McMath-Hulbert Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McMath-Hulbert Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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	13% 10% 4 9% 5% 5% 5% 4 6% 7% 33%	125002
	Night	14% 11% 4 9% 5% 5% 4 4 5% 7% 34%	123153
Transparency	Day	47% 8% 12% 20% 12% 1	128571
	Night	47% 7% 12% 20% 12% 1	127640
Seeing	Day	17% 13% 30% 22% 12% 5%	16413
	Night	18% 11% 31% 28% 11% 1	26110

By Month

Percentage Cloud Cover SE (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	10% 16% 6% 16% 7% 4 4 3 5% 5% 24%	3617
	11th to 20th	Day	13% 15% 6% 16% 10% 9% 7% 4 5% 5% 10%	3620
	21st to end	Day	20% 17% 6% 13% 6% 5% 6% 3 4 5% 15%	4395
February	1st to 10th	Day	13% 14% 5% 13% 9% 6% 5% 4 5% 5% 21%	1800
	11th to 20th	Day	13% 13% 5% 13% 8% 6% 5% 4 5% 5% 23%	1881
	21st to end	Day	12% 15% 5% 12% 6% 4 5% 4 5% 5% 25%	1650
March	1st to 10th	Day	14% 12% 4 11% 6% 5% 4 3 4 6% 30%	4605
	11th to 20th	Day	12% 11% 4 9% 6% 5% 5% 3 5% 8% 32%	4980
	21st to end	Day	17% 13% 5% 10% 6% 5% 4 3 4 5% 28%	5465
April	1st to 10th	Day	16% 13% 3 8% 5% 5% 4 4 5% 7% 30%	2322
	11th to 20th	Day	15% 10% 3 8% 5% 5% 4 4 6% 7% 34%	2542
	21st to end	Day	16% 9% 3 8% 5% 5% 4 4 5% 7% 35%	2600
May	1st to 10th	Day	18% 9% 3 7% 5% 4 4 4 5% 6% 34%	5862
	11th to 20th	Day	16% 8% 3 8% 6% 5% 5% 4 6% 7% 34%	5861
	21st to end	Day	13% 7% 3 7% 4 4 4 4 5% 7% 41%	6433
June	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 10% 40%	2904
	11th to 20th	Day	10% 7% 3 7% 4 5% 5% 5% 5% 9% 40%	3000
	21st to end	Day	10% 6% 3 7% 5% 6% 6% 5% 7% 10% 36%	2873
July	1st to 10th	Day	8% 5% 2 6% 5% 6% 7% 5% 7% 10% 38%	6360
	11th to 20th	Day	6% 5% 2 6% 4 5% 5% 4 7% 9% 45%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 9% 45%	6527
August	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 5% 6% 8% 41%	4156
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 5% 8% 9% 39%	4044
	21st to end	Day	8% 6% 2 8% 6% 6% 6% 5% 7% 9% 38%	4504
September	1st to 10th	Day	11% 7% 3 6% 5% 5% 7% 5% 6% 8% 40%	2146
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 8% 8% 46%	2090
	21st to end	Day	14% 8% 2 7% 6% 4 5% 4 7% 8% 34%	2090
October	1st to 10th	Day	13% 7% 3 6% 4 5% 5% 4 5% 6% 41%	2033
	11th to 20th	Day	16% 11% 2 8% 5% 4 5% 4 4 7% 33%	1900
	21st to end	Day	19% 12% 3 7% 4 5% 4 3 5% 6% 29%	1937
November	1st to 10th	Day	11% 9% 3 9% 5% 5% 5% 4 5% 8% 36%	1762
	11th to 20th	Day	15% 11% 4 10% 6% 5% 6% 3 6% 5% 29%	1800
	21st to end	Day	17% 12% 6% 10% 6% 4 4 3 4 6% 28%	1668
December	1st to 10th	Day	12% 14% 7% 12% 6% 5% 6% 3 6% 7% 22%	2600
	11th to 20th	Day	18% 19% 6% 9% 5% 4 5% 3 5% 4 20%	2753
	21st to end	Day	24% 21% 7% 12% 6% 4 4 2 3 5% 13%	3862
January	1st to 10th	Night	13% 13% 5% 12% 6% 5% 5% 4 5% 5% 27%	6348
	11th to 20th	Night	10% 15% 7% 15% 9% 7% 6% 4 6% 6% 15%	6360
	21st to end	Night	16% 15% 8% 12% 6% 5% 4 3 4 5% 22%	6562
