Shiloh School Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shiloh School Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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 6% 8% 43%	101126
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	97919
Transparency	Day	37% 7% 13% 24% 16% 3	104759
	Night	39% 6% 11% 23% 17% 3	102425
Seeing	Day	15% 7% 23% 30% 20% 5%	15187
	Night	16% 8% 26% 31% 16% 2	24686

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	16% 5% 2 3 3 3 4 3 4 7% 50%	2585
	11th to 20th	Day	12% 8% 3 6% 5% 4 4 4 5% 6% 44%	2840
	21st to end	Day	13% 10% 4 8% 5% 4 4 3 5% 6% 39%	3124
February	1st to 10th	Day	12% 8% 3 6% 5% 4 5% 4 6% 6% 40%	1710
	11th to 20th	Day	19% 5% 2 5% 3 3 4 3 3 7% 45%	1792
	21st to end	Day	16% 8% 3 5% 5% 5% 5% 4 6% 6% 37%	1570
March	1st to 10th	Day	21% 7% 3 7% 4 4 4 4 5% 7% 35%	3300
	11th to 20th	Day	10% 4 1 4 3 3 3 3 5% 7% 56%	3480
	21st to end	Day	19% 8% 2 4 4 3 4 3 5% 6% 41%	3937
April	1st to 10th	Day	12% 6% 2 5% 4 3 4 3 6% 6% 48%	2280
	11th to 20th	Day	9% 6% 2 5% 4 5% 4 3 5% 6% 50%	2254
	21st to end	Day	13% 7% 2 5% 4 4 4 3 4 7% 45%	2280
May	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 10% 31%	3815
	11th to 20th	Day	19% 7% 2 7% 4 4 5% 4 6% 7% 34%	4302
	21st to end	Day	9% 4 1 5% 4 4 4 4 7% 10% 47%	4823
June	1st to 10th	Day	10% 4 1 5% 4 4 4 4 8% 10% 44%	2660
	11th to 20th	Day	9% 4 1 5% 4 5% 5% 5% 7% 11% 44%	2660
	21st to end	Day	7% 4 1 5% 5% 4 6% 5% 7% 11% 46%	2660
July	1st to 10th	Day	5% 5% 2 5% 4 4 6% 5% 8% 13% 43%	4440
	11th to 20th	Day	11% 6% 2 7% 4 5% 5% 5% 7% 11% 37%	4314
	21st to end	Day	9% 5% 3 7% 6% 6% 6% 5% 9% 9% 35%	4466
August	1st to 10th	Day	13% 8% 3 7% 5% 4 5% 5% 8% 11% 32%	4104
	11th to 20th	Day	10% 5% 3 7% 6% 6% 5% 4 6% 9% 39%	3735
	21st to end	Day	10% 3 1 4 3 4 4 3 7% 10% 51%	4180
September	1st to 10th	Day	13% 5% 2 5% 3 4 4 5% 7% 8% 45%	2280
	11th to 20th	Day	11% 6% 3 7% 4 4 4 3 6% 8% 45%	2280
	21st to end	Day	19% 8% 3 6% 4 4 3 3 5% 7% 39%	2032
October	1st to 10th	Day	14% 5% 3 6% 4 4 4 3 5% 7% 45%	1900
	11th to 20th	Day	16% 5% 2 4 3 3 3 3 4 5% 52%	1900
	21st to end	Day	12% 6% 2 6% 4 4 4 3 4 6% 50%	2090
November	1st to 10th	Day	15% 5% 2 5% 3 3 3 3 4 6% 51%	1805
	11th to 20th	Day	13% 7% 2 5% 4 3 4 4 5% 7% 47%	1710
	21st to end	Day	14% 4 2 5% 4 3 4 3 6% 5% 50%	1616
December	1st to 10th	Day	15% 8% 2 5% 4 3 4 3 6% 8% 42%	1520
	11th to 20th	Day	16% 7% 2 5% 3 3 3 3 3 5% 50%	1910
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 7% 43%	2772
January	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 4 8% 44%	4356
	11th to 20th	Night	15% 8% 3 6% 4 4 3 3 4 5% 46%	4120
	21st to end	Night	17% 10% 3 7% 4 3 3 3 4 5% 41%	4532
February	1st to 10th	Night	15% 9% 2 6% 3 4 4 3 5% 6% 43%	2470
	11th to 20th	Night	19% 7% 2 4 4 4 3 3 4 5% 43%	2271
	21st to end	Night	18% 8% 2 5% 4 2 3 3 5% 6% 45%	1884
