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 2024-01-27.

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% 3 5% 5% 3 5% 5% 5% 7% 43%	74885
	Night	14% 6% 2 5% 4 3 4 4 5% 7% 46%	75172
Transparency	Day	39% 7% 12% 23% 16% 3	74594
	Night	37% 6% 11% 23% 18% 4	74984
Seeing	Day	15% 9% 25% 26% 18% 7%	14386
	Night	16% 8% 27% 31% 16% 2	24337

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	19% 9% 3 6% 4 3 4 3 4 6% 40%	1517
	11th to 20th	Day	20% 8% 3 6% 5% 3 4 4 4 5% 37%	1643
	21st to end	Day	16% 9% 2 7% 5% 3 3 4 4 5% 42%	1836
February	1st to 10th	Day	18% 9% 4 5% 5% 2 4 4 5% 5% 38%	1696
	11th to 20th	Day	18% 7% 4 5% 6% 3 4 5% 5% 6% 37%	1748
	21st to end	Day	18% 10% 3 5% 5% 3 4 4 5% 6% 36%	1480
March	1st to 10th	Day	18% 7% 2 5% 5% 3 5% 4 4 5% 42%	1800
	11th to 20th	Day	20% 8% 4 6% 5% 3 5% 4 4 6% 35%	1952
	21st to end	Day	21% 8% 4 6% 6% 3 4 4 5% 6% 33%	2326
April	1st to 10th	Day	13% 5% 2 4 4 3 4 5% 6% 8% 46%	2160
	11th to 20th	Day	17% 7% 3 6% 5% 3 4 4 4 6% 41%	2134
	21st to end	Day	18% 7% 3 6% 5% 3 4 5% 5% 7% 39%	2160
May	1st to 10th	Day	18% 5% 2 6% 5% 3 4 6% 6% 8% 38%	2219
	11th to 20th	Day	16% 5% 3 5% 5% 3 4 4 5% 8% 41%	2318
	21st to end	Day	10% 6% 3 6% 6% 4 7% 6% 6% 10% 36%	2725
June	1st to 10th	Day	9% 4 2 4 5% 3 6% 5% 6% 9% 46%	2520
	11th to 20th	Day	5% 3 2 4 5% 4 5% 6% 7% 8% 51%	2520
	21st to end	Day	5% 3 1 5% 5% 3 5% 5% 8% 10% 48%	2520
July	1st to 10th	Day	8% 5% 2 4 6% 3 6% 6% 7% 10% 41%	2520
	11th to 20th	Day	5% 3 2 4 5% 2 6% 6% 7% 11% 48%	2520
	21st to end	Day	6% 3 2 5% 5% 4 5% 6% 7% 10% 46%	2612
August	1st to 10th	Day	7% 4 2 5% 6% 3 6% 6% 7% 11% 43%	2224
	11th to 20th	Day	10% 4 2 5% 5% 3 5% 6% 7% 8% 44%	2125
	21st to end	Day	7% 4 2 5% 4 3 6% 6% 6% 8% 49%	2376
September	1st to 10th	Day	10% 5% 2 5% 4 3 4 4 6% 9% 48%	2160
	11th to 20th	Day	9% 5% 3 5% 5% 3 5% 4 5% 7% 50%	2097
	21st to end	Day	11% 4 2 4 4 2 3 3 5% 7% 54%	1980
October	1st to 10th	Day	9% 3 2 3 3 3 3 4 4 7% 59%	1873
	11th to 20th	Day	14% 5% 2 4 5% 2 3 4 4 6% 52%	1800
	21st to end	Day	19% 6% 2 4 4 2 3 3 4 5% 46%	1944
November	1st to 10th	Day	11% 5% 2 4 4 2 4 3 4 5% 54%	1672
	11th to 20th	Day	14% 8% 3 5% 4 3 3 3 4 6% 48%	1710
	21st to end	Day	20% 8% 2 5% 4 2 3 4 4 5% 43%	1710
December	1st to 10th	Day	16% 8% 2 5% 4 3 4 5% 4 7% 41%	1587
	11th to 20th	Day	18% 8% 3 6% 5% 4 5% 5% 5% 6% 38%	1918
	21st to end	Day	22% 12% 3 6% 5% 3 5% 3 4 6% 31%	2783
January	1st to 10th	Night	19% 7% 3 5% 4 2 3 4 4 6% 42%	2648
	11th to 20th	Night	20% 8% 3 6% 4 3 5% 3 5% 6% 38%	2532
	21st to end	Night	17% 7% 3 5% 5% 3 4 4 4 5% 42%	2665
February	1st to 10th	Night	20% 10% 5% 6% 5% 3 4 3 4 5% 36%	2261
	11th to 20th	Night	18% 7% 3 6% 4 3 3 3 4 6% 43%	2098
	21st to end	Night	20% 8% 4 5% 4 2 4 4 4 5% 39%	1776
March	1st to 10th	Night	17% 7% 3 5% 5% 2 4 4 5% 4 44%	2160
