ames went another meteorological summer without setting a record high temperature. One has to go back to 1988 to find the last record high for Ames during June, July, and August. For July, the youngest record for high temperature was back in 1955! No records are in the forecast for this week.

Voting:
Good = 18
Bad = 3

Tags:   climate   high   low  

The wind speed and direction can be broken into two components. These are a south to north (v wind) and west to east (u wind) component. [Those of you that failed geometry may wish to vote bad now]. The featured graph presents the daily averaged u and v wind speed component. This chart shows a clear annual cycle. Outside of the summer months, the jet stream is often nearby and tends to help promote dominant westerly winds. The v winds show us the dominant air temperature advection regimes with northerly (negative v values) during the winter time and warm southerly (positive v) in the summer time. September is shown as a transition period with westerlies increasing and southerlies decreasing.

Voting:
Good = 43
Bad = 6

Tags:   wind   climate  

The featured chart presents the frequency of precipitation events as measured by the number of consecuative hours at least one hundredth of an inch is reported by the Des Moines weather sensor. Events lasting 10 or more hours are highlighted with the white crosses. The chart shows that long duration events are extremely rare during the summer time when storms tend to be more isolated and not associated with large storm complexes. The largest concentration of long duration events is in March and April.

Voting:
Good = 51
Bad = 16

Tags:   climate   precip  

Our stretch of sunny days looks to continue today. The featured chart looks at the observed frequencies of having one day (black line) and three days in a row (blue line) of mostly clear or clear skies as observed at the Des Moines Airport. The chart shows September as having the highest frequencies with an interesting dip in the trend during the middle of the month. It is not clear what would be causing that, but this is another chart that would indicate perhaps fall is the best season of the year in Iowa!

Voting:
Good = 57
Bad = 15

Tags:   climate   clouds  

Yesterday's feature highlighted the rainfall recorded at Lamoni. The station reported more rainfall in a day than what on average falls during the entire month of June! The featured chart today displays the frequency of a climate site in Iowa picking up more rainfall in a day than what is average for the month. While the chart may seem counter intuitive, monthly precipitation averages during the winter months are much lower and so a bit easier to exceed than the summer months. More heavy rainfall is occurring this morning.

Voting:
Good = 77
Bad = 41

Tags:   climate   precip  

The featured graph looks at the frequency of hourly precipitation rates over a quarter of an inch (red bars) and the largest amount of precipitation reported during an hour (blue bars) from the Des Moines Airport since 1973. The lowest and less frequent rates occur during the 1 PM hour with maximums in the late afternoon and very early morning around sunrise. The heavy rainfall events tend to occur in the late afternoon forced by peak solar heating and early morning forced by low level jet activity. The middle of the day tends to be a lull in the forcing activity. For Iowa, peak rainfall rates usually do not exceed around 2 inches per hour.

Voting:
Good = 22
Bad = 11

Tags:   climate   precip  

The featured image displays a climatology of NWS issued tornado and severe thunderstorm warnings by month and by hour. In general, these warnings are issued during the spring and summer months in the late afternoon into the evening. The month of June sticks out as the most active month and the winter months the least.

Voting:
Good = 46
Bad = 12

Tags:   climate   nws  

Roughly a week ago, some places in Iowa were setting records for warmest temperatures and then with the most recent storm, record rainfall. The featured chart looks at the frequency of having a record temperature around the days of having a record daily rainfall based on data for Ames. You can see how record maximum high temperatures frequently occur before a record rainfall as very warm air masses would imply stronger fronts, which should help produce higher rainfall amounts (as what recently happened). Record minimum highs (cold) also appear to be a common after a record rainfall. The climatology for any of the days shown on the chart would be one record.

Voting:
Good = 29
Bad = 10

Tags:   climate   high   low  

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  

With a good portion of the state stuck in dense fog for the past few days, the featured chart looks at the hourly frequencies by month having observed visibilities below one mile for Des Moines. The largest values are shown during the morning hours in winter time and another maximum late summer just after sunrise. Overall frequencies are higher during the winter time due to the occurrence of visibility limiting snow fall.

