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 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	16% 7% 2 6% 4 4 4 4 6% 7% 40%	101380
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 41%	97720
Transparency	Day	41% 7% 12% 23% 15% 2	104940
	Night	41% 6% 12% 23% 16% 2	102248
Seeing	Day	23% 15% 28% 21% 11% 3	14964
	Night	20% 13% 29% 27% 10% 1	24244

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	15% 8% 2 6% 4 4 4 3 4 5% 44%	2517
	11th to 20th	Day	15% 10% 3 10% 6% 4 4 3 5% 6% 34%	2616
	21st to end	Day	20% 10% 3 7% 5% 4 4 3 4 5% 35%	3124
February	1st to 10th	Day	16% 7% 3 7% 6% 4 3 4 5% 8% 38%	1710
	11th to 20th	Day	15% 8% 3 7% 4 4 4 4 5% 6% 39%	1778
	21st to end	Day	19% 10% 4 6% 4 4 4 3 4 6% 37%	1570
March	1st to 10th	Day	26% 6% 2 5% 4 4 3 3 4 6% 36%	3166
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 8% 44%	3480
	21st to end	Day	22% 7% 2 5% 3 3 3 2 3 5% 43%	4021
April	1st to 10th	Day	16% 7% 2 6% 5% 4 4 4 5% 7% 39%	2280
	11th to 20th	Day	15% 7% 3 6% 4 3 4 4 5% 6% 42%	2254
	21st to end	Day	16% 5% 2 6% 5% 4 4 4 7% 7% 40%	2280
May	1st to 10th	Day	27% 7% 2 6% 4 4 4 4 5% 6% 29%	4091
	11th to 20th	Day	25% 9% 3 7% 5% 4 4 4 4 6% 28%	4398
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 9% 44%	4837
June	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 5% 8% 9% 39%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 6% 9% 38%	2660
	21st to end	Day	11% 5% 2 6% 4 5% 6% 4 6% 10% 41%	2660
July	1st to 10th	Day	7% 4 2 7% 5% 6% 6% 5% 8% 11% 38%	4440
	11th to 20th	Day	9% 5% 2 7% 4 5% 5% 5% 8% 10% 40%	4440
	21st to end	Day	10% 9% 3 8% 6% 5% 6% 5% 6% 8% 34%	4699
August	1st to 10th	Day	19% 5% 3 7% 5% 5% 5% 4 6% 8% 34%	4104
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 7% 9% 42%	3770
	21st to end	Day	12% 5% 2 5% 4 3 3 4 5% 9% 48%	4180
September	1st to 10th	Day	15% 6% 3 5% 3 4 5% 4 5% 6% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 5% 8% 50%	2266
	21st to end	Day	19% 7% 3 7% 5% 4 3 3 5% 5% 39%	2062
October	1st to 10th	Day	18% 7% 2 5% 3 3 4 3 6% 6% 43%	1900
	11th to 20th	Day	15% 6% 1 4 4 3 4 3 5% 7% 47%	1900
	21st to end	Day	20% 5% 2 5% 3 3 4 3 5% 6% 45%	2041
November	1st to 10th	Day	13% 7% 2 4 3 3 3 3 4 6% 52%	1734
	11th to 20th	Day	16% 5% 1 4 3 3 4 3 6% 6% 46%	1710
	21st to end	Day	15% 6% 2 4 4 4 4 4 5% 7% 44%	1554
December	1st to 10th	Day	18% 9% 2 7% 4 4 4 4 5% 7% 36%	1520
	11th to 20th	Day	20% 7% 2 5% 4 4 4 3 4 7% 40%	1906
	21st to end	Day	17% 7% 3 8% 6% 5% 5% 4 6% 7% 34%	2772
January	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 5% 7% 43%	4429
	11th to 20th	Night	18% 8% 2 6% 4 3 4 3 4 6% 41%	4340
	21st to end	Night	23% 9% 2 7% 4 3 3 3 4 5% 36%	4532
February	1st to 10th	Night	17% 8% 2 6% 4 4 3 3 5% 7% 40%	2470
	11th to 20th	Night	16% 8% 3 7% 4 4 4 3 6% 5% 40%	2287
	21st to end	Night	18% 8% 2 5% 3 4 4 4 5% 7% 41%	1884
