Rand Observatory II
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rand Observatory II clear sky chart from 2005-11-06 to 2026-05-02.

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% 13% 5% 12% 7% 6% 5% 4 5% 6% 25%	131722
	Night	12% 14% 6% 14% 8% 6% 5% 4 4 5% 22%	130775
Transparency	Day	56% 7% 10% 17% 9% 1	135177
	Night	59% 8% 10% 15% 8% 1	135394
Seeing	Day	26% 22% 28% 15% 6% 2	16476
	Night	23% 22% 26% 20% 8% 1	25625

By Month

Percentage Cloud Cover NE (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	6% 17% 8% 22% 14% 7% 5% 3 4 3 12%	4661
	11th to 20th	Day	4 12% 8% 24% 16% 11% 4 2 3 3 13%	4650
	21st to end	Day	7% 13% 7% 17% 10% 11% 9% 3 5% 4 15%	5166
February	1st to 10th	Day	9% 15% 7% 16% 12% 8% 7% 3 4 4 16%	1890
	11th to 20th	Day	6% 13% 8% 19% 10% 7% 5% 4 5% 5% 18%	1966
	21st to end	Day	8% 17% 6% 17% 9% 7% 5% 4 5% 4 18%	1730
March	1st to 10th	Day	12% 16% 7% 19% 7% 5% 4 3 3 4 20%	4576
	11th to 20th	Day	8% 16% 9% 14% 7% 5% 5% 3 4 5% 25%	4846
	21st to end	Day	13% 14% 5% 12% 7% 4 4 3 4 4 28%	6073
April	1st to 10th	Day	15% 19% 6% 10% 5% 4 5% 3 4 5% 25%	2520
	11th to 20th	Day	14% 18% 5% 9% 5% 4 4 3 5% 4 29%	2494
	21st to end	Day	19% 16% 4 9% 4 4 4 3 4 6% 27%	2643
May	1st to 10th	Day	25% 16% 4 9% 5% 5% 4 3 5% 5% 19%	6209
	11th to 20th	Day	16% 14% 4 9% 6% 5% 5% 4 6% 7% 24%	6318
	21st to end	Day	21% 11% 3 9% 6% 5% 5% 4 5% 6% 25%	6949
June	1st to 10th	Day	24% 10% 3 8% 5% 5% 4 4 5% 6% 25%	2800
	11th to 20th	Day	17% 9% 3 7% 6% 6% 5% 5% 5% 8% 28%	2800
	21st to end	Day	21% 10% 3 9% 5% 5% 5% 5% 5% 7% 25%	2800
July	1st to 10th	Day	12% 8% 3 7% 6% 6% 6% 5% 7% 9% 30%	6360
	11th to 20th	Day	10% 7% 2 8% 6% 6% 7% 6% 7% 9% 32%	6360
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 4 7% 8% 35%	6926
August	1st to 10th	Day	17% 8% 4 8% 5% 6% 6% 5% 6% 8% 28%	4563
	11th to 20th	Day	14% 8% 4 10% 6% 7% 6% 5% 6% 7% 26%	3987
	21st to end	Day	15% 8% 2 7% 5% 5% 6% 5% 6% 9% 33%	4312
September	1st to 10th	Day	15% 7% 3 7% 4 5% 5% 4 4 8% 39%	2400
	11th to 20th	Day	15% 8% 2 6% 3 3 5% 3 5% 8% 43%	2323
	21st to end	Day	15% 10% 3 6% 5% 4 6% 4 6% 7% 34%	2200
October	1st to 10th	Day	19% 13% 3 7% 5% 4 4 3 5% 6% 30%	2058
	11th to 20th	Day	16% 17% 3 9% 5% 3 4 3 4 6% 29%	1990
	21st to end	Day	19% 18% 6% 10% 5% 4 4 3 4 5% 23%	1980
November	1st to 10th	Day	11% 18% 7% 10% 6% 5% 4 4 6% 5% 24%	1851
	11th to 20th	Day	11% 16% 9% 14% 7% 5% 3 3 4 5% 24%	1847
