Chelsea
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chelsea clear sky chart from 2005-11-06 to 2026-03-18.

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% 10% 4 8% 5% 5% 5% 4 5% 7% 31%	131420
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 30%	130067
Transparency	Day	36% 35% 7% 15% 7% 1	134885
	Night	38% 34% 6% 13% 7% 1	134689
Seeing	Day	25% 16% 31% 19% 8% 2	16071
	Night	24% 13% 28% 25% 9% 1	25299

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	14% 16% 7% 15% 6% 5% 5% 3 4 6% 20%	4661
	11th to 20th	Day	11% 20% 7% 14% 7% 5% 4 3 4 5% 18%	4650
	21st to end	Day	8% 15% 7% 15% 8% 8% 5% 4 5% 5% 20%	5344
February	1st to 10th	Day	13% 15% 6% 13% 7% 5% 4 4 5% 5% 24%	1890
	11th to 20th	Day	12% 15% 6% 11% 6% 5% 4 4 5% 5% 27%	1843
	21st to end	Day	19% 13% 5% 10% 7% 5% 4 4 5% 4 24%	1730
March	1st to 10th	Day	20% 15% 4 10% 5% 5% 4 3 5% 5% 24%	4576
	11th to 20th	Day	14% 12% 5% 9% 5% 5% 5% 4 6% 6% 30%	5057
	21st to end	Day	18% 12% 3 8% 6% 4 3 3 4 5% 35%	5774
April	1st to 10th	Day	22% 12% 3 7% 4 4 4 2 4 6% 32%	2400
	11th to 20th	Day	22% 10% 3 7% 4 3 4 3 4 4 36%	2374
	21st to end	Day	22% 10% 3 8% 5% 5% 4 3 4 5% 31%	2550
May	1st to 10th	Day	23% 9% 3 8% 5% 5% 5% 4 6% 5% 26%	6346
	11th to 20th	Day	17% 9% 3 9% 5% 6% 4 4 5% 7% 32%	6318
	21st to end	Day	22% 9% 3 6% 5% 4 4 3 5% 6% 32%	6949
June	1st to 10th	Day	22% 8% 2 7% 6% 5% 5% 4 5% 6% 30%	2800
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	2800
	21st to end	Day	20% 6% 2 6% 5% 5% 6% 5% 7% 6% 32%	2800
July	1st to 10th	Day	14% 6% 2 7% 5% 5% 6% 5% 7% 9% 33%	6360
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 6% 7% 43%	6360
	21st to end	Day	9% 5% 2 5% 4 3 5% 4 6% 10% 46%	6926
August	1st to 10th	Day	13% 6% 3 8% 4 5% 5% 4 6% 8% 37%	4500
	11th to 20th	Day	17% 5% 2 6% 4 4 4 4 7% 9% 37%	4126
	21st to end	Day	13% 5% 2 6% 4 5% 5% 4 7% 9% 40%	4312
September	1st to 10th	Day	14% 5% 2 6% 5% 4 4 4 5% 8% 43%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 48%	2400
	21st to end	Day	20% 5% 2 6% 4 3 4 4 5% 8% 40%	2135
October	1st to 10th	Day	20% 8% 3 6% 3 4 4 4 6% 6% 36%	2000
	11th to 20th	Day	23% 9% 2 8% 5% 4 5% 4 5% 6% 29%	2000
	21st to end	Day	26% 9% 3 7% 4 5% 4 3 4 6% 28%	2038
November	1st to 10th	Day	16% 10% 4 7% 5% 4 5% 3 6% 7% 32%	1847
	11th to 20th	Day	19% 13% 4 7% 5% 5% 5% 4 6% 6% 26%	1680
	21st to end	Day	22% 15% 5% 8% 7% 4 4 4 4 6% 20%	1680
December	1st to 10th	Day	20% 21% 5% 12% 5% 4 4 4 6% 5% 14%	2680
	11th to 20th	Day	17% 16% 6% 11% 5% 5% 4 3 3 4 25%	3066
	21st to end	Day	23% 18% 5% 10% 5% 5% 4 3 3 4 20%	4048
January	1st to 10th	Night	13% 20% 7% 14% 6% 5% 4 3 4 5% 20%	8234
	11th to 20th	Night	11% 19% 7% 15% 7% 6% 5% 3 4 5% 19%	8235
	21st to end	Night	10% 17% 6% 12% 7% 7% 5% 4 5% 5% 22%	8835
February	1st to 10th	Night	13% 14% 5% 12% 7% 5% 5% 4 5% 5% 25%	2730
