Keeble Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Keeble Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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%	120032
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	119102
Transparency	Day	38% 6% 12% 22% 17% 4	123673
	Night	38% 5% 10% 23% 19% 5%	123599
Seeing	Day	18% 12% 28% 25% 13% 4	16008
	Night	17% 11% 29% 30% 12% 1	25640

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% 4 2 4 3 3 3 2 4 5% 53%	3938
	11th to 20th	Day	13% 8% 2 5% 3 3 3 3 4 4 51%	4060
	21st to end	Day	13% 8% 2 8% 4 4 4 3 4 5% 45%	4466
February	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 4 5% 7% 40%	1800
	11th to 20th	Day	21% 7% 2 5% 3 3 4 4 6% 4 41%	1818
	21st to end	Day	18% 7% 3 6% 5% 5% 4 3 5% 6% 39%	1650
March	1st to 10th	Day	17% 7% 2 7% 5% 4 5% 4 5% 6% 38%	4935
	11th to 20th	Day	11% 5% 2 5% 3 3 4 4 5% 7% 52%	4980
	21st to end	Day	17% 6% 2 6% 4 5% 4 3 5% 7% 41%	5505
April	1st to 10th	Day	10% 6% 2 6% 4 3 3 4 5% 7% 50%	2400
	11th to 20th	Day	12% 6% 2 4 4 4 4 3 4 8% 48%	2374
	21st to end	Day	15% 6% 2 6% 5% 4 4 3 5% 7% 42%	2400
May	1st to 10th	Day	14% 8% 2 7% 5% 5% 5% 5% 7% 10% 32%	5712
	11th to 20th	Day	19% 7% 2 7% 4 4 5% 4 7% 9% 31%	5924
	21st to end	Day	14% 5% 2 6% 4 4 4 4 6% 9% 41%	6934
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 5% 10% 44%	2800
	11th to 20th	Day	11% 4 1 4 5% 5% 5% 5% 7% 10% 43%	2800
	21st to end	Day	8% 5% 2 6% 5% 4 6% 5% 7% 11% 42%	2800
July	1st to 10th	Day	6% 4 2 6% 4 5% 5% 5% 9% 12% 41%	4554
	11th to 20th	Day	11% 5% 2 6% 5% 5% 6% 5% 9% 12% 35%	4250
	21st to end	Day	10% 7% 3 8% 5% 5% 5% 5% 7% 10% 35%	4466
August	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 5% 8% 11% 29%	4104
	11th to 20th	Day	9% 6% 3 7% 5% 5% 6% 4 7% 10% 39%	3853
	21st to end	Day	8% 4 2 5% 3 4 4 3 6% 11% 49%	4180
September	1st to 10th	Day	13% 5% 2 5% 4 5% 5% 4 6% 7% 45%	2280
	11th to 20th	Day	12% 7% 2 6% 4 3 4 3 5% 8% 46%	2280
	21st to end	Day	21% 7% 2 6% 4 4 4 3 4 6% 38%	1987
October	1st to 10th	Day	15% 5% 2 6% 5% 4 3 3 4 7% 47%	1900
	11th to 20th	Day	16% 4 1 4 3 3 3 3 5% 5% 53%	1900
	21st to end	Day	14% 5% 2 5% 3 2 3 2 4 6% 53%	2090
November	1st to 10th	Day	12% 5% 2 5% 3 4 4 3 4 6% 53%	1927
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 5% 7% 49%	1800
	21st to end	Day	14% 5% 2 4 4 3 2 4 4 5% 52%	1615
December	1st to 10th	Day	11% 6% 2 5% 3 3 4 3 5% 7% 52%	2600
	11th to 20th	Day	18% 7% 2 5% 3 3 3 2 4 6% 47%	2990
	21st to end	Day	14% 6% 2 6% 4 4 4 3 6% 7% 43%	3960
January	1st to 10th	Night	13% 7% 2 4 3 3 3 2 4 5% 54%	6003
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 4 55%	5900
	21st to end	Night	16% 7% 3 5% 3 3 3 3 4 5% 49%	6490
February	1st to 10th	Night	15% 7% 2 5% 4 3 5% 4 6% 6% 42%	2600
	11th to 20th	Night	20% 7% 2 5% 4 3 3 3 4 6% 42%	2468
	21st to end	Night	16% 6% 3 6% 4 4 3 3 5% 5% 46%	1975
