Eileen Collins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eileen Collins Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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	15% 11% 4 10% 6% 5% 5% 4 6% 7% 30%	126429
	Night	15% 11% 4 10% 6% 5% 4 3 5% 6% 30%	122289
Transparency	Day	52% 6% 11% 20% 10% 1	130000
	Night	53% 6% 11% 18% 10% 1	126804
Seeing	Day	25% 17% 28% 18% 9% 3	15739
	Night	23% 15% 28% 24% 10% 1	25003

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	11% 16% 9% 21% 6% 4 4 2 4 5% 17%	3597
	11th to 20th	Day	11% 15% 8% 21% 8% 6% 4 3 4 5% 15%	3731
	21st to end	Day	15% 13% 8% 12% 7% 5% 5% 4 5% 5% 21%	4466
February	1st to 10th	Day	14% 15% 5% 13% 9% 6% 5% 4 5% 4 21%	1800
	11th to 20th	Day	13% 14% 6% 15% 7% 4 6% 4 5% 4 22%	1756
	21st to end	Day	18% 14% 3 11% 7% 4 3 3 5% 5% 26%	1650
March	1st to 10th	Day	17% 13% 6% 11% 5% 4 4 2 3 4 30%	4740
	11th to 20th	Day	11% 15% 5% 9% 5% 4 4 3 4 6% 35%	4980
	21st to end	Day	13% 11% 4 13% 7% 5% 5% 4 5% 5% 28%	5616
April	1st to 10th	Day	16% 16% 3 9% 6% 4 4 3 5% 6% 29%	2400
	11th to 20th	Day	17% 12% 3 7% 5% 4 5% 5% 6% 7% 30%	2374
	21st to end	Day	17% 11% 3 7% 4 5% 4 3 5% 7% 33%	2460
May	1st to 10th	Day	27% 11% 3 7% 4 4 4 3 4 5% 28%	6067
	11th to 20th	Day	19% 11% 4 8% 5% 5% 5% 4 6% 7% 27%	6318
	21st to end	Day	19% 8% 3 8% 5% 5% 4 4 6% 7% 31%	6949
June	1st to 10th	Day	15% 8% 2 8% 5% 5% 6% 4 6% 8% 33%	2800
	11th to 20th	Day	14% 7% 2 7% 5% 6% 5% 4 6% 8% 35%	2800
	21st to end	Day	11% 7% 3 7% 5% 5% 6% 5% 6% 10% 35%	2800
July	1st to 10th	Day	7% 6% 3 7% 5% 6% 6% 5% 7% 10% 38%	6360
	11th to 20th	Day	6% 5% 2 9% 7% 7% 7% 6% 8% 12% 31%	6360
	21st to end	Day	9% 7% 2 7% 5% 5% 6% 5% 6% 8% 40%	6709
August	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 6% 9% 32%	4234
	11th to 20th	Day	15% 8% 3 8% 6% 6% 6% 5% 8% 9% 27%	4108
	21st to end	Day	13% 8% 3 6% 4 5% 5% 4 7% 9% 36%	4312
September	1st to 10th	Day	15% 6% 2 6% 4 5% 5% 4 5% 8% 39%	2316
	11th to 20th	Day	14% 7% 3 6% 4 5% 3 4 5% 8% 42%	2280
	21st to end	Day	20% 8% 3 6% 5% 5% 5% 4 6% 6% 33%	2003
October	1st to 10th	Day	15% 8% 3 7% 4 6% 5% 5% 6% 8% 33%	1900
	11th to 20th	Day	17% 10% 3 10% 5% 5% 4 4 4 6% 32%	1900
	21st to end	Day	20% 12% 4 8% 5% 5% 5% 4 4 7% 26%	2077
November	1st to 10th	Day	13% 11% 3 8% 5% 4 4 4 5% 6% 37%	1761
	11th to 20th	Day	14% 16% 4 8% 5% 4 4 3 6% 6% 28%	1659
	21st to end	Day	16% 16% 4 10% 7% 5% 4 3 5% 7% 24%	1600
December	1st to 10th	Day	16% 19% 8% 16% 6% 5% 4 3 4 5% 14%	2600
	11th to 20th	Day	18% 17% 8% 10% 5% 4 4 3 4 5% 23%	2986
	21st to end	Day	17% 17% 6% 17% 7% 5% 4 3 5% 5% 15%	3960
January	1st to 10th	Night	13% 13% 7% 20% 9% 5% 4 3 4 5% 18%	6349
	11th to 20th	Night	9% 15% 7% 20% 8% 7% 5% 3 5% 5% 17%	6150
	21st to end	Night	15% 14% 7% 14% 7% 5% 6% 3 4 4 20%	6490
February	1st to 10th	Night	14% 15% 4 12% 7% 6% 5% 4 5% 6% 23%	2600
