Edward Knight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Edward Knight 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	17% 7% 2 6% 4 4 4 4 5% 7% 38%	126496
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	123551
Transparency	Day	44% 6% 11% 22% 15% 3	130016
	Night	43% 6% 11% 22% 16% 3	128118
Seeing	Day	23% 16% 31% 19% 8% 2	16289
	Night	21% 14% 33% 25% 7% 1	25155

By Month

Percentage Cloud Cover NE (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	12% 7% 3 9% 5% 5% 5% 3 6% 5% 39%	3597
	11th to 20th	Day	13% 8% 3 6% 4 3 4 3 4 6% 46%	3570
	21st to end	Day	17% 8% 3 6% 4 4 3 3 4 4 44%	3960
February	1st to 10th	Day	18% 8% 3 5% 5% 5% 4 5% 5% 6% 36%	1849
	11th to 20th	Day	13% 8% 2 7% 5% 4 4 5% 5% 6% 39%	1948
	21st to end	Day	18% 11% 2 6% 5% 4 4 3 4 5% 37%	1650
March	1st to 10th	Day	21% 11% 3 7% 4 4 4 3 5% 5% 35%	4476
	11th to 20th	Day	16% 9% 2 6% 5% 4 4 4 6% 5% 40%	4748
	21st to end	Day	24% 9% 2 6% 4 3 4 3 6% 6% 32%	5803
April	1st to 10th	Day	19% 9% 3 7% 5% 4 4 3 4 6% 36%	2400
	11th to 20th	Day	18% 6% 2 5% 4 4 5% 4 6% 7% 39%	2384
	21st to end	Day	20% 7% 3 7% 5% 4 4 4 5% 5% 37%	2600
May	1st to 10th	Day	31% 10% 3 6% 4 4 4 3 5% 6% 24%	5886
	11th to 20th	Day	20% 8% 3 9% 6% 5% 4 3 4 7% 32%	6030
	21st to end	Day	19% 6% 2 6% 5% 4 4 4 6% 8% 35%	6949
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 5% 6% 8% 34%	2800
	11th to 20th	Day	17% 5% 2 6% 5% 5% 5% 5% 5% 8% 38%	2800
	21st to end	Day	17% 6% 3 7% 5% 4 4 4 7% 8% 36%	2800
July	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 5% 7% 9% 34%	6360
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 11% 38%	6360
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 8% 43%	6926
August	1st to 10th	Day	14% 7% 2 7% 5% 6% 5% 4 7% 9% 34%	4311
	11th to 20th	Day	14% 8% 2 7% 4 5% 6% 4 6% 7% 38%	4113
	21st to end	Day	7% 4 2 7% 5% 5% 5% 5% 7% 9% 44%	4312
September	1st to 10th	Day	13% 6% 2 5% 3 4 5% 4 6% 6% 47%	2400
	11th to 20th	Day	15% 5% 2 5% 3 2 3 3 4 6% 51%	2205
	21st to end	Day	16% 5% 2 6% 4 4 4 3 4 7% 43%	2200
October	1st to 10th	Day	22% 5% 2 6% 4 4 4 3 4 5% 41%	2200
	11th to 20th	Day	15% 5% 2 5% 4 3 4 4 5% 7% 46%	1944
	21st to end	Day	21% 6% 2 6% 4 3 4 3 4 5% 41%	1980
November	1st to 10th	Day	14% 7% 1 6% 4 4 3 2 4 5% 50%	1779
	11th to 20th	Day	18% 7% 2 5% 3 3 3 2 4 5% 49%	1800
	21st to end	Day	20% 7% 2 5% 4 3 4 4 4 5% 43%	1800
December	1st to 10th	Day	15% 8% 2 5% 3 3 3 3 5% 6% 45%	2610
	11th to 20th	Day	20% 7% 2 5% 3 3 3 2 5% 5% 46%	2986
	21st to end	Day	20% 10% 3 6% 4 4 4 3 4 5% 38%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 4 4 4 5% 6% 46%	6349
	11th to 20th	Night	13% 8% 3 9% 5% 4 5% 3 5% 5% 39%	6315
	21st to end	Night	16% 10% 3 7% 5% 5% 4 3 4 5% 38%	6996
February	1st to 10th	Night	16% 9% 3 6% 4 4 5% 4 5% 5% 39%	2545
	11th to 20th	Night	14% 10% 3 6% 5% 5% 5% 3 5% 5% 40%	2338
