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 2024-01-14.

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% 6% 2 5% 5% 3 4 5% 5% 7% 40%	74779
	Night	17% 7% 3 5% 4 3 4 4 5% 7% 42%	74964
Transparency	Day	42% 6% 12% 22% 16% 2	74484
	Night	42% 5% 11% 23% 16% 3	74793
Seeing	Day	21% 16% 31% 21% 9% 2	13871
	Night	21% 13% 32% 25% 9% 1	23091

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% 6% 2 5% 6% 3 4 5% 6% 6% 43%	1517
	11th to 20th	Day	17% 8% 3 6% 5% 4 4 5% 5% 6% 36%	1464
	21st to end	Day	14% 8% 2 6% 5% 3 5% 4 5% 8% 41%	1685
February	1st to 10th	Day	18% 8% 2 5% 5% 4 5% 4 5% 7% 36%	1644
	11th to 20th	Day	15% 7% 2 6% 6% 3 4 4 5% 7% 42%	1748
	21st to end	Day	21% 9% 3 6% 5% 2 4 4 4 5% 38%	1480
March	1st to 10th	Day	15% 7% 2 5% 5% 3 4 5% 6% 6% 42%	1797
	11th to 20th	Day	15% 7% 3 5% 6% 3 6% 5% 6% 8% 37%	1867
	21st to end	Day	21% 8% 3 5% 5% 2 4 4 3 5% 40%	2375
April	1st to 10th	Day	18% 9% 4 5% 4 2 4 4 5% 6% 39%	2160
	11th to 20th	Day	18% 6% 2 5% 5% 2 4 4 4 8% 42%	2134
	21st to end	Day	21% 5% 2 5% 4 3 3 5% 5% 6% 40%	2210
May	1st to 10th	Day	24% 7% 2 5% 4 3 4 5% 4 7% 35%	2340
	11th to 20th	Day	19% 7% 2 5% 4 3 5% 6% 5% 8% 37%	2442
	21st to end	Day	16% 6% 2 5% 5% 2 5% 5% 6% 8% 41%	2725
June	1st to 10th	Day	20% 5% 2 5% 5% 3 5% 5% 7% 9% 36%	2520
	11th to 20th	Day	17% 7% 3 6% 5% 3 5% 5% 6% 8% 36%	2520
	21st to end	Day	12% 6% 3 5% 6% 3 6% 5% 6% 9% 38%	2520
July	1st to 10th	Day	13% 6% 2 5% 6% 4 6% 6% 7% 8% 39%	2520
	11th to 20th	Day	12% 5% 2 5% 4 3 5% 7% 7% 9% 40%	2520
	21st to end	Day	13% 6% 2 6% 6% 4 6% 6% 6% 8% 38%	2744
August	1st to 10th	Day	12% 6% 2 5% 6% 4 6% 7% 7% 9% 37%	2397
	11th to 20th	Day	13% 6% 2 6% 5% 4 6% 5% 6% 8% 39%	2126
	21st to end	Day	13% 5% 3 6% 5% 3 5% 4 6% 9% 41%	2376
September	1st to 10th	Day	14% 5% 2 5% 5% 4 5% 5% 5% 6% 42%	2160
	11th to 20th	Day	14% 5% 2 4 4 2 4 4 4 7% 49%	2066
	21st to end	Day	17% 5% 3 6% 5% 3 4 4 5% 6% 42%	1980
October	1st to 10th	Day	23% 6% 2 4 4 3 4 4 4 5% 41%	1905
	11th to 20th	Day	15% 5% 2 5% 4 2 4 4 5% 8% 45%	1800
	21st to end	Day	22% 5% 2 4 5% 3 4 4 5% 5% 39%	1810
November	1st to 10th	Day	14% 6% 2 4 4 2 3 4 5% 6% 50%	1671
	11th to 20th	Day	20% 5% 2 3 3 2 4 4 5% 6% 46%	1710
	21st to end	Day	17% 5% 3 4 4 3 4 4 5% 6% 44%	1648
December	1st to 10th	Day	19% 7% 3 5% 7% 3 4 5% 6% 6% 36%	1520
	11th to 20th	Day	23% 6% 2 5% 3 2 4 3 4 5% 44%	1906
	21st to end	Day	19% 9% 4 7% 5% 3 5% 5% 5% 6% 32%	2772
January	1st to 10th	Night	16% 7% 3 5% 4 2 3 3 5% 7% 45%	2649
	11th to 20th	Night	20% 7% 2 5% 4 3 4 4 5% 7% 39%	2569
	21st to end	Night	14% 9% 4 5% 4 2 4 5% 5% 6% 44%	2668
February	1st to 10th	Night	17% 8% 4 6% 5% 2 4 4 4 6% 41%	2313
	11th to 20th	Night	17% 7% 3 6% 5% 3 4 4 5% 6% 41%	2098
