Greenwich Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greenwich Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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% 39%	127305
	Night	17% 7% 3 6% 4 3 4 3 5% 6% 42%	124384
Transparency	Day	42% 5% 12% 22% 16% 3	130819
	Night	42% 4 11% 22% 17% 3	128944
Seeing	Day	22% 16% 31% 21% 8% 2	16459
	Night	21% 13% 32% 25% 8% 1	25647

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 5% 4 4 4 3 5% 5% 46%	3677
	11th to 20th	Day	14% 10% 3 6% 5% 4 4 3 4 5% 41%	3618
	21st to end	Day	18% 9% 3 7% 4 5% 4 3 4 5% 39%	4215
February	1st to 10th	Day	17% 8% 2 7% 5% 4 6% 3 5% 7% 37%	1827
	11th to 20th	Day	15% 8% 2 7% 5% 4 3 3 4 6% 42%	1948
	21st to end	Day	20% 9% 3 6% 4 3 4 3 4 5% 39%	1650
March	1st to 10th	Day	23% 7% 2 5% 4 4 3 3 4 5% 41%	4476
	11th to 20th	Day	13% 6% 3 6% 5% 4 4 4 6% 7% 42%	4699
	21st to end	Day	19% 7% 2 6% 4 4 4 3 4 6% 39%	5969
April	1st to 10th	Day	19% 10% 3 6% 4 3 4 3 5% 6% 37%	2400
	11th to 20th	Day	18% 7% 2 6% 4 3 4 3 4 7% 40%	2374
	21st to end	Day	19% 6% 2 6% 4 4 4 4 6% 6% 40%	2456
May	1st to 10th	Day	30% 8% 2 5% 4 4 4 3 5% 8% 27%	5886
	11th to 20th	Day	22% 9% 3 6% 4 4 4 3 4 7% 34%	6226
	21st to end	Day	22% 6% 2 5% 4 4 5% 4 6% 7% 35%	6949
June	1st to 10th	Day	19% 6% 2 5% 4 4 5% 4 7% 8% 36%	2800
	11th to 20th	Day	17% 7% 3 6% 4 4 5% 4 6% 8% 37%	2800
	21st to end	Day	12% 7% 2 6% 5% 5% 5% 4 7% 9% 38%	2800
July	1st to 10th	Day	13% 7% 2 7% 5% 5% 6% 5% 7% 9% 34%	6360
	11th to 20th	Day	13% 7% 3 7% 5% 5% 6% 5% 8% 11% 32%	6360
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	6926
August	1st to 10th	Day	21% 6% 2 7% 4 5% 5% 4 7% 7% 32%	4351
	11th to 20th	Day	13% 6% 3 6% 4 4 4 5% 6% 9% 40%	3850
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 9% 46%	4312
September	1st to 10th	Day	14% 5% 2 6% 4 5% 5% 3 5% 6% 44%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 4 4 5% 7% 50%	2295
	21st to end	Day	17% 6% 3 7% 4 5% 4 3 5% 6% 39%	2200
October	1st to 10th	Day	21% 6% 2 5% 3 4 4 3 4 6% 43%	2116
	11th to 20th	Day	15% 5% 2 5% 3 4 4 3 5% 8% 46%	2000
	21st to end	Day	21% 5% 2 5% 4 3 4 3 5% 5% 42%	2010
November	1st to 10th	Day	14% 6% 2 4 4 3 3 3 5% 6% 50%	1851
	11th to 20th	Day	18% 6% 2 4 3 3 4 3 5% 6% 47%	1890
	21st to end	Day	17% 7% 3 5% 4 4 4 3 5% 6% 43%	1820
December	1st to 10th	Day	15% 7% 3 6% 5% 4 3 3 5% 6% 42%	2680
	11th to 20th	Day	22% 8% 2 5% 3 2 3 2 4 5% 45%	3066
	21st to end	Day	18% 9% 3 7% 4 4 4 3 4 6% 37%	4048
January	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 7% 47%	6489
	11th to 20th	Night	14% 8% 3 7% 4 3 4 3 4 6% 46%	6400
	21st to end	Night	17% 9% 3 6% 4 4 4 3 4 5% 40%	6738
February	1st to 10th	Night	16% 9% 3 6% 4 3 4 3 4 6% 41%	2570
	11th to 20th	Night	17% 8% 3 6% 4 4 3 3 5% 6% 41%	2338
	21st to end	Night	18% 8% 3 6% 4 4 4 3 4 5% 41%	1980