March	1st to 10th	Night	17% 5% 2 5% 3 3 4 3 4 6% 46%	4918
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 6% 51%	4980
	21st to end	Night	13% 7% 2 5% 4 4 4 3 4 6% 47%	5426
April	1st to 10th	Night	14% 8% 2 5% 5% 4 4 2 4 7% 45%	2000
	11th to 20th	Night	11% 6% 2 5% 3 4 4 4 5% 8% 49%	1975
	21st to end	Night	14% 6% 2 6% 4 3 4 4 6% 8% 44%	2000
May	1st to 10th	Night	15% 8% 3 8% 4 5% 5% 4 6% 9% 33%	4224
	11th to 20th	Night	21% 8% 3 7% 4 4 4 4 6% 7% 32%	3951
	21st to end	Night	15% 7% 2 6% 4 4 5% 4 7% 8% 38%	3920
June	1st to 10th	Night	10% 5% 2 6% 5% 6% 6% 5% 7% 9% 41%	1600
	11th to 20th	Night	11% 5% 2 6% 5% 5% 5% 4 7% 10% 40%	1600
	21st to end	Night	8% 4 2 5% 5% 5% 6% 5% 6% 11% 45%	1600
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 5% 7% 10% 40%	2622
	11th to 20th	Night	9% 6% 2 7% 5% 5% 6% 5% 9% 11% 35%	2930
	21st to end	Night	11% 8% 3 9% 7% 7% 6% 6% 7% 7% 30%	3087
August	1st to 10th	Night	15% 7% 3 8% 5% 5% 5% 5% 7% 10% 31%	2825
	11th to 20th	Night	10% 7% 2 7% 5% 5% 6% 5% 7% 9% 37%	2943
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 8% 47%	3476
September	1st to 10th	Night	13% 5% 2 6% 5% 4 4 4 6% 7% 43%	1900
	11th to 20th	Night	12% 6% 1 6% 4 3 5% 3 6% 8% 47%	1900
	21st to end	Night	21% 9% 3 6% 4 4 3 3 5% 6% 37%	2188
October	1st to 10th	Night	15% 6% 2 6% 3 3 4 3 5% 6% 47%	2280
	11th to 20th	Night	15% 6% 2 5% 4 4 3 2 4 6% 49%	2280
	21st to end	Night	13% 5% 2 5% 4 3 3 2 4 6% 53%	2508
November	1st to 10th	Night	12% 7% 2 6% 4 4 2 3 5% 5% 52%	2368
	11th to 20th	Night	15% 6% 3 5% 4 3 3 3 4 5% 50%	2600
	21st to end	Night	13% 4 2 4 3 4 4 4 5% 7% 51%	2784
December	1st to 10th	Night	10% 5% 2 4 3 3 3 3 5% 7% 55%	4580
	11th to 20th	Night	20% 7% 3 6% 4 4 4 3 5% 5% 40%	5205
	21st to end	Night	14% 7% 2 6% 3 4 3 4 5% 7% 45%	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	34% 3 5% 26% 22% 10%	3939
	11th to 20th	Day	36% 4 6% 20% 25% 9%	4060
	21st to end	Day	42% 4 9% 25% 17% 4	4466
February	1st to 10th	Day	42% 5% 11% 26% 14% 2	1800
	11th to 20th	Day	43% 5% 7% 22% 19% 5%	1827
	21st to end	Day	46% 5% 10% 17% 17% 4	1650
March	1st to 10th	Day	44% 5% 9% 17% 17% 8%	4935
	11th to 20th	Day	30% 5% 9% 24% 22% 9%	4980
	21st to end	Day	42% 5% 10% 17% 19% 6%	5505
April	1st to 10th	Day	34% 5% 12% 22% 19% 8%	2400
	11th to 20th	Day	35% 5% 11% 21% 22% 6%	2374
	21st to end	Day	41% 5% 11% 24% 14% 5%	2400
May	1st to 10th	Day	44% 9% 15% 19% 12% 1	5745
	11th to 20th	Day	47% 8% 13% 21% 10% 1	5928
	21st to end	Day	37% 7% 15% 22% 17% 2	6949
June	1st to 10th	Day	35% 7% 16% 25% 14% 1	2800
	11th to 20th	Day	35% 9% 16% 23% 16% 1	2800
	21st to end	Day	34% 8% 16% 24% 17% 2	2800
July	1st to 10th	Day	32% 10% 23% 18% 11% 5%	4484
	11th to 20th	Day	38% 8% 23% 22% 7% 1	4181
	21st to end	Day	43% 9% 13% 19% 16% 1	4324
August	1st to 10th	Day	46% 8% 18% 21% 6%	4091
	11th to 20th	Day	38% 8% 16% 25% 14%	3892
