Robert Reeves Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert Reeves Observatory clear sky chart from 2005-11-06 to 2026-06-16.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	14% 6% 2 5% 4 3 4 3 5% 7% 48%	137816
	Night	14% 6% 2 4 3 3 3 3 4 7% 52%	134587
Transparency	Day	35% 6% 9% 20% 19% 11%	141661
	Night	33% 5% 10% 19% 19% 14%	138797
Seeing	Day	17% 9% 25% 30% 17% 3	16872
	Night	17% 8% 25% 35% 14% 1	26856

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	11% 6% 2 4 2 2 3 2 4 5% 59%	5289
	11th to 20th	Day	12% 7% 2 4 2 2 2 2 4 4 58%	5508
	21st to end	Day	21% 6% 2 4 2 2 3 2 4 6% 47%	6468
February	1st to 10th	Day	15% 8% 2 5% 4 3 3 2 4 4 51%	2100
	11th to 20th	Day	16% 7% 2 5% 4 3 4 3 5% 5% 47%	2049
	21st to end	Day	15% 5% 1 3 2 3 3 3 4 6% 54%	1730
March	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 4 5% 55%	4773
	11th to 20th	Day	15% 7% 2 5% 3 2 3 2 3 4 54%	5090
	21st to end	Day	16% 7% 2 4 2 3 3 3 4 7% 49%	5583
April	1st to 10th	Day	16% 7% 3 5% 3 3 3 4 5% 6% 45%	2310
	11th to 20th	Day	20% 5% 2 5% 4 3 3 3 5% 6% 43%	2462
	21st to end	Day	18% 6% 3 6% 4 3 4 3 4 6% 45%	2546
May	1st to 10th	Day	23% 7% 2 6% 4 3 3 3 5% 7% 38%	7617
	11th to 20th	Day	15% 4 2 5% 4 4 4 4 6% 9% 44%	7627
	21st to end	Day	16% 7% 2 7% 5% 5% 5% 4 7% 8% 35%	8392
June	1st to 10th	Day	8% 5% 2 5% 4 3 5% 4 7% 10% 48%	2730
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 8% 45%	2666
	21st to end	Day	10% 5% 2 6% 5% 5% 7% 5% 7% 9% 39%	2600
July	1st to 10th	Day	17% 8% 2 6% 5% 5% 5% 4 5% 7% 37%	5900
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 4 6% 8% 45%	5900
	21st to end	Day	8% 5% 2 5% 5% 4 5% 4 7% 10% 45%	6384
August	1st to 10th	Day	2 2 1 3 3 3 3 3 5% 8% 67%	3905
	11th to 20th	Day	4 2 1 4 3 4 4 4 6% 9% 60%	3846
	21st to end	Day	5% 4 2 5% 5% 4 5% 5% 6% 10% 48%	4285
September	1st to 10th	Day	13% 6% 3 7% 5% 5% 6% 3 5% 7% 40%	2200
	11th to 20th	Day	10% 6% 3 6% 4 5% 5% 4 7% 7% 43%	2200
	21st to end	Day	10% 5% 2 6% 5% 5% 4 4 5% 7% 47%	2200
October	1st to 10th	Day	8% 4 2 5% 4 4 4 3 5% 6% 55%	2200
	11th to 20th	Day	14% 5% 2 4 3 3 3 3 5% 6% 52%	2178
	21st to end	Day	14% 5% 1 5% 3 2 3 2 3 4 58%	2200
November	1st to 10th	Day	16% 6% 2 6% 3 3 3 3 4 6% 48%	2000
	11th to 20th	Day	13% 8% 1 5% 3 3 3 3 4 6% 51%	1890
	21st to end	Day	19% 6% 3 5% 4 3 3 2 4 5% 48%	1890
December	1st to 10th	Day	23% 8% 3 6% 3 3 3 2 3 4 41%	3030
	11th to 20th	Day	16% 10% 2 5% 4 4 3 2 4 5% 45%	3481
	21st to end	Day	14% 6% 2 3 2 2 2 2 3 4 60%	4587
January	1st to 10th	Night	12% 4 2 4 2 2 3 2 4 6% 59%	7634
	11th to 20th	Night	10% 4 1 3 2 2 2 2 5% 5% 65%	7523
	21st to end	Night	26% 6% 2 5% 2 2 2 2 3 4 45%	7746
February	1st to 10th	Night	14% 7% 2 3 2 2 2 3 5% 6% 54%	2520
	11th to 20th	Night	16% 6% 2 4 3 2 3 3 4 6% 51%	2458
	21st to end	Night	16% 5% 1 5% 3 3 3 2 5% 6% 51%	2076
