Collins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Collins Observatory clear sky chart from 2005-11-06 to 2025-04-11.

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	18% 8% 3 7% 4 4 4 4 5% 7% 36%	112572
	Night	18% 9% 3 7% 4 4 4 4 5% 7% 36%	115713
Transparency	Day	46% 6% 12% 21% 13% 2	116127
	Night	46% 6% 12% 21% 13% 2	120323
Seeing	Day	24% 16% 31% 20% 8% 2	15898
	Night	23% 12% 33% 26% 7%	24617

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	14% 9% 4 8% 5% 5% 5% 4 5% 5% 36%	3597
	11th to 20th	Day	14% 8% 4 8% 4 4 3 3 5% 6% 42%	3570
	21st to end	Day	12% 10% 4 7% 4 4 4 3 4 4 43%	3960
February	1st to 10th	Day	17% 10% 3 6% 4 5% 4 4 6% 6% 35%	1771
	11th to 20th	Day	13% 10% 3 8% 5% 5% 6% 4 6% 6% 35%	1908
	21st to end	Day	15% 10% 3 8% 5% 5% 4 3 5% 6% 35%	1650
March	1st to 10th	Day	23% 12% 3 10% 5% 4 3 3 4 4 30%	4476
	11th to 20th	Day	19% 11% 3 7% 5% 4 4 3 5% 5% 35%	4928
	21st to end	Day	27% 8% 3 6% 3 3 3 3 5% 7% 31%	5550
April	1st to 10th	Day	20% 12% 3 7% 5% 3 3 3 5% 5% 35%	2400
	11th to 20th	Day	19% 7% 2 6% 4 4 4 3 6% 7% 38%	2380
	21st to end	Day	23% 9% 3 6% 5% 4 4 3 5% 5% 34%	2470
May	1st to 10th	Day	21% 10% 3 6% 5% 5% 5% 4 5% 6% 30%	4106
	11th to 20th	Day	18% 8% 3 8% 5% 6% 6% 4 6% 7% 30%	4139
	21st to end	Day	15% 5% 2 6% 5% 4 6% 5% 6% 9% 36%	4837
June	1st to 10th	Day	23% 8% 2 6% 4 5% 5% 3 6% 8% 30%	2817
	11th to 20th	Day	17% 7% 3 6% 5% 4 4 4 6% 7% 38%	2850
	21st to end	Day	24% 7% 2 5% 5% 4 4 4 6% 8% 30%	2802
July	1st to 10th	Day	14% 5% 2 7% 5% 5% 5% 5% 8% 9% 35%	4440
	11th to 20th	Day	13% 5% 2 6% 4 5% 6% 6% 7% 10% 36%	4440
	21st to end	Day	18% 7% 3 7% 5% 4 4 3 6% 8% 36%	4814
August	1st to 10th	Day	16% 6% 3 7% 5% 5% 5% 4 7% 9% 34%	4118
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 4 6% 7% 34%	4045
	21st to end	Day	11% 6% 2 5% 3 4 4 4 7% 9% 45%	4414
September	1st to 10th	Day	11% 6% 1 6% 4 4 5% 4 5% 7% 48%	2221
	11th to 20th	Day	12% 5% 2 5% 3 4 3 3 5% 6% 52%	2090
	21st to end	Day	17% 5% 2 5% 3 4 4 4 5% 8% 43%	2090
October	1st to 10th	Day	18% 7% 2 7% 4 5% 4 3 5% 5% 39%	2076
	11th to 20th	Day	20% 6% 2 6% 4 4 5% 3 5% 7% 40%	1835
	21st to end	Day	23% 7% 3 7% 4 4 3 3 4 5% 36%	1881
November	1st to 10th	Day	17% 6% 2 5% 5% 4 4 4 5% 7% 41%	1761
	11th to 20th	Day	20% 6% 2 5% 4 4 4 3 6% 6% 41%	1800
	21st to end	Day	22% 8% 2 5% 3 3 3 2 3 6% 42%	1660
December	1st to 10th	Day	18% 10% 2 6% 4 5% 4 4 6% 7% 35%	2318
	11th to 20th	Day	20% 7% 2 5% 3 3 3 3 4 6% 43%	2618
	21st to end	Day	21% 11% 3 5% 4 4 5% 3 4 5% 33%	3740
January	1st to 10th	Night	13% 9% 3 9% 5% 4 4 3 5% 6% 38%	6349
	11th to 20th	Night	13% 10% 4 10% 5% 5% 5% 4 6% 7% 29%	6315
	21st to end	Night	12% 10% 3 8% 5% 4 5% 4 5% 6% 38%	6996
February	1st to 10th	Night	15% 10% 4 8% 6% 5% 5% 4 5% 6% 33%	2627
	11th to 20th	Night	12% 10% 3 8% 5% 5% 5% 3 5% 5% 38%	2373