Voting:
Good = 24
Bad = 6

Tags:   climate   visibility  

The featured chart presents the earliest and latest occurrence of a temperature threshold after the first of the year. Many parts of the state have yet to reach 40 degrees, which is approaching a record as shown on the chart for Ames. It is interesting to note how the "latest" line flattens out around 32 F, which basically shows the influence of a deep snowpack in February and early March (much like what is happening this year). This chart also shows that some years, by this date, we have experienced temperatures above 70 degrees. Imagine what that must feel like!

Voting:
Good = 30
Bad = 8

Tags:   climate   high  

Ames has not seen a temperature at or above 40 degrees since the first of December (83 days). The featured graph looks at the longest periods of consecutive days above or below a certain high temperature threshold for Ames. The record for consecutive days below 40 appear is around 92, so we'll have to make it to March to set that record. The plot is truncated at 1000 days, but values at the extremes go well beyond 1000.

Voting:
Good = 27
Bad = 10

Tags:   climate   high  

The featured chart looks at the probability of having snow cover on a day during the winter contingent on having snow cover for a given date. Confused? In other words, if there is snow cover for this date, how likely was there also snow cover on March 1rst or December 25th of that same winter. One of the disappointing aspects of the chart are the clear month boundary effects as often the data was manually quality controlled a month at a time (the end of one month doesn't match the beginning of the next). The second week of March shows up nicely as an elevated chance of snow cover, but those events can then be associated with elevated probabilities in mid January.

Voting:
Good = 30
Bad = 26

Tags:   snowcover   climate  

The featured chart looks at the cumulative probability that by a given date, the min low, min high or maximum snowfall has occurred for the winter season. For example, by Jan 28th the lowest high temperature for the season occurred 75% of the time prior to the date (meaning the coldest highs are hopefully behind us). It is interesting to note the slope of the maximum snowfall line and how the current value is around 40%. Big snowfalls are still possible, especially as we transition to spring (imagine that).

Voting:
Good = 77
Bad = 13

Tags:   winter   climate  

The featured chart looks at the largest and smallest temperature range for a month based on all COOP observations in Iowa for a given year compared to what was observed in 2009. The months of September, October, and November had an exceptionally small amount of temperature range with November unofficially being the smallest on record. The smallest number on this chart is 44, which means that for any given month, we Iowans will probably experience a 44 degree difference between the warmest high and coldest low temperature.

Voting:
Good = 16
Bad = 8

Tags:   climate   2009  

The end of 2009 also brought the end of the first decade of the 21^st century. The featured graph looks at the number of days per decade our current set of daily high and low temperature records consists of. Roughly 1/3 of our current daily high temperature records occurred during the 1930s and most of our daily low temperature records occurred before the 1950s.

Voting:
Good = 15
Bad = 7

Tags:   climate   high   low  

The featured chart shows the frequency of low temperature thresholds for the fall period before the first freezing temperature. For instance, on average we would expect to see around 10 days with a low temperature at or below 45 degrees before the first freeze (blue dots). The maximum and minimum values are shown as well along with the current observation this fall. The moral of the story is that we probably should see a few more cool mornings before the first freeze with a day or two in the 30s and then finally that first fall freeze.

Voting:
Good = 25
Bad = 7

Tags:   freeze   climate  

It is commonly said that the first fall season freeze occurs during a full moon in late September or October. The featured plot shows that this simply is not the case in Iowa. The horizontal axis contains the number of days from the nearest full moon and the vertical axis is the number of days that observation was from the 6th of October (the median first freeze date). The contours indicate the relative frequency of the data. The important portion of the plot is the histogram in red at the top of the page clearly showing no full moon date dependence. Here is an Excel file with this data.

