Notre Dame Regional High School Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Notre Dame Regional High School Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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% 6% 2 6% 4 4 4 4 5% 7% 44%	131982
	Night	14% 6% 2 6% 4 4 4 3 5% 6% 46%	131400
Transparency	Day	37% 6% 11% 25% 17% 3	135602
	Night	37% 6% 11% 24% 19% 4	135858
Seeing	Day	15% 10% 25% 26% 17% 7%	17582
	Night	16% 8% 28% 31% 15% 2	27436

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% 9% 3 8% 4 4 4 3 5% 6% 40%	4691
	11th to 20th	Day	10% 6% 2 7% 4 4 4 3 5% 6% 50%	5107
	21st to end	Day	18% 9% 2 6% 4 3 3 3 4 5% 42%	5812
February	1st to 10th	Day	17% 10% 3 5% 4 3 4 3 4 6% 39%	1980
	11th to 20th	Day	18% 8% 4 6% 5% 4 4 4 4 6% 38%	2048
	21st to end	Day	16% 10% 2 6% 4 4 4 4 6% 5% 38%	1730
March	1st to 10th	Day	18% 10% 2 6% 4 4 4 3 4 4 41%	4580
	11th to 20th	Day	11% 6% 2 5% 3 3 4 3 5% 6% 53%	5032
	21st to end	Day	14% 6% 3 7% 4 4 4 3 5% 7% 42%	5990
April	1st to 10th	Day	15% 5% 2 5% 4 3 5% 3 6% 8% 44%	2520
	11th to 20th	Day	16% 7% 3 7% 5% 4 4 3 4 6% 42%	2494
	21st to end	Day	16% 7% 3 6% 4 4 4 4 5% 8% 38%	2520
May	1st to 10th	Day	16% 7% 2 8% 6% 5% 5% 5% 7% 9% 30%	5699
	11th to 20th	Day	17% 7% 3 8% 5% 4 5% 4 6% 8% 34%	6037
	21st to end	Day	15% 7% 2 7% 5% 4 5% 4 5% 7% 39%	7089
June	1st to 10th	Day	10% 5% 2 5% 4 4 6% 4 7% 9% 45%	2801
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 4 7% 8% 50%	2800
	21st to end	Day	5% 3 1 5% 4 5% 5% 4 9% 10% 48%	2800
July	1st to 10th	Day	8% 4 2 6% 5% 5% 5% 4 6% 9% 46%	6360
	11th to 20th	Day	6% 4 2 5% 4 5% 6% 4 7% 12% 46%	6360
	21st to end	Day	7% 5% 2 7% 5% 5% 5% 5% 7% 11% 40%	6572
August	1st to 10th	Day	4 3 1 4 4 4 5% 4 6% 10% 56%	4040
	11th to 20th	Day	11% 5% 2 5% 4 3 4 4 6% 8% 47%	3849
	21st to end	Day	5% 3 1 4 3 4 4 4 5% 8% 58%	4312
September	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 6% 8% 49%	2400
	11th to 20th	Day	9% 5% 2 5% 5% 4 4 4 5% 7% 50%	2329
	21st to end	Day	12% 4 2 5% 4 4 3 2 5% 7% 52%	2200
October	1st to 10th	Day	8% 3 1 3 3 3 3 3 5% 6% 61%	2081
	11th to 20th	Day	12% 5% 2 5% 4 3 3 3 4 6% 54%	2000
	21st to end	Day	19% 6% 2 4 4 3 3 3 4 5% 47%	2160
November	1st to 10th	Day	11% 6% 2 5% 5% 3 4 3 4 5% 52%	1852
	11th to 20th	Day	14% 8% 2 5% 4 3 3 3 4 7% 47%	1890
	21st to end	Day	20% 8% 2 6% 3 2 2 3 4 5% 43%	1890
December	1st to 10th	Day	11% 8% 2 5% 3 4 4 3 4 5% 51%	2799
	11th to 20th	Day	16% 9% 2 7% 4 4 4 3 4 6% 41%	3088
	21st to end	Day	26% 14% 4 7% 4 3 4 2 4 5% 27%	4070
January	1st to 10th	Night	18% 9% 3 7% 4 3 4 3 4 6% 39%	8232
	11th to 20th	Night	11% 8% 3 7% 4 4 4 3 5% 5% 47%	7878
	21st to end	Night	18% 7% 3 6% 4 4 4 3 5% 5% 41%	8402
February	1st to 10th	Night	19% 10% 4 7% 4 4 3 3 4 5% 37%	2637
	11th to 20th	Night	18% 8% 3 7% 4 3 3 2 4 6% 42%	2458
	21st to end	Night	18% 8% 3 6% 4 3 4 3 4 5% 42%	2076