	21st to end	Day	12% 19% 9% 14% 8% 5% 4 4 4 5% 18%	1680
December	1st to 10th	Day	7% 22% 9% 24% 10% 5% 4 3 3 3 9%	2680
	11th to 20th	Day	12% 16% 6% 20% 7% 5% 4 3 5% 5% 16%	3066
	21st to end	Day	12% 14% 7% 15% 8% 5% 5% 4 6% 5% 20%	4048
January	1st to 10th	Night	7% 16% 9% 22% 12% 9% 5% 3 4 3 10%	8234
	11th to 20th	Night	5% 13% 9% 27% 17% 10% 5% 3 3 2 7%	8235
	21st to end	Night	7% 10% 5% 18% 11% 12% 12% 6% 5% 3 10%	8997
February	1st to 10th	Night	8% 15% 7% 19% 13% 7% 6% 3 3 3 15%	2730
	11th to 20th	Night	7% 12% 8% 20% 10% 9% 6% 4 4 4 14%	2538
	21st to end	Night	8% 16% 7% 16% 10% 8% 5% 4 5% 4 17%	2076
March	1st to 10th	Night	12% 12% 5% 15% 10% 9% 7% 4 4 4 18%	5484
	11th to 20th	Night	9% 15% 8% 13% 6% 4 4 3 5% 6% 26%	5329
	21st to end	Night	15% 15% 7% 13% 8% 4 4 3 4 4 24%	5101
April	1st to 10th	Night	15% 17% 7% 12% 5% 4 3 3 4 5% 26%	2100
	11th to 20th	Night	16% 18% 5% 9% 6% 4 4 3 5% 4 26%	2075
	21st to end	Night	18% 13% 5% 8% 5% 3 4 2 5% 5% 31%	1975
May	1st to 10th	Night	23% 19% 4 9% 5% 5% 4 3 4 5% 18%	3798
	11th to 20th	Night	16% 14% 3 8% 5% 5% 5% 4 5% 7% 27%	3575
	21st to end	Night	19% 14% 3 8% 6% 5% 5% 3 5% 6% 27%	3926
June	1st to 10th	Night	22% 10% 3 7% 6% 4 5% 3 5% 6% 29%	1600
	11th to 20th	Night	17% 8% 2 7% 5% 4 4 4 5% 7% 35%	1600
	21st to end	Night	19% 10% 3 7% 5% 5% 5% 4 5% 7% 31%	1600
July	1st to 10th	Night	13% 7% 3 8% 6% 6% 5% 4 6% 9% 34%	3600
	11th to 20th	Night	15% 7% 3 7% 5% 5% 4 4 5% 8% 37%	3600
	21st to end	Night	11% 8% 2 7% 5% 4 5% 4 5% 7% 40%	3926
August	1st to 10th	Night	20% 9% 3 7% 4 4 3 3 5% 6% 36%	2586
	11th to 20th	Night	15% 9% 3 10% 5% 6% 5% 4 5% 7% 32%	3024
	21st to end	Night	13% 9% 2 6% 4 4 4 4 6% 8% 39%	3586
September	1st to 10th	Night	16% 8% 2 6% 5% 4 4 3 5% 6% 40%	2000
	11th to 20th	Night	13% 9% 3 6% 4 4 3 3 5% 6% 44%	2076
	21st to end	Night	17% 10% 2 5% 4 4 4 4 6% 7% 36%	2200
October	1st to 10th	Night	18% 11% 3 8% 4 5% 5% 4 5% 7% 30%	2339
	11th to 20th	Night	16% 16% 4 8% 4 5% 5% 3 4 6% 28%	2408
	21st to end	Night	20% 19% 5% 9% 5% 5% 5% 3 4 4 21%	2860
November	1st to 10th	Night	12% 19% 7% 12% 6% 4 4 3 4 4 24%	2667
	11th to 20th	Night	12% 16% 7% 16% 6% 4 4 3 4 5% 24%	2772
	21st to end	Night	10% 19% 8% 17% 8% 5% 4 4 4 5% 16%	2940
December	1st to 10th	Night	9% 18% 10% 28% 10% 5% 4 2 3 3 8%	4720
	11th to 20th	Night	11% 18% 10% 17% 9% 6% 4 3 4 3 15%	5348
	21st to end	Night	11% 17% 6% 17% 8% 6% 5% 4 5% 4 17%	7150

