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 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