March	1st to 10th	Night	23% 8% 2 6% 4 4 4 3 4 6% 36%	3551
	11th to 20th	Night	15% 6% 1 4 4 3 3 3 5% 7% 50%	3480
	21st to end	Night	17% 8% 3 6% 3 2 3 2 4 5% 47%	3693
April	1st to 10th	Night	14% 7% 2 6% 4 3 5% 4 6% 6% 42%	1900
	11th to 20th	Night	12% 5% 2 6% 4 3 3 3 5% 8% 49%	1875
	21st to end	Night	16% 6% 2 5% 4 4 4 4 5% 7% 44%	1900
May	1st to 10th	Night	19% 7% 2 8% 5% 4 5% 4 5% 7% 33%	3109
	11th to 20th	Night	23% 9% 3 6% 4 4 4 3 6% 6% 33%	2574
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 10% 45%	2732
June	1st to 10th	Night	13% 5% 2 6% 5% 4 5% 4 6% 9% 41%	1520
	11th to 20th	Night	10% 4 1 6% 5% 5% 6% 5% 8% 10% 41%	1520
	21st to end	Night	7% 5% 2 6% 5% 5% 6% 5% 6% 10% 43%	1520
July	1st to 10th	Night	8% 5% 2 8% 5% 5% 5% 5% 8% 11% 38%	2520
	11th to 20th	Night	9% 5% 2 6% 4 5% 5% 5% 8% 12% 38%	2640
	21st to end	Night	12% 8% 3 9% 6% 5% 5% 5% 7% 8% 33%	3087
August	1st to 10th	Night	15% 8% 3 7% 5% 5% 5% 4 7% 10% 30%	2825
	11th to 20th	Night	9% 6% 2 8% 5% 5% 5% 4 7% 9% 40%	3061
	21st to end	Night	10% 4 2 5% 4 4 4 3 5% 8% 51%	3476
September	1st to 10th	Night	13% 5% 2 6% 4 3 4 3 5% 7% 48%	1900
	11th to 20th	Night	10% 5% 2 7% 5% 4 5% 3 5% 9% 45%	1900
	21st to end	Night	20% 7% 3 6% 4 5% 4 3 5% 6% 37%	2142
October	1st to 10th	Night	15% 6% 2 5% 5% 4 4 4 5% 7% 44%	2280
	11th to 20th	Night	16% 5% 2 4 4 3 3 3 4 5% 51%	2280
	21st to end	Night	15% 5% 2 5% 3 3 2 3 4 7% 51%	2508
November	1st to 10th	Night	14% 7% 2 6% 3 3 3 3 4 6% 48%	2434
	11th to 20th	Night	16% 7% 2 5% 4 3 3 3 5% 5% 47%	2470
	21st to end	Night	12% 5% 2 4 3 4 4 4 5% 7% 50%	2560
December	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 5% 6% 41%	2660
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 4 7% 43%	3285
	21st to end	Night	14% 7% 2 6% 4 4 4 4 6% 7% 42%	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	34% 4 9% 31% 18% 3	2586
	11th to 20th	Day	41% 3 9% 21% 21% 4	2840
	21st to end	Day	45% 5% 10% 23% 14% 2	3124
February	1st to 10th	Day	42% 4 11% 27% 13% 3	1710
	11th to 20th	Day	41% 4 9% 23% 20% 4	1801
	21st to end	Day	45% 7% 12% 17% 15% 4	1570
March	1st to 10th	Day	47% 5% 10% 18% 15% 4	3300
	11th to 20th	Day	27% 5% 10% 27% 25% 7%	3480
	21st to end	Day	44% 6% 11% 19% 18% 3	3937
April	1st to 10th	Day	34% 6% 13% 22% 20% 6%	2280
	11th to 20th	Day	34% 6% 11% 24% 20% 5%	2254
	21st to end	Day	37% 6% 11% 24% 16% 5%	2280
May	1st to 10th	Day	47% 8% 13% 21% 8% 2	3850
	11th to 20th	Day	47% 9% 13% 22% 8% 1	4306
	21st to end	Day	29% 8% 15% 27% 17% 3	4837
June	1st to 10th	Day	33% 9% 17% 28% 12% 2	2660
	11th to 20th	Day	31% 9% 17% 26% 15% 2	2660
	21st to end	Day	32% 9% 16% 25% 17% 2	2660
July	1st to 10th	Day	30% 12% 25% 19% 12% 3	4370
	11th to 20th	Day	39% 9% 22% 23% 6% 1	4239
	21st to end	Day	40% 10% 16% 18% 14% 1	4324
August	1st to 10th	Day	43% 9% 18% 23% 6% 1	4091
	11th to 20th	Day	38% 9% 13% 25% 13% 1	3771