	21st to end	Night	13% 11% 3 8% 5% 5% 4 4 4 5% 39%	1980
March	1st to 10th	Night	26% 11% 3 7% 5% 5% 3 3 3 4 29%	5364
	11th to 20th	Night	17% 11% 4 7% 4 4 5% 4 6% 7% 32%	5030
	21st to end	Night	25% 10% 2 6% 4 4 4 4 6% 6% 30%	5380
April	1st to 10th	Night	21% 13% 3 6% 5% 3 4 3 5% 4 34%	2000
	11th to 20th	Night	19% 6% 2 6% 6% 4 5% 4 6% 6% 36%	1756
	21st to end	Night	25% 9% 2 7% 4 4 4 3 5% 5% 34%	1710
May	1st to 10th	Night	23% 8% 3 5% 4 4 4 4 5% 9% 31%	2834
	11th to 20th	Night	18% 8% 3 8% 6% 4 5% 5% 7% 8% 29%	2782
	21st to end	Night	15% 6% 3 6% 4 5% 4 4 6% 8% 39%	2738
June	1st to 10th	Night	21% 8% 3 6% 4 5% 5% 3 6% 7% 32%	1359
	11th to 20th	Night	16% 6% 2 7% 5% 5% 4 3 6% 10% 37%	1330
	21st to end	Night	23% 9% 2 6% 5% 3 4 4 5% 8% 30%	1372
July	1st to 10th	Night	12% 6% 2 7% 5% 5% 6% 5% 8% 9% 35%	2520
	11th to 20th	Night	14% 6% 3 8% 5% 4 4 3 6% 9% 37%	2520
	21st to end	Night	15% 5% 2 6% 4 4 5% 4 7% 8% 41%	2739
August	1st to 10th	Night	19% 8% 3 8% 5% 5% 6% 4 5% 6% 30%	2780
	11th to 20th	Night	15% 10% 3 6% 4 5% 5% 5% 6% 7% 34%	2782
	21st to end	Night	11% 5% 2 7% 5% 5% 5% 3 5% 7% 44%	3238
September	1st to 10th	Night	14% 5% 1 5% 3 3 5% 3 5% 7% 48%	1957
	11th to 20th	Night	15% 5% 2 5% 3 3 3 2 4 6% 52%	2090
	21st to end	Night	16% 7% 2 5% 4 4 4 3 5% 6% 44%	2090
October	1st to 10th	Night	18% 6% 3 6% 4 5% 4 3 4 7% 39%	2104
	11th to 20th	Night	17% 6% 2 5% 4 4 4 4 5% 7% 41%	2342
	21st to end	Night	24% 9% 2 6% 4 3 4 3 4 6% 36%	2717
November	1st to 10th	Night	18% 6% 2 6% 5% 3 4 3 5% 6% 42%	2537
	11th to 20th	Night	17% 5% 2 7% 4 4 4 4 5% 7% 42%	2600
	21st to end	Night	18% 8% 2 6% 5% 4 4 3 5% 7% 39%	2736
December	1st to 10th	Night	17% 10% 2 7% 4 4 5% 4 6% 7% 33%	4867
	11th to 20th	Night	24% 7% 2 5% 4 3 3 3 4 5% 39%	5591
	21st to end	Night	21% 11% 3 5% 3 3 4 3 5% 6% 36%	7208

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	49% 5% 11% 24% 11% 1	3598
	11th to 20th	Day	42% 5% 12% 28% 11% 3	3600
	21st to end	Day	44% 4 11% 18% 17% 6%	3960
February	1st to 10th	Day	48% 6% 10% 19% 15% 3	1771
	11th to 20th	Day	49% 6% 12% 18% 12% 3	1917
	21st to end	Day	49% 6% 10% 18% 11% 5%	1650
March	1st to 10th	Day	59% 4 7% 10% 14% 6%	4479
	11th to 20th	Day	50% 6% 14% 18% 11% 2	4928
	21st to end	Day	53% 4 6% 17% 16% 4	5550
April	1st to 10th	Day	52% 5% 8% 16% 14% 4	2400
	11th to 20th	Day	44% 7% 9% 23% 15% 2	2380
	21st to end	Day	52% 5% 8% 19% 13% 4	2470
May	1st to 10th	Day	51% 9% 12% 17% 8% 2	4120
	11th to 20th	Day	50% 9% 14% 17% 9% 1	4143
	21st to end	Day	40% 9% 15% 20% 14% 1	4837
June	1st to 10th	Day	51% 7% 13% 21% 8%	2817
	11th to 20th	Day	44% 7% 12% 19% 15% 2	2850
	21st to end	Day	52% 8% 13% 17% 10% 1	2802
July	1st to 10th	Day	42% 8% 14% 24% 11%	4370
	11th to 20th	Day	39% 9% 15% 26% 10% 1	4365
	21st to end	Day	46% 7% 12% 21% 14% 1	4661
August	1st to 10th	Day	47% 8% 14% 25% 7%	4104
	11th to 20th	Day	47% 8% 16% 22% 7%	4084