	11th to 20th	Night	12% 14% 5% 12% 7% 7% 5% 4 6% 5% 23%	2529
	21st to end	Night	16% 13% 5% 12% 7% 4 5% 3 6% 5% 23%	1980
March	1st to 10th	Night	15% 11% 6% 11% 8% 4 4 3 4 5% 28%	5098
	11th to 20th	Night	13% 15% 5% 8% 5% 3 3 3 4 6% 34%	4979
	21st to end	Night	16% 11% 4 11% 6% 4 5% 3 4 6% 30%	5315
April	1st to 10th	Night	19% 12% 4 9% 5% 4 4 3 4 6% 28%	2000
	11th to 20th	Night	18% 11% 3 6% 5% 4 4 3 5% 6% 34%	1975
	21st to end	Night	17% 10% 3 6% 4 4 5% 4 5% 6% 36%	1957
May	1st to 10th	Night	30% 11% 3 7% 4 3 4 3 4 5% 25%	3872
	11th to 20th	Night	20% 9% 3 7% 5% 4 5% 3 5% 7% 31%	3575
	21st to end	Night	18% 9% 3 7% 5% 4 5% 4 6% 6% 33%	3926
June	1st to 10th	Night	15% 8% 2 6% 5% 4 4 4 6% 9% 38%	1600
	11th to 20th	Night	12% 8% 2 6% 5% 4 4 5% 7% 8% 39%	1600
	21st to end	Night	10% 6% 2 7% 4 4 4 5% 6% 8% 44%	1600
July	1st to 10th	Night	9% 8% 3 8% 6% 5% 4 4 5% 10% 39%	3600
	11th to 20th	Night	8% 5% 2 7% 6% 5% 6% 4 8% 10% 38%	3600
	21st to end	Night	9% 6% 2 7% 4 4 5% 3 5% 7% 48%	4140
August	1st to 10th	Night	17% 7% 3 8% 4 4 4 4 6% 7% 36%	2915
	11th to 20th	Night	16% 7% 2 6% 4 4 3 3 6% 8% 40%	2907
	21st to end	Night	11% 6% 2 6% 4 5% 6% 4 7% 8% 40%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 6% 46%	1931
	11th to 20th	Night	12% 6% 3 6% 4 3 3 3 6% 7% 47%	1900
	21st to end	Night	18% 7% 2 7% 5% 4 4 4 5% 7% 38%	2175
October	1st to 10th	Night	16% 9% 2 8% 4 4 5% 3 6% 7% 36%	2280
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 6% 7% 35%	2280
	21st to end	Night	22% 12% 3 7% 5% 4 4 3 5% 6% 30%	2519
November	1st to 10th	Night	12% 10% 3 8% 5% 4 4 4 4 6% 40%	2537
	11th to 20th	Night	16% 13% 3 9% 6% 5% 4 3 5% 6% 30%	2740
	21st to end	Night	14% 15% 4 10% 6% 5% 5% 3 6% 6% 26%	2800
December	1st to 10th	Night	11% 15% 7% 15% 8% 6% 6% 4 5% 6% 17%	4580
	11th to 20th	Night	19% 16% 5% 11% 5% 4 4 3 4 5% 24%	5208
	21st to end	Night	20% 20% 6% 11% 5% 4 4 3 4 5% 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	70% 4 6% 14% 5% 1	3598
	11th to 20th	Day	72% 5% 7% 10% 5% 1	3762
	21st to end	Day	63% 5% 9% 15% 7%	4466
February	1st to 10th	Day	66% 5% 10% 13% 6% 1	1800
	11th to 20th	Day	63% 6% 6% 17% 8%	1765
	21st to end	Day	59% 5% 8% 18% 8% 2	1650
March	1st to 10th	Day	58% 3 10% 18% 10% 1	4743
	11th to 20th	Day	51% 4 17% 17% 8% 2	4980
	21st to end	Day	56% 4 7% 18% 12% 2	5616
April	1st to 10th	Day	56% 5% 8% 20% 10% 2	2400
	11th to 20th	Day	50% 7% 10% 18% 14% 1	2374
	21st to end	Day	50% 5% 9% 19% 14% 3	2460
May	1st to 10th	Day	58% 5% 9% 16% 11% 2	6076
	11th to 20th	Day	54% 8% 9% 19% 10% 1	6322
	21st to end	Day	51% 7% 10% 21% 10% 1	6949
June	1st to 10th	Day	47% 7% 12% 19% 15% 1	2800
	11th to 20th	Day	45% 7% 15% 21% 11% 1	2800
	21st to end	Day	42% 10% 13% 21% 13% 1	2800
July	1st to 10th	Day	40% 7% 14% 27% 12% 1	6290
	11th to 20th	Day	41% 8% 15% 23% 12%	6285
	21st to end	Day	40% 6% 12% 24% 17% 1	6516
