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 2024-10-24.

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 6% 4 4 4 4 6% 7% 36%	100971
	Night	18% 8% 3 7% 5% 4 4 4 5% 7% 36%	97801
Transparency	Day	46% 7% 12% 21% 12% 2	100729
	Night	47% 6% 12% 21% 12% 2	97664
Seeing	Day	24% 15% 31% 20% 8% 2	15396
	Night	23% 12% 32% 26% 7%	23908

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	15% 8% 3 7% 5% 4 4 4 6% 6% 39%	2517
	11th to 20th	Day	17% 8% 3 8% 4 4 4 3 4 6% 40%	2520
	21st to end	Day	16% 11% 5% 8% 4 4 4 4 5% 5% 35%	2772
February	1st to 10th	Day	18% 10% 3 6% 4 5% 4 4 6% 6% 35%	1682
	11th to 20th	Day	13% 10% 4 8% 5% 5% 6% 4 5% 6% 35%	1810
	21st to end	Day	15% 11% 3 8% 4 5% 4 3 6% 6% 35%	1570
March	1st to 10th	Day	25% 12% 3 7% 4 4 3 3 4 4 29%	3116
	11th to 20th	Day	18% 9% 2 6% 6% 5% 5% 3 5% 6% 35%	3442
	21st to end	Day	25% 9% 3 6% 3 3 3 3 6% 7% 32%	3872
April	1st to 10th	Day	20% 12% 4 7% 5% 3 3 3 5% 5% 34%	2280
	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% 8% 3 7% 4 3 3 3 5% 5% 35%	1809
November	1st to 10th	Day	18% 6% 2 5% 5% 4 4 4 5% 7% 41%	1671
	11th to 20th	Day	21% 6% 2 5% 4 4 4 3 6% 6% 40%	1710
	21st to end	Day	21% 8% 2 5% 3 3 3 2 3 6% 43%	1577
December	1st to 10th	Day	22% 11% 3 7% 4 5% 3 3 4 6% 33%	1354
	11th to 20th	Day	23% 9% 3 6% 3 3 3 3 4 6% 37%	1668
	21st to end	Day	26% 12% 2 6% 5% 4 5% 3 4 5% 29%	2617
January	1st to 10th	Night	14% 9% 3 8% 5% 4 4 3 5% 6% 39%	4429
	11th to 20th	Night	13% 9% 4 10% 6% 5% 4 4 6% 7% 32%	4440
	21st to end	Night	17% 12% 4 9% 5% 4 4 4 5% 6% 32%	4884
February	1st to 10th	Night	16% 9% 4 8% 6% 5% 5% 4 5% 6% 32%	2496
	11th to 20th	Night	13% 11% 3 8% 5% 5% 5% 3 5% 5% 38%	2252
	21st to end	Night	13% 12% 3 7% 5% 5% 4 4 4 5% 38%	1884
March	1st to 10th	Night	29% 12% 2 5% 4 4 3 3 3 4 31%	3724
	11th to 20th	Night	16% 10% 4 7% 5% 5% 6% 4 6% 6% 31%	3516
	21st to end	Night	26% 10% 3 6% 4 4 4 4 6% 6% 28%	3758
April	1st to 10th	Night	21% 12% 3 6% 5% 3 3 3 5% 4 35%	1900
	11th to 20th	Night	19% 6% 2 6% 6% 4 5% 4 6% 6% 36%	1749
	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	25% 8% 2 6% 4 3 4 3 4 6% 35%	2614
November	1st to 10th	Night	19% 6% 2 6% 4 3 4 3 5% 6% 42%	2407
	11th to 20th	Night	17% 5% 2 7% 4 4 4 4 5% 7% 41%	2470
	21st to end	Night	17% 8% 2 6% 5% 4 4 3 5% 7% 39%	2599
December	1st to 10th	Night	22% 10% 2 7% 5% 4 4 3 5% 6% 32%	2821
	11th to 20th	Night	22% 8% 3 6% 4 4 3 3 4 6% 37%	3531
	21st to end	Night	24% 10% 3 5% 3 4 4 3 5% 6% 33%	5040