	21st to end	Day	33% 8% 15% 25% 18% 2	4414
September	1st to 10th	Day	35% 7% 14% 26% 17% 1	2221
	11th to 20th	Day	33% 5% 10% 29% 21% 2	2090
	21st to end	Day	38% 6% 12% 27% 16% 1	2090
October	1st to 10th	Day	51% 7% 11% 19% 11% 1	3453
	11th to 20th	Day	35% 5% 11% 29% 19%	3025
	21st to end	Day	37% 4 13% 24% 19% 3	3081
November	1st to 10th	Day	43% 6% 14% 21% 15% 2	1761
	11th to 20th	Day	41% 6% 10% 21% 18% 4	1800
	21st to end	Day	45% 4 16% 23% 10% 1	1660
December	1st to 10th	Day	46% 5% 16% 20% 12% 1	2318
	11th to 20th	Day	43% 4 13% 17% 16% 6%	2618
	21st to end	Day	53% 5% 8% 18% 13% 2	3740
January	1st to 10th	Night	46% 5% 13% 26% 10% 1	6349
	11th to 20th	Night	51% 6% 14% 18% 8% 2	6360
	21st to end	Night	45% 6% 8% 20% 16% 5%	6996
February	1st to 10th	Night	51% 6% 11% 18% 12% 3	2627
	11th to 20th	Night	47% 5% 11% 19% 15% 3	2375
	21st to end	Night	47% 5% 9% 20% 14% 4	1980
March	1st to 10th	Night	58% 4 7% 12% 13% 6%	5374
	11th to 20th	Night	49% 7% 17% 18% 7% 2	5031
	21st to end	Night	54% 4 7% 15% 17% 3	5380
April	1st to 10th	Night	53% 5% 8% 16% 15% 3	2000
	11th to 20th	Night	47% 6% 10% 21% 16% 1	1756
	21st to end	Night	51% 5% 11% 15% 14% 3	1710
May	1st to 10th	Night	48% 8% 18% 18% 8% 1	2840
	11th to 20th	Night	49% 9% 16% 17% 7% 1	2783
	21st to end	Night	41% 9% 16% 21% 12% 1	2739
June	1st to 10th	Night	52% 5% 13% 21% 8% 1	1359
	11th to 20th	Night	42% 8% 15% 21% 12% 2	1330
	21st to end	Night	51% 8% 13% 17% 9% 2	1372
July	1st to 10th	Night	41% 10% 15% 22% 11%	2480
	11th to 20th	Night	42% 8% 15% 25% 9%	2472
	21st to end	Night	39% 7% 16% 25% 12% 1	2651
August	1st to 10th	Night	53% 8% 13% 19% 6%	2770
	11th to 20th	Night	45% 9% 17% 18% 10%	2809
	21st to end	Night	36% 8% 15% 23% 16% 2	3238
September	1st to 10th	Night	34% 7% 14% 29% 16%	1957
	11th to 20th	Night	34% 6% 10% 30% 19% 2	2090
	21st to end	Night	40% 6% 9% 29% 14% 2	2090
October	1st to 10th	Night	44% 6% 15% 22% 12% 1	3507
	11th to 20th	Night	35% 6% 11% 29% 19% 1	3891
	21st to end	Night	39% 4 12% 27% 15% 2	4467
November	1st to 10th	Night	42% 5% 13% 25% 14% 1	2537
	11th to 20th	Night	43% 5% 11% 23% 16% 2	2600
	21st to end	Night	46% 5% 15% 20% 14% 1	2736
December	1st to 10th	Night	47% 6% 11% 26% 10%	4867
	11th to 20th	Night	48% 4 10% 18% 18% 2	5592
	21st to end	Night	50% 6% 10% 17% 16% 1	7208

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	30% 21% 28% 17% 5%	200
	11th to 20th	Day	30% 22% 28% 16% 3 2	200
	21st to end	Day	19% 27% 33% 20% 2	220
February	1st to 10th	Day	29% 19% 29% 19% 4	371
	11th to 20th	Day	28% 23% 34% 13% 2	476
	21st to end	Day	23% 21% 35% 16% 4	414
March	1st to 10th	Day	32% 18% 32% 14% 4	500
	11th to 20th	Day	27% 23% 32% 14% 3 1	500
	21st to end	Day	31% 21% 28% 13% 5% 2	550
April	1st to 10th	Day	36% 20% 28% 10% 5% 1	500
	11th to 20th	Day	28% 25% 27% 13% 5% 1	452