March	1st to 10th	Night	18% 9% 3 7% 5% 4 4 3 4 4 38%	5493
	11th to 20th	Night	13% 6% 2 6% 4 4 3 3 4 6% 49%	5190
	21st to end	Night	14% 6% 2 7% 5% 5% 4 4 5% 7% 42%	5179
April	1st to 10th	Night	12% 6% 2 4 3 4 4 4 6% 7% 48%	2100
	11th to 20th	Night	15% 5% 2 5% 4 4 4 3 6% 7% 44%	2075
	21st to end	Night	20% 6% 2 5% 4 4 4 4 5% 6% 40%	2100
May	1st to 10th	Night	17% 7% 2 7% 5% 5% 5% 4 6% 8% 35%	4413
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 3 5% 7% 38%	4062
	21st to end	Night	16% 6% 3 7% 5% 4 5% 3 6% 7% 38%	4030
June	1st to 10th	Night	9% 5% 2 6% 4 5% 4 4 7% 9% 46%	1600
	11th to 20th	Night	5% 3 1 6% 3 4 4 3 5% 10% 54%	1600
	21st to end	Night	4 2 1 4 3 4 4 5% 9% 11% 54%	1600
July	1st to 10th	Night	9% 4 2 5% 4 4 4 3 6% 8% 51%	3618
	11th to 20th	Night	5% 4 2 6% 4 5% 5% 5% 7% 8% 49%	3617
	21st to end	Night	6% 6% 2 6% 3 4 4 4 7% 12% 47%	4281
August	1st to 10th	Night	4 2 1 4 3 3 4 3 5% 8% 62%	3112
	11th to 20th	Night	8% 4 2 6% 3 4 4 4 5% 8% 53%	3200
	21st to end	Night	3 3 1 4 3 3 3 4 6% 9% 61%	3586
September	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 5% 6% 54%	2000
	11th to 20th	Night	8% 4 2 5% 4 4 3 4 5% 7% 53%	2050
	21st to end	Night	12% 4 2 4 3 4 4 3 5% 6% 55%	2200
October	1st to 10th	Night	7% 3 2 4 3 3 3 3 4 6% 63%	2315
	11th to 20th	Night	12% 4 2 4 3 3 3 3 4 5% 57%	2400
	21st to end	Night	18% 6% 2 5% 3 2 3 2 4 5% 49%	2679
November	1st to 10th	Night	12% 5% 2 5% 3 2 2 3 5% 6% 56%	2666
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 4 7% 52%	2730
	21st to end	Night	19% 7% 2 5% 3 2 3 2 4 5% 47%	2730
December	1st to 10th	Night	16% 7% 2 6% 3 3 4 3 4 5% 47%	4602
	11th to 20th	Night	18% 7% 2 5% 4 3 4 3 3 5% 45%	5339
	21st to end	Night	25% 11% 3 7% 4 3 3 3 4 5% 32%	7150

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	45% 4 15% 26% 9% 2	4692
	11th to 20th	Day	35% 4 7% 30% 19% 4	5107
	21st to end	Day	45% 4 11% 20% 12% 8%	5812
February	1st to 10th	Day	46% 5% 13% 21% 13% 1	1980
	11th to 20th	Day	47% 5% 10% 20% 13% 5%	2058
	21st to end	Day	45% 6% 9% 21% 12% 5%	1730
March	1st to 10th	Day	47% 5% 5% 21% 17% 6%	4580
	11th to 20th	Day	33% 4 9% 28% 20% 6%	5032
	21st to end	Day	41% 6% 13% 29% 12%	5992
April	1st to 10th	Day	36% 7% 12% 25% 18% 2	2520
	11th to 20th	Day	43% 6% 12% 20% 16% 4	2494
	21st to end	Day	43% 7% 11% 24% 13% 1	2520
May	1st to 10th	Day	47% 10% 12% 19% 11% 1	5734
	11th to 20th	Day	46% 9% 12% 20% 11% 1	6041
	21st to end	Day	42% 8% 10% 20% 18% 3	7103
June	1st to 10th	Day	33% 8% 14% 24% 19% 2	2801
	11th to 20th	Day	29% 8% 12% 26% 21% 3	2800
	21st to end	Day	26% 10% 18% 24% 17% 5%	2800
July	1st to 10th	Day	33% 8% 12% 26% 19% 3	6291
	11th to 20th	Day	29% 10% 17% 31% 12% 2	6285
	21st to end	Day	36% 9% 17% 29% 8% 1	6386
August	1st to 10th	Day	22% 7% 15% 32% 23% 1	4026
	11th to 20th	Day	34% 7% 12% 27% 18% 2	3885
