April TSP

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    This year’s April TSP had cool and sometimes cold nightime temperatures, but we were rewarded with clear skies every night. Indeed, this was the best TSP since 2003 in terms of offering clear skies. To some degree, weather is a matter of luck, but it is also true that April typically has less humidity, precipitation, and cloudy skies than what you get in May and June in Fort Davis. In my own experience, TSP’s that take place before May 10 generally offer clearer skies than those that take place after this date. I wonder if it is even possible to consistently schedule TSP’s for April or early May and how others would feel about doing this?


    Steve, yes indeed we had a great TSP this April! Clear skies are vital to our observing! But this happened amid circumstances that can happen at any time April-June.

    This year, post-La Nina effects likely aided our premium skies. But these are highly influenced by the *timing* of the bands of clouds streaming up from the Intertropical Convergence Zone (ITCZ), which was actually responsible for our “perfect” anti-storm. A few of our nights were also blessed by the passage of a cool front from the north, whose timing was perfect to allow the jet stream (and its turbulence!) to be pushed south of us, allowing sub-arc second seeing and very transparent skies.

    BUT… if TSP had been a week earlier this April, we would have had only 2-3 nights. (No precip, just clouds!)

    While TSP observing is impacted by El Nino and La Nina, we have had exceptions simply due to the timing of cloud ITCZ bands and low-pressure systems carried by the Baja Jet.

    An informative article on El Nino/La Nina can be found at:

    Another factor, “the Baja Jet” also influences our cloudiness. This is a southern jet-stream that carries small weather systems (and their clouds) across Baja which are uplifted by the mountains in central Mexico (plus instability from typical desert highland heating), and then move over our region.

    And lastly, another factor is the position of the “dry line” that typically forms over west Texas. The dry line’s position is highly variable regardless which month. My observation is that this dry line tends to form roughly in the same place (+- 100 miles) during any TSP week. But the average position that week could be west/east of us or right over us. When east of us, it can still impact us with backwash cloudiness or lightning from storms over 150 miles away! Sometimes there is no dryline feature at all.

    There is another complication about April TSPs at out 5000+’ elevation… we can often have temps in the 20s, sometimes with frozen precip. Our limited power resources would be even more strained when 400-500 people find ways to keep warm…. including the numerous electric blankets like we found plugged-in this year. Folks are not happy when their observing equipment is on a circuit whose breaker trips in the dead of night.

    I can assure you that *everyone* has something to say about the weather at TSP, especially when we get many clear (or many cloudy) nights! To help guide our decisions, we have used NOAA climate data to help understand the weather situation using real data, rather than rely merely on varied opinions/impressions.

    The data tells us that while cloudiness averages might be somewhat better in earlier months, the probability distribution for any particular date is so wide, that it swamps any slight advantage of one month over another.

    The TSP policy was decided a while back (after much discussion over the past 30+ years): TSPs will be scheduled to start after April 15 and ending no later than June 10. This reflects a balance of several factors including… Sub-freezing temps in April, hot temps (in tents) in May/June, available evening views of galaxies in Leo vs the spectacular Milky Way, opportunities to attend by folks (and young people) in school/college, avoiding Memorial Day (if possible), and the timing of the moon phase… especially if new moon is on a weekend or early in the week (impacting early and/or late observers).

    For over 25 years, we have asked our attendees to tell us about their month preferences using their paper Registration Form (at least 95% participation!). Regardless of which month TSP is held for the survey, over 2/3rds have consistently told us they prefer May, with April and June coming next.

    While it is not likely that we will schedule a lot of TSPs in late-May/early June… we will have some to accommodate these attendees.

    And of course the moon position on any potential week is a primary consideration.

    This year with the transformation of the TSP website, we intend on a more substantial electronic survey form. We’ll be sending out survey invitations in the near future, so please be sure to send yours in!


    To continue this discussion, very belatedly, your point that climate in Fort Davis is highly variable is well taken.  You can have clear or cloudy skies at any time of year.  However, it is also a fact that there is on average almost 3 times more precipitation and twice as many days with precipitation in May than there is in April.   Along with that, there is more cloudiness and higher humidity levels in May.  June is worse still.  Statistically speaking, the odds of clear and transparent skies with low humidity are significantly better in April. 

    With respect to the 2/3 of people in your survey who express preference for May, I wonder what percentage of them are actually aware of the climate statistics including temperature?  My guess is that few are.  If you prefaced your survey by informing people of the NOAA weather statistics for Fort Davis in April, May, and June, I suspect the preferences that people express about when they would like to have the star party would be different. 

    I understand your concerns about the increased likelihood of cold weather in April and the strains that this could impose on electrical power at the Prude Ranch.  I wonder if it is possible to impose a ban on the use of electric blankets or heaters at TSP?  After all, it is easy to sleep warm or even hot in 20 degree weather with warms blankets or warm clothes or a moderately warm sleeping bag.


    Steve, you said:

    However, it is also a fact that there is on average almost 3 times more precipitation and twice as many days with precipitation in May than there is in April.   Along with that, there is more cloudiness and higher humidity levels in May.

    I have seen no long-term climate statistics that support your statement, or at least to any useful degree (any variation is within range of normal weather probabilities).

    Further, increased precipitation is merely a reflection of thunderstorm style downpours typical of summer instead of usual rain (from lower clouds).  All this means is that when it does rain, it is more intense.  In this sense, the inference that higher rainfall totals means less observing is incorrect. 

    What really matters when evaluating weather for astronomy statistically is cloudcover, not rainfall.   Of course lingering humidity from any rainfall, or smoke from Mexican agricultural burning (or nearby wildfires!) affect transparency… but so does atmospheric instability after the passage of a coldfront or when a jetstream component is overhead. 

    While you may be advocating April-only TSPs, the point is that folks are varied in their preferences on when they want to attend TSP.   This is why you will find TSP not scheduled only in one month… April, or in May, or in June.

    Thanks for the discussion of the weather, but as my earlier post points out there are many factors that go into the decisions of scheduling TSPs.  You can be assured however, that we will continue to have TSPs in April!



    I will happily take a TSP at any time. I wish I were able to attend every year, regardless of the month it is held. Weather is always a gamble on planet Earth!


    Forecast is looking great at this point.  Sure glad it was not going on this week, 27* and windy for tonight!!!

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