SNOMI – The Summer Nighttime Outdoor Misery Index

During the summer academic geologists commonly spend time in the field doing research; it’s one of the great things about studying the Earth.  I’m fortunate to work on projects from Virginia’s Blue Ridge Mountains to the deserts of Oman, and at many of our field sites we camp while conducting research.

Tents nestled amongst the aspens at our summer base camp, Fish Lake, Utah.

Tents nestled amongst the aspens at our summer base camp, Fish Lake, Utah.

This summer, with the Buckmarlson Banshees, our base camp was near the central Virginia town of Howardsville.  Most nights I’d crawl into the tent, lie on top of my sleeping bag, and stare out at the darkness—uncomfortable, cranky, and awake for far longer than I’d like to have been after a day’s fieldwork.  Many nights it was miserable to be outdoors.

What conditions make for misery while sleeping outdoors in the summer?

Certainly pesky insects, boorish camping companions, and wayward nocturnal mammals can and do contribute to camping misery, but I’m interested in the meteorological conditions that add up to a poor night’s sleep while in the field.

Air temperature is obvious enough.  When it’s too hot, it’s uncomfortable.

Humidity also plays a role, as moist air is cloying and far less refreshing than dry air.

How best to measure humidity? Relative humidity, a commonly used meteorological term, is a function of both the air temperature and the dewpoint temperature (the temperature at which air is saturated with water vapor).  Relative humidity varies as a function of air temperature.  For example, in the cool early morning hours the relative humidity is typically high, but as the temperature rises during the day the relative humidity decreases, even though the amount of water vapor in the air remains the same.  Dewpoint temperature is a better metric for how moist or dry a particular parcel of air is.  A dewpoint above 20 ˚C (~68 ˚F) indicates air that’s moist and humid, whereas a low dewpoint, near 0 ˚C (~32 ˚F), is air that’s quite dry.

Collectively, these two factors work together to make things miserable.  If both the temperature and dewpoint temperature are high then it’s commonly not good sleeping weather out-of-doors.  For instance on June 3rd at 10 p.m., about the time we laid our weary heads to rest, the air temperature in Howardsville was 21.7 ˚C (71 ˚F) and the dewpoint was 18.3 ˚C (65 ˚F): a typical humid and sticky mid-summer night in Virginia.

A quantitative measure of the Summer Nighttime Outdoor Misery Index (SNOMI) could simply involve adding up the air temperature and the dewpoint temperature; for Howardsville on the night of June 3rd  that sums to 40.0 ˚C.  Compare that with the comfortable night I spent at Fish Lake, Utah on June 10th 2013, when at 10 p.m. the temperature was 16.7 ˚C (62 ˚F) and the dewpoint 1.1 ˚C (34 ˚F), which sums to 17.8 ˚C.

The trade winds freshen things up on Barbados' eastern coast.

The trade winds freshen things up on Barbados’ eastern coast.

But summing the air temperature and the dewpoint temperature does not adequately encapsulate nighttime misery.  As an example, on July 19th 2013 I was near the town of Bathsheba on the eastern coast of Barbados, and at 10 p.m. the air temperature registered a sultry 27.2˚ C (81˚ F) and the dewpoint a hefty 23.9 ˚C (75 ˚F), equaling 51.2 ˚C on my simple additive scale.  Yet, it was a perfectly comfortable evening to be outdoors and that’s because the trade winds were blowing a fair and steady breeze at nearly 9 meters per second (~20 mph).  Moving air facilitates evaporation, and evaporation is a cooling process that humans use for thermoregulation.

To better quantify nighttime outdoor misery three main factors need to be considered: 1) air temperature, 2) dewpoint temperature, and 3) wind speed.  I formulated a numeric relationship that first involves dividing the product of the air temperature and the dewpoint temperature by the square root of the wind speed plus 1, and secondly multiplying that quantity by 0.01.  Yikes, that’s a cumbersome sentence; it is far easier to just write out the equation.

SNOMI in equation form.

SNOMI in equation form. Note when the dewpoint drops below 1˚C the value in the equation defaults to 1˚C.

My unpleasant night at Howardsville came in with a SNOMI of 3.8, while the Fish Lake and Barbados evenings registered 0.1 and 2.1 respectively.  I also tabulated data for some of my other travels over the past two summers.  The winner (or is that the loser?) was a muggy night in Wachapreague, Virginia at 5.2.  From this limited data set I’d argue that a SNOMI of >3 makes for misery.

Table data for SNOMI relevant parameters from my travels over the past two summers.

Table data for SNOMI-relevant parameters from my travels over the past two summers.

The 2014-15 academic year starts this week, and one of the courses I’m teaching is our Weather, Climate, & Change course (GEOL 312).  As the class explores the workings of the atmosphere we’ll discuss topics like humidity and dewpoint temperature, as well as consider how wind impacts both one’s comfort and the climate.

To get in the proper mood for the new semester I should be asking as many questions as I answer, so here it goes…

  • What are the units for SNOMI or is it a dimensionless number?
  • In the SNOMI equation why add 1 to the wind speed and then take its square root?
  • What are weaknesses in the SNOMI calculation?
  • What’s the difference between the commonly used heat index and SNOMI?
  • Why go to the trouble of calculating SNOMI, as the National Weather Service dutifully reports the heat index when temperatures exceed 27 ˚C (80 ˚F)?
  • Where, globally, would the highest SNOMIs likely occur?
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