	21st to end	Day	24% 6% 13% 36% 18% 3	4311
September	1st to 10th	Day	33% 6% 12% 25% 20% 4	2400
	11th to 20th	Day	33% 6% 10% 22% 26% 2	2329
	21st to end	Day	32% 5% 8% 25% 27% 4	2200
October	1st to 10th	Day	15% 3 6% 20% 44% 12%	3393
	11th to 20th	Day	22% 3 7% 22% 36% 10%	3240
	21st to end	Day	31% 4 5% 27% 28% 5%	3474
November	1st to 10th	Day	34% 5% 9% 26% 20% 6%	1852
	11th to 20th	Day	37% 4 9% 23% 22% 5%	1890
	21st to end	Day	44% 4 8% 23% 19% 2	1890
December	1st to 10th	Day	36% 4 8% 35% 16% 1	2799
	11th to 20th	Day	44% 5% 10% 24% 15% 2	3085
	21st to end	Day	59% 6% 11% 15% 8% 1	4070
January	1st to 10th	Night	45% 5% 13% 26% 10% 1	8233
	11th to 20th	Night	37% 5% 8% 25% 21% 3	7878
	21st to end	Night	44% 5% 11% 22% 15% 3	8402
February	1st to 10th	Night	49% 4 11% 21% 13% 2	2637
	11th to 20th	Night	44% 4 9% 22% 17% 4	2459
	21st to end	Night	44% 5% 9% 20% 17% 4	2076
March	1st to 10th	Night	48% 5% 7% 17% 17% 6%	5493
	11th to 20th	Night	36% 6% 9% 21% 19% 10%	5190
	21st to end	Night	40% 7% 12% 27% 13% 1	5179
April	1st to 10th	Night	34% 6% 13% 25% 19% 4	2100
	11th to 20th	Night	38% 6% 11% 21% 19% 5%	2075
	21st to end	Night	43% 7% 9% 24% 16% 2	2100
May	1st to 10th	Night	44% 9% 16% 21% 9% 1	4443
	11th to 20th	Night	45% 8% 14% 19% 13% 2	4063
	21st to end	Night	42% 8% 11% 21% 14% 3	4038
June	1st to 10th	Night	32% 10% 13% 24% 20% 2	1600
	11th to 20th	Night	24% 8% 13% 25% 24% 6%	1600
	21st to end	Night	20% 10% 17% 27% 18% 8%	1600
July	1st to 10th	Night	32% 7% 11% 23% 24% 3	3580
	11th to 20th	Night	30% 9% 15% 30% 13% 3	3572
	21st to end	Night	29% 7% 23% 25% 15% 1	4156
August	1st to 10th	Night	19% 7% 18% 34% 21% 1	3102
	11th to 20th	Night	29% 6% 16% 27% 18% 5%	3230
	21st to end	Night	21% 6% 11% 36% 24% 3	3586
September	1st to 10th	Night	30% 6% 9% 27% 21% 7%	2000
	11th to 20th	Night	29% 6% 10% 26% 24% 4	2050
	21st to end	Night	31% 6% 10% 20% 27% 7%	2200
October	1st to 10th	Night	15% 3 6% 16% 43% 17%	3783
	11th to 20th	Night	21% 3 6% 18% 42% 10%	3899
	21st to end	Night	30% 3 6% 24% 29% 8%	4315
November	1st to 10th	Night	29% 4 10% 27% 24% 6%	2666
	11th to 20th	Night	33% 4 8% 25% 22% 7%	2730
	21st to end	Night	40% 4 9% 24% 21% 2	2730
December	1st to 10th	Night	40% 4 9% 28% 17% 1	4602
	11th to 20th	Night	41% 5% 12% 21% 20% 1	5341
	21st to end	Night	54% 6% 11% 19% 9% 1	7150

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	25% 14% 35% 21% 4	330
	11th to 20th	Day	21% 20% 43% 15% 1	330
	21st to end	Day	22% 18% 41% 17% 1	363
February	1st to 10th	Day	27% 14% 41% 13% 5%	330
	11th to 20th	Day	23% 25% 37% 12% 2	330
	21st to end	Day	25% 23% 33% 16% 3	273
March	1st to 10th	Day	20% 21% 38% 15% 5% 1	330
	11th to 20th	Day	22% 24% 37% 15% 2	330
	21st to end	Day	24% 16% 37% 18% 5%	537
April	1st to 10th	Day	19% 15% 33% 25% 8%	540
	11th to 20th	Day	21% 21% 28% 20% 9% 1	532
