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 2025-01-07.

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% 11% 6% 5% 5% 4 5% 6% 25%	105092
	Night	13% 14% 6% 13% 7% 5% 5% 3 4 5% 24%	103946
Transparency	Day	54% 8% 11% 17% 9% 1	108565
	Night	58% 8% 10% 15% 8% 1	108549
Seeing	Day	25% 22% 28% 16% 7% 2	14810
	Night	23% 22% 26% 20% 8% 1	23924

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	8% 19% 9% 20% 10% 5% 4 2 4 3 15%	2565
	11th to 20th	Day	7% 14% 8% 22% 15% 8% 3 2 3 3 14%	2520
	21st to end	Day	10% 17% 9% 17% 8% 6% 6% 3 5% 4 14%	2786
February	1st to 10th	Day	9% 16% 7% 16% 11% 7% 6% 3 4 4 16%	1710
	11th to 20th	Day	6% 14% 9% 20% 9% 7% 5% 4 5% 5% 18%	1774
	21st to end	Day	9% 18% 5% 16% 8% 7% 5% 3 5% 4 19%	1570
March	1st to 10th	Day	16% 18% 7% 14% 6% 5% 4 3 4 5% 19%	3116
	11th to 20th	Day	8% 20% 12% 15% 6% 4 4 3 3 4 21%	3311
	21st to end	Day	12% 15% 5% 11% 8% 4 3 3 4 5% 31%	4152
April	1st to 10th	Day	15% 19% 6% 9% 5% 3 5% 3 4 5% 26%	2280
	11th to 20th	Day	14% 18% 4 9% 5% 4 4 3 5% 4 30%	2254
	21st to end	Day	19% 17% 4 9% 4 4 4 2 4 5% 26%	2391
May	1st to 10th	Day	20% 15% 4 8% 4 5% 4 3 5% 6% 25%	4318
	11th to 20th	Day	17% 11% 4 9% 6% 5% 5% 4 6% 7% 25%	4398
	21st to end	Day	16% 8% 3 8% 5% 4 5% 4 6% 8% 33%	4837
June	1st to 10th	Day	25% 10% 3 8% 5% 5% 4 4 5% 6% 25%	2660
	11th to 20th	Day	18% 9% 3 7% 5% 5% 4 5% 5% 8% 29%	2660
	21st to end	Day	21% 10% 3 9% 5% 6% 5% 5% 5% 7% 25%	2660
July	1st to 10th	Day	14% 7% 2 7% 6% 6% 6% 5% 8% 9% 31%	4440
	11th to 20th	Day	12% 8% 2 8% 6% 6% 7% 6% 8% 9% 28%	4440
	21st to end	Day	13% 8% 3 8% 6% 5% 6% 4 7% 8% 31%	4814
August	1st to 10th	Day	18% 9% 4 8% 5% 6% 6% 5% 6% 8% 27%	4423
	11th to 20th	Day	14% 8% 4 10% 6% 7% 6% 5% 6% 7% 26%	3863
	21st to end	Day	16% 8% 2 7% 5% 5% 6% 5% 6% 9% 33%	4180
September	1st to 10th	Day	15% 7% 3 7% 4 5% 5% 4 4 8% 38%	2280
	11th to 20th	Day	16% 8% 2 6% 3 3 5% 3 5% 7% 42%	2207
	21st to end	Day	15% 10% 3 6% 5% 4 6% 4 6% 7% 34%	2090
October	1st to 10th	Day	20% 13% 3 7% 5% 4 5% 3 5% 6% 28%	1955
	11th to 20th	Day	16% 17% 4 10% 5% 3 4 3 4 5% 29%	1891
	21st to end	Day	19% 19% 6% 10% 5% 4 4 3 4 5% 23%	1881
November	1st to 10th	Day	11% 17% 6% 10% 7% 5% 4 4 6% 5% 25%	1761
	11th to 20th	Day	12% 16% 9% 13% 7% 5% 3 3 4 5% 24%	1759
	21st to end	Day	12% 19% 8% 13% 7% 5% 4 4 4 5% 19%	1600
December	1st to 10th	Day	7% 22% 9% 24% 10% 5% 4 3 3 3 9%	2600
	11th to 20th	Day	12% 16% 6% 20% 7% 5% 4 3 5% 5% 16%	2986
	21st to end	Day	12% 15% 7% 14% 7% 5% 5% 4 6% 5% 20%	3960
January	1st to 10th	Night	9% 18% 9% 23% 8% 6% 5% 3 3 3 13%	4514
	11th to 20th	Night	7% 14% 8% 24% 15% 9% 5% 3 3 3 10%	4440
	21st to end	Night	12% 16% 7% 19% 9% 6% 7% 4 4 4 13%	4852
February	1st to 10th	Night	8% 16% 7% 19% 13% 7% 5% 3 3 3 15%	2470
	11th to 20th	Night	7% 12% 8% 19% 10% 9% 6% 4 4 5% 15%	2291