	11th to 20th	Night	13% 14% 6% 11% 8% 5% 4 3 4 6% 28%	2663
	21st to end	Night	16% 14% 5% 8% 6% 5% 5% 3 5% 5% 28%	2076
March	1st to 10th	Night	22% 14% 4 9% 5% 4 4 3 3 3 28%	5467
	11th to 20th	Night	12% 13% 4 9% 5% 5% 5% 4 6% 8% 29%	5057
	21st to end	Night	18% 11% 3 9% 5% 4 3 3 4 6% 33%	5157
April	1st to 10th	Night	21% 11% 4 8% 4 4 4 3 4 5% 32%	2000
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 3 4 6% 36%	1975
	21st to end	Night	22% 9% 3 6% 4 4 4 3 5% 6% 34%	1866
May	1st to 10th	Night	23% 9% 3 9% 4 5% 6% 4 6% 6% 25%	3594
	11th to 20th	Night	17% 7% 2 7% 5% 4 5% 3 5% 6% 38%	3575
	21st to end	Night	20% 8% 3 8% 5% 5% 5% 4 6% 5% 32%	3926
June	1st to 10th	Night	16% 6% 3 8% 5% 5% 4 5% 6% 7% 35%	1600
	11th to 20th	Night	14% 6% 2 7% 4 4 3 4 6% 7% 43%	1600
	21st to end	Night	19% 7% 2 5% 5% 5% 5% 3 5% 8% 35%	1600
July	1st to 10th	Night	14% 6% 3 8% 5% 6% 6% 5% 6% 8% 34%	3600
	11th to 20th	Night	13% 6% 2 6% 5% 4 5% 4 5% 8% 43%	3600
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 7% 7% 48%	3926
August	1st to 10th	Night	12% 7% 3 7% 4 4 5% 4 6% 8% 40%	2646
	11th to 20th	Night	15% 6% 2 6% 3 4 4 4 5% 8% 42%	2889
	21st to end	Night	14% 5% 2 6% 5% 5% 4 4 6% 7% 41%	3586
September	1st to 10th	Night	12% 5% 2 6% 4 3 4 4 5% 7% 48%	2000
	11th to 20th	Night	15% 4 2 5% 3 3 4 4 5% 6% 51%	2000
	21st to end	Night	18% 6% 2 7% 5% 3 4 2 4 6% 44%	2264
October	1st to 10th	Night	19% 6% 2 6% 4 4 4 3 4 6% 41%	2400
	11th to 20th	Night	21% 7% 2 7% 6% 5% 5% 4 5% 5% 33%	2400
	21st to end	Night	23% 10% 2 8% 5% 5% 4 3 4 7% 29%	2801
November	1st to 10th	Night	19% 11% 3 7% 4 4 4 5% 4 6% 32%	2667
	11th to 20th	Night	19% 11% 4 9% 5% 5% 5% 4 4 5% 30%	2940
	21st to end	Night	21% 13% 5% 9% 6% 5% 4 3 5% 6% 24%	2940
December	1st to 10th	Night	19% 17% 5% 11% 6% 5% 5% 5% 5% 6% 16%	4720
	11th to 20th	Night	23% 14% 5% 9% 6% 4 3 2 3 4 26%	5348
	21st to end	Night	25% 18% 4 10% 5% 3 3 3 4 4 22%	7150

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	55% 19% 7% 13% 5%	4662
	11th to 20th	Day	59% 19% 5% 8% 8%	4680
	21st to end	Day	56% 21% 6% 9% 7% 1	5344
February	1st to 10th	Day	28% 58% 3 8% 3 1	1890
	11th to 20th	Day	27% 57% 3 9% 5%	1852
	21st to end	Day	26% 58% 4 4 6% 2	1730
March	1st to 10th	Day	49% 27% 4 7% 10% 3	4579
	11th to 20th	Day	41% 27% 11% 11% 8% 1	5057
	21st to end	Day	41% 27% 2 14% 15% 1	5774
April	1st to 10th	Day	22% 62% 2 6% 7% 1	2400
	11th to 20th	Day	22% 62% 1 9% 6% 1	2374
	21st to end	Day	22% 61% 3 7% 6%	2550
May	1st to 10th	Day	47% 27% 4 16% 6% 1	6357
	11th to 20th	Day	41% 27% 11% 15% 7%	6322
	21st to end	Day	43% 27% 7% 15% 8%	6949
June	1st to 10th	Day	20% 61% 3 10% 5% 1	2800
	11th to 20th	Day	18% 62% 5% 10% 5% 1	2800
	21st to end	Day	19% 60% 4 12% 5%	2800
July	1st to 10th	Day	37% 24% 11% 21% 7%	6290
	11th to 20th	Day	30% 24% 9% 28% 8%	6285
	21st to end	Day	25% 24% 12% 31% 8%	6728