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	44% 6% 15% 24% 11% 1	2518
	11th to 20th	Day	44% 5% 12% 25% 11% 2	2520
	21st to end	Day	50% 5% 14% 17% 13% 1	2772
February	1st to 10th	Day	49% 6% 10% 19% 14% 2	1682
	11th to 20th	Day	49% 6% 11% 18% 13% 3	1819
	21st to end	Day	49% 6% 11% 19% 10% 6%	1570
March	1st to 10th	Day	59% 4 9% 11% 14% 3	3119
	11th to 20th	Day	47% 7% 11% 19% 13% 3	3442
	21st to end	Day	51% 4 7% 16% 17% 4	3872
April	1st to 10th	Day	53% 5% 8% 16% 14% 4	2280
	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	45% 6% 11% 23% 14% 1	2076
	11th to 20th	Day	43% 6% 13% 26% 11% 1	1835
	21st to end	Day	52% 5% 10% 19% 13% 1	1809
November	1st to 10th	Day	42% 5% 14% 21% 15% 2	1671
	11th to 20th	Day	42% 6% 11% 20% 17% 3	1710
	21st to end	Day	45% 4 16% 23% 11% 1	1577
December	1st to 10th	Day	53% 5% 10% 20% 10% 1	1354
	11th to 20th	Day	49% 5% 10% 19% 15% 3	1668
	21st to end	Day	58% 5% 9% 18% 8% 2	2617
January	1st to 10th	Night	46% 5% 13% 25% 9% 1	4429
	11th to 20th	Night	49% 7% 14% 18% 10% 3	4440
	21st to end	Night	52% 6% 9% 19% 12% 1	4884
February	1st to 10th	Night	51% 6% 12% 18% 11% 3	2496
	11th to 20th	Night	47% 5% 10% 20% 15% 3	2254
	21st to end	Night	48% 5% 9% 19% 14% 5%	1884
March	1st to 10th	Night	58% 4 9% 15% 10% 4	3734
	11th to 20th	Night	50% 8% 13% 18% 9% 2	3517
	21st to end	Night	56% 3 7% 16% 15% 3	3758
April	1st to 10th	Night	52% 5% 8% 16% 15% 4	1900
	11th to 20th	Night	47% 6% 10% 21% 16% 1	1749
	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% 12% 23% 13% 1	2104
	11th to 20th	Night	42% 6% 14% 25% 11% 1	2342
	21st to end	Night	50% 5% 11% 20% 13% 1	2614
November	1st to 10th	Night	43% 5% 12% 24% 14% 1	2407
	11th to 20th	Night	43% 5% 11% 24% 15% 2	2470
	21st to end	Night	45% 5% 15% 20% 14% 1	2599
December	1st to 10th	Night	53% 5% 10% 20% 11% 1	2821
	11th to 20th	Night	49% 5% 10% 19% 17% 1	3532
	21st to end	Night	51% 6% 12% 18% 11% 1	5040

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	29% 18% 29% 18% 6%	180
	11th to 20th	Day	32% 22% 27% 15% 3 2	180
	21st to end	Day	18% 25% 34% 21% 2	198
February	1st to 10th	Day	30% 18% 29% 19% 4	342
	11th to 20th	Day	27% 24% 34% 14% 2	438
	21st to end	Day	24% 20% 35% 17% 5%	382
March	1st to 10th	Day	30% 18% 33% 15% 4	460
	11th to 20th	Day	26% 23% 33% 14% 3 1	460
	21st to end	Day	30% 22% 27% 14% 6% 2	506
April	1st to 10th	Day	35% 20% 28% 11% 5% 1	460
	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% 11% 29% 19% 7% 1	405
November	1st to 10th	Day	24% 14% 31% 21% 8% 1	365
	11th to 20th	Day	31% 14% 28% 21% 5% 1	370
	21st to end	Day	29% 17% 26% 21% 6% 1	370
December	1st to 10th	Day	28% 17% 33% 16% 6%	204
	11th to 20th	Day	35% 21% 33% 8% 3	180
	21st to end	Day	36% 17% 26% 14% 7% 1	198
January	1st to 10th	Night	24% 13% 36% 24% 2	760
	11th to 20th	Night	25% 16% 34% 20% 4	760
	21st to end	Night	19% 17% 40% 20% 4	836
February	1st to 10th	Night	23% 16% 32% 25% 3	760
	11th to 20th	Night	21% 19% 38% 20% 2	743
	21st to end	Night	20% 16% 38% 22% 4	628
March	1st to 10th	Night	26% 15% 36% 20% 1	760
	11th to 20th	Night	22% 17% 34% 22% 5%	760
	21st to end	Night	25% 17% 32% 21% 5%	836
April	1st to 10th	Night	29% 17% 30% 21% 3	760
	11th to 20th	Night	24% 15% 37% 21% 4	625
	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	30% 13% 29% 22% 5%	805
November	1st to 10th	Night	23% 10% 32% 25% 8% 1	740
	11th to 20th	Night	22% 13% 32% 24% 7% 1	760
	21st to end	Night	24% 16% 32% 22% 6%	821
December	1st to 10th	Night	29% 13% 35% 19% 5%	850
	11th to 20th	Night	26% 15% 36% 20% 3	850
	21st to end	Night	29% 13% 35% 20% 3	879

Page generated at 2024-10-21 02:20:01. Version ac07

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