	21st to end	Day	26% 8% 16% 29% 19% 2	4180
September	1st to 10th	Day	35% 9% 15% 20% 17% 4	2280
	11th to 20th	Day	36% 7% 10% 26% 19% 2	2280
	21st to end	Day	45% 6% 7% 22% 16% 3	2032
October	1st to 10th	Day	31% 6% 10% 24% 24% 4	3160
	11th to 20th	Day	25% 4 7% 21% 34% 9%	3140
	21st to end	Day	23% 4 8% 29% 27% 10%	3428
November	1st to 10th	Day	35% 4 9% 28% 19% 4	1805
	11th to 20th	Day	37% 5% 12% 19% 22% 6%	1710
	21st to end	Day	36% 4 10% 27% 20% 4	1616
December	1st to 10th	Day	41% 5% 13% 25% 14% 3	1520
	11th to 20th	Day	38% 4 8% 28% 16% 6%	1906
	21st to end	Day	41% 5% 11% 27% 14% 1	2772
January	1st to 10th	Night	39% 4 9% 26% 18% 4	4356
	11th to 20th	Night	42% 4 8% 24% 19% 3	4120
	21st to end	Night	46% 3 8% 23% 17% 3	4532
February	1st to 10th	Night	41% 4 11% 25% 16% 2	2470
	11th to 20th	Night	43% 3 8% 22% 19% 5%	2273
	21st to end	Night	41% 4 10% 21% 19% 5%	1884
March	1st to 10th	Night	49% 4 8% 22% 15% 2	3551
	11th to 20th	Night	35% 5% 8% 23% 23% 6%	3480
	21st to end	Night	42% 4 7% 24% 19% 5%	3693
April	1st to 10th	Night	40% 6% 10% 17% 21% 4	1900
	11th to 20th	Night	34% 5% 11% 23% 22% 5%	1875
	21st to end	Night	39% 5% 13% 23% 16% 4	1900
May	1st to 10th	Night	49% 7% 12% 21% 9% 2	3139
	11th to 20th	Night	50% 7% 12% 22% 8% 1	2575
	21st to end	Night	33% 9% 16% 25% 17% 1	2739
June	1st to 10th	Night	38% 8% 13% 24% 15% 1	1520
	11th to 20th	Night	35% 9% 15% 23% 15% 2	1520
	21st to end	Night	34% 9% 14% 24% 16% 3	1520
July	1st to 10th	Night	38% 12% 18% 20% 11% 2	2480
	11th to 20th	Night	36% 10% 24% 24% 6% 1	2592
	21st to end	Night	47% 10% 12% 19% 11%	2988
August	1st to 10th	Night	47% 9% 17% 21% 6%	2814
	11th to 20th	Night	38% 8% 17% 24% 12%	3091
	21st to end	Night	31% 6% 10% 26% 24% 1	3476
September	1st to 10th	Night	35% 8% 16% 19% 17% 5%	1900
	11th to 20th	Night	36% 7% 10% 25% 20% 2	1900
	21st to end	Night	47% 7% 10% 17% 17% 3	2142
October	1st to 10th	Night	37% 5% 10% 22% 21% 4	3800
	11th to 20th	Night	26% 3 8% 23% 32% 8%	3779
	21st to end	Night	22% 3 7% 33% 25% 10%	4120
November	1st to 10th	Night	38% 4 11% 25% 19% 4	2434
	11th to 20th	Night	39% 4 10% 21% 19% 7%	2470
	21st to end	Night	34% 4 9% 27% 22% 4	2560
December	1st to 10th	Night	44% 4 11% 23% 14% 3	2660
	11th to 20th	Night	41% 4 10% 22% 18% 4	3288
	21st to end	Night	39% 5% 13% 27% 14% 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	19% 17% 32% 28% 6%	200
	11th to 20th	Day	17% 20% 39% 19% 5%	270
	21st to end	Day	19% 10% 36% 29% 6%	297
February	1st to 10th	Day	16% 13% 35% 29% 7%	270
	11th to 20th	Day	26% 10% 40% 21% 4	400
	21st to end	Day	23% 9% 40% 23% 5%	382
March	1st to 10th	Day	17% 13% 33% 28% 7% 1	460
	11th to 20th	Day	17% 12% 37% 26% 7% 1	460
	21st to end	Day	17% 10% 33% 32% 7% 1	506
April	1st to 10th	Day	14% 12% 36% 28% 9% 1	460
	11th to 20th	Day	10% 10% 28% 38% 12% 3	452
