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 2024-11-04.

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	13% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	100976
	Night	14% 10% 4 8% 5% 4 4 4 5% 7% 35%	98029
Transparency	Day	46% 8% 13% 20% 12% 1	104589
	Night	46% 7% 12% 21% 12% 1	102542
Seeing	Day	17% 13% 30% 22% 13% 5%	15418
	Night	18% 11% 30% 28% 11% 1	25120

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	14% 18% 7% 12% 7% 4 4 3 5% 5% 21%	2527
	11th to 20th	Day	14% 18% 6% 13% 9% 8% 7% 4 5% 5% 12%	2530
	21st to end	Day	25% 20% 7% 11% 6% 4 4 3 3 4 14%	3066
February	1st to 10th	Day	13% 14% 5% 13% 9% 6% 6% 4 5% 5% 21%	1710
	11th to 20th	Day	13% 13% 5% 12% 8% 6% 5% 4 5% 5% 24%	1784
	21st to end	Day	13% 15% 5% 12% 6% 4 4 4 5% 5% 26%	1570
March	1st to 10th	Day	17% 15% 5% 10% 5% 5% 4 3 4 5% 27%	3203
	11th to 20th	Day	12% 11% 4 9% 7% 6% 5% 3 6% 7% 30%	3480
	21st to end	Day	18% 12% 4 10% 6% 5% 4 4 4 5% 28%	3815
April	1st to 10th	Day	15% 13% 3 8% 5% 5% 4 4 5% 7% 30%	2206
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 6% 7% 34%	2413
	21st to end	Day	17% 9% 3 8% 5% 5% 4 4 5% 7% 34%	2470
May	1st to 10th	Day	16% 9% 4 7% 5% 5% 4 4 5% 7% 35%	4082
	11th to 20th	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 35%	4081
	21st to end	Day	10% 6% 2 7% 4 4 4 4 5% 8% 45%	4476
June	1st to 10th	Day	10% 5% 2 7% 5% 5% 5% 4 6% 10% 40%	2758
	11th to 20th	Day	10% 7% 3 7% 4 6% 4 5% 5% 9% 41%	2850
	21st to end	Day	10% 6% 3 7% 5% 6% 6% 5% 8% 10% 36%	2730
July	1st to 10th	Day	10% 5% 3 7% 6% 6% 6% 5% 7% 9% 36%	4440
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 5% 8% 10% 45%	4440
	21st to end	Day	6% 5% 2 7% 5% 6% 6% 5% 7% 9% 41%	4541
August	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 5% 6% 8% 40%	4104
	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 8% 5% 6% 5% 5% 5% 8% 35%	1699
	11th to 20th	Day	14% 11% 4 9% 7% 5% 5% 3 6% 5% 31%	1710
	21st to end	Day	17% 12% 5% 9% 6% 4 4 3 4 7% 29%	1585
December	1st to 10th	Day	13% 15% 6% 13% 7% 5% 5% 3 5% 6% 22%	1520
	11th to 20th	Day	16% 15% 5% 11% 5% 4 5% 3 7% 6% 23%	1751
	21st to end	Day	24% 22% 6% 12% 6% 4 4 3 3 5% 11%	2691
January	1st to 10th	Night	16% 14% 6% 11% 5% 5% 4 4 4 5% 25%	4428
	11th to 20th	Night	13% 17% 7% 14% 7% 6% 6% 4 6% 6% 15%	4440
	21st to end	Night	22% 16% 9% 12% 5% 4 3 3 3 5% 19%	4590
February	1st to 10th	Night	14% 16% 5% 10% 7% 6% 5% 4 5% 6% 23%	2470
	11th to 20th	Night	12% 13% 5% 10% 8% 6% 5% 4 5% 6% 27%	2279
	21st to end	Night	17% 12% 4 10% 5% 4 3 4 5% 6% 29%	1884
March	1st to 10th	Night	14% 14% 4 10% 4 4 4 2 4 6% 35%	3649
	11th to 20th	Night	9% 8% 3 6% 5% 5% 6% 4 6% 7% 41%	3480
	21st to end	Night	15% 8% 3 10% 5% 4 5% 3 4 5% 37%	3817
April	1st to 10th	Night	16% 11% 2 7% 5% 4 3 4 5% 7% 35%	1990
	11th to 20th	Night	18% 8% 2 7% 5% 3 4 3 5% 8% 37%	1715
	21st to end	Night	16% 9% 2 5% 4 4 4 4 5% 8% 38%	1710
May	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 6% 8% 37%	2813
	11th to 20th	Night	14% 7% 3 7% 4 3 4 4 5% 7% 42%	2812
	21st to end	Night	10% 6% 2 5% 4 4 4 4 5% 8% 49%	3079
June	1st to 10th	Night	11% 6% 2 5% 5% 3 4 4 6% 8% 47%	1408
	11th to 20th	Night	10% 5% 2 7% 4 4 4 5% 7% 8% 45%	1330
	21st to end	Night	9% 6% 3 5% 4 5% 4 3 5% 8% 48%	1440
July	1st to 10th	Night	8% 4 2 5% 4 4 4 4 7% 10% 48%	2530
	11th to 20th	Night	9% 4 1 4 3 3 3 3 6% 9% 54%	2530
	21st to end	Night	5% 4 2 6% 4 6% 6% 5% 7% 10% 46%	3013
August	1st to 10th	Night	10% 6% 3 6% 3 4 4 4 6% 9% 46%	2825
	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% 8% 3 6% 5% 4 4 3 5% 7% 43%	2452
	11th to 20th	Night	15% 10% 3 9% 6% 5% 4 5% 6% 6% 31%	2470
	21st to end	Night	17% 14% 5% 8% 4 4 4 4 5% 6% 28%	2590
December	1st to 10th	Night	16% 15% 6% 12% 6% 5% 4 4 5% 5% 22%	2660
	11th to 20th	Night	13% 16% 5% 11% 6% 5% 5% 4 5% 6% 24%	3441
	21st to end	Night	26% 24% 6% 10% 5% 4 4 3 3 4 12%	4971

