Old Oak Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Old Oak 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	16% 7% 2 6% 4 4 4 4 6% 8% 40%	126844
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	123169
Transparency	Day	41% 7% 12% 23% 16% 2	130386
	Night	40% 6% 11% 23% 17% 3	127712
Seeing	Day	23% 15% 27% 21% 11% 3	16244
	Night	20% 14% 28% 27% 10% 1	25502

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	13% 7% 2 6% 4 4 4 3 5% 6% 48%	3597
	11th to 20th	Day	14% 9% 3 9% 6% 4 4 3 5% 6% 38%	3708
	21st to end	Day	16% 8% 3 7% 5% 4 4 3 4 6% 40%	4466
February	1st to 10th	Day	15% 7% 4 7% 5% 4 4 4 5% 7% 38%	1800
	11th to 20th	Day	16% 8% 3 7% 4 4 4 3 5% 6% 38%	1875
	21st to end	Day	18% 10% 4 6% 4 4 4 3 4 6% 38%	1650
March	1st to 10th	Day	21% 4 2 4 4 4 4 3 4 6% 44%	4540
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 5% 7% 43%	4980
	21st to end	Day	18% 6% 2 6% 3 4 4 3 5% 6% 43%	5755
April	1st to 10th	Day	17% 7% 2 6% 5% 4 4 4 5% 7% 39%	2400
	11th to 20th	Day	15% 6% 3 7% 4 3 4 4 6% 6% 41%	2374
	21st to end	Day	16% 5% 2 6% 5% 4 4 4 7% 7% 41%	2400
May	1st to 10th	Day	26% 7% 2 6% 4 4 5% 4 6% 7% 29%	5857
	11th to 20th	Day	25% 8% 3 6% 4 3 4 3 5% 7% 32%	6318
	21st to end	Day	19% 6% 2 5% 4 4 4 4 6% 8% 37%	6949
June	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 5% 7% 9% 39%	2800
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 9% 38%	2800
	21st to end	Day	11% 5% 2 6% 4 5% 6% 4 6% 10% 42%	2800
July	1st to 10th	Day	8% 5% 3 7% 5% 5% 6% 5% 7% 11% 39%	6360
	11th to 20th	Day	10% 7% 3 8% 5% 5% 6% 5% 8% 11% 32%	6360
	21st to end	Day	9% 8% 2 7% 5% 4 5% 4 6% 9% 40%	6755
August	1st to 10th	Day	19% 5% 3 7% 5% 5% 5% 4 6% 8% 35%	4234
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 7% 9% 41%	3891
	21st to end	Day	12% 5% 2 5% 4 3 3 4 5% 9% 48%	4312
September	1st to 10th	Day	15% 6% 2 5% 3 4 4 4 5% 6% 46%	2400
	11th to 20th	Day	13% 5% 1 5% 3 3 3 3 5% 8% 50%	2385
	21st to end	Day	19% 7% 3 7% 5% 4 4 3 5% 5% 38%	2170
October	1st to 10th	Day	17% 7% 2 5% 3 3 4 4 6% 6% 44%	2000
	11th to 20th	Day	16% 6% 1 4 4 4 4 3 5% 7% 47%	2000
	21st to end	Day	19% 5% 2 5% 3 4 4 3 5% 6% 45%	2148
November	1st to 10th	Day	13% 7% 2 4 3 3 4 3 4 6% 52%	1779
	11th to 20th	Day	16% 6% 1 5% 3 3 4 3 5% 6% 47%	1800
	21st to end	Day	15% 7% 2 4 4 4 4 5% 5% 7% 44%	1635
December	1st to 10th	Day	14% 9% 2 5% 4 4 4 4 6% 7% 42%	2600
	11th to 20th	Day	21% 8% 2 4 3 3 3 3 4 6% 42%	2986
	21st to end	Day	17% 6% 2 7% 5% 4 5% 3 6% 8% 37%	3960
January	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 5% 6% 48%	6349
	11th to 20th	Night	14% 7% 2 5% 4 3 4 3 5% 7% 46%	6160
	21st to end	Night	17% 8% 2 6% 4 3 3 3 4 6% 42%	6490
February	1st to 10th	Night	17% 8% 2 6% 4 5% 3 3 5% 6% 40%	2600
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 6% 5% 40%	2410
	21st to end	Night	17% 8% 2 6% 3 4 4 4 5% 7% 40%	1980