February	1st to 10th	Night	14% 16% 5% 11% 8% 6% 5% 4 5% 6% 23%	2600
	11th to 20th	Night	13% 12% 4 11% 8% 6% 4 4 5% 6% 27%	2402
	21st to end	Night	17% 12% 4 10% 6% 5% 4 4 5% 6% 29%	1980
March	1st to 10th	Night	14% 12% 4 10% 4 4 4 3 5% 7% 34%	5247
	11th to 20th	Night	10% 8% 3 7% 4 5% 5% 4 6% 7% 41%	4980
	21st to end	Night	15% 9% 3 9% 5% 4 5% 3 4 5% 38%	5467
April	1st to 10th	Night	17% 11% 3 7% 5% 4 4 4 5% 7% 35%	2095
	11th to 20th	Night	17% 8% 2 7% 5% 3 4 4 5% 8% 37%	1806
	21st to end	Night	15% 9% 2 6% 4 4 5% 4 5% 8% 38%	1800
May	1st to 10th	Night	20% 10% 3 8% 5% 4 4 3 5% 6% 33%	4033
	11th to 20th	Night	17% 9% 3 7% 4 3 4 3 4 7% 40%	4032
	21st to end	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	4421
June	1st to 10th	Night	11% 5% 2 5% 5% 3 4 4 6% 8% 47%	1482
	11th to 20th	Night	10% 5% 2 6% 3 4 4 5% 7% 8% 44%	1400
	21st to end	Night	8% 7% 2 5% 4 5% 4 3 5% 8% 49%	1516
July	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 7% 10% 47%	3618
	11th to 20th	Night	8% 5% 1 6% 4 4 4 4 6% 9% 50%	3617
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 6% 8% 47%	4314
August	1st to 10th	Night	10% 6% 3 6% 3 4 4 4 6% 9% 47%	2861
	11th to 20th	Night	8% 7% 2 5% 4 3 5% 4 6% 8% 48%	2785
	21st to end	Night	8% 6% 2 7% 5% 4 4 3 6% 7% 48%	3148
September	1st to 10th	Night	9% 7% 3 6% 4 5% 5% 4 5% 7% 47%	2014
	11th to 20th	Night	9% 5% 1 5% 4 4 4 3 5% 7% 55%	2090
	21st to end	Night	15% 6% 2 7% 4 3 4 3 5% 7% 42%	2090
October	1st to 10th	Night	13% 7% 3 6% 4 3 3 3 5% 6% 46%	2147
	11th to 20th	Night	18% 9% 3 8% 4 5% 4 3 5% 6% 35%	2280
	21st to end	Night	20% 10% 3 8% 5% 5% 5% 3 5% 6% 32%	2659
November	1st to 10th	Night	13% 7% 3 6% 5% 4 4 3 5% 7% 43%	2536
	11th to 20th	Night	15% 11% 3 10% 6% 5% 4 5% 6% 6% 30%	2600
	21st to end	Night	17% 14% 5% 9% 5% 4 4 4 5% 6% 27%	2727
December	1st to 10th	Night	11% 13% 5% 14% 6% 5% 5% 4 6% 7% 22%	4580
	11th to 20th	Night	14% 20% 6% 12% 7% 5% 4 3 5% 5% 19%	5445
	21st to end	Night	24% 22% 7% 10% 4 4 4 3 3 4 15%	7111

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	61% 8% 10% 14% 6% 1	3618
	11th to 20th	Day	71% 10% 9% 6% 4 1	3620
	21st to end	Day	70% 6% 11% 8% 4	4395
February	1st to 10th	Day	63% 7% 9% 13% 7%	1800
	11th to 20th	Day	61% 7% 9% 15% 8% 1	1890
	21st to end	Day	58% 8% 11% 15% 7% 2	1650
March	1st to 10th	Day	54% 5% 14% 17% 9% 2	4605
	11th to 20th	Day	47% 8% 15% 21% 8% 2	4980
	21st to end	Day	56% 7% 11% 14% 10% 2	5465
April	1st to 10th	Day	52% 8% 11% 18% 10% 1	2322
	11th to 20th	Day	48% 7% 11% 20% 13% 2	2542
	21st to end	Day	47% 7% 14% 18% 13% 1	2600
May	1st to 10th	Day	47% 9% 11% 18% 12% 4	5897
	11th to 20th	Day	45% 9% 13% 20% 12% 1	5865
	21st to end	Day	40% 6% 11% 20% 19% 4	6447
June	1st to 10th	Day	37% 8% 14% 25% 14% 2	2904
	11th to 20th	Day	39% 8% 13% 23% 15% 2	3000
	21st to end	Day	39% 9% 13% 22% 15% 2	2873
July	1st to 10th	Day	36% 9% 14% 24% 15% 1	6291
	11th to 20th	Day	31% 8% 15% 28% 17% 1	6285