August	1st to 10th	Day	31% 35% 12% 17% 6%	4486
	11th to 20th	Day	31% 35% 9% 21% 5%	4165
	21st to end	Day	27% 35% 11% 19% 8%	4312
September	1st to 10th	Day	17% 56% 8% 13% 6%	2400
	11th to 20th	Day	14% 57% 5% 18% 7%	2400
	21st to end	Day	17% 56% 6% 13% 8%	2135
October	1st to 10th	Day	33% 34% 6% 23% 4	3260
	11th to 20th	Day	29% 36% 8% 19% 8%	3240
	21st to end	Day	33% 35% 14% 15% 4	3263
November	1st to 10th	Day	30% 53% 5% 9% 3 1	1847
	11th to 20th	Day	27% 54% 4 10% 4 1	1680
	21st to end	Day	33% 54% 5% 4 4	1680
December	1st to 10th	Day	49% 34% 7% 8% 1	2680
	11th to 20th	Day	49% 30% 5% 10% 6%	3066
	21st to end	Day	52% 25% 3 15% 5%	4048
January	1st to 10th	Night	57% 19% 8% 11% 5%	8234
	11th to 20th	Night	58% 19% 6% 11% 5%	8280
	21st to end	Night	55% 21% 5% 9% 10%	8835
February	1st to 10th	Night	25% 58% 3 10% 4	2730
	11th to 20th	Night	25% 57% 3 11% 5%	2665
	21st to end	Night	25% 58% 3 8% 6%	2076
March	1st to 10th	Night	48% 27% 4 9% 10% 3	5477
	11th to 20th	Night	41% 28% 11% 12% 6% 2	5058
	21st to end	Night	42% 27% 3 16% 10% 2	5157
April	1st to 10th	Night	22% 61% 3 8% 6% 1	2000
	11th to 20th	Night	20% 61% 2 9% 7%	1975
	21st to end	Night	20% 61% 4 7% 7% 1	1866
May	1st to 10th	Night	46% 27% 5% 16% 4 1	3600
	11th to 20th	Night	37% 27% 10% 15% 10%	3576
	21st to end	Night	44% 27% 7% 13% 8%	3927
June	1st to 10th	Night	18% 62% 2 12% 5% 1	1600
	11th to 20th	Night	14% 62% 3 11% 8% 1	1600
	21st to end	Night	19% 60% 3 11% 7%	1600
July	1st to 10th	Night	39% 24% 13% 15% 8%	3560
	11th to 20th	Night	31% 24% 10% 27% 8%	3552
	21st to end	Night	26% 24% 10% 29% 11%	3809
August	1st to 10th	Night	29% 35% 10% 19% 6%	2636
	11th to 20th	Night	31% 35% 10% 20% 4	2916
	21st to end	Night	30% 35% 8% 18% 9% 2	3586
September	1st to 10th	Night	13% 57% 6% 17% 6%	2000
	11th to 20th	Night	14% 57% 5% 17% 6% 1	2000
	21st to end	Night	17% 56% 4 15% 8%	2264
October	1st to 10th	Night	25% 34% 8% 23% 9%	3920
	11th to 20th	Night	27% 34% 10% 21% 7%	3899
	21st to end	Night	32% 35% 12% 14% 7%	4526
November	1st to 10th	Night	28% 52% 6% 9% 4	2667
	11th to 20th	Night	27% 54% 4 10% 5%	2940
	21st to end	Night	30% 54% 5% 8% 4	2940
December	1st to 10th	Night	45% 33% 9% 10% 2	4720
	11th to 20th	Night	47% 29% 3 10% 10% 1	5348
	21st to end	Night	53% 24% 2 11% 10%	7150

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	27% 16% 37% 15% 5%	220
	11th to 20th	Day	29% 19% 38% 11% 2 1	220
	21st to end	Day	21% 21% 36% 18% 4	283
February	1st to 10th	Day	34% 22% 32% 10% 2	330
	11th to 20th	Day	29% 28% 36% 7% 1	330
	21st to end	Day	31% 22% 32% 13% 2	446
March	1st to 10th	Day	29% 20% 33% 13% 5%	540
	11th to 20th	Day	31% 20% 32% 14% 3 1	528
	21st to end	Day	33% 18% 30% 13% 5% 1	550
April	1st to 10th	Day	33% 23% 26% 14% 4 1	500
	11th to 20th	Day	36% 19% 23% 13% 7% 1	492
	21st to end	Day	32% 22% 29% 11% 5% 1	500