March	1st to 10th	Night	14% 7% 2 4 3 3 3 3 5% 6% 50%	5311
	11th to 20th	Night	13% 7% 2 5% 3 3 2 2 3 5% 54%	5090
	21st to end	Night	17% 9% 2 4 2 2 3 3 4 5% 49%	5588
April	1st to 10th	Night	12% 7% 2 5% 4 3 3 4 5% 6% 50%	2310
	11th to 20th	Night	20% 5% 2 6% 3 3 3 3 4 5% 46%	2103
	21st to end	Night	21% 6% 2 4 4 3 3 2 4 5% 45%	2072
May	1st to 10th	Night	23% 8% 2 5% 3 3 3 3 4 6% 39%	5266
	11th to 20th	Night	12% 5% 2 5% 4 4 4 3 6% 8% 47%	5265
	21st to end	Night	16% 8% 2 5% 3 4 4 4 6% 10% 38%	5793
June	1st to 10th	Night	6% 4 1 3 3 2 3 3 6% 9% 58%	1890
	11th to 20th	Night	8% 4 2 3 3 3 4 4 5% 8% 55%	1845
	21st to end	Night	9% 5% 2 5% 5% 3 3 4 5% 10% 51%	1800
July	1st to 10th	Night	15% 6% 2 5% 4 4 4 4 6% 9% 41%	4078
	11th to 20th	Night	8% 5% 2 5% 3 3 4 4 6% 9% 51%	4077
	21st to end	Night	10% 6% 2 5% 3 3 4 4 5% 9% 49%	4482
August	1st to 10th	Night	2 2 1 3 2 2 3 3 4 7% 71%	3255
	11th to 20th	Night	4 2 1 3 3 3 3 4 6% 9% 62%	3213
	21st to end	Night	6% 5% 2 4 3 3 4 3 7% 11% 52%	3620
September	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 6% 8% 47%	2200
	11th to 20th	Night	12% 6% 2 6% 4 3 4 3 5% 9% 47%	2200
	21st to end	Night	9% 4 1 5% 4 4 4 3 5% 7% 53%	2200
October	1st to 10th	Night	8% 5% 2 4 3 2 3 3 4 6% 61%	2200
	11th to 20th	Night	14% 5% 1 4 2 3 2 3 4 6% 56%	2200
	21st to end	Night	15% 4 1 3 2 2 2 1 2 4 64%	2640
November	1st to 10th	Night	14% 6% 2 4 3 2 2 2 4 5% 56%	2513
	11th to 20th	Night	11% 6% 2 4 3 2 3 2 5% 5% 57%	2730
	21st to end	Night	18% 5% 2 4 3 3 3 3 5% 6% 49%	2730
December	1st to 10th	Night	25% 7% 2 4 3 3 3 3 4 5% 41%	4380
	11th to 20th	Night	21% 8% 2 4 3 3 3 2 3 4 49%	4946
	21st to end	Night	17% 6% 1 3 2 2 2 2 3 5% 58%	6633

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	29% 3 6% 20% 27% 16%	5290
	11th to 20th	Day	32% 2 4 14% 25% 24%	5508
	21st to end	Day	40% 3 4 14% 27% 11%	6468
February	1st to 10th	Day	39% 4 6% 12% 21% 19%	2100
	11th to 20th	Day	40% 3 6% 17% 19% 14%	2059
	21st to end	Day	34% 4 6% 17% 22% 18%	1730
March	1st to 10th	Day	34% 3 5% 15% 17% 27%	4773
	11th to 20th	Day	36% 3 4 9% 15% 33%	5090
	21st to end	Day	36% 5% 7% 18% 19% 15%	5585
April	1st to 10th	Day	40% 5% 8% 15% 17% 15%	2310
	11th to 20th	Day	41% 5% 7% 17% 16% 14%	2462
	21st to end	Day	42% 5% 8% 17% 16% 12%	2546
May	1st to 10th	Day	45% 6% 9% 19% 15% 5%	7655
	11th to 20th	Day	36% 6% 10% 18% 21% 9%	7631
	21st to end	Day	43% 8% 11% 24% 12% 2	8405
June	1st to 10th	Day	31% 8% 13% 26% 18% 3	2730
	11th to 20th	Day	35% 7% 11% 24% 17% 5%	2666
	21st to end	Day	39% 8% 12% 23% 15% 4	2600
July	1st to 10th	Day	43% 8% 16% 27% 6%	5836
	11th to 20th	Day	34% 8% 15% 30% 12% 1	5831
	21st to end	Day	30% 9% 18% 25% 17% 1	6202
August	1st to 10th	Day	15% 6% 11% 40% 27%	3892
	11th to 20th	Day	20% 7% 22% 38% 11% 2	3882
	21st to end	Day	28% 11% 16% 24% 19% 2	4284