	21st to end	Day	33% 8% 14% 22% 22% 1	6341
August	1st to 10th	Day	36% 10% 13% 24% 16% 1	4143
	11th to 20th	Day	37% 10% 12% 30% 10% 1	4083
	21st to end	Day	38% 9% 14% 26% 12% 1	4503
September	1st to 10th	Day	39% 9% 15% 23% 12% 2	2146
	11th to 20th	Day	32% 7% 13% 32% 14% 1	2090
	21st to end	Day	44% 8% 13% 23% 12%	2090
October	1st to 10th	Day	31% 6% 12% 30% 20% 1	3384
	11th to 20th	Day	39% 7% 12% 24% 18%	3140
	21st to end	Day	45% 6% 14% 23% 11% 1	3161
November	1st to 10th	Day	46% 7% 13% 23% 11% 1	1762
	11th to 20th	Day	54% 7% 12% 17% 9% 1	1800
	21st to end	Day	57% 6% 13% 16% 6% 1	1668
December	1st to 10th	Day	60% 8% 16% 13% 3	2600
	11th to 20th	Day	65% 6% 11% 12% 6%	2749
	21st to end	Day	74% 6% 8% 7% 5%	3862
January	1st to 10th	Night	55% 8% 11% 18% 6% 2	6348
	11th to 20th	Night	64% 9% 11% 10% 5%	6360
	21st to end	Night	63% 6% 10% 14% 7%	6562
February	1st to 10th	Night	61% 8% 10% 14% 7% 1	2600
	11th to 20th	Night	57% 6% 10% 18% 8%	2403
	21st to end	Night	54% 6% 13% 17% 8% 1	1980
March	1st to 10th	Night	48% 7% 14% 21% 9% 1	5247
	11th to 20th	Night	39% 8% 15% 24% 14% 1	4980
	21st to end	Night	47% 7% 11% 19% 14% 2	5467
April	1st to 10th	Night	49% 5% 11% 20% 12% 2	2095
	11th to 20th	Night	44% 7% 12% 18% 18% 2	1806
	21st to end	Night	43% 8% 12% 19% 16% 3	1800
May	1st to 10th	Night	50% 7% 10% 15% 13% 5%	4060
	11th to 20th	Night	44% 6% 13% 23% 11% 3	4033
	21st to end	Night	36% 8% 12% 23% 17% 5%	4429
June	1st to 10th	Night	34% 7% 12% 27% 19% 2	1482
	11th to 20th	Night	34% 9% 14% 25% 17% 2	1400
	21st to end	Night	33% 7% 13% 25% 20% 1	1516
July	1st to 10th	Night	30% 8% 14% 28% 16% 4	3580
	11th to 20th	Night	30% 7% 15% 29% 17% 2	3572
	21st to end	Night	32% 9% 14% 21% 23% 2	4187
August	1st to 10th	Night	33% 8% 14% 27% 17% 1	2850
	11th to 20th	Night	31% 9% 13% 34% 11% 2	2812
	21st to end	Night	33% 9% 13% 32% 14% 1	3148
September	1st to 10th	Night	34% 9% 13% 26% 16% 2	2014
	11th to 20th	Night	28% 7% 13% 29% 21% 2	2090
	21st to end	Night	40% 6% 13% 28% 13% 1	2090
October	1st to 10th	Night	26% 5% 10% 32% 26% 1	3576
	11th to 20th	Night	42% 6% 11% 23% 17%	3779
	21st to end	Night	42% 6% 11% 28% 12% 1	4372
November	1st to 10th	Night	41% 6% 13% 26% 14% 1	2536
	11th to 20th	Night	51% 8% 13% 18% 9% 1	2600
	21st to end	Night	56% 6% 13% 18% 6%	2727
December	1st to 10th	Night	58% 8% 16% 15% 3	4580
	11th to 20th	Night	64% 8% 10% 13% 5% 1	5448
	21st to end	Night	72% 7% 8% 7% 6%	7111

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	22% 22% 38% 13% 4	300
	11th to 20th	Day	22% 19% 45% 12% 1 1	300
	21st to end	Day	28% 16% 35% 16% 4 1	330
February	1st to 10th	Day	24% 24% 36% 13% 3 1	300
	11th to 20th	Day	25% 22% 37% 16% 1	300
	21st to end	Day	25% 29% 35% 10% 2	249
March	1st to 10th	Day	27% 22% 35% 13% 3	347
	11th to 20th	Day	24% 14% 34% 22% 5% 1	500
	21st to end	Day	27% 17% 33% 19% 4 1	550
April	1st to 10th	Day	19% 18% 37% 18% 7% 1	500
	11th to 20th	Day	26% 19% 30% 14% 7% 4	492