	11th to 20th	Night	19% 8% 3 5% 5% 3 3 4 4 5% 41%	2022
	21st to end	Night	20% 7% 2 6% 5% 3 3 5% 5% 6% 37%	2028
April	1st to 10th	Night	11% 6% 2 4 3 3 4 4 5% 8% 49%	1800
	11th to 20th	Night	15% 5% 2 5% 4 3 4 4 6% 8% 44%	1775
	21st to end	Night	20% 6% 2 4 4 3 4 5% 6% 6% 40%	1800
May	1st to 10th	Night	17% 5% 1 4 4 3 5% 4 4 7% 46%	1738
	11th to 20th	Night	14% 6% 4 5% 6% 3 4 3 5% 6% 44%	1572
	21st to end	Night	7% 5% 2 6% 7% 4 6% 5% 6% 10% 41%	1551
June	1st to 10th	Night	9% 5% 2 5% 5% 3 4 4 7% 9% 47%	1440
	11th to 20th	Night	4 2 2 4 4 3 4 4 5% 10% 57%	1440
	21st to end	Night	5% 1 1 3 4 3 4 6% 9% 12% 53%	1440
July	1st to 10th	Night	7% 2 1 4 5% 3 5% 4 7% 9% 53%	1440
	11th to 20th	Night	6% 3 2 4 4 3 5% 4 6% 8% 54%	1440
	21st to end	Night	5% 4 2 4 4 3 4 5% 7% 10% 52%	1702
August	1st to 10th	Night	7% 3 1 4 4 3 4 6% 7% 10% 51%	1713
	11th to 20th	Night	8% 4 2 4 4 2 5% 5% 5% 8% 54%	1776
	21st to end	Night	6% 4 1 5% 4 3 4 5% 7% 9% 53%	1980
September	1st to 10th	Night	10% 5% 3 4 5% 2 3 3 5% 6% 54%	1800
	11th to 20th	Night	8% 4 2 4 5% 3 4 4 5% 7% 54%	1844
	21st to end	Night	10% 3 2 3 4 3 3 3 5% 6% 57%	1980
October	1st to 10th	Night	7% 3 2 4 4 3 4 4 4 6% 61%	2083
	11th to 20th	Night	13% 4 2 4 3 2 3 4 4 5% 55%	2160
	21st to end	Night	18% 6% 2 4 4 2 3 3 4 5% 48%	2411
November	1st to 10th	Night	12% 5% 2 4 3 2 2 3 5% 6% 58%	2406
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 6% 52%	2470
	21st to end	Night	20% 6% 2 5% 3 1 3 3 4 5% 48%	2470
December	1st to 10th	Night	19% 7% 3 5% 4 3 4 4 5% 7% 41%	2588
	11th to 20th	Night	19% 6% 3 5% 5% 2 4 4 3 5% 43%	3279
	21st to end	Night	18% 11% 4 8% 7% 3 4 4 4 5% 33%	4884

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	47% 4 10% 23% 13% 3	1518
	11th to 20th	Day	48% 6% 10% 24% 11% 1	1643
	21st to end	Day	45% 5% 12% 23% 12% 2	1836
February	1st to 10th	Day	47% 6% 12% 21% 13% 1	1696
	11th to 20th	Day	47% 5% 11% 20% 12% 4	1758
	21st to end	Day	48% 6% 10% 20% 11% 5%	1480
March	1st to 10th	Day	44% 6% 7% 24% 16% 4	1800
	11th to 20th	Day	51% 6% 11% 17% 12% 3	1952
	21st to end	Day	52% 6% 12% 19% 9% 1	2326
April	1st to 10th	Day	35% 6% 12% 27% 18% 1	2160
	11th to 20th	Day	44% 5% 11% 20% 16% 4	2134
	21st to end	Day	44% 7% 11% 23% 14% 1	2160
May	1st to 10th	Day	44% 9% 12% 23% 11% 1	2219
	11th to 20th	Day	41% 8% 14% 20% 15% 2	2322
	21st to end	Day	41% 10% 16% 20% 11% 2	2725
June	1st to 10th	Day	33% 8% 15% 24% 20% 1	2520
	11th to 20th	Day	28% 9% 13% 27% 21% 3	2520
	21st to end	Day	26% 11% 17% 24% 17% 5%	2520
July	1st to 10th	Day	34% 11% 16% 22% 14% 4	2451
	11th to 20th	Day	28% 10% 15% 27% 16% 4	2445
	21st to end	Day	31% 11% 15% 30% 12% 2	2467
August	1st to 10th	Day	32% 11% 15% 26% 15% 1	2210
	11th to 20th	Day	35% 10% 13% 26% 14% 2	2125
	21st to end	Day	30% 9% 16% 23% 17% 5%	2376