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% 8% 10% 12% 5% 1	2528
	11th to 20th	Day	69% 11% 11% 5% 3	2530
	21st to end	Day	74% 6% 10% 6% 3 1	3066
February	1st to 10th	Day	63% 7% 9% 13% 7%	1710
	11th to 20th	Day	60% 7% 9% 15% 8% 1	1793
	21st to end	Day	58% 8% 11% 15% 7% 2	1570
March	1st to 10th	Day	58% 6% 13% 17% 6% 1	3203
	11th to 20th	Day	50% 9% 13% 19% 7% 2	3480
	21st to end	Day	55% 8% 12% 13% 9% 2	3815
April	1st to 10th	Day	51% 8% 11% 18% 10% 1	2206
	11th to 20th	Day	48% 8% 11% 19% 12% 2	2413
	21st to end	Day	48% 7% 14% 16% 13% 1	2470
May	1st to 10th	Day	44% 10% 12% 21% 10% 2	4120
	11th to 20th	Day	43% 10% 16% 22% 9% 1	4085
	21st to end	Day	35% 8% 12% 17% 23% 5%	4489
June	1st to 10th	Day	38% 8% 14% 24% 14% 2	2758
	11th to 20th	Day	38% 8% 13% 23% 16% 2	2850
	21st to end	Day	40% 9% 13% 21% 15% 2	2730
July	1st to 10th	Day	39% 9% 15% 20% 15% 2	4371
	11th to 20th	Day	29% 8% 15% 32% 15% 1	4365
	21st to end	Day	35% 10% 15% 17% 21% 2	4397
August	1st to 10th	Day	37% 10% 13% 24% 15% 1	4091
	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	45% 7% 14% 22% 11% 1	1699
	11th to 20th	Day	53% 7% 13% 17% 9% 1	1710
	21st to end	Day	55% 6% 14% 17% 7% 1	1585
December	1st to 10th	Day	63% 7% 11% 15% 4	1520
	11th to 20th	Day	59% 8% 14% 14% 5%	1747
	21st to end	Day	75% 7% 9% 6% 3	2691
January	1st to 10th	Night	59% 8% 12% 14% 5% 1	4428
	11th to 20th	Night	67% 9% 11% 9% 4	4440
	21st to end	Night	69% 6% 9% 12% 3	4590
February	1st to 10th	Night	61% 8% 10% 14% 7% 1	2470
	11th to 20th	Night	56% 7% 11% 17% 9%	2280
	21st to end	Night	54% 6% 13% 17% 8% 1	1884
March	1st to 10th	Night	50% 6% 13% 23% 7% 1	3649
	11th to 20th	Night	37% 9% 15% 21% 17% 1	3480
	21st to end	Night	47% 8% 12% 22% 11% 1	3817
April	1st to 10th	Night	49% 5% 11% 21% 12% 2	1990
	11th to 20th	Night	44% 7% 12% 17% 17% 2	1715
	21st to end	Night	43% 7% 12% 19% 16% 3	1710
May	1st to 10th	Night	44% 8% 11% 21% 13% 4	2840
	11th to 20th	Night	39% 7% 14% 25% 12% 2	2813
	21st to end	Night	31% 10% 12% 21% 20% 6%	3087
June	1st to 10th	Night	34% 7% 12% 28% 17% 2	1408
	11th to 20th	Night	33% 9% 14% 25% 17% 2	1330
	21st to end	Night	33% 7% 13% 26% 20% 1	1440
July	1st to 10th	Night	29% 8% 13% 29% 15% 6%	2490
	11th to 20th	Night	27% 6% 14% 33% 18% 2	2482
	21st to end	Night	30% 11% 18% 19% 20% 2	2917
August	1st to 10th	Night	33% 8% 14% 28% 16% 1	2814
	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	2452
	11th to 20th	Night	50% 8% 14% 19% 8% 1	2470
	21st to end	Night	55% 6% 13% 19% 6%	2590
December	1st to 10th	Night	63% 7% 11% 14% 5%	2660
	11th to 20th	Night	58% 10% 13% 14% 6% 1	3444
	21st to end	Night	75% 7% 9% 6% 2	4971

