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North Carolina Cloud Survey

Joseph Mack

jmack (at) wm7d (dot) net
member Raleigh Astronomy Club(RAC)

Mar 2004

Table of Contents

1. Is October the best time of year to hold a star party?
2. Anecdotal View of the best observing time
3. The Data
3.1. % Average Cloud
3.2. % Clear Sky
3.3. % Precipitation
4. Best Dates
4.1. Criteria for best dates
4.2. Star Party in Raleigh
4.3. Star Party at Pisgah
5. Conclusion


Reviews of historical cloud data show that October is the best time of year to hold a star party in central North Carolina.

1. Is October the best time of year to hold a star party?

The Mid Atlantic Star Party (MASP) ( is a gathering of amateur astronomers at the Boy Scout Camp Reeves site, south of Raleigh, NC, in October. Although in the past, observing had been good at MASP, for the last 3 years observing has been poor because of cloud, rain or fog.

I was looking for a reasonable expectation of the weather at MASP, and to see if there was a better time of year to hold a star party. We should at least know that in the chosen month we have X% chance of skies clear of cloud. If the best time of year is only 10% better than the average, then we shouldn't be picking the date by weather, but by some other criterion.

Another site, the Pisgah Astronomical Research Institute ( in the mountains in the western part of the state, was under consideration for a star party.

2. Anecdotal View of the best observing time

The weather up and down the mid east coast (from CT to at least NC) is much the same. From attempts to set up club observing sessions we know:

  • winter:

    winter is too cold for most of us (most nights below freezing, frost is common). Winter cloud cover is variable; in early 2004 we had many nights of clear sky, however I remember winters when I didn't see the sun or any blue sky for 6 weeks.

  • spring:

    spring is usually rainy (except for a drought in 2002). Usually weather is cool until summer arrives, when it's hot. There are a few weeks in between when the temperatures alternate daily between hot and cold (you can't predict the weather a day ahead). There isn't a smooth gradual transition in the weather and by the time people realise that the cool is over, it's too late, summer is here.

  • summer:

    It's too hot and humid to be outside (even at night) and most of us are living behind air conditioners. There isn't a breath of air for 3 months, courtesy of the Bermuda high (affects the whole east coast). As a result the lower 500-1000ft of the atmosphere is an impenetrable white haze of humidity, sulphates from power stations and pollution from cars. (Photographs from 50yrs ago show good visibility. It's unfortunate that the Smokey Mountains are the most visited National Park in USA, but you no longer can see anything from them.) Even in the absence of clouds in summer, you don't see blue sky. Those who venture out at night have to handle the mosquitoes (in the temperatures, long sleeved clothes or long pants aren't acceptable). You put your scope away for summer.

  • fall:

    fall is the last resort. Historical data shows that October is the driest month of the year (good for planning weddings). The temperatures drop gradually from the summer to winter allowing you to reasonably predict the temperature a few days ahead (not possible in the spring). If you're going to do something social in the outdoors, this is the time of year to do it.

    With the disappearance of the summer haze, you can see blue sky again, and after 3 months of observers's cabin fever, everyone rushes out, with hopes high, to observe. In some years, we had continuous blue skies for weeks. In 2001, I saw Mercury for 10 days in row against blue morning skies by locating it in proximity to Venus.

What you've heard about North Carolina blue skies must have come from an advertising agency. You get blue skies in the fall, but not continuously. For most of the year, dew is heavy within an hour or two after sunset, making observing difficult (as is true for the whole east coast).

3. The Data

Joe Pedit, who probably spends more time on the ground than any of us, said that anytime between Sept and Jan would be good.

Attilla Danko of the Cleardarksky clock ( sent me the following

I suggest you try Ignatius G. Rigor's home page (

  1. click on "land cloud atlas"
  2. click on "total cloud cover"
  3. click on "average cloud amount"
  4. look at the "night" maps for each month.

The spatial resolution of the maps are quite low -- not enough to distinguish Pisgah ARI from Camp Reeves. However, it should help you confirm that you're holding the party in the best month.

(you may also find the "complete clear sky" choice in step 2. Personally, I find it too depressing.)

The data I used from this webpage is averaged from 1971-96 (cloud) or from 1982-1991 (rain). Although not discussed on Ignatius's webpage, the deviations from average can be large: we recently had a drought of almost 3yrs length and then for almost all of last year, rain cancelled almost half the soccer games for my son's league, a rarity in previous years.

The maps show the earth in 5deg squares. There is a large amount of data here and considering the number of agencies involved in collecting the data, it represents quite an effort in collation.

The points of interest happen to be in the corners of the squares and it's possible that the data averaged over the whole square might not accurately represent the data in the corner. Raleigh is at the SW corner of one of the gridsquares, which extends to the MD-PA border. The adjacent gridsquare to the south is in the ocean and has no content, and so doesn't give us any hints as to differences in that direction. Pisgah is in the next gridsquare to the west. The Pisgah gridsquare includes WV, which is mostly mountains, and areas of flatter piedmont, which are likely to have different weather. The gridsquare to the south of Pisgah is in the flatter GA and northern FL. Despite these differences, the weather on the east coast is qualitatively similar from CT, through MD to NC (places I have lived) from the point of view of astronomy (summers are dominated by the Bermuda high). Below we find that the results from Pisgah and Raleigh are similar, and I will assume that the whole gridsquare is sufficiently representative of conditions in the corners.

Here I show results for the Raleigh (Camp Reeves) area and for the Pisgah Astronomical Research Inst (PARI) site. Results shown are for night (data is available for day, night, or total 24hr day).

3.1. % Average Cloud

Table 1. Average Cloud, % (presumably % coverage)

year average5862

Conclusion: Oct has the lowest % cloud cover for Raleigh. We can expect half the sky to be covered at night. Even so Oct isn't much better than any other time of year. August is the best time for Pisgah, but the variation isn't large and the best time isn't as good as Raleigh in Oct.

