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 2025-11-03.

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	12% 6% 2 6% 4 4 4 4 6% 8% 43%	126502
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	123538
Transparency	Day	36% 7% 13% 23% 17% 4	130091
	Night	38% 6% 11% 23% 18% 4	128018
Seeing	Day	15% 7% 22% 30% 20% 5%	16487
	Night	16% 8% 26% 31% 16% 2	25966

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% 4 1 3 3 3 4 3 4 6% 55%	3707
	11th to 20th	Day	11% 7% 2 6% 4 4 3 3 5% 5% 50%	4060
	21st to end	Day	11% 8% 3 8% 5% 4 4 3 5% 6% 43%	4466
February	1st to 10th	Day	13% 8% 3 6% 5% 4 5% 4 6% 7% 40%	1800
	11th to 20th	Day	20% 6% 2 5% 3 4 4 3 3 7% 44%	1890
	21st to end	Day	16% 8% 3 5% 4 4 5% 4 5% 6% 39%	1650
March	1st to 10th	Day	17% 5% 3 6% 4 4 4 4 6% 7% 39%	4744
	11th to 20th	Day	10% 7% 2 5% 3 3 3 3 5% 7% 52%	4980
	21st to end	Day	15% 8% 3 5% 4 4 4 3 5% 7% 43%	5643
April	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 6% 7% 48%	2400
	11th to 20th	Day	10% 6% 2 5% 4 5% 5% 3 5% 6% 49%	2374
	21st to end	Day	13% 7% 2 6% 4 4 4 4 4 8% 45%	2400
May	1st to 10th	Day	15% 8% 3 7% 4 4 5% 4 6% 11% 33%	5469
	11th to 20th	Day	18% 7% 2 7% 4 5% 5% 4 6% 9% 33%	6180
	21st to end	Day	14% 4 2 6% 4 4 4 4 7% 10% 41%	6934
June	1st to 10th	Day	10% 4 2 5% 4 4 4 4 8% 10% 44%	2800
	11th to 20th	Day	10% 4 1 5% 4 5% 5% 5% 7% 11% 44%	2800
	21st to end	Day	7% 4 1 5% 5% 4 6% 4 7% 11% 47%	2800
July	1st to 10th	Day	6% 5% 2 5% 4 4 6% 5% 8% 12% 43%	6360
	11th to 20th	Day	10% 6% 3 8% 5% 6% 6% 5% 8% 12% 31%	6178
	21st to end	Day	7% 4 2 5% 5% 5% 5% 5% 8% 11% 41%	6424
August	1st to 10th	Day	13% 8% 3 7% 4 4 5% 5% 8% 11% 32%	4234
	11th to 20th	Day	10% 5% 3 7% 6% 6% 5% 5% 6% 9% 39%	3855
	21st to end	Day	10% 3 1 4 3 4 4 3 6% 10% 51%	4312
September	1st to 10th	Day	13% 4 2 5% 3 4 4 5% 7% 8% 45%	2400
	11th to 20th	Day	12% 6% 3 7% 4 4 4 3 6% 8% 44%	2400
	21st to end	Day	19% 8% 3 6% 4 4 4 3 5% 7% 38%	2139
October	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 7% 46%	2000
	11th to 20th	Day	17% 5% 2 4 3 3 3 3 4 5% 52%	2000
	21st to end	Day	12% 5% 2 6% 4 4 4 3 5% 6% 50%	2200
November	1st to 10th	Day	14% 5% 2 5% 3 3 3 3 4 6% 51%	1852
	11th to 20th	Day	13% 7% 2 4 4 3 4 4 5% 7% 47%	1800
	21st to end	Day	14% 4 2 5% 4 3 4 3 5% 5% 51%	1701
December	1st to 10th	Day	10% 6% 2 5% 3 3 4 3 5% 7% 54%	2600
	11th to 20th	Day	18% 8% 3 6% 3 3 3 2 3 5% 46%	2990
	21st to end	Day	12% 6% 2 5% 4 4 3 3 5% 7% 49%	3960
January	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 4 6% 51%	6234
	11th to 20th	Night	12% 7% 2 5% 3 3 3 2 4 4 54%	5900
	21st to end	Night	14% 8% 3 6% 3 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	16% 8% 2 6% 3 4 4 3 5% 6% 43%	2600
	11th to 20th	Night	20% 7% 2 4 4 4 3 3 4 5% 43%	2393
	21st to end	Night	17% 7% 2 5% 4 2 3 3 5% 6% 46%	1980