Voting:
Good = 20
Bad = 7

Tags:   climate   moon   freeze  

On Friday, the featured plot shown two seasonal maxima of difference between the daily average computed by taking the high and low temperature to that of taken by averaging hourly observations. The question remains as to why these appear... Today's featured plot displays the composite diurnal temperature cycle for four months of interest. The plots are normalized by their respective daily average temperature computed by averaging the high and low. The plot means are displayed in the legend for each month. It may take another feature plot before this mini-mystery is resolved!

Voting:
Good = 16
Bad = 4

Tags:   climate   temperature  

Yesterday, the months of the year battled for the being warmer than the others. Today's feature looks at the frequency of having a month being wetter than another. This time there is no clear winner. The summer months are generally wetter, but fall and spring months can top the summer months.

Voting:
Good = 19
Bad = 8

Tags:   climate   precip  

The featured chart compares the average temperature for a month with the other months for the same year. The number represents the years for which the given month was warmer than the other. So far June has been warmer than July, which is somewhat rare (happening only 3 times since 1951). There is time for July to make a comeback and assume its rightful place as warmest month this year.

Voting:
Good = 22
Bad = 12

Tags:   climate  

Building off of yesterday's feature, today's feature looks at the mean amount of rainfall in three different 7 day periods for a given high temperature in June for all sites in Iowa. For a given day, these 7 day periods are the 7 days prior to the high temperature, the 7 days centered on the high temperature, and the 7 days after. The plot indicates an interesting regime change from when we have highs in the 70s versus highs in the 90s. Notice how in the 70s, the centered and after lines mostly track, while in the 90s the prior and centered lines track. One interuptation would be that when temperatures warm up, more rainfall comes in the forecast and effectively knocks temperatures back down into what is shown in the 70s with more precipitation in the prior period. The overall decrease in precipitation for increasing temperatures is also important (the sun can more effectively warm the ground when it is drier [less precip] ). Rain is in the forecast for this weekend with some strong storms possible.

Voting:
Good = 18
Bad = 12

Tags:   climate   high   precip  

The featured graph displays the frequency of rainfall observations a day after a given high temperature for Ames. The three lines are the frequency of given minimum precipitation thresholds. While there are a number of things going on to make this plot appear the way it does, it generally shows that the frequency of rainfall events increases as our temperature increases (more rainfall in the summer). The interesting detail is to note what happens when temperatures reach the 90s. Our chances of rainfall decrease as the temperature gets even warmer. One could speculate that when our temperatures get really warm, the air mass is often drier and soil moisture state is drier as well allowing more efficient warm ups. Rewording, having temperatures in the 90s for Ames is probably associated with a dry weather pattern.

Voting:
Good = 15
Bad = 6

Tags:   climate   precip   highs  

The featured graph shows the contribution of various daily rainfall total amounts to the monthly average for Ames. The red portion of the bar chart represents when an inch plus of rain falls. If you were to eliminate the red (stop daily rainfalls over an inch) portion of this chart, our wettest month would only see a bit over 2 inches of rain. Another interesting part of this graph is this insignificance of less than 0.05 inch rainfalls to the overall totals (can not see the blues). Heavy rainfall is currently occuring in Iowa today with some locations definitely picking up more than an inch.

Voting:
Good = 21
Bad = 6

Tags:   climate   precip  

The featured image presents an hourly climatology of the frequency of having a relative humidity observation below 60 percent. Since relative humidity is a function of temperature, as the air warms during the daytime the rh value will drop. The plot indicates two maximum in the late springtime and fall. These times of year have strong heating and less profilic sources of evapotranspiration. July into August are a time of maximum corn and soy plant transpiration which helps to keep relative humidity values even while temperatures warm.

Voting:
Good = 18
Bad = 4

Tags:   climate   rh  

The featured chart is the percentage of days per month that experience either high or low temperatures at least 1 degree warmer than the previous day. April has the largest value for high temperature, but is just a mere few percentage points larger than October which only equates to approximately 1 more day per month. This chart would imply that climatology does not have a strong influence on actually seeing our day to day temperatures warm. Day to day temperatures are more strongly influenced by the passage of air masses.