March	1st to 10th	Night	20% 9% 2 5% 3 3 3 3 4 6% 42%	5364
	11th to 20th	Night	15% 8% 4 6% 4 4 4 3 4 6% 44%	5256
	21st to end	Night	18% 10% 3 6% 4 3 3 3 4 5% 40%	4962
April	1st to 10th	Night	19% 8% 2 6% 5% 4 4 3 5% 6% 37%	2000
	11th to 20th	Night	20% 6% 2 6% 4 3 3 3 4 7% 39%	1975
	21st to end	Night	19% 7% 2 7% 4 4 3 3 5% 6% 41%	1941
May	1st to 10th	Night	29% 9% 3 7% 4 4 3 3 5% 7% 26%	4054
	11th to 20th	Night	21% 10% 3 6% 4 4 4 3 5% 6% 34%	3695
	21st to end	Night	24% 8% 2 5% 4 4 3 2 5% 7% 37%	3926
June	1st to 10th	Night	18% 5% 2 6% 4 4 4 4 5% 8% 41%	1600
	11th to 20th	Night	15% 7% 3 6% 5% 3 4 4 5% 8% 41%	1600
	21st to end	Night	12% 6% 2 5% 4 4 4 4 6% 9% 44%	1600
July	1st to 10th	Night	13% 7% 2 8% 5% 5% 5% 4 7% 8% 37%	3600
	11th to 20th	Night	17% 9% 3 7% 5% 4 4 4 6% 9% 33%	3600
	21st to end	Night	15% 6% 2 5% 4 3 5% 3 6% 7% 45%	3926
August	1st to 10th	Night	23% 6% 3 7% 4 5% 4 4 6% 6% 32%	2761
	11th to 20th	Night	15% 6% 2 7% 5% 4 4 4 5% 7% 40%	3197
	21st to end	Night	11% 5% 2 4 3 3 4 3 5% 8% 53%	3586
September	1st to 10th	Night	15% 5% 3 5% 4 4 4 3 5% 6% 46%	2000
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 6% 52%	2105
	21st to end	Night	18% 8% 3 5% 2 4 4 2 4 6% 44%	2200
October	1st to 10th	Night	19% 7% 2 6% 4 3 3 2 4 5% 44%	2280
	11th to 20th	Night	16% 6% 2 5% 3 3 3 2 5% 7% 48%	2400
	21st to end	Night	21% 7% 2 5% 3 2 3 3 5% 6% 42%	2828
November	1st to 10th	Night	12% 6% 2 6% 3 3 4 3 5% 6% 50%	2667
	11th to 20th	Night	18% 6% 2 5% 3 2 3 3 5% 5% 48%	2730
	21st to end	Night	15% 6% 2 5% 4 3 3 4 5% 7% 46%	2798
December	1st to 10th	Night	16% 7% 2 6% 4 3 3 2 4 5% 48%	4720
	11th to 20th	Night	24% 8% 2 4 3 3 3 3 4 4 42%	5348
	21st to end	Night	19% 8% 3 6% 4 3 4 3 4 5% 41%	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	36% 4 8% 30% 19% 3	3678
	11th to 20th	Day	44% 3 10% 22% 15% 6%	3648
	21st to end	Day	47% 5% 9% 24% 14% 1	4215
February	1st to 10th	Day	46% 4 13% 22% 14% 2	1827
	11th to 20th	Day	42% 4 9% 24% 18% 3	1957
	21st to end	Day	46% 4 11% 22% 15% 3	1650
March	1st to 10th	Day	44% 3 12% 19% 13% 9%	4479
	11th to 20th	Day	38% 6% 14% 28% 11% 3	4699
	21st to end	Day	45% 4 9% 13% 23% 6%	5969
April	1st to 10th	Day	46% 4 10% 20% 16% 4	2400
	11th to 20th	Day	43% 5% 11% 20% 19% 3	2374
	21st to end	Day	43% 5% 11% 21% 17% 4	2456
May	1st to 10th	Day	55% 6% 14% 14% 11% 1	5897
	11th to 20th	Day	48% 6% 11% 19% 14% 2	6230
	21st to end	Day	45% 6% 11% 22% 14% 1	6949
June	1st to 10th	Day	43% 7% 13% 21% 15% 1	2800
	11th to 20th	Day	43% 7% 13% 24% 13% 2	2800
	21st to end	Day	41% 7% 13% 22% 15% 1	2800
July	1st to 10th	Day	44% 8% 11% 20% 16% 1	6290
	11th to 20th	Day	43% 7% 18% 22% 10% 1	6285
	21st to end	Day	42% 6% 11% 20% 19% 2	6728
August	1st to 10th	Day	48% 7% 14% 22% 9% 1	4337
	11th to 20th	Day	38% 6% 13% 30% 11% 1	3886
	21st to end	Day	34% 6% 11% 27% 20% 1	4312