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	69% 10% 7% 11% 3	4662
	11th to 20th	Day	73% 6% 3 6% 9% 2	4680
	21st to end	Day	67% 8% 7% 10% 7% 1	5166
February	1st to 10th	Day	69% 7% 6% 12% 5% 2	1890
	11th to 20th	Day	65% 6% 9% 11% 8% 1	1975
	21st to end	Day	62% 8% 10% 10% 8% 2	1730
March	1st to 10th	Day	65% 5% 10% 8% 9% 3	4579
	11th to 20th	Day	57% 7% 13% 12% 7% 3	4846
	21st to end	Day	56% 5% 8% 12% 17% 2	6073
April	1st to 10th	Day	59% 6% 9% 15% 10% 2	2520
	11th to 20th	Day	56% 5% 8% 17% 13% 1	2494
	21st to end	Day	58% 5% 7% 16% 11% 2	2643
May	1st to 10th	Day	66% 6% 7% 14% 6% 1	6220
	11th to 20th	Day	55% 9% 12% 15% 8% 1	6322
	21st to end	Day	55% 8% 10% 19% 7% 1	6949
June	1st to 10th	Day	56% 8% 10% 16% 9%	2800
	11th to 20th	Day	50% 9% 10% 21% 9% 1	2800
	21st to end	Day	56% 7% 11% 17% 9% 1	2800
July	1st to 10th	Day	46% 8% 14% 22% 10%	6290
	11th to 20th	Day	41% 10% 13% 26% 9%	6285
	21st to end	Day	42% 8% 12% 33% 5%	6728
August	1st to 10th	Day	51% 8% 12% 20% 9%	4549
	11th to 20th	Day	49% 10% 15% 21% 5%	4027
	21st to end	Day	43% 9% 15% 22% 9%	4312
September	1st to 10th	Day	42% 7% 11% 28% 12% 1	2400
	11th to 20th	Day	39% 5% 12% 29% 15% 1	2323
	21st to end	Day	45% 8% 13% 21% 11% 2	2200
October	1st to 10th	Day	47% 5% 11% 24% 13%	3344
	11th to 20th	Day	49% 6% 11% 18% 14% 1	3218
	21st to end	Day	50% 5% 13% 25% 6% 1	3180
November	1st to 10th	Day	58% 7% 11% 15% 9%	1851
	11th to 20th	Day	61% 5% 11% 11% 9% 3	1847
	21st to end	Day	65% 7% 11% 12% 4 1	1680
December	1st to 10th	Day	78% 6% 6% 7% 3	2680
	11th to 20th	Day	65% 8% 8% 11% 6%	3066
	21st to end	Day	61% 8% 6% 10% 13% 2	4048
January	1st to 10th	Night	74% 8% 7% 8% 4	8234
	11th to 20th	Night	77% 7% 5% 6% 5% 1	8280
	21st to end	Night	68% 8% 8% 8% 7% 1	8997
February	1st to 10th	Night	68% 8% 7% 9% 7% 1	2730
	11th to 20th	Night	67% 7% 9% 10% 7%	2540
	21st to end	Night	63% 7% 9% 12% 8% 1	2076
March	1st to 10th	Night	63% 7% 9% 10% 9% 1	5494
	11th to 20th	Night	53% 9% 15% 14% 7% 3	5330
	21st to end	Night	60% 6% 5% 14% 13% 1	5101
April	1st to 10th	Night	59% 6% 8% 15% 12% 1	2100
	11th to 20th	Night	58% 7% 9% 16% 10%	2075
	21st to end	Night	54% 5% 8% 18% 12% 2	1975
May	1st to 10th	Night	65% 9% 7% 12% 7% 1	3804
	11th to 20th	Night	51% 10% 10% 20% 8% 1	3576
	21st to end	Night	55% 8% 10% 18% 9%	3927
June	1st to 10th	Night	52% 8% 10% 20% 9% 1	1600
	11th to 20th	Night	45% 7% 14% 21% 12% 1	1600
	21st to end	Night	48% 10% 12% 22% 8%	1600
July	1st to 10th	Night	44% 6% 18% 23% 9%	3560
	11th to 20th	Night	41% 8% 13% 27% 10%	3552
	21st to end	Night	38% 10% 14% 31% 7%	3809
August	1st to 10th	Night	47% 7% 15% 22% 8%	2576
	11th to 20th	Night	46% 10% 16% 21% 7%	3050
	21st to end	Night	39% 8% 15% 28% 9% 1	3586
September	1st to 10th	Night	40% 8% 14% 27% 11% 1	2000
	11th to 20th	Night	39% 6% 13% 30% 12%	2076
	21st to end	Night	43% 8% 11% 23% 13% 2	2200
October	1st to 10th	Night	51% 7% 10% 22% 10%	3820
	11th to 20th	Night	51% 6% 10% 19% 10% 5%	3919
	21st to end	Night	55% 7% 14% 18% 6% 1	4610
November	1st to 10th	Night	60% 7% 9% 12% 9% 2	2667
	11th to 20th	Night	60% 6% 9% 13% 8% 3	2772
	21st to end	Night	65% 9% 10% 11% 5% 1	2940
December	1st to 10th	Night	78% 6% 6% 7% 3	4720
	11th to 20th	Night	70% 8% 6% 8% 9%	5348
	21st to end	Night	67% 8% 6% 9% 8% 4	7150