Voting:
Good = 22
Bad = 18

Tags:   climate   high   low  

The featured chart is the yearly difference between the number of days with a high temperature above average versus below for Ames. Negative numbers represent having more days below average than above. 1932 has the largest positive value while 1993 the lowest. For our recent decade, 2008 was the only year with a considerable number of more days below average. The number thus far for 2009 is a mere -4. Our weather is expected to creep above average later this week.

Voting:
Good = 27
Bad = 4

Tags:   highs   yearly   climate  

The featured plot is of daily high temperature percentiles from Ames based on data for the past 100 years. The plot provides a colorful look at the climatology of Ames. The black line is the 32°F contour. For February, this contour rapidly decreases, which implies that our chance for having a high temperature above freezing is rapidly increasing. The forecast calls for temperatures above freezing for most of the state and even thunder showers expected next week.

Voting:
Good = 15
Bad = 11

Tags:   percentile   climate  

Like yesterday's feature on snow pack, today's feature presents a similiar plot for high temperature. The plot summarizes the day to day change in high temperature based on 100 years of data from Ames. The green area represents when the next day high temperature is within 5 degrees of the previous high. The red area represents when the high temperature change is greater than 5 degrees and blue area less then negative five degrees. The white lines denote 25, 50, and 75 years. The plot indicates that our next day high temperature is within 5 degrees of the previous high roughly 50% of the time. An interesting note is to notice where the blue and red areas exceed the 25 and 75 thresholds. These are times of year marked by warming (springtime) and cooling (fall).

Voting:
Good = 23
Bad = 14

Tags:   climate   hightemp  

Continuing in the series of climate features, today's plot is of the hourly frequency of an observed 10+ mph wind speed by month based on archived data from Des Moines. Those of us that live in Central and Northwest Iowa notice the many wind farms that are being built. Obviously, these turbines need reliable wind speeds to generate power. This plot indicates that outside of the afternoon hours in the spring and fall, having 10+ mph winds is roughly a 50% or lower chance on a daily basis.

Voting:
Good = 17
Bad = 5

Tags:   climate   wind  

The featured chart is the daily difference between average high and low temperature climatology from Ames. The largest values occur during the spring thru fall months while the sun is more able to warm us up during the day due to the increased incidence angle.

Voting:
Good = 20
Bad = 11

Tags:   climate  

The featured graph shows the daily frequency of observed sub freezing high and low temperatures for Ames based on data since 1893. For this time of year, roughly a 45% chance exists that the high temperature will be below freezing. On the other hand, low temperatures are almost always below freezing this time of year. Our high temperature today may creep above freezing to help balance out our day yesterday below freezing.

Voting:
Good = 20
Bad = 10

Tags:   climate   below-freezing  

The featured plot continues the series of hourly climatology plots. This plot is of average hourly wind speeds by hour and by month. The windiest time of year appears to be 4 PM in April and the least in the early morning in August. This plot nicely shows two maximums in the spring and fall. These are a result of the season transition periods as warm and cold air fights over Iowa! Large temperature gradients result in stronger winds.

Voting:
Good = 18
Bad = 14

Tags:   climate   wind  

The fun never stops with these hourly climatology plots. Today's feature is a simple hourly climatology of the average relative humidity observed at Des Moines. This plot shows a strong annual and diurnal signal! While dew point temperature is not a function of air temperature, relative humidity is! While the atmosphere cools during the night, relative humidities rise (air temperature drops, dew point remains nearly constant). This plot also shows the least humid time of year being in April, when temperatures are warming nicely and vegetation is not transpiring yet.

Voting:
Good = 29
Bad = 10

Tags:   climate   rh  

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  

Keeping with the recent features, today's feature is a chart of the hour during which the low temperature occurs. Low temperatures tend to occur just after sunrise when net radiation finally becomes positive (downward). This chart shows this trend with lows happening earlier in the morning during the summer. You can also see a secondary maximum around 11 PM, which is often the case when clear skies at night follow a cloudy day allowing temperatures to drop rapidly below levels during the cloudy morning.