	21st to end	Day	30% 21% 31% 13% 5% 1	460
May	1st to 10th	Day	34% 16% 29% 17% 2 2	463
	11th to 20th	Day	23% 18% 34% 18% 5% 2	487
	21st to end	Day	21% 17% 38% 18% 6%	529
June	1st to 10th	Day	25% 14% 31% 20% 9% 1	647
	11th to 20th	Day	21% 13% 30% 21% 13% 2	680
	21st to end	Day	26% 11% 28% 26% 6% 2	632
July	1st to 10th	Day	19% 11% 36% 26% 8% 1	485
	11th to 20th	Day	15% 8% 40% 22% 12% 4	484
	21st to end	Day	15% 9% 39% 25% 10% 3	532
August	1st to 10th	Day	13% 9% 33% 31% 13% 1	486
	11th to 20th	Day	17% 9% 28% 29% 14% 4	479
	21st to end	Day	14% 12% 31% 27% 12% 4	539
September	1st to 10th	Day	13% 11% 32% 25% 14% 3	490
	11th to 20th	Day	19% 10% 28% 22% 13% 9%	490
	21st to end	Day	17% 13% 30% 24% 11% 5%	490
October	1st to 10th	Day	23% 14% 26% 22% 12% 3	483
	11th to 20th	Day	26% 14% 27% 19% 11% 2	390
	21st to end	Day	33% 12% 29% 19% 7% 1	429
November	1st to 10th	Day	26% 13% 31% 21% 8% 1	395
	11th to 20th	Day	31% 16% 29% 20% 5% 1	400
	21st to end	Day	30% 17% 25% 21% 6% 1	400
December	1st to 10th	Day	28% 16% 33% 17% 6%	225
	11th to 20th	Day	35% 20% 33% 10% 3	200
	21st to end	Day	34% 16% 27% 15% 7%	220
January	1st to 10th	Night	23% 14% 36% 24% 2	800
	11th to 20th	Night	25% 16% 34% 20% 5%	800
	21st to end	Night	18% 17% 40% 21% 4	880
February	1st to 10th	Night	24% 17% 32% 24% 3	800
	11th to 20th	Night	22% 18% 39% 19% 2	783
	21st to end	Night	20% 16% 38% 22% 4	660
March	1st to 10th	Night	27% 15% 36% 20% 2	800
	11th to 20th	Night	23% 16% 34% 22% 5%	800
	21st to end	Night	25% 17% 32% 21% 5%	880
April	1st to 10th	Night	30% 18% 29% 20% 3	800
	11th to 20th	Night	24% 14% 37% 21% 4	628
	21st to end	Night	30% 13% 33% 20% 4	570
May	1st to 10th	Night	26% 12% 31% 26% 5%	570
	11th to 20th	Night	22% 12% 37% 24% 5%	544
	21st to end	Night	21% 7% 35% 33% 5%	411
June	1st to 10th	Night	26% 8% 31% 28% 6%	380
	11th to 20th	Night	17% 7% 29% 37% 10%	380
	21st to end	Night	26% 4 31% 34% 6%	380
July	1st to 10th	Night	18% 4 31% 38% 10%	370
	11th to 20th	Night	16% 4 28% 40% 11%	368
	21st to end	Night	16% 3 27% 39% 14% 1	416
August	1st to 10th	Night	14% 5% 29% 40% 12%	535
	11th to 20th	Night	15% 7% 27% 36% 14% 1	562
	21st to end	Night	15% 8% 26% 37% 14% 1	689
September	1st to 10th	Night	16% 4 34% 32% 13% 1	760
	11th to 20th	Night	17% 7% 27% 29% 16% 3	760
	21st to end	Night	20% 8% 30% 28% 13% 2	760
October	1st to 10th	Night	22% 9% 25% 29% 14%	760
	11th to 20th	Night	20% 10% 32% 27% 9% 1	760
	21st to end	Night	29% 14% 30% 22% 5%	836
November	1st to 10th	Night	23% 11% 33% 25% 8% 1	780
	11th to 20th	Night	22% 14% 33% 24% 7% 1	800
	21st to end	Night	25% 16% 32% 21% 5%	868
December	1st to 10th	Night	28% 12% 35% 20% 5%	900
	11th to 20th	Night	26% 16% 35% 20% 3	900
	21st to end	Night	28% 13% 34% 20% 4	927

Page generated at 2025-04-08 09:17:29. Version ac07

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