September	1st to 10th	Day	34% 7% 12% 26% 18% 4	2160
	11th to 20th	Day	33% 7% 10% 22% 25% 2	2097
	21st to end	Day	29% 5% 9% 27% 26% 4	1980
October	1st to 10th	Day	26% 5% 11% 26% 27% 5%	1873
	11th to 20th	Day	34% 4 8% 24% 21% 7%	1800
	21st to end	Day	40% 6% 7% 25% 18% 4	1944
November	1st to 10th	Day	33% 5% 10% 26% 20% 6%	1672
	11th to 20th	Day	38% 4 9% 22% 22% 5%	1710
	21st to end	Day	44% 5% 8% 23% 19% 2	1710
December	1st to 10th	Day	43% 6% 11% 24% 15% 2	1587
	11th to 20th	Day	46% 6% 9% 23% 14% 1	1915
	21st to end	Day	53% 8% 11% 16% 11% 2	2783
January	1st to 10th	Night	42% 6% 10% 24% 16% 2	2648
	11th to 20th	Night	47% 6% 10% 23% 13% 1	2532
	21st to end	Night	44% 4 12% 24% 14% 2	2665
February	1st to 10th	Night	51% 5% 10% 20% 13% 2	2261
	11th to 20th	Night	44% 4 10% 21% 17% 3	2099
	21st to end	Night	47% 6% 10% 19% 15% 4	1776
March	1st to 10th	Night	43% 5% 9% 18% 19% 6%	2160
	11th to 20th	Night	45% 6% 12% 19% 14% 3	2022
	21st to end	Night	48% 6% 12% 21% 11% 2	2028
April	1st to 10th	Night	33% 6% 12% 26% 19% 4	1800
	11th to 20th	Night	37% 6% 11% 21% 20% 5%	1775
	21st to end	Night	43% 7% 9% 24% 16% 3	1800
May	1st to 10th	Night	38% 7% 12% 25% 15% 2	1738
	11th to 20th	Night	42% 7% 12% 20% 17% 2	1573
	21st to end	Night	36% 10% 14% 22% 15% 3	1552
June	1st to 10th	Night	32% 9% 13% 24% 21% 2	1440
	11th to 20th	Night	22% 8% 13% 27% 23% 6%	1440
	21st to end	Night	20% 10% 17% 27% 17% 8%	1440
July	1st to 10th	Night	26% 10% 17% 27% 15% 5%	1400
	11th to 20th	Night	26% 8% 14% 29% 16% 6%	1392
	21st to end	Night	25% 8% 17% 31% 18% 1	1607
August	1st to 10th	Night	25% 10% 18% 29% 16% 2	1703
	11th to 20th	Night	27% 8% 13% 27% 21% 3	1776
	21st to end	Night	26% 8% 15% 26% 19% 6%	1980
September	1st to 10th	Night	31% 7% 9% 28% 19% 6%	1800
	11th to 20th	Night	29% 6% 10% 26% 24% 4	1844
	21st to end	Night	28% 6% 10% 21% 28% 7%	1980
October	1st to 10th	Night	25% 5% 11% 23% 30% 6%	2083
	11th to 20th	Night	31% 4 8% 23% 26% 7%	2160
	21st to end	Night	39% 4 8% 23% 20% 6%	2411
November	1st to 10th	Night	28% 4 10% 26% 25% 6%	2406
	11th to 20th	Night	34% 4 8% 24% 23% 8%	2470
	21st to end	Night	40% 4 9% 23% 22% 2	2470
December	1st to 10th	Night	43% 5% 12% 22% 15% 2	2588
	11th to 20th	Night	42% 7% 10% 22% 18% 2	3281
	21st to end	Night	50% 8% 10% 18% 12% 2	4884

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	27% 14% 34% 20% 4	270
	11th to 20th	Day	24% 18% 42% 14% 1	270
	21st to end	Day	24% 17% 40% 17% 1	282
February	1st to 10th	Day	28% 15% 42% 10% 5%	240
	11th to 20th	Day	25% 23% 38% 12% 3	240
	21st to end	Day	27% 22% 33% 14% 3 1	198
March	1st to 10th	Day	18% 22% 40% 16% 5%	240
	11th to 20th	Day	25% 21% 35% 16% 3	240
	21st to end	Day	25% 17% 33% 19% 6%	412
April	1st to 10th	Day	14% 16% 35% 26% 8%	420
	11th to 20th	Day	21% 17% 29% 22% 10% 1	412