September	1st to 10th	Day	38% 6% 11% 23% 19% 2	2400
	11th to 20th	Day	33% 5% 10% 25% 25% 2	2295
	21st to end	Day	44% 6% 10% 19% 18% 3	2200
October	1st to 10th	Day	36% 4 10% 20% 24% 6%	3441
	11th to 20th	Day	28% 4 10% 23% 30% 5%	3240
	21st to end	Day	30% 3 14% 24% 25% 4	3222
November	1st to 10th	Day	35% 4 13% 23% 22% 3	1851
	11th to 20th	Day	37% 4 9% 23% 22% 4	1890
	21st to end	Day	41% 4 13% 27% 13% 2	1820
December	1st to 10th	Day	41% 4 15% 27% 12% 1	2680
	11th to 20th	Day	43% 3 13% 22% 17% 3	3066
	21st to end	Day	48% 5% 7% 21% 16% 3	4048
January	1st to 10th	Night	35% 3 9% 26% 23% 4	6489
	11th to 20th	Night	40% 3 13% 21% 16% 6%	6445
	21st to end	Night	46% 4 10% 20% 16% 5%	6738
February	1st to 10th	Night	44% 5% 11% 24% 13% 2	2570
	11th to 20th	Night	44% 5% 9% 20% 18% 5%	2340
	21st to end	Night	46% 3 8% 22% 17% 4	1980
March	1st to 10th	Night	44% 4 10% 20% 14% 9%	5374
	11th to 20th	Night	41% 4 15% 24% 12% 4	5257
	21st to end	Night	47% 3 7% 14% 23% 5%	4962
April	1st to 10th	Night	47% 5% 9% 19% 16% 4	2000
	11th to 20th	Night	45% 5% 10% 19% 21% 2	1975
	21st to end	Night	44% 5% 9% 21% 18% 4	1941
May	1st to 10th	Night	58% 5% 12% 14% 10% 1	4060
	11th to 20th	Night	50% 6% 13% 14% 14% 2	3696
	21st to end	Night	48% 4 13% 22% 10% 2	3927
June	1st to 10th	Night	42% 7% 10% 21% 19% 2	1600
	11th to 20th	Night	41% 7% 13% 27% 11% 1	1600
	21st to end	Night	36% 8% 14% 25% 15% 2	1600
July	1st to 10th	Night	44% 8% 12% 21% 13% 2	3560
	11th to 20th	Night	48% 6% 16% 20% 11% 1	3552
	21st to end	Night	38% 6% 13% 23% 19% 1	3809
August	1st to 10th	Night	51% 7% 14% 17% 10% 1	2751
	11th to 20th	Night	43% 7% 11% 29% 9% 1	3227
	21st to end	Night	30% 6% 12% 32% 19% 1	3586
September	1st to 10th	Night	40% 6% 9% 27% 18% 1	2000
	11th to 20th	Night	33% 5% 11% 24% 25% 3	2105
	21st to end	Night	43% 4 8% 22% 20% 3	2200
October	1st to 10th	Night	39% 4 10% 23% 19% 3	3722
	11th to 20th	Night	26% 3 9% 28% 30% 4	3899
	21st to end	Night	32% 4 12% 28% 21% 3	4566
November	1st to 10th	Night	35% 4 11% 23% 22% 4	2667
	11th to 20th	Night	39% 3 9% 24% 21% 5%	2730
	21st to end	Night	38% 5% 12% 30% 14% 1	2798
December	1st to 10th	Night	39% 3 11% 30% 14% 2	4720
	11th to 20th	Night	46% 3 10% 20% 18% 3	5348
	21st to end	Night	46% 4 11% 19% 17% 5%	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% 31% 25% 13% 4	220
	11th to 20th	Day	28% 26% 34% 9% 2	212
	21st to end	Day	23% 26% 36% 11% 4	277
February	1st to 10th	Day	29% 27% 34% 10% 1	327
	11th to 20th	Day	28% 25% 35% 11% 1	496
	21st to end	Day	30% 20% 35% 13% 2	414
March	1st to 10th	Day	25% 24% 34% 15% 2	500
	11th to 20th	Day	22% 25% 34% 15% 3 1	500
	21st to end	Day	30% 23% 31% 12% 3 1	550
April	1st to 10th	Day	32% 23% 27% 12% 5% 1	500
	11th to 20th	Day	27% 23% 28% 12% 8% 1	492
	21st to end	Day	28% 18% 34% 15% 4 1	500