March	1st to 10th	Night	17% 7% 2 5% 4 4 3 3 4 6% 46%	5107
	11th to 20th	Night	13% 7% 2 5% 4 3 3 3 5% 6% 49%	4980
	21st to end	Night	14% 7% 3 6% 4 3 3 3 5% 6% 46%	5287
April	1st to 10th	Night	15% 7% 2 6% 4 4 5% 4 6% 6% 41%	2000
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 5% 8% 48%	1975
	21st to end	Night	15% 6% 2 5% 4 4 4 4 5% 7% 44%	2000
May	1st to 10th	Night	15% 7% 3 9% 5% 5% 6% 4 6% 9% 32%	4455
	11th to 20th	Night	21% 9% 3 7% 4 4 4 3 6% 7% 32%	3696
	21st to end	Night	15% 7% 2 6% 4 4 4 4 6% 9% 39%	3920
June	1st to 10th	Night	12% 5% 2 6% 5% 4 5% 4 7% 9% 41%	1600
	11th to 20th	Night	11% 4 1 6% 5% 5% 6% 4 8% 9% 40%	1600
	21st to end	Night	7% 5% 2 6% 5% 5% 6% 5% 6% 10% 44%	1600
July	1st to 10th	Night	9% 5% 2 7% 5% 5% 5% 5% 8% 10% 37%	3598
	11th to 20th	Night	10% 6% 3 7% 5% 5% 5% 6% 9% 13% 32%	3773
	21st to end	Night	9% 6% 3 8% 5% 5% 6% 5% 8% 9% 37%	4427
August	1st to 10th	Night	16% 8% 3 7% 5% 5% 5% 4 7% 10% 30%	2915
	11th to 20th	Night	9% 6% 2 8% 5% 5% 5% 4 7% 9% 40%	3160
	21st to end	Night	10% 4 2 5% 4 4 4 3 5% 8% 51%	3586
September	1st to 10th	Night	13% 5% 2 6% 3 3 4 3 5% 7% 48%	2000
	11th to 20th	Night	11% 5% 2 7% 4 4 5% 3 5% 9% 45%	2000
	21st to end	Night	21% 7% 3 6% 4 5% 4 3 5% 6% 35%	2255
October	1st to 10th	Night	15% 6% 2 5% 4 4 4 4 5% 7% 45%	2400
	11th to 20th	Night	18% 5% 2 4 4 3 3 3 4 5% 50%	2400
	21st to end	Night	15% 5% 2 5% 3 3 2 3 4 7% 51%	2640
November	1st to 10th	Night	13% 7% 2 5% 4 3 3 3 4 6% 49%	2491
	11th to 20th	Night	16% 7% 2 5% 4 3 3 3 5% 5% 47%	2600
	21st to end	Night	12% 5% 2 4 3 4 3 4 5% 7% 51%	2695
December	1st to 10th	Night	10% 5% 2 4 3 4 4 3 5% 7% 54%	4580
	11th to 20th	Night	18% 7% 2 6% 5% 4 4 3 4 6% 38%	5205
	21st to end	Night	12% 7% 3 6% 4 3 4 3 5% 7% 46%	6996

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	31% 3 7% 26% 24% 9%	3708
	11th to 20th	Day	37% 3 7% 22% 25% 7%	4060
	21st to end	Day	42% 5% 9% 29% 14% 2	4466
February	1st to 10th	Day	42% 4 12% 27% 13% 2	1800
	11th to 20th	Day	42% 4 9% 22% 19% 4	1899
	21st to end	Day	43% 7% 12% 18% 16% 4	1650
March	1st to 10th	Day	42% 5% 10% 18% 19% 6%	4744
	11th to 20th	Day	31% 5% 10% 23% 24% 8%	4980
	21st to end	Day	40% 6% 11% 18% 19% 6%	5643
April	1st to 10th	Day	35% 6% 13% 22% 20% 6%	2400
	11th to 20th	Day	34% 6% 11% 24% 20% 5%	2374
	21st to end	Day	37% 6% 11% 24% 16% 5%	2400
May	1st to 10th	Day	44% 9% 16% 20% 10% 1	5501
	11th to 20th	Day	45% 9% 14% 22% 9% 1	6184
	21st to end	Day	36% 8% 14% 26% 14% 2	6949
June	1st to 10th	Day	33% 9% 16% 27% 13% 2	2800
	11th to 20th	Day	32% 9% 17% 25% 15% 2	2800
	21st to end	Day	31% 9% 16% 25% 17% 2	2800
July	1st to 10th	Day	31% 11% 22% 21% 13% 2	6290
	11th to 20th	Day	42% 12% 23% 18% 5% 1	6103
	21st to end	Day	33% 10% 17% 21% 19% 1	6240
August	1st to 10th	Day	43% 9% 18% 23% 6% 1	4221
	11th to 20th	Day	39% 9% 14% 25% 13% 1	3891