	21st to end	Day	17% 8% 28% 34% 13% 1	460
May	1st to 10th	Day	16% 8% 25% 34% 15% 2	463
	11th to 20th	Day	20% 3 23% 32% 18% 2	487
	21st to end	Day	18% 3 15% 31% 26% 7%	529
June	1st to 10th	Day	12% 2 14% 38% 28% 6%	490
	11th to 20th	Day	11% 2 16% 35% 26% 10%	490
	21st to end	Day	9% 4 13% 37% 32% 6%	490
July	1st to 10th	Day	9% 2 10% 30% 37% 12%	485
	11th to 20th	Day	6% 1 6% 31% 47% 9%	484
	21st to end	Day	10% 1 12% 35% 32% 9%	532
August	1st to 10th	Day	10% 1 14% 33% 34% 8%	486
	11th to 20th	Day	13% 2 11% 30% 35% 10%	479
	21st to end	Day	10% 2 10% 29% 34% 15%	539
September	1st to 10th	Day	16% 2 11% 28% 30% 13%	490
	11th to 20th	Day	14% 3 13% 29% 30% 10%	490
	21st to end	Day	25% 7% 20% 25% 19% 5%	490
October	1st to 10th	Day	18% 3 20% 26% 24% 10%	490
	11th to 20th	Day	16% 7% 23% 28% 21% 4	490
	21st to end	Day	12% 10% 27% 31% 15% 6%	539
November	1st to 10th	Day	18% 8% 24% 27% 18% 5%	423
	11th to 20th	Day	15% 15% 28% 31% 10% 1	370
	21st to end	Day	17% 14% 25% 28% 14% 2	276
December	1st to 10th	Day	23% 8% 35% 24% 8% 1	180
	11th to 20th	Day	21% 8% 36% 27% 8%	180
	21st to end	Day	20% 10% 30% 30% 10% 1	198
January	1st to 10th	Night	17% 16% 37% 26% 4	760
	11th to 20th	Night	20% 14% 33% 26% 7%	760
	21st to end	Night	20% 14% 35% 28% 3	836
February	1st to 10th	Night	19% 10% 39% 29% 4	760
	11th to 20th	Night	22% 16% 39% 20% 2	743
	21st to end	Night	20% 14% 34% 29% 4	628
March	1st to 10th	Night	18% 13% 33% 29% 7%	760
	11th to 20th	Night	21% 10% 34% 27% 7% 1	760
	21st to end	Night	18% 10% 36% 29% 7%	836
April	1st to 10th	Night	18% 11% 39% 26% 7%	760
	11th to 20th	Night	13% 8% 32% 36% 9% 1	751
	21st to end	Night	18% 7% 26% 38% 10% 1	760
May	1st to 10th	Night	18% 8% 28% 35% 11% 1	752
	11th to 20th	Night	18% 5% 25% 37% 14% 2	427
	21st to end	Night	14% 2 19% 32% 29% 4	411
June	1st to 10th	Night	13% 2 17% 38% 26% 2	380
	11th to 20th	Night	9% 3 16% 39% 29% 4	380
	21st to end	Night	7% 3 17% 43% 28% 3	380
July	1st to 10th	Night	10% 2 11% 41% 32% 4	370
	11th to 20th	Night	6% 1 9% 37% 41% 6%	436
	21st to end	Night	8% 2 11% 43% 32% 3	624
August	1st to 10th	Night	11% 2 10% 38% 34% 5%	567
	11th to 20th	Night	14% 2 11% 34% 35% 4	726
	21st to end	Night	11% 3 13% 31% 35% 8%	836
September	1st to 10th	Night	15% 2 11% 33% 32% 7%	760
	11th to 20th	Night	13% 2 15% 31% 33% 7%	760
	21st to end	Night	23% 5% 16% 32% 22% 2	760
October	1st to 10th	Night	18% 5% 20% 28% 24% 7%	760
	11th to 20th	Night	18% 9% 23% 30% 18% 2	760
	21st to end	Night	18% 10% 25% 28% 17% 3	836
November	1st to 10th	Night	17% 7% 25% 30% 18% 3	771
	11th to 20th	Night	18% 12% 29% 28% 12% 1	760
	21st to end	Night	14% 13% 30% 30% 12% 2	760
December	1st to 10th	Night	19% 11% 36% 23% 10% 2	760
	11th to 20th	Night	18% 10% 38% 25% 8%	760
	21st to end	Night	18% 10% 31% 33% 7%	836

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