March	1st to 10th	Night	18% 8% 2 5% 4 3 3 3 4 5% 43%	5283
	11th to 20th	Night	11% 7% 2 5% 4 4 4 3 4 7% 50%	4979
	21st to end	Night	15% 8% 3 7% 4 3 3 3 5% 6% 44%	5175
April	1st to 10th	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 38%	2000
	11th to 20th	Night	15% 6% 3 6% 5% 4 4 3 5% 8% 41%	1975
	21st to end	Night	18% 6% 2 5% 3 3 4 3 5% 7% 44%	2000
May	1st to 10th	Night	24% 8% 3 7% 4 4 5% 4 6% 7% 28%	4113
	11th to 20th	Night	24% 8% 4 6% 4 4 3 3 5% 7% 31%	3575
	21st to end	Night	22% 6% 2 6% 3 3 4 4 5% 8% 37%	3926
June	1st to 10th	Night	15% 5% 3 6% 5% 4 4 4 6% 8% 42%	1600
	11th to 20th	Night	13% 5% 2 5% 4 4 5% 5% 8% 9% 40%	1600
	21st to end	Night	11% 5% 1 7% 4 4 5% 4 7% 9% 44%	1600
July	1st to 10th	Night	8% 6% 3 8% 5% 6% 6% 5% 7% 11% 37%	3600
	11th to 20th	Night	11% 9% 3 8% 5% 6% 6% 5% 8% 9% 30%	3600
	21st to end	Night	11% 6% 2 5% 4 4 5% 4 6% 9% 44%	4093
August	1st to 10th	Night	19% 5% 2 6% 4 5% 5% 3 7% 9% 34%	2915
	11th to 20th	Night	13% 7% 3 7% 5% 5% 4 3 6% 7% 40%	3122
	21st to end	Night	11% 5% 2 5% 4 4 5% 3 6% 8% 48%	3586
September	1st to 10th	Night	14% 6% 2 7% 4 3 4 3 4 6% 47%	2000
	11th to 20th	Night	12% 5% 2 4 3 3 4 4 5% 6% 52%	2014
	21st to end	Night	19% 7% 2 5% 4 4 4 3 5% 7% 40%	2225
October	1st to 10th	Night	18% 7% 2 5% 4 3 4 2 4 5% 45%	2400
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 4 6% 50%	2400
	21st to end	Night	18% 6% 1 4 3 3 3 3 4 6% 48%	2690
November	1st to 10th	Night	11% 5% 2 4 3 4 3 4 6% 6% 53%	2564
	11th to 20th	Night	16% 8% 2 5% 3 3 4 3 4 5% 47%	2600
	21st to end	Night	14% 6% 2 6% 5% 4 4 3 6% 7% 45%	2761
December	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 6% 45%	4580
	11th to 20th	Night	22% 7% 2 5% 3 3 4 3 5% 6% 41%	5208
	21st to end	Night	19% 7% 2 7% 4 4 4 3 5% 6% 39%	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	37% 4 10% 26% 21% 2	3598
	11th to 20th	Day	47% 5% 8% 21% 14% 4	3738
	21st to end	Day	45% 6% 12% 22% 14% 1	4466
February	1st to 10th	Day	45% 7% 13% 23% 11% 1	1800
	11th to 20th	Day	46% 5% 9% 24% 14% 2	1884
	21st to end	Day	47% 5% 10% 23% 12% 3	1650
March	1st to 10th	Day	41% 5% 9% 23% 15% 7%	4543
	11th to 20th	Day	38% 6% 12% 27% 12% 5%	4980
	21st to end	Day	41% 5% 11% 16% 22% 5%	5755
April	1st to 10th	Day	43% 6% 10% 21% 15% 5%	2400
	11th to 20th	Day	41% 6% 12% 21% 16% 4	2374
	21st to end	Day	41% 6% 11% 21% 17% 4	2400
May	1st to 10th	Day	51% 7% 12% 18% 10% 2	5866
	11th to 20th	Day	51% 6% 9% 22% 11% 1	6322
	21st to end	Day	43% 7% 12% 23% 14% 1	6949
June	1st to 10th	Day	39% 9% 13% 23% 15% 1	2800
	11th to 20th	Day	41% 9% 14% 24% 10% 3	2800
	21st to end	Day	37% 9% 14% 25% 15% 1	2800
July	1st to 10th	Day	37% 11% 14% 22% 15% 2	6290
	11th to 20th	Day	41% 11% 17% 20% 9% 1	6285
	21st to end	Day	39% 8% 10% 21% 22% 1	6560
August	1st to 10th	Day	46% 8% 12% 23% 10%	4221
	11th to 20th	Day	37% 7% 13% 29% 13% 1	3928