May	1st to 10th	Day	27% 17% 32% 15% 6% 2	503
	11th to 20th	Day	19% 18% 35% 19% 7% 2	527
	21st to end	Day	25% 14% 32% 20% 8% 2	573
June	1st to 10th	Day	27% 12% 34% 19% 7% 1	530
	11th to 20th	Day	19% 14% 32% 21% 11% 2	530
	21st to end	Day	25% 12% 32% 19% 9% 2	530
July	1st to 10th	Day	15% 8% 33% 31% 10% 3	525
	11th to 20th	Day	14% 10% 36% 27% 10% 3	524
	21st to end	Day	13% 9% 36% 28% 11% 3	576
August	1st to 10th	Day	15% 9% 28% 32% 14% 3	526
	11th to 20th	Day	19% 4 28% 29% 13% 7%	519
	21st to end	Day	17% 11% 29% 25% 13% 5%	583
September	1st to 10th	Day	18% 11% 31% 21% 13% 6%	530
	11th to 20th	Day	14% 9% 26% 21% 20% 10%	530
	21st to end	Day	22% 12% 28% 21% 13% 4	530
October	1st to 10th	Day	24% 16% 24% 22% 10% 3	530
	11th to 20th	Day	27% 18% 33% 15% 6% 1	530
	21st to end	Day	32% 16% 29% 20% 3	493
November	1st to 10th	Day	29% 18% 30% 17% 6% 1	421
	11th to 20th	Day	27% 25% 31% 13% 4	220
	21st to end	Day	33% 22% 30% 11% 2 2	220
December	1st to 10th	Day	34% 13% 39% 12% 2	220
	11th to 20th	Day	28% 21% 40% 9% 2	220
	21st to end	Day	34% 19% 32% 12% 2 1	242
January	1st to 10th	Night	28% 16% 34% 19% 3	840
	11th to 20th	Night	31% 17% 25% 21% 5%	840
	21st to end	Night	20% 15% 35% 24% 5%	924
February	1st to 10th	Night	21% 20% 31% 24% 4	840
	11th to 20th	Night	22% 21% 35% 18% 4	823
	21st to end	Night	25% 18% 29% 23% 4	692
March	1st to 10th	Night	28% 16% 32% 21% 4	840
	11th to 20th	Night	26% 19% 31% 20% 5%	829
	21st to end	Night	27% 17% 28% 21% 7%	880
April	1st to 10th	Night	30% 16% 27% 24% 4	800
	11th to 20th	Night	25% 19% 28% 20% 7% 1	791
	21st to end	Night	28% 15% 27% 22% 8%	666
May	1st to 10th	Night	26% 10% 27% 29% 8% 2	400
	11th to 20th	Night	23% 9% 28% 30% 10%	394
	21st to end	Night	20% 7% 35% 27% 10% 1	433
June	1st to 10th	Night	20% 9% 28% 35% 9%	400
	11th to 20th	Night	16% 10% 27% 33% 14% 1	400
	21st to end	Night	21% 7% 24% 35% 14%	400
July	1st to 10th	Night	13% 3 28% 38% 18%	390
	11th to 20th	Night	13% 5% 22% 44% 16% 1	388
	21st to end	Night	10% 5% 24% 42% 17% 2	438
August	1st to 10th	Night	12% 4 19% 42% 22% 1	432
	11th to 20th	Night	14% 5% 17% 36% 24% 3	595
	21st to end	Night	15% 6% 25% 34% 18% 2	880
September	1st to 10th	Night	15% 7% 29% 29% 20% 1	800
	11th to 20th	Night	17% 6% 20% 31% 23% 4	800
	21st to end	Night	21% 8% 26% 26% 18% 2	800
October	1st to 10th	Night	23% 11% 26% 24% 15% 1	800
	11th to 20th	Night	26% 12% 28% 25% 8% 1	800
	21st to end	Night	29% 14% 28% 24% 5%	880
November	1st to 10th	Night	28% 15% 23% 25% 8% 1	820
	11th to 20th	Night	26% 17% 31% 17% 9%	840
	21st to end	Night	30% 19% 24% 22% 5%	840
December	1st to 10th	Night	29% 16% 30% 20% 4	840
	11th to 20th	Night	29% 19% 27% 19% 4 1	840
	21st to end	Night	35% 15% 30% 16% 4	924

Page generated at 2026-03-15 02:23:51. Version ac07

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