3.2. % Clear Sky

Table 2. Completely clear sky, % (I assume % of time there are no clouds at all)

year average2020

Conclusion: Oct has the highest probability of completely clear sky for both Raleigh and Pisgah - we should have 1 night in 3 clear. As we all know, the change in sky from summer to fall in Raleigh is dramatic, with the number of completely clear nights doubling. If you spend 3 nights at MASP, you can expect 1 cloudless night.

The conventional wisdom, that Oct is the best time to hold a star party, is presumably based on the larger time that we have cloudless skies. However 1 night in 3 isn't great when you have to bring all your gear with you and can't go home if the weather's bad.

3.3. % Precipitation

Table 3. Precipitation: Frequence of occurence, % (I assume % of time that it's raining)

year average1013

Conclusion: Oct is only average for rain at Raleigh. We'd have half the chance of rain if we had our star party in August at Pisgah.

4. Best Dates

4.1. Criteria for best dates

My criteria were in this order

  1. % cloud cover: This is the most useful metric for planning a star party, where you commit ahead of time to observe on a particular date.
  2. % cloudless skies: Although everyone is always impressed by a completely cloudless sky, blue in the day and stary at night, these don't come real often and to take advantage of them, you rely on being able to pick your nights. At a star party you can't go home on nights you don't like.
  3. % time raining: Rain is not really an issue, as long as it's not too much. Covering gear, and trying to prepare dinner in the rain is no fun. You don't want to be tromping through mud. From my anecdotal observation, the variation in rain between years in a particular month is larger than the variation between months in a year.

The strategies for choosing observing nights are different when based at home and when based at a camp. At home, you can take take advantage of a good night with minimal preparation. If you have 1 good night in 3 and you can choose your nights in a weekend, then most weekends you will get one night, satisfying many people. If the night is bad, you stay at home, and can do something else and nothing is lost. When going to a star party, you declare ahead of time that on certain dates, you will observe no matter what. You schlep your gear a long way from home, take a week off from work (costing you about $1000) and you would like as much observing time as you can physically take. To arrive at the camp and find yourself sitting in the cold and overcast dark evening, soaked with dew, exchanging tall stories about previous observing sessions with your fellow star partiers over a cup of tea, while waiting for the 1 night in 3 when you can observe, just doesn't cut it. On top of that you can't do anything useful with your day time, unless there's an excellent set of scheduled presentations; you can't mow the lawn or go to work. Lost nights at a star party cost a lot, while a lost night at home doesn't cost you much at all.

For a quick comparison to see if we were even in the right ballpark: the conditions in Baja California are great in all respects, way better even than Arizona. It seems pointless to even bother with a local star party - we should travel to Baja :-) (Iraq is also pretty good, the only problem might be that it's a little harder to get to. It's no wonder astronomy started in that part of the world.)

4.2. Star Party in Raleigh

The best month from the point of % cloud is Oct, with 49% coverage. This is not a whole lot better than the year average of 58%. The worst time of year, Jan and again in Jul, has 65% cloud cover. No matter what time of year then, we are going to have 50-66% cloud cover (or only 1/2 to 1/3 of the sky is going to be visible). The best you can get then on average is seeing half of the sky (or observing half of the time).

From the point of view of totally cloudless skies, Oct it is 50% better than average and almost 3 times better than in summer. In Oct, we should expect cloudless skies 30% of the time. It is clear why this time of year is favored by astronomers. However you have to be able to pick your date and go out observing on a cloudless night. If you can't pick your date, then you are operating in the % cloud regime, with a 50% chance of cloud and an average chance of rain.

The date of October then probably has been chosen for the higher probability of blue/clear skies, but when we get to the campsite, we're puzzled to find that we're operating with average cloud cover and average rain.

I suggest that Oct still is the best date for a star party in the Raleigh area, but we must understand the limitations: for 3 nights viewing we should expect a total of one night's worth of clear sky, and with average cloud cover and rain.

4.3. Star Party at Pisgah

Cloud cover is much the same all year giving no reason to pick one date over another. Cloud cover is heavier than in the Raleigh area. In Oct cloud cover is 64% which is about the same as the worst time of year in Raleigh and compares to 49% for Raleigh in Oct. At Pisgah in Oct, we should expect a smaller observing time than at Camp Reeves. The number of clear nights peaks in Oct and % time raining in Oct is about the same as at the Raleigh site. The best date for Pisgah then is also Oct, with 1 night in 3 clear, but otherwise it will be 2/3 cloud.

5. Conclusion

  • Raleigh:

    best time Oct, 1 night in 3 completely clear, other days 50% sky visible.

  • Pisgah:

    best time Oct, 1 night in 3 completely clear, other days 33% sky visible.

The choice of October comes from optimizing two parameters [1]

  • temperature: not too hot and not too cold (Goldilocks' criterion).
  • cloud cover: not great, but it's the best there is (Voltaire's criterion - the best of all worlds).

People should expect to be able to observe about half the time. The next question is whether to arrange for some activity for the other half of the time. On bad nights, people go to bed early or sit around and talk. Energetic people entertain themselves with light cannons. Movies have been played at the presentation tent, but few people go. People seem to be prepared for bad nights already.

From talking to people at other star parties, it seems like the weather and dew problems are much the same anywhere east of the Mississippi R., so a large effort would be required to escape these problems. The escapes from east coast weather are west to Texas or north to Maine, both a 2-3 day drive from NC. The skies in Maine are nice in summer, but observing time is short, and there also aren't any established week-long star parties. The Texas star party is one of the well known star parties in the country, but has a lottery to attend.

[1] thanks to Paul Rust

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