	21st to end	Night	19% 7% 3 6% 4 3 4 4 4 5% 42%	1776
March	1st to 10th	Night	17% 6% 2 4 3 2 4 4 5% 7% 45%	2150
	11th to 20th	Night	17% 6% 3 6% 5% 3 4 4 5% 7% 40%	2088
	21st to end	Night	18% 7% 3 5% 5% 3 3 4 4 6% 42%	1980
April	1st to 10th	Night	18% 8% 3 5% 5% 3 3 5% 5% 6% 39%	1800
	11th to 20th	Night	20% 6% 3 6% 5% 2 4 4 4 7% 41%	1775
	21st to end	Night	20% 7% 3 6% 5% 2 4 3 4 5% 40%	1747
May	1st to 10th	Night	23% 7% 3 6% 6% 3 4 4 5% 7% 34%	1620
	11th to 20th	Night	20% 8% 3 5% 4 3 3 4 6% 6% 37%	1465
	21st to end	Night	18% 7% 3 5% 5% 3 4 3 5% 8% 41%	1550
June	1st to 10th	Night	19% 5% 2 5% 4 2 5% 4 4 8% 41%	1440
	11th to 20th	Night	15% 6% 3 5% 5% 2 4 5% 5% 9% 41%	1440
	21st to end	Night	12% 5% 2 4 4 3 4 5% 6% 9% 45%	1440
July	1st to 10th	Night	13% 5% 2 6% 6% 4 5% 5% 6% 9% 39%	1440
	11th to 20th	Night	13% 6% 3 5% 5% 4 5% 5% 6% 9% 40%	1440
	21st to end	Night	12% 4 2 5% 5% 3 6% 5% 7% 9% 41%	1574
August	1st to 10th	Night	9% 5% 3 6% 5% 4 5% 6% 6% 9% 44%	1518
	11th to 20th	Night	12% 6% 3 7% 7% 5% 5% 6% 5% 8% 36%	1774
	21st to end	Night	14% 5% 2 4 5% 2 4 4 5% 8% 45%	1980
September	1st to 10th	Night	16% 5% 3 5% 5% 3 4 3 4 7% 45%	1800
	11th to 20th	Night	14% 5% 2 4 4 2 3 4 4 6% 52%	1894
	21st to end	Night	17% 7% 3 5% 3 2 5% 3 4 6% 45%	1980
October	1st to 10th	Night	20% 7% 2 6% 4 2 4 3 4 5% 43%	2052
	11th to 20th	Night	16% 6% 2 4 4 2 4 3 5% 7% 47%	2160
	21st to end	Night	23% 7% 2 5% 4 2 3 4 5% 6% 40%	2544
November	1st to 10th	Night	13% 5% 2 5% 4 3 4 4 5% 6% 49%	2407
	11th to 20th	Night	20% 6% 2 4 3 2 3 3 4 6% 47%	2470
	21st to end	Night	15% 5% 3 4 4 2 3 4 5% 7% 46%	2531
December	1st to 10th	Night	20% 7% 3 5% 5% 3 4 3 4 6% 40%	2660
	11th to 20th	Night	20% 7% 2 4 5% 2 4 5% 5% 6% 41%	3288
	21st to end	Night	21% 8% 3 5% 4 3 4 4 4 6% 37%	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	40% 5% 10% 26% 14% 4	1518
	11th to 20th	Day	47% 5% 13% 20% 13% 2	1464
	21st to end	Day	40% 6% 11% 25% 15% 3	1685
February	1st to 10th	Day	47% 4 13% 21% 13% 2	1644
	11th to 20th	Day	42% 4 10% 24% 17% 3	1757
	21st to end	Day	48% 4 10% 21% 14% 3	1480
March	1st to 10th	Day	40% 5% 14% 23% 15% 3	1800
	11th to 20th	Day	43% 6% 12% 22% 15% 2	1867
	21st to end	Day	47% 4 8% 18% 18% 5%	2375
April	1st to 10th	Day	45% 4 10% 21% 16% 4	2160
	11th to 20th	Day	42% 5% 11% 20% 19% 3	2134
	21st to end	Day	44% 5% 11% 20% 16% 4	2210
May	1st to 10th	Day	48% 6% 9% 19% 16% 2	2340
	11th to 20th	Day	44% 7% 11% 20% 16% 3	2446
	21st to end	Day	40% 8% 16% 20% 14% 2	2725
June	1st to 10th	Day	43% 7% 13% 21% 15% 1	2520
	11th to 20th	Day	44% 7% 12% 23% 13% 2	2520
	21st to end	Day	40% 8% 13% 23% 15% 1	2520
July	1st to 10th	Day	39% 9% 12% 21% 16% 2	2450
	11th to 20th	Day	37% 7% 16% 24% 14% 2	2445
	21st to end	Day	41% 8% 13% 19% 16% 3	2591