	21st to end	Night	9% 16% 6% 15% 10% 7% 6% 4 5% 4 18%	1884
March	1st to 10th	Night	15% 12% 5% 11% 8% 7% 5% 5% 4 5% 22%	3724
	11th to 20th	Night	9% 18% 10% 15% 6% 4 3 3 4 5% 24%	3644
	21st to end	Night	12% 15% 7% 11% 7% 4 4 3 4 4 29%	3480
April	1st to 10th	Night	15% 17% 7% 12% 5% 4 3 3 4 4 26%	1900
	11th to 20th	Night	15% 18% 5% 9% 6% 4 4 3 5% 4 26%	1875
	21st to end	Night	20% 13% 5% 8% 5% 3 4 2 4 5% 30%	1787
May	1st to 10th	Night	19% 16% 4 9% 6% 5% 5% 4 4 6% 23%	2652
	11th to 20th	Night	17% 13% 4 9% 5% 5% 5% 3 5% 5% 30%	2495
	21st to end	Night	16% 8% 3 7% 4 4 5% 4 5% 8% 35%	2738
June	1st to 10th	Night	22% 9% 3 7% 6% 4 5% 4 6% 7% 29%	1520
	11th to 20th	Night	18% 8% 2 7% 5% 4 4 4 5% 7% 36%	1520
	21st to end	Night	20% 9% 3 7% 5% 5% 4 4 6% 7% 31%	1520
July	1st to 10th	Night	14% 7% 3 8% 6% 6% 5% 4 5% 7% 35%	2520
	11th to 20th	Night	15% 8% 3 7% 4 5% 5% 4 5% 7% 37%	2520
	21st to end	Night	13% 9% 3 8% 5% 5% 5% 4 5% 7% 37%	2739
August	1st to 10th	Night	21% 9% 3 7% 4 4 3 3 5% 6% 35%	2506
	11th to 20th	Night	15% 9% 3 10% 5% 6% 5% 4 5% 7% 31%	2930
	21st to end	Night	13% 9% 2 5% 4 4 4 4 6% 9% 39%	3476
September	1st to 10th	Night	16% 8% 2 6% 5% 4 4 3 5% 6% 40%	1900
	11th to 20th	Night	13% 9% 3 6% 3 4 3 3 5% 6% 44%	1972
	21st to end	Night	17% 10% 2 5% 4 4 4 4 7% 7% 36%	2090
October	1st to 10th	Night	19% 12% 4 8% 5% 5% 5% 4 5% 7% 28%	2222
	11th to 20th	Night	17% 16% 4 8% 4 5% 5% 3 4 6% 28%	2287
	21st to end	Night	21% 19% 5% 9% 5% 5% 4 3 4 4 21%	2717
November	1st to 10th	Night	12% 19% 6% 12% 6% 4 4 3 4 5% 24%	2537
	11th to 20th	Night	12% 16% 7% 15% 6% 5% 4 3 4 5% 24%	2640
	21st to end	Night	11% 19% 8% 16% 8% 4 4 4 4 5% 17%	2800
December	1st to 10th	Night	9% 19% 10% 29% 10% 5% 4 2 3 3 8%	4580
	11th to 20th	Night	11% 18% 10% 17% 8% 5% 4 3 4 3 15%	5208
	21st to end	Night	11% 17% 6% 17% 8% 6% 6% 4 5% 4 17%	6996

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	64% 14% 9% 11% 3	2566
	11th to 20th	Day	72% 9% 5% 8% 5% 1	2520
	21st to end	Day	67% 10% 9% 8% 5% 1	2786
February	1st to 10th	Day	69% 7% 6% 12% 4 2	1710
	11th to 20th	Day	65% 7% 9% 10% 8% 1	1783
	21st to end	Day	62% 8% 10% 10% 8% 2	1570
March	1st to 10th	Day	65% 5% 12% 9% 8% 1	3119
	11th to 20th	Day	62% 8% 12% 10% 7% 2	3311
	21st to end	Day	52% 5% 10% 14% 17% 2	4152
April	1st to 10th	Day	58% 5% 9% 15% 10% 2	2280
	11th to 20th	Day	55% 5% 8% 17% 13% 1	2254
	21st to end	Day	60% 5% 8% 14% 11% 3	2391
May	1st to 10th	Day	58% 7% 9% 18% 7% 1	4332
	11th to 20th	Day	53% 10% 13% 16% 7% 1	4402
	21st to end	Day	44% 9% 12% 24% 10% 1	4837
June	1st to 10th	Day	57% 8% 11% 15% 9%	2660
	11th to 20th	Day	50% 8% 10% 21% 9% 2	2660
	21st to end	Day	57% 8% 11% 16% 8% 1	2660
July	1st to 10th	Day	44% 8% 14% 24% 10%	4370
	11th to 20th	Day	43% 10% 15% 27% 5%	4365
	21st to end	Day	46% 9% 12% 27% 6%	4661
August	1st to 10th	Day	51% 8% 12% 19% 9%	4409