	21st to end	Day	30% 6% 16% 28% 18% 2	4180
September	1st to 10th	Day	37% 7% 15% 16% 21% 4	2280
	11th to 20th	Day	37% 6% 9% 25% 19% 4	2280
	21st to end	Day	48% 5% 9% 20% 15% 4	1987
October	1st to 10th	Day	36% 5% 8% 19% 28% 5%	3160
	11th to 20th	Day	25% 4 6% 20% 33% 13%	3140
	21st to end	Day	20% 3 9% 29% 26% 13%	3428
November	1st to 10th	Day	33% 4 9% 25% 25% 4	1927
	11th to 20th	Day	36% 4 10% 21% 22% 7%	1800
	21st to end	Day	34% 4 8% 25% 24% 5%	1615
December	1st to 10th	Day	32% 5% 9% 28% 18% 8%	2600
	11th to 20th	Day	41% 3 12% 24% 16% 4	2986
	21st to end	Day	39% 6% 12% 26% 16% 1	3960
January	1st to 10th	Night	33% 4 6% 21% 27% 10%	6003
	11th to 20th	Night	34% 3 5% 23% 27% 8%	5900
	21st to end	Night	39% 3 6% 27% 21% 4	6490
February	1st to 10th	Night	41% 5% 11% 24% 16% 2	2600
	11th to 20th	Night	44% 4 7% 20% 19% 6%	2470
	21st to end	Night	40% 4 9% 21% 17% 8%	1975
March	1st to 10th	Night	38% 4 7% 22% 22% 7%	4918
	11th to 20th	Night	33% 4 7% 19% 25% 10%	4980
	21st to end	Night	39% 4 7% 22% 21% 6%	5426
April	1st to 10th	Night	40% 5% 8% 18% 21% 6%	2000
	11th to 20th	Night	34% 5% 12% 22% 21% 6%	1975
	21st to end	Night	39% 6% 11% 23% 17% 5%	2000
May	1st to 10th	Night	46% 8% 16% 18% 10% 2	4254
	11th to 20th	Night	49% 8% 11% 21% 10% 1	3952
	21st to end	Night	41% 8% 15% 23% 13% 1	3927
June	1st to 10th	Night	37% 9% 13% 24% 16% 1	1600
	11th to 20th	Night	36% 9% 17% 20% 15% 1	1600
	21st to end	Night	32% 9% 14% 23% 18% 3	1600
July	1st to 10th	Night	36% 12% 19% 19% 12% 3	2582
	11th to 20th	Night	38% 10% 24% 20% 6% 1	2876
	21st to end	Night	49% 10% 12% 16% 12%	2988
August	1st to 10th	Night	46% 9% 16% 22% 7%	2814
	11th to 20th	Night	39% 10% 16% 22% 13%	2970
	21st to end	Night	33% 7% 11% 28% 19% 1	3476
September	1st to 10th	Night	38% 8% 15% 19% 15% 6%	1900
	11th to 20th	Night	35% 6% 10% 25% 21% 3	1900
	21st to end	Night	47% 6% 9% 18% 16% 4	2188
October	1st to 10th	Night	39% 5% 10% 19% 22% 5%	3800
	11th to 20th	Night	25% 3 8% 22% 31% 12%	3779
	21st to end	Night	21% 3 6% 31% 26% 13%	4120
November	1st to 10th	Night	35% 4 9% 22% 25% 5%	2368
	11th to 20th	Night	37% 4 8% 20% 23% 9%	2600
	21st to end	Night	33% 4 9% 26% 23% 5%	2784
December	1st to 10th	Night	29% 4 9% 28% 25% 5%	4580
	11th to 20th	Night	46% 4 10% 18% 15% 6%	5208
	21st to end	Night	38% 5% 11% 28% 17% 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	28% 24% 32% 13% 4	275
	11th to 20th	Day	29% 28% 30% 10% 3	300
	21st to end	Day	24% 24% 39% 12% 1	330
February	1st to 10th	Day	22% 28% 36% 11% 3	300
	11th to 20th	Day	35% 24% 31% 10% 1	365
	21st to end	Day	28% 18% 32% 18% 3	414
March	1st to 10th	Day	23% 21% 34% 18% 5%	500
	11th to 20th	Day	22% 22% 34% 17% 4 1	500
	21st to end	Day	25% 15% 39% 15% 4 1	550
April	1st to 10th	Day	23% 22% 30% 18% 6% 1	500
	11th to 20th	Day	18% 15% 35% 21% 8% 3	492
	21st to end	Day	21% 14% 35% 22% 6% 2	500