March	1st to 10th	Night	22% 10% 3 6% 4 4 3 3 4 5% 37%	3671
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 6% 49%	3479
	21st to end	Night	16% 9% 3 7% 4 2 2 2 3 5% 46%	3609
April	1st to 10th	Night	17% 7% 2 6% 4 4 5% 4 6% 7% 39%	1900
	11th to 20th	Night	15% 6% 3 7% 5% 4 4 4 4 8% 41%	1875
	21st to end	Night	19% 6% 2 5% 3 3 4 4 5% 7% 43%	1900
May	1st to 10th	Night	25% 9% 2 6% 4 4 5% 4 5% 7% 30%	2865
	11th to 20th	Night	26% 9% 4 6% 5% 4 3 3 6% 6% 29%	2495
	21st to end	Night	13% 5% 2 6% 4 4 4 4 5% 10% 41%	2738
June	1st to 10th	Night	15% 5% 3 6% 5% 4 3 4 6% 8% 42%	1520
	11th to 20th	Night	12% 5% 2 5% 4 4 6% 4 8% 9% 41%	1520
	21st to end	Night	11% 4 1 7% 4 4 5% 4 7% 9% 44%	1520
July	1st to 10th	Night	7% 5% 2 8% 5% 6% 6% 5% 7% 11% 37%	2520
	11th to 20th	Night	8% 7% 3 7% 5% 5% 6% 5% 8% 10% 37%	2520
	21st to end	Night	14% 8% 3 6% 5% 4 5% 4 6% 8% 37%	2850
August	1st to 10th	Night	20% 5% 2 6% 4 5% 5% 3 7% 9% 33%	2825
	11th to 20th	Night	14% 7% 3 7% 4 5% 4 3 6% 7% 40%	3024
	21st to end	Night	11% 5% 2 5% 4 4 5% 3 6% 8% 47%	3476
September	1st to 10th	Night	15% 6% 2 7% 4 3 3 3 4 6% 47%	1900
	11th to 20th	Night	12% 5% 2 4 3 3 3 4 5% 6% 53%	1913
	21st to end	Night	19% 7% 2 5% 4 4 4 3 5% 6% 41%	2114
October	1st to 10th	Night	19% 7% 2 6% 4 3 4 2 3 5% 43%	2280
	11th to 20th	Night	16% 5% 2 4 3 3 4 3 4 7% 50%	2280
	21st to end	Night	18% 7% 2 4 3 3 3 3 4 6% 48%	2555
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 4 5% 6% 52%	2505
	11th to 20th	Night	16% 8% 2 5% 3 3 4 3 4 5% 47%	2470
	21st to end	Night	14% 5% 2 5% 5% 4 4 3 6% 7% 46%	2622
December	1st to 10th	Night	19% 7% 2 6% 5% 4 4 4 5% 6% 38%	2660
	11th to 20th	Night	17% 6% 2 6% 4 4 5% 4 6% 7% 40%	3288
	21st to end	Night	18% 8% 2 7% 4 4 4 3 6% 7% 36%	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	41% 5% 14% 25% 13% 3	2518
	11th to 20th	Day	50% 6% 10% 20% 12% 2	2616
	21st to end	Day	50% 6% 12% 21% 9% 1	3124
February	1st to 10th	Day	44% 7% 13% 23% 11% 1	1710
	11th to 20th	Day	45% 5% 9% 24% 14% 2	1787
	21st to end	Day	49% 5% 10% 21% 12% 3	1570
March	1st to 10th	Day	48% 6% 9% 26% 9% 2	3169
	11th to 20th	Day	35% 7% 12% 28% 15% 3	3480
	21st to end	Day	44% 4 9% 19% 20% 3	4021
April	1st to 10th	Day	42% 6% 11% 21% 15% 5%	2280
	11th to 20th	Day	40% 6% 12% 22% 16% 4	2254
	21st to end	Day	41% 7% 12% 20% 16% 4	2280
May	1st to 10th	Day	53% 7% 10% 18% 10% 2	4103
	11th to 20th	Day	55% 7% 12% 17% 8% 1	4402
	21st to end	Day	34% 8% 14% 26% 16% 2	4837
June	1st to 10th	Day	38% 9% 13% 24% 15% 1	2660
	11th to 20th	Day	40% 9% 14% 24% 10% 3	2660
	21st to end	Day	37% 9% 14% 25% 14% 1	2660
July	1st to 10th	Day	37% 11% 15% 22% 13% 2	4370
	11th to 20th	Day	35% 10% 17% 24% 13% 1	4365
	21st to end	Day	45% 9% 10% 18% 16% 2	4549
August	1st to 10th	Day	47% 8% 12% 23% 9%	4091
	11th to 20th	Day	37% 7% 13% 30% 13% 1	3807