	11th to 20th	Day	49% 10% 15% 20% 5%	3903
	21st to end	Day	43% 9% 15% 22% 9%	4180
September	1st to 10th	Day	43% 7% 11% 27% 12% 1	2280
	11th to 20th	Day	40% 5% 12% 28% 14% 1	2207
	21st to end	Day	45% 8% 14% 21% 10% 2	2090
October	1st to 10th	Day	48% 5% 11% 23% 13%	3241
	11th to 20th	Day	49% 6% 12% 18% 14% 1	3119
	21st to end	Day	49% 5% 13% 26% 6% 1	3081
November	1st to 10th	Day	57% 7% 12% 15% 9%	1761
	11th to 20th	Day	60% 5% 11% 11% 10% 3	1759
	21st to end	Day	65% 8% 11% 12% 5% 1	1600
December	1st to 10th	Day	78% 7% 6% 7% 2	2600
	11th to 20th	Day	65% 8% 8% 11% 6%	2986
	21st to end	Day	61% 8% 6% 11% 13% 2	3960
January	1st to 10th	Night	69% 11% 8% 8% 4	4514
	11th to 20th	Night	74% 8% 7% 7% 3 1	4440
	21st to end	Night	70% 9% 7% 8% 5% 1	4852
February	1st to 10th	Night	69% 8% 7% 8% 7% 1	2470
	11th to 20th	Night	66% 7% 9% 10% 7%	2293
	21st to end	Night	62% 8% 9% 12% 8% 1	1884
March	1st to 10th	Night	59% 8% 10% 12% 10% 1	3734
	11th to 20th	Night	58% 9% 12% 14% 7% 1	3645
	21st to end	Night	55% 5% 7% 16% 15% 2	3480
April	1st to 10th	Night	59% 6% 8% 15% 11% 1	1900
	11th to 20th	Night	57% 7% 9% 16% 10%	1875
	21st to end	Night	55% 5% 9% 17% 11% 3	1787
May	1st to 10th	Night	58% 9% 8% 16% 8% 1	2658
	11th to 20th	Night	51% 11% 10% 19% 8% 1	2496
	21st to end	Night	44% 9% 12% 21% 13% 1	2739
June	1st to 10th	Night	51% 9% 11% 20% 8% 1	1520
	11th to 20th	Night	45% 7% 14% 21% 12% 1	1520
	21st to end	Night	48% 9% 12% 22% 8%	1520
July	1st to 10th	Night	46% 6% 14% 22% 11% 1	2480
	11th to 20th	Night	41% 9% 14% 28% 7%	2472
	21st to end	Night	42% 10% 15% 25% 8%	2651
August	1st to 10th	Night	48% 7% 15% 22% 8%	2496
	11th to 20th	Night	46% 10% 17% 20% 7%	2956
	21st to end	Night	38% 8% 15% 28% 9% 1	3476
September	1st to 10th	Night	41% 8% 14% 26% 11% 1	1900
	11th to 20th	Night	39% 5% 13% 30% 12% 1	1972
	21st to end	Night	43% 8% 12% 23% 12% 2	2090
October	1st to 10th	Night	52% 7% 10% 22% 9%	3703
	11th to 20th	Night	51% 6% 10% 19% 9% 5%	3798
	21st to end	Night	54% 7% 14% 18% 5% 1	4467
November	1st to 10th	Night	60% 7% 10% 12% 9% 2	2537
	11th to 20th	Night	59% 7% 9% 13% 9% 3	2640
	21st to end	Night	65% 9% 10% 11% 5% 1	2800
December	1st to 10th	Night	79% 6% 6% 7% 2	4580
	11th to 20th	Night	69% 8% 6% 8% 9%	5208
	21st to end	Night	66% 8% 6% 8% 8% 4	6996

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% 29% 33% 12% 5% 1	192
	11th to 20th	Day	21% 33% 36% 8% 1 2	180
	21st to end	Day	19% 34% 32% 11% 3	204
February	1st to 10th	Day	36% 36% 20% 8% 1	270
	11th to 20th	Day	32% 31% 30% 6% 1	382
	21st to end	Day	33% 26% 28% 10% 2	382
March	1st to 10th	Day	30% 24% 28% 14% 3	460
	11th to 20th	Day	32% 25% 27% 12% 2 1	460
	21st to end	Day	30% 29% 23% 11% 6% 1	506
April	1st to 10th	Day	32% 23% 27% 12% 5% 1	460
	11th to 20th	Day	34% 24% 22% 13% 6% 1	452
	21st to end	Day	35% 20% 26% 11% 6% 2	460