	21st to end	Night	17% 9% 2 7% 4 4 3 4 5% 5% 40%	1980
March	1st to 10th	Night	23% 9% 2 5% 4 3 3 3 5% 6% 37%	5364
	11th to 20th	Night	15% 10% 3 7% 4 4 4 3 4 6% 38%	5210
	21st to end	Night	25% 9% 3 5% 4 4 4 3 5% 6% 33%	5114
April	1st to 10th	Night	19% 8% 3 6% 5% 5% 4 3 5% 6% 35%	2000
	11th to 20th	Night	20% 8% 2 6% 4 4 3 3 5% 5% 39%	1965
	21st to end	Night	20% 8% 2 5% 5% 3 4 3 5% 6% 40%	1800
May	1st to 10th	Night	33% 10% 3 5% 4 3 4 3 4 6% 25%	4054
	11th to 20th	Night	17% 9% 3 7% 5% 5% 6% 4 6% 7% 29%	3905
	21st to end	Night	20% 8% 3 8% 4 4 4 4 5% 6% 33%	3926
June	1st to 10th	Night	18% 8% 2 7% 5% 4 4 3 5% 6% 39%	1600
	11th to 20th	Night	19% 7% 2 6% 4 4 5% 4 5% 7% 38%	1600
	21st to end	Night	16% 6% 2 5% 5% 4 4 3 6% 8% 39%	1600
July	1st to 10th	Night	12% 6% 3 7% 5% 5% 5% 4 7% 9% 39%	3600
	11th to 20th	Night	10% 7% 3 7% 5% 5% 5% 4 6% 9% 39%	3600
	21st to end	Night	11% 7% 3 5% 4 4 4 4 5% 6% 47%	3926
August	1st to 10th	Night	19% 6% 3 8% 5% 5% 5% 3 6% 8% 33%	2805
	11th to 20th	Night	10% 7% 3 7% 5% 6% 7% 4 7% 7% 37%	2931
	21st to end	Night	9% 3 2 5% 3 3 4 3 5% 8% 55%	3586
September	1st to 10th	Night	13% 6% 1 5% 3 4 3 3 5% 7% 50%	2000
	11th to 20th	Night	13% 5% 2 4 3 3 4 2 4 7% 53%	2195
	21st to end	Night	17% 5% 2 6% 4 3 4 3 5% 6% 45%	2200
October	1st to 10th	Night	20% 6% 2 6% 4 3 4 3 4 5% 43%	2200
	11th to 20th	Night	17% 4 1 5% 4 3 4 3 6% 7% 47%	2453
	21st to end	Night	22% 7% 2 6% 3 3 4 3 5% 6% 39%	2860
November	1st to 10th	Night	15% 6% 2 5% 3 4 3 3 5% 5% 49%	2564
	11th to 20th	Night	18% 7% 2 5% 3 3 3 3 4 5% 47%	2600
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 6% 44%	2600
December	1st to 10th	Night	18% 9% 2 6% 4 4 4 3 4 5% 40%	4566
	11th to 20th	Night	22% 8% 2 5% 3 3 3 3 4 5% 41%	5208
	21st to end	Night	18% 10% 3 6% 4 4 4 3 5% 5% 38%	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	43% 5% 8% 26% 16% 2	3598
	11th to 20th	Day	40% 5% 10% 22% 19% 4	3600
	21st to end	Day	42% 5% 8% 22% 20% 3	3960
February	1st to 10th	Day	47% 6% 12% 20% 13% 3	1849
	11th to 20th	Day	44% 6% 11% 21% 15% 3	1957
	21st to end	Day	49% 5% 9% 20% 15% 3	1650
March	1st to 10th	Day	51% 4 10% 14% 11% 10%	4479
	11th to 20th	Day	43% 5% 13% 25% 11% 2	4748
	21st to end	Day	53% 3 6% 15% 15% 7%	5803
April	1st to 10th	Day	49% 4 9% 18% 16% 4	2400
	11th to 20th	Day	43% 6% 10% 19% 18% 3	2384
	21st to end	Day	48% 6% 7% 18% 18% 4	2600
May	1st to 10th	Day	59% 7% 12% 13% 9% 1	5897
	11th to 20th	Day	51% 7% 13% 17% 10% 1	6034
	21st to end	Day	45% 7% 11% 21% 14% 1	6949
June	1st to 10th	Day	45% 8% 13% 22% 12% 1	2800
	11th to 20th	Day	42% 8% 12% 19% 16% 3	2800
	21st to end	Day	45% 7% 12% 21% 14% 1	2800
July	1st to 10th	Day	45% 8% 12% 20% 14% 1	6290
	11th to 20th	Day	37% 8% 16% 25% 13% 1	6285
	21st to end	Day	39% 5% 12% 26% 16% 1	6728
August	1st to 10th	Day	44% 9% 15% 22% 9% 1	4297
	11th to 20th	Day	43% 7% 14% 26% 10% 1	4152