May	1st to 10th	Day	32% 18% 28% 15% 6% 1	503
	11th to 20th	Day	21% 17% 35% 20% 6% 1	527
	21st to end	Day	21% 10% 31% 25% 11% 2	573
June	1st to 10th	Day	19% 13% 29% 27% 11% 2	530
	11th to 20th	Day	20% 12% 33% 25% 8% 1	530
	21st to end	Day	15% 10% 31% 30% 12% 2	530
July	1st to 10th	Day	15% 9% 26% 31% 15% 5%	525
	11th to 20th	Day	10% 6% 29% 32% 18% 4	524
	21st to end	Day	14% 9% 29% 34% 13% 1	576
August	1st to 10th	Day	14% 10% 26% 31% 13% 6%	526
	11th to 20th	Day	14% 6% 26% 33% 15% 5%	519
	21st to end	Day	10% 10% 30% 30% 12% 8%	583
September	1st to 10th	Day	18% 10% 28% 24% 16% 4	530
	11th to 20th	Day	14% 7% 32% 25% 13% 9%	530
	21st to end	Day	21% 11% 32% 22% 11% 4	530
October	1st to 10th	Day	25% 9% 28% 21% 13% 4	530
	11th to 20th	Day	18% 19% 32% 20% 9% 2	530
	21st to end	Day	24% 17% 30% 20% 8% 1	478
November	1st to 10th	Day	18% 16% 33% 22% 8% 2	425
	11th to 20th	Day	24% 20% 34% 17% 5%	430
	21st to end	Day	25% 20% 32% 19% 4 1	360
December	1st to 10th	Day	26% 24% 32% 13% 5%	220
	11th to 20th	Day	34% 25% 28% 10% 3	220
	21st to end	Day	31% 18% 32% 15% 4	242
January	1st to 10th	Night	23% 20% 37% 18% 3	840
	11th to 20th	Night	26% 19% 33% 20% 3	825
	21st to end	Night	20% 20% 40% 18% 2	880
February	1st to 10th	Night	22% 18% 39% 19% 2	800
	11th to 20th	Night	26% 23% 35% 15% 2	783
	21st to end	Night	25% 19% 36% 17% 3	660
March	1st to 10th	Night	24% 18% 37% 18% 3	800
	11th to 20th	Night	22% 18% 39% 17% 4 1	800
	21st to end	Night	24% 19% 36% 17% 4	880
April	1st to 10th	Night	25% 18% 36% 17% 4	800
	11th to 20th	Night	24% 15% 36% 21% 5%	791
	21st to end	Night	24% 14% 35% 21% 6% 1	741
May	1st to 10th	Night	25% 14% 32% 22% 7%	600
	11th to 20th	Night	24% 14% 32% 24% 6%	449
	21st to end	Night	23% 7% 22% 38% 10% 1	433
June	1st to 10th	Night	19% 6% 32% 32% 11% 1	400
	11th to 20th	Night	15% 5% 28% 39% 13%	400
	21st to end	Night	11% 5% 26% 45% 13% 1	400
July	1st to 10th	Night	14% 4 22% 41% 19% 1	390
	11th to 20th	Night	15% 4 20% 41% 18% 1	388
	21st to end	Night	14% 4 22% 42% 17% 1	438
August	1st to 10th	Night	13% 3 21% 45% 17% 1	495
	11th to 20th	Night	15% 4 21% 36% 23% 2	790
	21st to end	Night	15% 6% 22% 33% 21% 3	880
September	1st to 10th	Night	17% 6% 28% 31% 16% 3	800
	11th to 20th	Night	16% 6% 23% 34% 18% 4	800
	21st to end	Night	22% 6% 24% 34% 13% 2	800
October	1st to 10th	Night	23% 10% 28% 23% 14% 2	800
	11th to 20th	Night	19% 12% 31% 26% 11%	800
	21st to end	Night	25% 14% 33% 21% 7%	880
November	1st to 10th	Night	16% 12% 36% 25% 10% 1	820
	11th to 20th	Night	24% 16% 33% 20% 6% 1	840
	21st to end	Night	21% 18% 30% 24% 6% 1	840
December	1st to 10th	Night	25% 18% 35% 16% 5% 1	840
	11th to 20th	Night	26% 16% 35% 20% 3	840
	21st to end	Night	27% 18% 35% 18% 3	924

Page generated at 2026-01-14 01:14:26. Version ac07

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