	21st to end	Day	33% 7% 11% 29% 19% 2	4312
September	1st to 10th	Day	39% 7% 12% 20% 20% 3	2400
	11th to 20th	Day	32% 5% 11% 26% 23% 4	2385
	21st to end	Day	48% 5% 9% 17% 19% 2	2170
October	1st to 10th	Day	36% 5% 9% 18% 27% 5%	3260
	11th to 20th	Day	26% 5% 12% 27% 25% 6%	3240
	21st to end	Day	25% 4 12% 26% 29% 4	3450
November	1st to 10th	Day	33% 5% 14% 25% 19% 4	1779
	11th to 20th	Day	37% 5% 10% 25% 20% 2	1800
	21st to end	Day	38% 6% 14% 29% 10% 2	1635
December	1st to 10th	Day	42% 5% 16% 24% 11% 2	2600
	11th to 20th	Day	44% 4 13% 28% 9% 1	2986
	21st to end	Day	44% 6% 14% 21% 14% 1	3960
January	1st to 10th	Night	36% 5% 11% 24% 20% 5%	6349
	11th to 20th	Night	38% 4 13% 23% 16% 6%	6204
	21st to end	Night	43% 4 10% 23% 17% 2	6490
February	1st to 10th	Night	45% 5% 13% 25% 11% 2	2600
	11th to 20th	Night	46% 4 9% 21% 17% 3	2412
	21st to end	Night	43% 5% 9% 22% 17% 4	1980
March	1st to 10th	Night	42% 5% 10% 23% 12% 8%	5293
	11th to 20th	Night	34% 6% 14% 27% 14% 5%	4980
	21st to end	Night	42% 5% 8% 17% 25% 4	5175
April	1st to 10th	Night	44% 6% 11% 20% 16% 4	2000
	11th to 20th	Night	41% 6% 10% 20% 20% 3	1975
	21st to end	Night	39% 5% 10% 22% 18% 5%	2000
May	1st to 10th	Night	53% 7% 11% 16% 11% 2	4120
	11th to 20th	Night	52% 7% 11% 17% 12% 1	3576
	21st to end	Night	44% 8% 11% 22% 13% 2	3927
June	1st to 10th	Night	40% 7% 10% 22% 19% 1	1600
	11th to 20th	Night	38% 9% 14% 23% 14% 1	1600
	21st to end	Night	34% 9% 15% 27% 14% 2	1600
July	1st to 10th	Night	40% 11% 15% 19% 14% 2	3560
	11th to 20th	Night	46% 11% 15% 18% 9% 1	3552
	21st to end	Night	35% 8% 12% 24% 20% 1	3973
August	1st to 10th	Night	45% 9% 19% 18% 9% 1	2904
	11th to 20th	Night	41% 6% 12% 23% 15% 1	3151
	21st to end	Night	33% 6% 12% 28% 20% 1	3586
September	1st to 10th	Night	39% 6% 10% 21% 22% 3	2000
	11th to 20th	Night	32% 6% 10% 23% 26% 3	2014
	21st to end	Night	44% 6% 9% 20% 20% 2	2225
October	1st to 10th	Night	42% 5% 9% 21% 21% 2	3920
	11th to 20th	Night	25% 5% 11% 29% 27% 4	3899
	21st to end	Night	26% 4 11% 32% 24% 3	4338
November	1st to 10th	Night	31% 5% 12% 25% 21% 5%	2564
	11th to 20th	Night	40% 4 9% 23% 20% 4	2600
	21st to end	Night	38% 5% 13% 27% 15% 1	2761
December	1st to 10th	Night	40% 4 12% 30% 12% 1	4580
	11th to 20th	Night	45% 4 11% 25% 12% 3	5208
	21st to end	Night	45% 6% 11% 19% 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	36% 24% 23% 14% 4	200
	11th to 20th	Day	40% 25% 26% 8% 1	230
	21st to end	Day	29% 29% 27% 12% 3 1	330
February	1st to 10th	Day	29% 26% 33% 9% 3 1	300
	11th to 20th	Day	35% 26% 30% 9% 1	423
	21st to end	Day	34% 21% 28% 14% 2	414
March	1st to 10th	Day	34% 17% 30% 15% 4	500
	11th to 20th	Day	30% 24% 29% 13% 3 1	500
	21st to end	Day	30% 24% 31% 10% 4 1	550
April	1st to 10th	Day	29% 24% 28% 14% 4 1	500
	11th to 20th	Day	27% 23% 27% 15% 6% 2	492
	21st to end	Day	27% 18% 34% 15% 5% 2	500