	21st to end	Day	32% 8% 13% 28% 18% 2	4312
September	1st to 10th	Day	36% 6% 12% 25% 20% 2	2400
	11th to 20th	Day	35% 4 8% 26% 24% 3	2205
	21st to end	Day	40% 6% 8% 22% 21% 2	2200
October	1st to 10th	Day	41% 6% 8% 22% 20% 3	3590
	11th to 20th	Day	31% 4 9% 28% 24% 3	3134
	21st to end	Day	33% 5% 9% 31% 19% 3	3180
November	1st to 10th	Day	38% 3 12% 27% 18% 2	1779
	11th to 20th	Day	38% 4 9% 22% 23% 3	1800
	21st to end	Day	42% 6% 14% 24% 13% 2	1800
December	1st to 10th	Day	40% 5% 14% 28% 12% 1	2610
	11th to 20th	Day	41% 4 13% 18% 19% 5%	2986
	21st to end	Day	49% 4 8% 20% 14% 5%	3960
January	1st to 10th	Night	36% 6% 11% 30% 14% 2	6349
	11th to 20th	Night	45% 6% 12% 21% 13% 4	6360
	21st to end	Night	48% 5% 9% 17% 17% 5%	6996
February	1st to 10th	Night	45% 6% 13% 21% 13% 2	2545
	11th to 20th	Night	44% 5% 11% 21% 16% 4	2340
	21st to end	Night	45% 4 8% 23% 15% 4	1980
March	1st to 10th	Night	49% 4 8% 17% 15% 7%	5374
	11th to 20th	Night	47% 6% 16% 17% 12% 2	5211
	21st to end	Night	52% 4 5% 13% 21% 6%	5114
April	1st to 10th	Night	48% 5% 10% 18% 16% 3	2000
	11th to 20th	Night	46% 4 9% 19% 19% 2	1965
	21st to end	Night	45% 5% 10% 18% 17% 4	1800
May	1st to 10th	Night	60% 6% 12% 12% 8% 2	4060
	11th to 20th	Night	51% 9% 14% 16% 9% 2	3906
	21st to end	Night	50% 7% 12% 18% 11% 1	3927
June	1st to 10th	Night	46% 7% 11% 23% 13% 1	1600
	11th to 20th	Night	44% 8% 12% 20% 14% 2	1600
	21st to end	Night	42% 7% 14% 22% 14% 1	1600
July	1st to 10th	Night	41% 7% 13% 23% 15%	3560
	11th to 20th	Night	40% 8% 18% 19% 15% 1	3552
	21st to end	Night	36% 6% 11% 30% 15% 1	3809
August	1st to 10th	Night	48% 8% 12% 19% 11% 1	2795
	11th to 20th	Night	42% 9% 14% 25% 10%	2958
	21st to end	Night	27% 6% 15% 30% 21% 1	3586
September	1st to 10th	Night	35% 5% 12% 26% 20% 1	2000
	11th to 20th	Night	33% 4 10% 24% 27% 2	2195
	21st to end	Night	40% 6% 9% 25% 19% 3	2200
October	1st to 10th	Night	38% 6% 15% 23% 15% 2	3590
	11th to 20th	Night	27% 5% 11% 27% 27% 3	4002
	21st to end	Night	35% 4 12% 32% 16% 2	4610
November	1st to 10th	Night	37% 6% 10% 23% 21% 3	2564
	11th to 20th	Night	40% 4 10% 22% 21% 4	2600
	21st to end	Night	41% 5% 13% 25% 14% 2	2600
December	1st to 10th	Night	45% 5% 11% 27% 11% 1	4566
	11th to 20th	Night	46% 4 14% 14% 20% 1	5208
	21st to end	Night	47% 4 8% 22% 16% 3	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	30% 26% 28% 13% 4	200
	11th to 20th	Day	33% 22% 32% 10% 3 1	200
	21st to end	Day	27% 26% 33% 10% 3	220
February	1st to 10th	Day	33% 21% 33% 11% 2	387
	11th to 20th	Day	30% 28% 30% 10% 2	496
	21st to end	Day	28% 21% 36% 12% 3	414
March	1st to 10th	Day	30% 23% 31% 13% 2	500
	11th to 20th	Day	29% 24% 31% 12% 3 1	500
	21st to end	Day	32% 22% 30% 10% 4 2	550
April	1st to 10th	Day	32% 22% 27% 13% 5% 1	500
	11th to 20th	Day	31% 24% 29% 10% 6% 1	492
	21st to end	Day	29% 18% 33% 16% 4 2	500