September	1st to 10th	Day	43% 7% 10% 17% 17% 7%	2200
	11th to 20th	Day	37% 9% 12% 21% 17% 5%	2200
	21st to end	Day	36% 6% 9% 19% 18% 12%	2200
October	1st to 10th	Day	19% 3 6% 14% 24% 33%	3590
	11th to 20th	Day	29% 4 7% 17% 20% 22%	3522
	21st to end	Day	28% 4 5% 15% 34% 13%	3538
November	1st to 10th	Day	38% 5% 8% 18% 19% 12%	2000
	11th to 20th	Day	36% 3 6% 17% 19% 18%	1890
	21st to end	Day	41% 4 4 16% 20% 14%	1890
December	1st to 10th	Day	49% 3 6% 15% 16% 11%	3030
	11th to 20th	Day	42% 5% 6% 14% 19% 15%	3479
	21st to end	Day	30% 3 4 15% 29% 19%	4587
January	1st to 10th	Night	27% 3 6% 18% 28% 17%	7635
	11th to 20th	Night	24% 2 4 15% 29% 26%	7523
	21st to end	Night	45% 2 3 10% 22% 17%	7746
February	1st to 10th	Night	32% 3 7% 14% 22% 22%	2520
	11th to 20th	Night	36% 3 7% 19% 19% 17%	2458
	21st to end	Night	35% 4 8% 14% 22% 18%	2076
March	1st to 10th	Night	35% 5% 6% 17% 17% 19%	5311
	11th to 20th	Night	34% 5% 5% 11% 15% 30%	5090
	21st to end	Night	38% 5% 7% 18% 17% 15%	5588
April	1st to 10th	Night	35% 6% 10% 16% 16% 17%	2310
	11th to 20th	Night	41% 5% 9% 14% 16% 16%	2103
	21st to end	Night	43% 5% 9% 16% 15% 12%	2072
May	1st to 10th	Night	45% 7% 9% 17% 12% 9%	5293
	11th to 20th	Night	34% 6% 11% 18% 20% 11%	5266
	21st to end	Night	40% 8% 16% 23% 9% 3	5805
June	1st to 10th	Night	22% 9% 18% 31% 19% 2	1890
	11th to 20th	Night	27% 7% 15% 26% 20% 5%	1845
	21st to end	Night	29% 7% 19% 26% 16% 3	1800
July	1st to 10th	Night	39% 9% 21% 26% 4	4035
	11th to 20th	Night	29% 10% 20% 28% 13% 1	4026
	21st to end	Night	31% 8% 20% 25% 14% 2	4353
August	1st to 10th	Night	13% 6% 15% 43% 22%	3244
	11th to 20th	Night	18% 8% 28% 35% 9% 1	3243
	21st to end	Night	26% 11% 19% 26% 16% 2	3620
September	1st to 10th	Night	35% 7% 13% 21% 17% 7%	2200
	11th to 20th	Night	35% 7% 18% 21% 14% 6%	2200
	21st to end	Night	31% 7% 11% 21% 19% 11%	2200
October	1st to 10th	Night	17% 4 7% 16% 21% 34%	3590
	11th to 20th	Night	26% 3 8% 14% 29% 20%	3570
	21st to end	Night	26% 3 5% 16% 34% 16%	4252
November	1st to 10th	Night	32% 4 7% 18% 23% 15%	2513
	11th to 20th	Night	30% 4 8% 19% 21% 18%	2730
	21st to end	Night	36% 4 8% 16% 20% 16%	2730
December	1st to 10th	Night	47% 4 8% 14% 14% 14%	4380
	11th to 20th	Night	40% 4 5% 12% 21% 18%	4947
	21st to end	Night	32% 3 4 15% 27% 19%	6633

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	23% 15% 37% 18% 6% 1	330
	11th to 20th	Day	26% 14% 33% 19% 7% 1	330
	21st to end	Day	25% 20% 32% 18% 6%	363
February	1st to 10th	Day	29% 13% 29% 21% 7% 1	330
	11th to 20th	Day	26% 17% 36% 16% 5%	330
	21st to end	Day	23% 19% 40% 15% 3	273
March	1st to 10th	Day	28% 19% 38% 14% 2	330
	11th to 20th	Day	24% 21% 38% 14% 3	330
	21st to end	Day	22% 18% 41% 16% 2 1	363
April	1st to 10th	Day	27% 17% 35% 20% 1	330
	11th to 20th	Day	22% 21% 38% 16% 3	500
	21st to end	Day	20% 16% 44% 17% 3	566