May	1st to 10th	Day	35% 17% 26% 14% 6% 2	503
	11th to 20th	Day	24% 15% 31% 19% 9% 2	527
	21st to end	Day	20% 12% 27% 23% 15% 3	573
June	1st to 10th	Day	17% 13% 31% 22% 13% 3	530
	11th to 20th	Day	19% 11% 29% 25% 14% 3	530
	21st to end	Day	13% 11% 29% 27% 15% 4	530
July	1st to 10th	Day	12% 5% 24% 32% 21% 7%	525
	11th to 20th	Day	10% 4 24% 34% 22% 6%	524
	21st to end	Day	12% 6% 29% 32% 18% 3	576
August	1st to 10th	Day	12% 6% 24% 30% 21% 6%	526
	11th to 20th	Day	13% 5% 20% 35% 19% 8%	519
	21st to end	Day	14% 7% 23% 29% 17% 10%	583
September	1st to 10th	Day	18% 9% 22% 25% 17% 8%	530
	11th to 20th	Day	16% 5% 25% 27% 18% 10%	530
	21st to end	Day	23% 14% 22% 25% 12% 4	530
October	1st to 10th	Day	24% 8% 26% 20% 15% 7%	530
	11th to 20th	Day	21% 18% 26% 22% 12% 2	530
	21st to end	Day	20% 18% 33% 16% 10% 3	553
November	1st to 10th	Day	20% 14% 31% 21% 11% 3	401
	11th to 20th	Day	23% 22% 30% 19% 6% 1	400
	21st to end	Day	27% 20% 31% 18% 3 1	235
December	1st to 10th	Day	31% 25% 29% 11% 5% 1	200
	11th to 20th	Day	37% 25% 24% 12% 3	200
	21st to end	Day	28% 20% 30% 13% 8% 1	220
January	1st to 10th	Night	21% 24% 36% 16% 3	800
	11th to 20th	Night	27% 18% 29% 22% 3	800
	21st to end	Night	19% 21% 37% 21% 2	880
February	1st to 10th	Night	24% 19% 35% 22% 1	800
	11th to 20th	Night	29% 24% 32% 13% 1	783
	21st to end	Night	26% 20% 35% 17% 3	660
March	1st to 10th	Night	25% 21% 30% 21% 4	800
	11th to 20th	Night	24% 22% 31% 17% 6%	800
	21st to end	Night	21% 17% 38% 19% 5%	880
April	1st to 10th	Night	22% 23% 33% 18% 4	800
	11th to 20th	Night	20% 15% 37% 22% 6%	791
	21st to end	Night	22% 16% 30% 26% 6%	800
May	1st to 10th	Night	25% 11% 31% 26% 6%	634
	11th to 20th	Night	20% 9% 33% 31% 7% 1	394
	21st to end	Night	18% 8% 23% 39% 12% 1	433
June	1st to 10th	Night	15% 6% 26% 40% 13%	400
	11th to 20th	Night	13% 6% 27% 41% 13% 1	400
	21st to end	Night	10% 5% 26% 43% 15% 1	400
July	1st to 10th	Night	11% 4 17% 45% 21% 2	390
	11th to 20th	Night	8% 3 21% 40% 27% 2	388
	21st to end	Night	11% 4 18% 43% 22% 2	504
August	1st to 10th	Night	11% 3 15% 43% 26% 2	597
	11th to 20th	Night	14% 4 17% 37% 26% 2	739
	21st to end	Night	13% 4 18% 39% 24% 3	880
September	1st to 10th	Night	17% 4 25% 33% 19% 3	800
	11th to 20th	Night	14% 5% 18% 38% 23% 4	800
	21st to end	Night	22% 7% 22% 30% 19% 1	800
October	1st to 10th	Night	22% 7% 24% 25% 18% 3	800
	11th to 20th	Night	19% 11% 30% 28% 13%	800
	21st to end	Night	23% 13% 28% 26% 9%	880
November	1st to 10th	Night	16% 12% 31% 29% 12% 1	789
	11th to 20th	Night	22% 17% 27% 27% 7% 1	800
	21st to end	Night	19% 19% 31% 24% 7%	800
December	1st to 10th	Night	26% 17% 31% 21% 5% 1	800
	11th to 20th	Night	22% 18% 33% 22% 5%	800
	21st to end	Night	23% 18% 31% 25% 3	880

Page generated at 2025-10-31 02:02:43. Version ac07

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