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	21% 31% 31% 12% 4	220
	11th to 20th	Day	24% 34% 33% 6% 1 1	220
	21st to end	Day	23% 33% 31% 10% 3	250
February	1st to 10th	Day	34% 35% 22% 9% 1	330
	11th to 20th	Day	33% 29% 31% 6%	454
	21st to end	Day	32% 28% 29% 9% 2	446
March	1st to 10th	Day	30% 26% 28% 13% 3	540
	11th to 20th	Day	34% 25% 27% 11% 2 1	540
	21st to end	Day	31% 31% 22% 10% 5% 1	594
April	1st to 10th	Day	34% 24% 26% 11% 5% 1	540
	11th to 20th	Day	36% 24% 22% 12% 5% 1	532
	21st to end	Day	35% 21% 27% 10% 6% 1	540
May	1st to 10th	Day	33% 22% 26% 12% 4 3	507
	11th to 20th	Day	23% 22% 31% 17% 6% 2	527
	21st to end	Day	24% 20% 28% 20% 7% 1	573
June	1st to 10th	Day	31% 21% 27% 15% 4 2	530
	11th to 20th	Day	22% 17% 35% 17% 6% 2	530
	21st to end	Day	27% 22% 29% 15% 5% 2	530
July	1st to 10th	Day	17% 14% 37% 22% 10% 1	525
	11th to 20th	Day	15% 16% 36% 25% 7% 2	524
	21st to end	Day	15% 15% 43% 20% 6% 1	576
August	1st to 10th	Day	16% 15% 30% 24% 11% 4	526
	11th to 20th	Day	15% 12% 34% 21% 13% 4	519
	21st to end	Day	14% 20% 29% 22% 11% 4	583
September	1st to 10th	Day	18% 18% 29% 17% 12% 6%	530
	11th to 20th	Day	15% 15% 23% 22% 16% 9%	530
	21st to end	Day	19% 17% 29% 19% 11% 6%	530
October	1st to 10th	Day	27% 18% 22% 18% 11% 4	530
	11th to 20th	Day	27% 26% 23% 15% 8% 1	524
	21st to end	Day	31% 23% 25% 16% 4	462
November	1st to 10th	Day	31% 21% 26% 16% 5%	425
	11th to 20th	Day	29% 27% 23% 14% 5% 1	387
	21st to end	Day	33% 29% 30% 8% 1	220
December	1st to 10th	Day	31% 26% 28% 12% 3	220
	11th to 20th	Day	35% 30% 26% 7% 1	220
	21st to end	Day	30% 30% 26% 10% 4	242
January	1st to 10th	Night	24% 30% 31% 13% 2	840
	11th to 20th	Night	23% 30% 31% 13% 3	840
	21st to end	Night	16% 32% 31% 17% 4 1	924
February	1st to 10th	Night	21% 32% 32% 14% 1	840
	11th to 20th	Night	24% 30% 31% 13% 2	823
	21st to end	Night	23% 29% 33% 13% 2	692
March	1st to 10th	Night	27% 26% 28% 16% 3	840
	11th to 20th	Night	29% 26% 26% 14% 3 1	840
	21st to end	Night	27% 24% 26% 17% 5% 1	924
April	1st to 10th	Night	28% 26% 27% 17% 3	840
	11th to 20th	Night	28% 25% 24% 17% 5%	831
	21st to end	Night	29% 21% 25% 19% 5%	715
May	1st to 10th	Night	31% 18% 23% 19% 8%	494
	11th to 20th	Night	19% 18% 30% 27% 7% 1	394
	21st to end	Night	17% 16% 28% 26% 11% 1	433
June	1st to 10th	Night	24% 11% 31% 24% 10%	400
	11th to 20th	Night	20% 9% 28% 32% 10% 1	400
	21st to end	Night	20% 13% 28% 29% 9% 1	400
July	1st to 10th	Night	14% 10% 28% 34% 13% 1	390
	11th to 20th	Night	11% 11% 26% 33% 17% 1	388
	21st to end	Night	13% 10% 30% 36% 11% 1	438
August	1st to 10th	Night	14% 9% 22% 39% 15% 1	398
	11th to 20th	Night	16% 10% 22% 31% 19% 3	677
	21st to end	Night	17% 11% 26% 28% 16% 2	880
September	1st to 10th	Night	18% 11% 25% 29% 15% 2	800
	11th to 20th	Night	16% 9% 22% 27% 22% 4	800
	21st to end	Night	20% 16% 25% 20% 18% 2	800
October	1st to 10th	Night	27% 16% 22% 20% 14% 2	800
	11th to 20th	Night	26% 20% 23% 22% 9% 1	800
	21st to end	Night	30% 26% 21% 18% 4	880
November	1st to 10th	Night	29% 22% 26% 17% 6%	820
	11th to 20th	Night	27% 27% 23% 14% 7% 2	840
	21st to end	Night	25% 31% 23% 15% 5% 1	840
December	1st to 10th	Night	27% 34% 24% 12% 3	840
	11th to 20th	Night	27% 27% 27% 14% 3 1	840
	21st to end	Night	27% 30% 26% 14% 3	924

Page generated at 2026-04-29 02:21:47. Version ac07

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