	21st to end	Day	27% 8% 16% 29% 19% 2	4312
September	1st to 10th	Day	35% 8% 15% 21% 17% 3	2400
	11th to 20th	Day	37% 7% 10% 25% 19% 2	2400
	21st to end	Day	46% 6% 8% 22% 15% 3	2139
October	1st to 10th	Day	31% 6% 10% 24% 25% 4	3260
	11th to 20th	Day	26% 4 6% 21% 34% 9%	3240
	21st to end	Day	23% 4 8% 28% 27% 10%	3538
November	1st to 10th	Day	35% 4 9% 29% 19% 4	1852
	11th to 20th	Day	37% 5% 12% 19% 22% 6%	1800
	21st to end	Day	35% 4 11% 27% 20% 4	1701
December	1st to 10th	Day	31% 5% 10% 30% 20% 4	2600
	11th to 20th	Day	42% 4 12% 22% 16% 4	2986
	21st to end	Day	35% 5% 14% 30% 15% 1	3960
January	1st to 10th	Night	35% 3 7% 22% 26% 7%	6234
	11th to 20th	Night	35% 3 6% 26% 25% 6%	5900
	21st to end	Night	39% 3 8% 29% 18% 3	6490
February	1st to 10th	Night	42% 4 11% 25% 16% 2	2600
	11th to 20th	Night	44% 3 8% 21% 19% 5%	2395
	21st to end	Night	40% 4 10% 21% 19% 6%	1980
March	1st to 10th	Night	40% 4 7% 22% 19% 8%	5107
	11th to 20th	Night	35% 5% 9% 20% 23% 8%	4980
	21st to end	Night	40% 5% 7% 24% 18% 6%	5287
April	1st to 10th	Night	41% 6% 10% 17% 21% 4	2000
	11th to 20th	Night	35% 5% 11% 23% 22% 5%	1975
	21st to end	Night	39% 5% 13% 22% 17% 4	2000
May	1st to 10th	Night	47% 8% 14% 21% 8% 1	4485
	11th to 20th	Night	49% 7% 13% 21% 9% 1	3697
	21st to end	Night	41% 8% 13% 24% 13%	3927
June	1st to 10th	Night	38% 8% 13% 24% 16% 1	1600
	11th to 20th	Night	37% 9% 15% 23% 14% 2	1600
	21st to end	Night	33% 9% 15% 24% 16% 3	1600
July	1st to 10th	Night	38% 11% 18% 19% 13% 1	3560
	11th to 20th	Night	40% 12% 25% 18% 4	3728
	21st to end	Night	40% 10% 13% 23% 14%	4297
August	1st to 10th	Night	47% 9% 17% 21% 6%	2904
	11th to 20th	Night	38% 9% 17% 24% 12%	3190
	21st to end	Night	31% 7% 10% 26% 24% 1	3586
September	1st to 10th	Night	35% 8% 16% 20% 17% 4	2000
	11th to 20th	Night	36% 7% 10% 25% 21% 2	2000
	21st to end	Night	48% 7% 11% 16% 16% 3	2255
October	1st to 10th	Night	37% 5% 10% 22% 22% 4	3920
	11th to 20th	Night	27% 3 8% 23% 31% 8%	3899
	21st to end	Night	22% 3 7% 33% 25% 10%	4252
November	1st to 10th	Night	37% 4 10% 25% 20% 4	2491
	11th to 20th	Night	40% 4 10% 21% 18% 7%	2600
	21st to end	Night	34% 4 9% 27% 22% 4	2695
December	1st to 10th	Night	31% 4 10% 28% 24% 4	4580
	11th to 20th	Night	47% 4 12% 20% 13% 5%	5208
	21st to end	Night	36% 5% 13% 29% 15% 1	6996

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	18% 16% 35% 27% 5%	223
	11th to 20th	Day	17% 18% 39% 21% 5%	300
	21st to end	Day	18% 9% 34% 34% 5%	330
February	1st to 10th	Day	17% 12% 36% 29% 7%	300
	11th to 20th	Day	27% 10% 39% 20% 4	438
	21st to end	Day	21% 8% 38% 25% 6%	414
March	1st to 10th	Day	16% 12% 33% 31% 7% 1	500
	11th to 20th	Day	17% 11% 36% 27% 8% 1	500
	21st to end	Day	16% 10% 33% 34% 6% 1	550
April	1st to 10th	Day	14% 13% 36% 28% 9% 1	500
	11th to 20th	Day	10% 10% 28% 39% 11% 3	492