	21st to end	Day	20% 13% 31% 25% 8% 2	420
May	1st to 10th	Day	18% 13% 32% 23% 10% 2	423
	11th to 20th	Day	16% 8% 28% 25% 16% 6%	464
	21st to end	Day	11% 5% 26% 33% 18% 7%	679
June	1st to 10th	Day	10% 6% 20% 25% 27% 13%	630
	11th to 20th	Day	6% 2 18% 41% 24% 10%	630
	21st to end	Day	6% 3 24% 37% 20% 10%	630
July	1st to 10th	Day	9% 2 14% 25% 30% 19%	621
	11th to 20th	Day	6% 3 15% 32% 31% 14%	618
	21st to end	Day	8% 2 13% 33% 31% 14%	684
August	1st to 10th	Day	8% 2 10% 35% 32% 13%	522
	11th to 20th	Day	15% 2 15% 27% 29% 12%	439
	21st to end	Day	9% 1 9% 27% 35% 18%	495
September	1st to 10th	Day	8% 5% 14% 27% 34% 11%	450
	11th to 20th	Day	8% 2 10% 31% 34% 15%	450
	21st to end	Day	12% 7% 18% 27% 26% 10%	450
October	1st to 10th	Day	10% 6% 22% 38% 15% 8%	343
	11th to 20th	Day	18% 11% 35% 22% 11% 3	270
	21st to end	Day	27% 13% 31% 21% 9%	297
November	1st to 10th	Day	12% 12% 33% 26% 13% 3	270
	11th to 20th	Day	12% 20% 32% 24% 11% 1	270
	21st to end	Day	24% 13% 30% 24% 8% 1	270
December	1st to 10th	Day	14% 15% 38% 21% 9% 3	270
	11th to 20th	Day	24% 17% 29% 22% 8%	270
	21st to end	Day	26% 17% 37% 15% 4	297
January	1st to 10th	Night	23% 14% 35% 24% 4	950
	11th to 20th	Night	24% 15% 37% 20% 4	950
	21st to end	Night	20% 11% 41% 26% 2	1025
February	1st to 10th	Night	24% 14% 37% 23% 2	821
	11th to 20th	Night	22% 16% 37% 23% 2	703
	21st to end	Night	25% 15% 36% 23% 2	592
March	1st to 10th	Night	22% 15% 35% 25% 4	720
	11th to 20th	Night	20% 13% 35% 27% 5%	582
	21st to end	Night	23% 10% 32% 28% 7%	594
April	1st to 10th	Night	13% 11% 40% 31% 6%	540
	11th to 20th	Night	17% 12% 30% 29% 12% 1	533
	21st to end	Night	21% 7% 33% 30% 9%	540
May	1st to 10th	Night	17% 5% 30% 36% 12% 1	540
	11th to 20th	Night	15% 6% 26% 35% 17% 2	531
	21st to end	Night	10% 2 22% 41% 23% 3	584
June	1st to 10th	Night	10% 4 14% 38% 31% 3	540
	11th to 20th	Night	4 1 14% 50% 27% 2	540
	21st to end	Night	4 1 17% 43% 32% 2	540
July	1st to 10th	Night	8% 9% 33% 41% 9%	525
	11th to 20th	Night	5% 2 10% 37% 38% 7%	522
	21st to end	Night	6% 2 11% 39% 38% 5%	591
August	1st to 10th	Night	7% 1 8% 40% 38% 6%	537
	11th to 20th	Night	9% 1 10% 35% 41% 4	534
	21st to end	Night	7% 1 9% 32% 44% 7%	594
September	1st to 10th	Night	11% 2 11% 35% 35% 6%	540
	11th to 20th	Night	10% 1 14% 31% 39% 6%	584
	21st to end	Night	10% 4 20% 33% 29% 3	720
October	1st to 10th	Night	7% 5% 21% 38% 25% 4	720
	11th to 20th	Night	15% 8% 30% 30% 16% 2	720
	21st to end	Night	21% 9% 29% 31% 10% 1	792
November	1st to 10th	Night	14% 8% 31% 34% 14%	740
	11th to 20th	Night	16% 8% 35% 30% 11%	760
	21st to end	Night	22% 10% 30% 30% 9%	760
December	1st to 10th	Night	21% 10% 35% 29% 5%	878
	11th to 20th	Night	22% 10% 31% 30% 6%	950
	21st to end	Night	24% 13% 36% 24% 4	1045

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