May	1st to 10th	Day	33% 19% 28% 14% 4 2	503
	11th to 20th	Day	23% 16% 40% 17% 4 2	527
	21st to end	Day	19% 13% 38% 21% 7% 2	573
June	1st to 10th	Day	18% 15% 38% 19% 9% 2	530
	11th to 20th	Day	18% 15% 38% 20% 8% 1	530
	21st to end	Day	19% 13% 35% 21% 8% 2	530
July	1st to 10th	Day	18% 10% 29% 28% 12% 2	525
	11th to 20th	Day	14% 7% 33% 29% 15% 3	524
	21st to end	Day	14% 10% 35% 30% 10% 2	576
August	1st to 10th	Day	11% 10% 30% 32% 14% 3	526
	11th to 20th	Day	17% 9% 27% 28% 16% 4	519
	21st to end	Day	11% 9% 35% 26% 13% 6%	583
September	1st to 10th	Day	18% 9% 29% 22% 15% 6%	530
	11th to 20th	Day	19% 11% 25% 22% 13% 10%	530
	21st to end	Day	19% 12% 31% 23% 10% 5%	530
October	1st to 10th	Day	27% 13% 24% 21% 12% 5%	530
	11th to 20th	Day	21% 16% 27% 24% 10% 2	471
	21st to end	Day	29% 14% 30% 19% 8% 1	462
November	1st to 10th	Day	22% 14% 33% 20% 9% 2	401
	11th to 20th	Day	30% 19% 27% 19% 5% 1	400
	21st to end	Day	30% 17% 30% 16% 5% 1	400
December	1st to 10th	Day	21% 27% 30% 18% 4	210
	11th to 20th	Day	33% 25% 29% 11% 3	200
	21st to end	Day	22% 26% 30% 16% 4 1	220
January	1st to 10th	Night	22% 20% 37% 18% 3	800
	11th to 20th	Night	26% 17% 36% 18% 3	800
	21st to end	Night	20% 19% 38% 20% 3	880
February	1st to 10th	Night	23% 18% 37% 21% 2	800
	11th to 20th	Night	24% 18% 39% 17% 2	783
	21st to end	Night	25% 19% 33% 20% 3	660
March	1st to 10th	Night	25% 20% 34% 19% 3	800
	11th to 20th	Night	23% 21% 35% 17% 4	800
	21st to end	Night	26% 19% 32% 18% 4	880
April	1st to 10th	Night	25% 18% 33% 20% 3	800
	11th to 20th	Night	26% 19% 32% 18% 5%	781
	21st to end	Night	23% 18% 35% 21% 3	600
May	1st to 10th	Night	29% 13% 33% 22% 4	600
	11th to 20th	Night	21% 15% 36% 22% 6%	542
	21st to end	Night	19% 10% 33% 32% 6%	433
June	1st to 10th	Night	22% 7% 35% 28% 8% 1	400
	11th to 20th	Night	19% 10% 32% 32% 7% 1	400
	21st to end	Night	18% 4 38% 33% 9%	400
July	1st to 10th	Night	15% 4 28% 43% 9% 1	390
	11th to 20th	Night	11% 4 28% 45% 11% 1	388
	21st to end	Night	13% 3 31% 43% 10%	438
August	1st to 10th	Night	13% 5% 26% 42% 13% 1	526
	11th to 20th	Night	12% 5% 29% 39% 12% 1	625
	21st to end	Night	14% 5% 27% 37% 16% 2	880
September	1st to 10th	Night	16% 6% 31% 33% 14% 1	800
	11th to 20th	Night	17% 7% 25% 30% 18% 3	800
	21st to end	Night	20% 8% 28% 29% 14% 2	800
October	1st to 10th	Night	23% 11% 25% 27% 13% 2	800
	11th to 20th	Night	19% 12% 32% 26% 11% 1	800
	21st to end	Night	26% 14% 32% 23% 5%	880
November	1st to 10th	Night	19% 14% 32% 27% 8% 1	789
	11th to 20th	Night	23% 16% 33% 21% 7% 1	800
	21st to end	Night	24% 18% 33% 19% 5% 1	800
December	1st to 10th	Night	25% 18% 35% 18% 5%	800
	11th to 20th	Night	24% 18% 32% 23% 3	800
	21st to end	Night	24% 18% 36% 20% 3	880

Page generated at 2025-10-31 02:02:41. Version ac07

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