May	1st to 10th	Day	32% 22% 27% 12% 4 3	463
	11th to 20th	Day	22% 22% 31% 17% 6% 2	487
	21st to end	Day	23% 20% 29% 20% 7% 1	529
June	1st to 10th	Day	31% 21% 26% 16% 4 2	490
	11th to 20th	Day	23% 17% 34% 18% 7% 2	490
	21st to end	Day	28% 22% 27% 16% 5% 2	490
July	1st to 10th	Day	17% 13% 36% 23% 10% 1	485
	11th to 20th	Day	15% 17% 36% 24% 6% 2	484
	21st to end	Day	15% 15% 42% 20% 5% 2	532
August	1st to 10th	Day	17% 16% 30% 22% 11% 3	486
	11th to 20th	Day	16% 13% 33% 21% 13% 4	479
	21st to end	Day	15% 19% 28% 22% 11% 5%	539
September	1st to 10th	Day	18% 18% 29% 17% 11% 7%	490
	11th to 20th	Day	15% 16% 23% 21% 16% 9%	490
	21st to end	Day	18% 17% 29% 20% 11% 6%	490
October	1st to 10th	Day	29% 19% 21% 17% 11% 4	490
	11th to 20th	Day	26% 26% 23% 15% 9% 1	485
	21st to end	Day	32% 23% 24% 16% 4	429
November	1st to 10th	Day	31% 19% 27% 17% 6%	395
	11th to 20th	Day	30% 27% 22% 15% 5% 1	359
	21st to end	Day	32% 29% 31% 9% 1	200
December	1st to 10th	Day	31% 26% 29% 12% 3	200
	11th to 20th	Day	34% 30% 27% 8% 2	200
	21st to end	Day	30% 30% 25% 11% 4	220
January	1st to 10th	Night	23% 30% 31% 13% 2	785
	11th to 20th	Night	23% 29% 32% 13% 3	760
	21st to end	Night	14% 31% 33% 17% 4 1	836
February	1st to 10th	Night	21% 34% 33% 12% 1	760
	11th to 20th	Night	23% 31% 31% 13% 2	743
	21st to end	Night	23% 29% 32% 14% 3	628
March	1st to 10th	Night	26% 25% 29% 17% 3	760
	11th to 20th	Night	28% 27% 25% 16% 4 1	760
	21st to end	Night	25% 25% 25% 19% 5% 1	836
April	1st to 10th	Night	26% 26% 27% 18% 3	760
	11th to 20th	Night	27% 25% 24% 18% 6%	751
	21st to end	Night	28% 22% 25% 19% 6%	647
May	1st to 10th	Night	31% 16% 24% 20% 9%	466
	11th to 20th	Night	18% 18% 29% 27% 7% 1	374
	21st to end	Night	17% 16% 29% 26% 12% 1	411
June	1st to 10th	Night	24% 11% 31% 24% 10%	380
	11th to 20th	Night	20% 8% 28% 33% 10% 1	380
	21st to end	Night	20% 13% 28% 29% 10% 1	380
July	1st to 10th	Night	14% 9% 28% 34% 14% 1	370
	11th to 20th	Night	11% 12% 26% 32% 18% 1	368
	21st to end	Night	13% 9% 31% 36% 11% 1	416
August	1st to 10th	Night	15% 9% 22% 38% 15% 1	378
	11th to 20th	Night	15% 10% 23% 31% 18% 3	643
	21st to end	Night	17% 10% 27% 28% 17% 2	836
September	1st to 10th	Night	18% 11% 25% 29% 14% 3	760
	11th to 20th	Night	16% 9% 23% 26% 21% 4	760
	21st to end	Night	20% 15% 24% 20% 18% 2	760
October	1st to 10th	Night	27% 16% 22% 19% 14% 2	760
	11th to 20th	Night	26% 21% 21% 22% 9% 1	760
	21st to end	Night	31% 26% 21% 18% 4	836
November	1st to 10th	Night	27% 23% 27% 17% 6%	780
	11th to 20th	Night	26% 26% 24% 15% 7% 2	800
	21st to end	Night	25% 31% 23% 15% 5% 1	800
December	1st to 10th	Night	27% 34% 23% 13% 4	800
	11th to 20th	Night	28% 26% 28% 15% 4 1	800
	21st to end	Night	26% 30% 27% 14% 3	880

Page generated at 2025-01-04 03:23:58. Version ac07

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