August	1st to 10th	Day	48% 7% 13% 21% 11% 1	4221
	11th to 20th	Day	48% 9% 14% 23% 6%	4147
	21st to end	Day	41% 7% 11% 31% 9% 1	4312
September	1st to 10th	Day	42% 7% 10% 26% 13% 1	2316
	11th to 20th	Day	40% 7% 11% 25% 17% 1	2280
	21st to end	Day	50% 5% 10% 21% 12% 1	2003
October	1st to 10th	Day	42% 7% 12% 23% 16% 1	3160
	11th to 20th	Day	46% 5% 11% 20% 17% 1	3140
	21st to end	Day	41% 5% 15% 27% 11% 1	3403
November	1st to 10th	Day	47% 6% 12% 21% 12% 2	1761
	11th to 20th	Day	56% 5% 10% 19% 8% 2	1659
	21st to end	Day	59% 7% 12% 17% 5%	1600
December	1st to 10th	Day	71% 6% 8% 12% 3	2600
	11th to 20th	Day	63% 5% 11% 14% 7%	2986
	21st to end	Day	70% 6% 10% 10% 4	3960
January	1st to 10th	Night	67% 5% 9% 11% 7%	6349
	11th to 20th	Night	67% 6% 9% 12% 5% 1	6194
	21st to end	Night	67% 5% 7% 10% 11% 1	6490
February	1st to 10th	Night	60% 7% 11% 15% 7%	2600
	11th to 20th	Night	61% 5% 11% 15% 7% 1	2531
	21st to end	Night	60% 5% 10% 14% 9% 2	1980
March	1st to 10th	Night	58% 5% 13% 15% 9% 1	5108
	11th to 20th	Night	50% 5% 16% 18% 6% 4	4980
	21st to end	Night	55% 4 8% 17% 14% 2	5315
April	1st to 10th	Night	56% 5% 9% 17% 10% 2	2000
	11th to 20th	Night	51% 6% 11% 18% 14% 1	1975
	21st to end	Night	47% 7% 10% 20% 14% 2	1957
May	1st to 10th	Night	61% 5% 8% 17% 7% 2	3879
	11th to 20th	Night	50% 9% 12% 18% 9% 1	3576
	21st to end	Night	49% 8% 11% 18% 12% 2	3927
June	1st to 10th	Night	43% 7% 10% 25% 15% 1	1600
	11th to 20th	Night	41% 8% 14% 23% 12% 2	1600
	21st to end	Night	36% 9% 14% 26% 14% 1	1600
July	1st to 10th	Night	42% 6% 14% 25% 13% 1	3560
	11th to 20th	Night	38% 9% 16% 26% 11%	3552
	21st to end	Night	35% 6% 12% 25% 21% 1	4018
August	1st to 10th	Night	46% 7% 15% 22% 9% 1	2904
	11th to 20th	Night	42% 6% 16% 27% 9%	2934
	21st to end	Night	39% 7% 12% 31% 10% 1	3586
September	1st to 10th	Night	38% 6% 12% 29% 12% 2	1931
	11th to 20th	Night	35% 7% 13% 26% 18% 1	1900
	21st to end	Night	45% 6% 12% 22% 13% 1	2175
October	1st to 10th	Night	48% 7% 10% 22% 13%	3800
	11th to 20th	Night	41% 5% 12% 22% 16% 4	3779
	21st to end	Night	41% 5% 15% 28% 10% 1	4143
November	1st to 10th	Night	44% 6% 12% 23% 13% 2	2537
	11th to 20th	Night	54% 5% 11% 18% 9% 2	2740
	21st to end	Night	56% 7% 14% 18% 5%	2800
December	1st to 10th	Night	65% 6% 12% 14% 2	4580
	11th to 20th	Night	62% 4 10% 13% 10%	5208
	21st to end	Night	68% 6% 8% 12% 6%	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	36% 21% 30% 10% 4 1	200
	11th to 20th	Day	35% 29% 27% 8%	236
	21st to end	Day	29% 32% 27% 8% 4	330
February	1st to 10th	Day	36% 32% 26% 5% 1	300
	11th to 20th	Day	39% 27% 29% 5%	303
	21st to end	Day	39% 20% 29% 10% 1	414
March	1st to 10th	Day	35% 21% 28% 13% 3	500
	11th to 20th	Day	34% 23% 28% 11% 4	500
	21st to end	Day	33% 23% 27% 11% 4 1	550
April	1st to 10th	Day	33% 23% 27% 13% 3 2	500
	11th to 20th	Day	31% 23% 27% 11% 5% 3	492