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	23% 21% 38% 12% 5%	270
	11th to 20th	Day	22% 16% 46% 13% 1 1	270
	21st to end	Day	29% 15% 34% 17% 4 1	297
February	1st to 10th	Day	23% 23% 36% 13% 3 1	270
	11th to 20th	Day	23% 21% 39% 17% 1	270
	21st to end	Day	24% 29% 35% 10% 2	225
March	1st to 10th	Day	27% 21% 33% 14% 4	314
	11th to 20th	Day	24% 13% 35% 22% 5% 1	460
	21st to end	Day	27% 15% 34% 19% 4 1	506
April	1st to 10th	Day	17% 19% 37% 19% 7% 1	460
	11th to 20th	Day	26% 19% 30% 15% 7% 4	452
	21st to end	Day	24% 16% 30% 19% 9% 2	460
May	1st to 10th	Day	22% 15% 28% 21% 11% 2	463
	11th to 20th	Day	21% 9% 29% 23% 13% 6%	487
	21st to end	Day	15% 7% 25% 28% 17% 8%	529
June	1st to 10th	Day	12% 10% 31% 29% 15% 3	674
	11th to 20th	Day	12% 9% 33% 26% 17% 3	680
	21st to end	Day	13% 9% 31% 29% 15% 4	680
July	1st to 10th	Day	9% 6% 22% 32% 20% 11%	671
	11th to 20th	Day	9% 6% 30% 28% 19% 7%	668
	21st to end	Day	8% 5% 25% 31% 22% 9%	581
August	1st to 10th	Day	9% 4 24% 25% 25% 12%	486
	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	15% 21% 37% 17% 8% 1	279
	11th to 20th	Day	21% 23% 34% 16% 5% 1	270
	21st to end	Day	23% 19% 40% 14% 4	210
December	1st to 10th	Day	22% 27% 32% 13% 7%	180
	11th to 20th	Day	26% 27% 34% 9% 3 1	180
	21st to end	Day	24% 27% 36% 13% 1	272
January	1st to 10th	Night	21% 15% 38% 20% 5% 1	950
	11th to 20th	Night	24% 14% 36% 22% 3 1	950
	21st to end	Night	19% 13% 35% 26% 5%	868
February	1st to 10th	Night	23% 16% 37% 22% 2	760
	11th to 20th	Night	21% 15% 36% 24% 4	743
	21st to end	Night	25% 17% 38% 18% 2	628
March	1st to 10th	Night	20% 14% 36% 25% 5%	760
	11th to 20th	Night	17% 16% 36% 25% 5% 1	760
	21st to end	Night	21% 13% 31% 27% 7% 1	836
April	1st to 10th	Night	20% 15% 35% 26% 4	660
	11th to 20th	Night	22% 12% 34% 24% 6% 2	563
	21st to end	Night	19% 10% 33% 30% 8%	570
May	1st to 10th	Night	18% 9% 31% 32% 9% 1	570
	11th to 20th	Night	21% 4 28% 34% 12% 1	561
	21st to end	Night	11% 4 25% 41% 18% 2	617
June	1st to 10th	Night	12% 4 27% 43% 13%	456
	11th to 20th	Night	10% 5% 26% 48% 11%	380
	21st to end	Night	10% 5% 23% 45% 16% 1	490
July	1st to 10th	Night	8% 3 20% 40% 26% 3	555
	11th to 20th	Night	9% 3 22% 40% 22% 4	552
	21st to end	Night	5% 2 19% 49% 22% 2	624
August	1st to 10th	Night	7% 3 18% 41% 27% 3	567
	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% 27% 12% 1	755
	11th to 20th	Night	21% 16% 35% 21% 6% 1	760
	21st to end	Night	24% 15% 36% 21% 5%	760
December	1st to 10th	Night	23% 16% 36% 19% 6%	760
	11th to 20th	Night	22% 15% 39% 21% 3	869
	21st to end	Night	30% 14% 37% 17% 2	1045

Page generated at 2024-11-01 22:47:49. Version ac07

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