Voting:
Good = 23
Bad = 12

Tags:   climate   lows  

Last Friday, we featured the favored times of rainfall. Today we featured the favored times of day when the high temperature occurs. This plot has a number of interesting features including the increased chance of the high temperature coming at midnight during the winter season. This is often associated with air mass changes with a steady feed of colder air during the day. Another interesting feature is the shift of high temperature shown between Feb/Mar and Oct/Nov. This is due to the change of daylight savings time. The hourly time axis on the plot is in local (standard/daylight) time depending on the time of year.

Voting:
Good = 27
Bad = 7

Tags:   climate   highs  

The featured chart shows the hourly frequency of rainfall observations based on observations from the Des Moines Airport since 1973. The units of this chart are average number of hourly observations per month per year for that hour. Confused? For example, the red square represents a number around 4. This means that during the month of May, approximately 4 days during May have rainfall reported during the 7 oclock hour, which is the rainiest time of the year. According to this plot, the driest time is roughly 5 PM in August. There are many other interesting things in this plot. Compare October and November versus July and August. During the summer, rainfall comes during the nighttime. During the fall, it mostly comes during the afternoon.

Voting:
Good = 42
Bad = 24

Tags:   climate   precip  

The featured chart shows the frequency of a certain 10 day period of a month being the warmest of that month. Rewording, which 10 day period of a month is usually the warmest. This is based on observations from Ames since 1893. For example, for October the first 10 days are the warmest for roughly 70% of the years. This will appear to be the case for us this year. This chart also shows interesting asymmetry during the spring and fall months.

Voting:
Good = 16
Bad = 14

Tags:   climate   temps  

The featured chart shows the percentage of years since 1893 that a certain temperature threshold was exceeded by a certain date in the fall for the Ames climate site. For instance, the chance (observational frequency) of a sub-33 temperature by this time of year is roughly 50%. This means half of the years prior to this one have seen a sub-33 temperature by October 4th.

Voting:
Good = 42
Bad = 24

Tags:   freeze   climate  

The featured chart is the number of days with measurable precip per year for the long term climate sites. This chart would indicate that the number of rainy days is increasing at a rate of roughly 1 day per 5 years. 2008 will continue this trend with more rainy days in the forecast today and this weekend.

Voting:
Good = 41
Bad = 14

Tags:   precip   climate  

The featured chart presents the longest consecutive day periods where the high temperature was less than 90 degrees. Our current streak has a long ways to go to catch the 1000 day streak back in the early 90s. The current forecast does not look to end this anytime soon with highs back in the 70s tomorrow.

Voting:
Good = 27
Bad = 9

Tags:   climate  

After a cold start on Wednesday, temperatures warmed 30+ degrees to comfortable afternoon values. The featured plot graphs the daily average temperature range, which can be thought of as the amount of warming that occurs. You can see that September and October experience the largest temperature swings, so cold mornings are more often recovered into nice afternoons. The reason for this is the decreasing humidity levels allowing more efficient warming and ground temperatures which are still very warm.

Voting:
Good = 23
Bad = 3

Tags:   climate  

The featured bar chart is of monthly precipitation climatology for Ames. The blue bars are the average monthly accumulation and red bars are the average number of days during the month which have measurable precip. So for June, we average roughly 5 inches of rainfall spread over 10 days. In other words, we average 0.50 inches of rain every 3 days. Our recent weather for this month has been rather dry thanks to slow moving ridge that has kept the rainfall west of the state.

Voting:
Good = 12
Bad = 3

Tags:   climate   precip  

The featured graph is the distribution of hourly rainfall observations from the Des Moines Airport for the month of June (red) and for the year (blue) since 1973. The graph shows two clear modes for rainfall in Iowa. The first being around 6 AM due to the night time thunderstorm complexes feed by the low level jet in the summertime. The second being around 7 PM due to storms fueled by the heating of the day. In general, the night time is clearly the rainiest period during June with the other summer months showing a similiar distribution.

Voting:
Good = 13
Bad = 3

Tags:   climate   precip