	21st to end	Day	16% 8% 26% 34% 15% 1	500
May	1st to 10th	Day	16% 8% 24% 35% 15% 2	503
	11th to 20th	Day	20% 3 24% 32% 19% 3	527
	21st to end	Day	19% 3 15% 32% 24% 7%	573
June	1st to 10th	Day	12% 2 14% 39% 27% 6%	530
	11th to 20th	Day	11% 2 15% 34% 28% 10%	530
	21st to end	Day	9% 3 12% 35% 33% 8%	530
July	1st to 10th	Day	9% 2 9% 29% 38% 12%	525
	11th to 20th	Day	6% 1 5% 30% 47% 11%	524
	21st to end	Day	9% 1 11% 34% 33% 12%	576
August	1st to 10th	Day	11% 2 13% 32% 34% 8%	526
	11th to 20th	Day	13% 2 11% 28% 37% 10%	519
	21st to end	Day	10% 2 10% 30% 34% 14%	583
September	1st to 10th	Day	16% 2 11% 29% 30% 12%	530
	11th to 20th	Day	15% 4 13% 28% 31% 10%	530
	21st to end	Day	25% 7% 19% 25% 19% 5%	530
October	1st to 10th	Day	17% 3 19% 25% 24% 12%	530
	11th to 20th	Day	18% 7% 23% 28% 20% 4	530
	21st to end	Day	13% 9% 28% 31% 14% 5%	583
November	1st to 10th	Day	17% 8% 24% 29% 17% 5%	440
	11th to 20th	Day	15% 16% 28% 32% 10% 1	400
	21st to end	Day	16% 14% 27% 29% 14% 2	301
December	1st to 10th	Day	22% 8% 34% 27% 9% 1	200
	11th to 20th	Day	22% 8% 34% 29% 8%	200
	21st to end	Day	18% 10% 29% 31% 11%	220
January	1st to 10th	Night	17% 16% 38% 25% 4	800
	11th to 20th	Night	19% 14% 33% 27% 7%	800
	21st to end	Night	20% 13% 35% 29% 3	880
February	1st to 10th	Night	19% 10% 38% 29% 4	800
	11th to 20th	Night	23% 15% 39% 20% 2	783
	21st to end	Night	19% 14% 33% 29% 4	660
March	1st to 10th	Night	17% 13% 34% 29% 7%	800
	11th to 20th	Night	21% 10% 34% 27% 8% 1	800
	21st to end	Night	18% 10% 37% 29% 7%	880
April	1st to 10th	Night	18% 11% 38% 27% 7%	800
	11th to 20th	Night	14% 8% 32% 37% 9% 1	791
	21st to end	Night	18% 7% 25% 39% 10% 1	800
May	1st to 10th	Night	17% 8% 27% 36% 11% 1	791
	11th to 20th	Night	18% 5% 25% 37% 13% 2	450
	21st to end	Night	15% 3 20% 31% 27% 3	433
June	1st to 10th	Night	13% 3 17% 39% 26% 2	400
	11th to 20th	Night	11% 3 16% 38% 29% 4	400
	21st to end	Night	7% 3 16% 42% 29% 5%	400
July	1st to 10th	Night	11% 2 10% 41% 32% 5%	390
	11th to 20th	Night	6% 1 9% 36% 41% 7%	460
	21st to end	Night	8% 2 11% 42% 34% 4	657
August	1st to 10th	Night	11% 3 10% 38% 33% 5%	597
	11th to 20th	Night	14% 2 10% 33% 36% 5%	765
	21st to end	Night	11% 3 13% 31% 35% 8%	880
September	1st to 10th	Night	15% 2 11% 34% 31% 7%	800
	11th to 20th	Night	13% 2 14% 31% 33% 7%	800
	21st to end	Night	23% 6% 16% 32% 23% 2	800
October	1st to 10th	Night	18% 5% 19% 27% 24% 8%	800
	11th to 20th	Night	19% 9% 23% 30% 17% 2	800
	21st to end	Night	18% 10% 25% 28% 17% 3	880
November	1st to 10th	Night	17% 7% 25% 30% 18% 3	789
	11th to 20th	Night	19% 11% 29% 29% 12% 1	800
	21st to end	Night	14% 13% 30% 30% 11% 2	800
December	1st to 10th	Night	18% 11% 36% 23% 9% 2	800
	11th to 20th	Night	18% 10% 37% 26% 8%	800
	21st to end	Night	18% 10% 31% 33% 8%	880

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