	21st to end	Day	24% 16% 29% 20% 9% 2	500
May	1st to 10th	Day	22% 16% 29% 20% 10% 2	503
	11th to 20th	Day	21% 9% 30% 23% 12% 6%	527
	21st to end	Day	15% 10% 26% 26% 16% 7%	573
June	1st to 10th	Day	12% 11% 32% 28% 14% 3	724
	11th to 20th	Day	12% 9% 34% 26% 16% 3	730
	21st to end	Day	12% 8% 31% 30% 15% 4	730
July	1st to 10th	Day	9% 6% 23% 32% 20% 11%	721
	11th to 20th	Day	9% 6% 30% 28% 19% 7%	718
	21st to end	Day	8% 4 26% 30% 22% 9%	627
August	1st to 10th	Day	9% 4 24% 25% 25% 13%	502
	11th to 20th	Day	11% 5% 21% 29% 21% 13%	479
	21st to end	Day	11% 6% 23% 29% 20% 12%	539
September	1st to 10th	Day	12% 7% 25% 24% 22% 11%	490
	11th to 20th	Day	9% 4 20% 24% 27% 15%	490
	21st to end	Day	17% 11% 25% 23% 18% 7%	490
October	1st to 10th	Day	16% 11% 28% 21% 17% 7%	490
	11th to 20th	Day	19% 17% 30% 24% 8% 2	490
	21st to end	Day	25% 19% 37% 15% 5% 1	386
November	1st to 10th	Day	17% 21% 38% 16% 8% 1	300
	11th to 20th	Day	22% 23% 33% 15% 5% 1	300
	21st to end	Day	23% 21% 39% 14% 4	233
December	1st to 10th	Day	21% 28% 31% 13% 8%	200
	11th to 20th	Day	28% 26% 34% 10% 3 1	200
	21st to end	Day	25% 26% 34% 13% 1	303
January	1st to 10th	Night	20% 16% 39% 20% 5% 1	1000
	11th to 20th	Night	23% 15% 37% 22% 3 1	1000
	21st to end	Night	20% 14% 35% 26% 5%	913
February	1st to 10th	Night	23% 16% 37% 22% 2	800
	11th to 20th	Night	21% 16% 36% 23% 4	783
	21st to end	Night	24% 18% 38% 18% 2	660
March	1st to 10th	Night	20% 15% 36% 24% 5%	800
	11th to 20th	Night	17% 16% 37% 25% 5% 1	800
	21st to end	Night	21% 14% 32% 26% 7% 1	880
April	1st to 10th	Night	21% 15% 34% 25% 4	695
	11th to 20th	Night	21% 13% 33% 24% 6% 2	593
	21st to end	Night	19% 11% 33% 29% 8%	600
May	1st to 10th	Night	19% 9% 31% 32% 8% 1	600
	11th to 20th	Night	21% 6% 28% 32% 12% 1	591
	21st to end	Night	12% 4 26% 40% 17% 2	650
June	1st to 10th	Night	12% 5% 27% 42% 13%	480
	11th to 20th	Night	11% 5% 26% 47% 11%	400
	21st to end	Night	10% 5% 23% 45% 17% 1	516
July	1st to 10th	Night	8% 3 19% 40% 26% 3	585
	11th to 20th	Night	9% 3 21% 40% 22% 4	582
	21st to end	Night	6% 2 19% 49% 22% 2	657
August	1st to 10th	Night	7% 3 18% 41% 28% 3	579
	11th to 20th	Night	8% 2 21% 35% 30% 4	564
	21st to end	Night	9% 4 20% 33% 31% 3	627
September	1st to 10th	Night	11% 5% 21% 34% 23% 5%	684
	11th to 20th	Night	10% 5% 18% 34% 29% 5%	760
	21st to end	Night	16% 7% 23% 29% 19% 5%	760
October	1st to 10th	Night	16% 9% 26% 29% 17% 3	760
	11th to 20th	Night	22% 16% 27% 26% 9% 1	760
	21st to end	Night	25% 13% 34% 24% 5%	836
November	1st to 10th	Night	19% 13% 29% 26% 12% 1	780
	11th to 20th	Night	21% 16% 35% 21% 6% 1	800
	21st to end	Night	24% 16% 36% 20% 5%	800
December	1st to 10th	Night	23% 16% 36% 20% 6%	800
	11th to 20th	Night	22% 15% 38% 21% 3	915
	21st to end	Night	30% 14% 36% 18% 2	1100

Page generated at 2025-08-02 01:32:22. Version ac07

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