	21st to end	Day	18% 15% 31% 25% 9% 1	540
May	1st to 10th	Day	17% 15% 33% 23% 10% 2	543
	11th to 20th	Day	16% 9% 29% 25% 15% 6%	587
	21st to end	Day	14% 6% 26% 31% 17% 6%	844
June	1st to 10th	Day	11% 6% 21% 24% 26% 12%	731
	11th to 20th	Day	8% 2 17% 39% 24% 10%	730
	21st to end	Day	6% 3 22% 37% 22% 10%	730
July	1st to 10th	Day	9% 3 14% 27% 29% 18%	721
	11th to 20th	Day	6% 3 14% 33% 30% 13%	718
	21st to end	Day	8% 3 13% 31% 31% 14%	794
August	1st to 10th	Day	7% 1 12% 35% 31% 14%	610
	11th to 20th	Day	13% 2 15% 28% 27% 15%	519
	21st to end	Day	9% 1 9% 29% 35% 17%	583
September	1st to 10th	Day	8% 5% 15% 29% 34% 10%	530
	11th to 20th	Day	9% 2 9% 30% 35% 15%	530
	21st to end	Day	13% 8% 19% 26% 25% 9%	530
October	1st to 10th	Day	9% 5% 20% 38% 19% 9%	411
	11th to 20th	Day	16% 11% 31% 23% 15% 4	330
	21st to end	Day	26% 12% 32% 21% 8%	363
November	1st to 10th	Day	14% 13% 33% 26% 11% 3	330
	11th to 20th	Day	12% 20% 33% 24% 11% 1	330
	21st to end	Day	24% 14% 30% 23% 7% 1	330
December	1st to 10th	Day	14% 16% 38% 22% 7% 2	330
	11th to 20th	Day	23% 18% 29% 22% 7%	330
	21st to end	Day	27% 16% 36% 17% 4	363
January	1st to 10th	Night	22% 14% 35% 25% 4	1050
	11th to 20th	Night	23% 14% 39% 20% 3	1050
	21st to end	Night	19% 11% 42% 25% 2	1155
February	1st to 10th	Night	23% 13% 38% 24% 2	957
	11th to 20th	Night	23% 16% 37% 22% 2	823
	21st to end	Night	23% 15% 38% 23% 2	692
March	1st to 10th	Night	22% 16% 35% 25% 4	840
	11th to 20th	Night	19% 14% 36% 26% 5%	678
	21st to end	Night	21% 12% 33% 27% 7%	693
April	1st to 10th	Night	14% 11% 40% 29% 5%	630
	11th to 20th	Night	17% 13% 30% 28% 10% 1	623
	21st to end	Night	20% 8% 33% 30% 9%	630
May	1st to 10th	Night	17% 5% 31% 35% 10% 1	630
	11th to 20th	Night	15% 6% 26% 35% 16% 2	621
	21st to end	Night	12% 2 23% 40% 20% 3	683
June	1st to 10th	Night	11% 4 15% 39% 29% 3	600
	11th to 20th	Night	5% 2 15% 49% 28% 2	600
	21st to end	Night	4 1 17% 42% 33% 3	600
July	1st to 10th	Night	9% 1 9% 33% 39% 9%	585
	11th to 20th	Night	5% 2 10% 38% 38% 7%	582
	21st to end	Night	7% 2 10% 37% 39% 5%	657
August	1st to 10th	Night	6% 2 8% 40% 38% 5%	597
	11th to 20th	Night	8% 1 10% 34% 41% 5%	594
	21st to end	Night	7% 1 8% 34% 44% 6%	660
September	1st to 10th	Night	11% 2 11% 37% 34% 5%	600
	11th to 20th	Night	10% 1 13% 32% 38% 5%	650
	21st to end	Night	12% 4 20% 33% 28% 3	800
October	1st to 10th	Night	7% 5% 20% 39% 26% 4	800
	11th to 20th	Night	14% 8% 29% 30% 19% 2	800
	21st to end	Night	20% 9% 30% 31% 10% 1	880
November	1st to 10th	Night	14% 8% 31% 34% 13%	820
	11th to 20th	Night	16% 8% 35% 31% 10%	840
	21st to end	Night	22% 10% 30% 29% 8%	840
December	1st to 10th	Night	21% 11% 35% 28% 5%	971
	11th to 20th	Night	22% 10% 33% 29% 6%	1050
	21st to end	Night	25% 13% 34% 24% 4	1155

Page generated at 2026-05-29 00:53:28. Version ac07

Copyright © 2026 Attilla Danko.