May	1st to 10th	Day	19% 16% 33% 21% 9% 2	503
	11th to 20th	Day	22% 9% 30% 26% 11% 2	527
	21st to end	Day	20% 6% 24% 30% 16% 4	573
June	1st to 10th	Day	10% 6% 30% 30% 20% 4	530
	11th to 20th	Day	13% 5% 25% 34% 17% 6%	530
	21st to end	Day	10% 6% 24% 32% 23% 5%	530
July	1st to 10th	Day	10% 3 18% 34% 28% 7%	525
	11th to 20th	Day	6% 2 16% 41% 26% 10%	524
	21st to end	Day	8% 4 23% 34% 22% 9%	532
August	1st to 10th	Day	12% 4 20% 31% 25% 8%	486
	11th to 20th	Day	11% 3 19% 32% 26% 9%	479
	21st to end	Day	9% 5% 17% 31% 23% 15%	539
September	1st to 10th	Day	14% 4 17% 29% 20% 15%	490
	11th to 20th	Day	15% 4 19% 30% 21% 11%	490
	21st to end	Day	30% 8% 21% 27% 11% 3	490
October	1st to 10th	Day	19% 7% 22% 27% 17% 8%	490
	11th to 20th	Day	17% 11% 24% 27% 18% 2	490
	21st to end	Day	15% 11% 34% 27% 9% 5%	539
November	1st to 10th	Day	16% 9% 34% 25% 12% 4	480
	11th to 20th	Day	18% 15% 34% 24% 10% 1	400
	21st to end	Day	20% 19% 31% 20% 9% 1	215
December	1st to 10th	Day	21% 20% 38% 15% 7% 1	200
	11th to 20th	Day	21% 18% 35% 18% 9% 1	200
	21st to end	Day	23% 18% 30% 18% 10% 1	220
January	1st to 10th	Night	21% 18% 39% 19% 3	800
	11th to 20th	Night	21% 17% 36% 23% 4	800
	21st to end	Night	20% 16% 39% 23% 1	880
February	1st to 10th	Night	20% 15% 38% 24% 3	800
	11th to 20th	Night	26% 20% 36% 17% 1	783
	21st to end	Night	21% 19% 37% 21% 3	660
March	1st to 10th	Night	19% 18% 36% 24% 3	800
	11th to 20th	Night	23% 18% 33% 20% 5% 1	800
	21st to end	Night	18% 16% 40% 22% 4	880
April	1st to 10th	Night	19% 19% 38% 21% 3	800
	11th to 20th	Night	15% 14% 38% 25% 7%	791
	21st to end	Night	17% 10% 37% 30% 7%	800
May	1st to 10th	Night	17% 11% 36% 28% 7%	690
	11th to 20th	Night	19% 5% 33% 31% 11% 1	562
	21st to end	Night	15% 3 24% 37% 21% 1	433
June	1st to 10th	Night	11% 2 24% 43% 20%	400
	11th to 20th	Night	11% 4 22% 38% 23% 2	400
	21st to end	Night	8% 3 22% 47% 20% 1	400
July	1st to 10th	Night	10% 3 18% 40% 26% 3	399
	11th to 20th	Night	5% 1 13% 41% 34% 6%	582
	21st to end	Night	8% 2 20% 42% 25% 4	624
August	1st to 10th	Night	12% 2 13% 43% 28% 3	567
	11th to 20th	Night	13% 5% 12% 42% 25% 3	657
	21st to end	Night	11% 2 15% 37% 30% 5%	836
September	1st to 10th	Night	15% 2 14% 37% 28% 5%	760
	11th to 20th	Night	14% 4 14% 36% 29% 3	760
	21st to end	Night	26% 4 20% 31% 18% 1	760
October	1st to 10th	Night	18% 4 22% 31% 22% 4	760
	11th to 20th	Night	17% 9% 27% 31% 14% 2	760
	21st to end	Night	15% 10% 30% 29% 14% 1	836
November	1st to 10th	Night	17% 9% 29% 31% 12% 2	780
	11th to 20th	Night	18% 12% 32% 29% 9% 1	800
	21st to end	Night	14% 14% 34% 29% 8% 1	800
December	1st to 10th	Night	21% 13% 36% 23% 7% 1	800
	11th to 20th	Night	20% 11% 34% 29% 6%	800
	21st to end	Night	19% 14% 30% 33% 4	880

Page generated at 2025-07-18 01:10:00. Version ac07

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