	21st to end	Day	33% 7% 11% 28% 19% 2	4180
September	1st to 10th	Day	39% 7% 11% 20% 20% 3	2280
	11th to 20th	Day	32% 5% 11% 26% 23% 4	2266
	21st to end	Day	47% 4 9% 18% 20% 2	2062
October	1st to 10th	Day	37% 5% 9% 18% 26% 5%	3160
	11th to 20th	Day	25% 5% 13% 27% 25% 6%	3140
	21st to end	Day	25% 4 12% 26% 29% 4	3343
November	1st to 10th	Day	34% 5% 14% 26% 19% 4	1734
	11th to 20th	Day	38% 6% 10% 25% 20% 2	1710
	21st to end	Day	38% 6% 14% 30% 10% 2	1554
December	1st to 10th	Day	47% 6% 15% 19% 12% 1	1520
	11th to 20th	Day	44% 5% 15% 26% 9% 1	1906
	21st to end	Day	48% 7% 14% 20% 9% 1	2772
January	1st to 10th	Night	41% 5% 13% 26% 12% 3	4429
	11th to 20th	Night	44% 4 13% 24% 12% 2	4340
	21st to end	Night	50% 4 9% 25% 10% 1	4532
February	1st to 10th	Night	45% 5% 13% 25% 11% 2	2470
	11th to 20th	Night	46% 4 9% 20% 18% 3	2289
	21st to end	Night	43% 5% 9% 23% 16% 4	1884
March	1st to 10th	Night	48% 6% 10% 22% 12% 2	3681
	11th to 20th	Night	34% 7% 12% 29% 16% 2	3480
	21st to end	Night	42% 4 7% 18% 23% 5%	3609
April	1st to 10th	Night	43% 6% 11% 21% 16% 4	1900
	11th to 20th	Night	41% 7% 10% 20% 20% 3	1875
	21st to end	Night	40% 6% 11% 22% 17% 5%	1900
May	1st to 10th	Night	53% 8% 9% 19% 10% 2	2872
	11th to 20th	Night	54% 8% 13% 12% 10% 2	2496
	21st to end	Night	36% 10% 14% 22% 15% 3	2739
June	1st to 10th	Night	39% 7% 11% 23% 19% 1	1520
	11th to 20th	Night	37% 9% 15% 24% 15% 1	1520
	21st to end	Night	34% 9% 14% 28% 13% 2	1520
July	1st to 10th	Night	38% 12% 17% 16% 14% 3	2480
	11th to 20th	Night	38% 11% 16% 22% 12% 1	2472
	21st to end	Night	42% 9% 13% 19% 16% 1	2759
August	1st to 10th	Night	45% 9% 19% 17% 9% 1	2814
	11th to 20th	Night	41% 6% 13% 23% 15% 1	3053
	21st to end	Night	33% 6% 12% 28% 19% 1	3476
September	1st to 10th	Night	39% 6% 10% 21% 22% 3	1900
	11th to 20th	Night	31% 6% 10% 22% 27% 4	1913
	21st to end	Night	43% 6% 8% 21% 21% 2	2114
October	1st to 10th	Night	43% 5% 9% 21% 20% 2	3800
	11th to 20th	Night	25% 5% 10% 29% 27% 4	3779
	21st to end	Night	26% 4 12% 32% 23% 3	4203
November	1st to 10th	Night	32% 6% 12% 26% 20% 5%	2505
	11th to 20th	Night	40% 4 10% 23% 19% 4	2470
	21st to end	Night	37% 5% 13% 28% 15% 2	2622
December	1st to 10th	Night	47% 6% 12% 21% 13% 1	2660
	11th to 20th	Night	43% 6% 12% 21% 15% 3	3288
	21st to end	Night	45% 8% 14% 22% 10% 1	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	34% 22% 24% 16% 4	180
	11th to 20th	Day	39% 25% 26% 9% 1	207
	21st to end	Day	29% 29% 26% 13% 3 1	297
February	1st to 10th	Day	29% 26% 33% 9% 3 1	270
	11th to 20th	Day	32% 26% 32% 9% 1	386
	21st to end	Day	36% 20% 26% 14% 2	382
March	1st to 10th	Day	34% 17% 29% 16% 4	460
	11th to 20th	Day	31% 25% 28% 12% 3 1	460
	21st to end	Day	31% 23% 30% 11% 4 1	506
April	1st to 10th	Day	27% 24% 29% 14% 4 1	460
	11th to 20th	Day	26% 23% 27% 16% 6% 3	452
	21st to end	Day	28% 17% 34% 15% 4 2	460