	21st to end	Day	29% 21% 28% 15% 6% 1	500
May	1st to 10th	Day	31% 19% 26% 14% 6% 4	503
	11th to 20th	Day	24% 17% 30% 17% 10% 3	527
	21st to end	Day	21% 11% 28% 23% 11% 5%	573
June	1st to 10th	Day	17% 18% 31% 21% 10% 3	530
	11th to 20th	Day	17% 13% 31% 25% 12% 2	530
	21st to end	Day	14% 15% 35% 24% 9% 3	530
July	1st to 10th	Day	11% 8% 31% 31% 14% 5%	525
	11th to 20th	Day	9% 10% 27% 31% 16% 6%	524
	21st to end	Day	11% 8% 36% 30% 12% 4	576
August	1st to 10th	Day	12% 7% 28% 27% 19% 8%	526
	11th to 20th	Day	16% 7% 26% 28% 16% 7%	519
	21st to end	Day	15% 11% 28% 23% 15% 9%	583
September	1st to 10th	Day	19% 10% 20% 25% 17% 9%	502
	11th to 20th	Day	16% 7% 25% 22% 19% 12%	490
	21st to end	Day	26% 13% 26% 19% 13% 4	490
October	1st to 10th	Day	26% 12% 23% 21% 13% 5%	490
	11th to 20th	Day	25% 17% 29% 20% 7% 2	490
	21st to end	Day	29% 18% 29% 19% 5% 1	532
November	1st to 10th	Day	26% 19% 28% 16% 9% 2	395
	11th to 20th	Day	33% 22% 28% 14% 3 1	259
	21st to end	Day	37% 24% 31% 8% 1	200
December	1st to 10th	Day	35% 27% 28% 6% 4 1	200
	11th to 20th	Day	37% 25% 28% 8% 2 1	200
	21st to end	Day	31% 25% 32% 10% 1	220
January	1st to 10th	Night	25% 24% 34% 15% 2	800
	11th to 20th	Night	25% 23% 30% 20% 3	800
	21st to end	Night	23% 24% 33% 18% 3	880
February	1st to 10th	Night	28% 21% 31% 19% 1	800
	11th to 20th	Night	29% 27% 29% 14% 1	783
	21st to end	Night	30% 23% 28% 16% 3	660
March	1st to 10th	Night	24% 26% 31% 17% 2	800
	11th to 20th	Night	28% 22% 29% 17% 4	800
	21st to end	Night	28% 20% 30% 16% 5% 1	880
April	1st to 10th	Night	28% 18% 31% 21% 3	800
	11th to 20th	Night	27% 19% 28% 18% 7% 1	791
	21st to end	Night	23% 13% 30% 26% 7%	757
May	1st to 10th	Night	27% 12% 23% 28% 10% 1	523
	11th to 20th	Night	17% 10% 33% 28% 11%	394
	21st to end	Night	14% 8% 25% 38% 15%	433
June	1st to 10th	Night	18% 8% 26% 35% 14% 1	400
	11th to 20th	Night	16% 8% 26% 37% 15%	400
	21st to end	Night	12% 7% 27% 38% 16%	400
July	1st to 10th	Night	9% 5% 23% 42% 20% 2	390
	11th to 20th	Night	10% 4 25% 37% 22% 2	388
	21st to end	Night	12% 4 19% 43% 21% 2	525
August	1st to 10th	Night	9% 3 21% 40% 25% 2	597
	11th to 20th	Night	13% 4 19% 33% 28% 3	613
	21st to end	Night	13% 6% 18% 38% 23% 3	880
September	1st to 10th	Night	18% 7% 22% 30% 20% 3	773
	11th to 20th	Night	15% 6% 20% 27% 26% 5%	760
	21st to end	Night	20% 10% 25% 25% 17% 3	760
October	1st to 10th	Night	23% 9% 25% 23% 18% 2	760
	11th to 20th	Night	23% 14% 29% 26% 7% 1	760
	21st to end	Night	29% 15% 28% 22% 5%	836
November	1st to 10th	Night	20% 15% 32% 23% 9%	780
	11th to 20th	Night	28% 18% 28% 18% 6% 1	800
	21st to end	Night	26% 19% 32% 20% 4 1	800
December	1st to 10th	Night	29% 20% 32% 15% 4	800
	11th to 20th	Night	29% 21% 31% 17% 3	800
	21st to end	Night	29% 19% 33% 16% 2	880

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