May	1st to 10th	Day	24% 11% 40% 20% 5%	753
	11th to 20th	Day	20% 10% 36% 26% 6% 1	775
	21st to end	Day	20% 9% 35% 26% 9% 1	844
June	1st to 10th	Day	9% 4 24% 43% 17% 1	780
	11th to 20th	Day	11% 4 25% 44% 15% 2	756
	21st to end	Day	11% 3 18% 38% 25% 5%	730
July	1st to 10th	Day	16% 4 12% 36% 27% 5%	721
	11th to 20th	Day	9% 1 8% 43% 33% 5%	718
	21st to end	Day	7% 2 10% 41% 35% 6%	781
August	1st to 10th	Day	2 1 7% 47% 36% 7%	527
	11th to 20th	Day	7% 1 7% 42% 35% 8%	517
	21st to end	Day	7% 3 5% 38% 39% 9%	569
September	1st to 10th	Day	12% 1 12% 31% 37% 8%	330
	11th to 20th	Day	6% 1 8% 35% 43% 6%	330
	21st to end	Day	9% 3 11% 37% 35% 5%	330
October	1st to 10th	Day	8% 4 13% 42% 28% 7%	330
	11th to 20th	Day	12% 5% 23% 39% 18% 2	330
	21st to end	Day	22% 12% 26% 26% 12% 2	363
November	1st to 10th	Day	21% 6% 34% 26% 12% 1	330
	11th to 20th	Day	21% 9% 31% 26% 9% 4	330
	21st to end	Day	28% 15% 31% 15% 9% 2	330
December	1st to 10th	Day	23% 11% 31% 25% 8% 1	330
	11th to 20th	Day	25% 14% 30% 21% 9% 1	330
	21st to end	Day	22% 12% 31% 26% 7% 1	363
January	1st to 10th	Night	18% 11% 27% 35% 8% 1	1050
	11th to 20th	Night	21% 11% 29% 32% 7%	997
	21st to end	Night	27% 9% 27% 30% 7%	924
February	1st to 10th	Night	22% 10% 30% 30% 6% 1	840
	11th to 20th	Night	24% 13% 31% 27% 4	824
	21st to end	Night	22% 12% 37% 26% 2	692
March	1st to 10th	Night	25% 13% 32% 25% 4	761
	11th to 20th	Night	21% 11% 34% 28% 5%	630
	21st to end	Night	20% 11% 41% 24% 4	693
April	1st to 10th	Night	16% 15% 42% 25% 3	630
	11th to 20th	Night	22% 16% 40% 20% 2	623
	21st to end	Night	23% 12% 41% 20% 4	630
May	1st to 10th	Night	18% 13% 41% 24% 4	630
	11th to 20th	Night	19% 10% 33% 32% 6%	621
	21st to end	Night	18% 9% 33% 32% 8%	683
June	1st to 10th	Night	7% 3 31% 44% 14%	630
	11th to 20th	Night	10% 3 36% 43% 8%	615
	21st to end	Night	10% 4 25% 42% 18% 1	600
July	1st to 10th	Night	11% 4 15% 49% 20% 1	585
	11th to 20th	Night	7% 2 15% 49% 25% 2	582
	21st to end	Night	7% 2 11% 50% 28% 1	657
August	1st to 10th	Night	3 1 9% 52% 34% 2	597
	11th to 20th	Night	5% 2 7% 50% 34% 2	594
	21st to end	Night	7% 2 9% 43% 35% 3	660
September	1st to 10th	Night	9% 3 8% 43% 35% 3	600
	11th to 20th	Night	12% 2 9% 42% 34% 2	600
	21st to end	Night	10% 3 14% 40% 31% 4	600
October	1st to 10th	Night	8% 3 13% 45% 30% 2	600
	11th to 20th	Night	16% 5% 20% 36% 22% 2	600
	21st to end	Night	17% 8% 24% 29% 20% 2	880
November	1st to 10th	Night	17% 6% 20% 37% 18% 2	873
	11th to 20th	Night	15% 7% 23% 37% 15% 2	1050
	21st to end	Night	22% 10% 23% 33% 11% 1	1050
December	1st to 10th	Night	20% 7% 26% 35% 11% 1	1050
	11th to 20th	Night	26% 10% 24% 31% 9% 1	1050
	21st to end	Night	19% 10% 26% 36% 8%	1155

Page generated at 2026-06-13 01:31:41. Version ac07

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