May	1st to 10th	Day	35% 17% 26% 14% 6% 2	463
	11th to 20th	Day	24% 14% 31% 20% 9% 2	487
	21st to end	Day	18% 11% 28% 24% 16% 3	529
June	1st to 10th	Day	17% 13% 31% 22% 13% 3	490
	11th to 20th	Day	17% 11% 29% 26% 14% 3	490
	21st to end	Day	13% 12% 30% 27% 14% 4	490
July	1st to 10th	Day	12% 5% 24% 33% 21% 7%	485
	11th to 20th	Day	10% 4 24% 36% 22% 5%	484
	21st to end	Day	12% 6% 30% 30% 18% 3	532
August	1st to 10th	Day	12% 6% 25% 30% 21% 7%	486
	11th to 20th	Day	13% 5% 21% 34% 19% 7%	479
	21st to end	Day	14% 6% 23% 29% 17% 11%	539
September	1st to 10th	Day	19% 9% 21% 26% 16% 9%	490
	11th to 20th	Day	15% 5% 26% 27% 17% 10%	490
	21st to end	Day	24% 15% 22% 24% 12% 4	490
October	1st to 10th	Day	26% 8% 27% 20% 15% 5%	490
	11th to 20th	Day	19% 18% 26% 23% 12% 2	490
	21st to end	Day	20% 18% 32% 16% 11% 3	512
November	1st to 10th	Day	20% 14% 31% 21% 11% 3	386
	11th to 20th	Day	24% 22% 29% 19% 6% 1	370
	21st to end	Day	23% 21% 33% 19% 3 1	214
December	1st to 10th	Day	32% 24% 29% 9% 5% 1	180
	11th to 20th	Day	36% 26% 24% 12% 3	180
	21st to end	Day	29% 21% 30% 13% 6% 1	198
January	1st to 10th	Night	21% 23% 37% 17% 3	760
	11th to 20th	Night	28% 18% 30% 22% 3	760
	21st to end	Night	19% 22% 36% 21% 2	836
February	1st to 10th	Night	23% 19% 34% 22% 1	760
	11th to 20th	Night	28% 25% 33% 13% 1	743
	21st to end	Night	26% 21% 34% 16% 3	628
March	1st to 10th	Night	25% 20% 30% 21% 4	760
	11th to 20th	Night	24% 22% 31% 17% 5%	760
	21st to end	Night	22% 17% 37% 19% 5%	836
April	1st to 10th	Night	22% 23% 34% 17% 4	760
	11th to 20th	Night	20% 15% 37% 22% 7%	751
	21st to end	Night	22% 16% 30% 25% 6%	760
May	1st to 10th	Night	25% 12% 31% 26% 6%	602
	11th to 20th	Night	20% 8% 34% 31% 7% 1	374
	21st to end	Night	17% 7% 23% 39% 12% 1	411
June	1st to 10th	Night	15% 6% 26% 39% 13%	380
	11th to 20th	Night	11% 6% 28% 42% 12% 1	380
	21st to end	Night	10% 4 26% 44% 14% 1	380
July	1st to 10th	Night	11% 4 18% 45% 20% 2	370
	11th to 20th	Night	8% 3 20% 41% 27% 2	368
	21st to end	Night	12% 4 19% 42% 21% 2	478
August	1st to 10th	Night	11% 3 16% 43% 26% 2	567
	11th to 20th	Night	14% 4 17% 37% 26% 3	701
	21st to end	Night	13% 4 18% 39% 24% 3	836
September	1st to 10th	Night	17% 4 25% 32% 18% 3	760
	11th to 20th	Night	14% 5% 18% 36% 23% 4	760
	21st to end	Night	22% 7% 22% 29% 18% 1	760
October	1st to 10th	Night	23% 8% 24% 25% 17% 3	760
	11th to 20th	Night	19% 10% 30% 28% 13%	760
	21st to end	Night	24% 13% 27% 26% 10%	836
November	1st to 10th	Night	15% 12% 32% 28% 12% 1	771
	11th to 20th	Night	23% 17% 26% 26% 8% 1	760
	21st to end	Night	18% 18% 31% 25% 7%	760
December	1st to 10th	Night	26% 18% 30% 21% 5% 1	760
	11th to 20th	Night	21% 18% 34% 22% 5%	760
	21st to end	Night	23% 18% 32% 24% 3	836

Page generated at 2024-11-05 08:15:53. Version ac07

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