Past IEM Features tagged: mixingratio
Our somewhat strange early August weather continued on Wednesday with mild temperatures and low humidity levels. The featured chart displays a climatology of surface mixing ratio, which is a direct measure of the amount of moisture in the air. The daily averages for this year are placed in the context of long term daily averages and ranges. The bottom panel displays a departure from average. You can see how recent stretch has been well below average and running near the bottom of the range of what humidity levels have been during previous years.
The dew point temperature and relative humidity values are commonly used to express the humidity levels present in our atmosphere. So why does the featured chart display this value in terms of mixing ratio? Mixing ratio is an absolute measure of humidity as it compares the mass of water vs the mass of the dry air holding the water and can be directly compared with mixing ratio values at other times of the year. On average, mixing ratio values are 3 times larger in mid July than currently for the end of March.
The featured chart presents a climatology of water vapor mixing ratio for Des Moines. This value is an expression of humidity, just like dew point and relative humidity is, but has a nice property of being on a linear scale so two values are easily comparable. For example, the difference between a 50 and 60 degree dew point is much different than the difference between 60 and 70 degrees. Anyway, this past weekend's surge in humidity shows up in the bottom graph as a large departure from climatology. The drought period during this year shows up as well during July and August with most of the departures drier than average. Very dry air is expected to arrive this weekend allowing for very chilly temperatures and highs well below freezing expected on Saturday!
While much needed rain showers have recently graced much of Iowa, most of this summer has been very dry. The lack of rainfall produces water availability stress in plants and their production of water vapor by transpiration is decreased. The result should show up in decreased air humidities as shown by the featured chart of Des Moines daily mean mixing ratio (the amount of water in the air versus total air). In general, this has been the case for most of the summer with humidity values running near or below long term average. The warm and humid weather we had in March is probably the most noticeable feature in this chart. The initial wet period we had to start May also shows up as an elevated period of higher humidities.
Tags: 2012 mixingratio