August	1st to 10th	Day	39% 8% 14% 24% 14% 1	2383
	11th to 20th	Day	40% 7% 15% 24% 13% 1	2126
	21st to end	Day	38% 7% 12% 22% 19% 2	2376
September	1st to 10th	Day	39% 7% 12% 22% 18% 2	2160
	11th to 20th	Day	33% 5% 11% 24% 25% 2	2066
	21st to end	Day	42% 6% 9% 20% 19% 3	1980
October	1st to 10th	Day	44% 4 10% 22% 17% 3	1905
	11th to 20th	Day	38% 5% 12% 26% 18% 2	1800
	21st to end	Day	46% 5% 9% 21% 17% 3	1810
November	1st to 10th	Day	36% 4 13% 24% 21% 3	1671
	11th to 20th	Day	39% 4 10% 23% 21% 4	1710
	21st to end	Day	39% 5% 13% 27% 14% 3	1648
December	1st to 10th	Day	47% 5% 14% 18% 14% 1	1520
	11th to 20th	Day	42% 4 15% 22% 15% 2	1906
	21st to end	Day	52% 6% 10% 22% 9% 1	2772
January	1st to 10th	Night	40% 3 11% 25% 17% 4	2649
	11th to 20th	Night	45% 5% 13% 23% 10% 2	2569
	21st to end	Night	39% 5% 10% 23% 20% 3	2668
February	1st to 10th	Night	46% 5% 12% 23% 13% 2	2313
	11th to 20th	Night	45% 5% 10% 19% 16% 5%	2100
	21st to end	Night	45% 3 8% 24% 16% 3	1776
March	1st to 10th	Night	39% 5% 11% 24% 16% 4	2160
	11th to 20th	Night	43% 5% 10% 22% 16% 4	2089
	21st to end	Night	43% 4 10% 18% 18% 5%	1980
April	1st to 10th	Night	45% 5% 10% 19% 17% 3	1800
	11th to 20th	Night	44% 4 10% 19% 21% 1	1775
	21st to end	Night	45% 5% 9% 21% 16% 4	1747
May	1st to 10th	Night	51% 5% 10% 18% 14% 2	1620
	11th to 20th	Night	47% 6% 10% 17% 18% 3	1466
	21st to end	Night	43% 7% 15% 22% 11% 2	1551
June	1st to 10th	Night	42% 7% 9% 21% 18% 2	1440
	11th to 20th	Night	41% 7% 13% 26% 11% 1	1440
	21st to end	Night	35% 8% 15% 26% 15% 1	1440
July	1st to 10th	Night	42% 8% 13% 20% 15% 3	1400
	11th to 20th	Night	39% 8% 14% 26% 12% 1	1392
	21st to end	Night	35% 8% 16% 22% 15% 3	1486
August	1st to 10th	Night	36% 9% 15% 23% 16% 1	1508
	11th to 20th	Night	42% 10% 12% 25% 10% 1	1774
	21st to end	Night	37% 8% 13% 21% 20% 2	1980
September	1st to 10th	Night	41% 6% 10% 26% 16% 1	1800
	11th to 20th	Night	34% 5% 11% 24% 24% 3	1894
	21st to end	Night	41% 4 7% 23% 22% 3	1980
October	1st to 10th	Night	46% 6% 9% 23% 14% 3	2052
	11th to 20th	Night	38% 4 12% 28% 16% 1	2160
	21st to end	Night	45% 5% 10% 22% 17% 1	2544
November	1st to 10th	Night	36% 5% 12% 22% 21% 4	2407
	11th to 20th	Night	40% 3 9% 24% 19% 5%	2470
	21st to end	Night	37% 5% 12% 30% 14% 2	2531
December	1st to 10th	Night	46% 5% 11% 22% 15% 1	2660
	11th to 20th	Night	43% 4 13% 18% 18% 3	3288
	21st to end	Night	48% 5% 13% 22% 12% 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	26% 30% 26% 14% 4	180
	11th to 20th	Day	27% 25% 35% 11% 2	166
	21st to end	Day	21% 25% 37% 12% 5%	222
February	1st to 10th	Day	33% 27% 29% 11% 1	264
	11th to 20th	Day	25% 26% 36% 12% 1	416
	21st to end	Day	31% 20% 34% 13% 2	346
March	1st to 10th	Day	22% 24% 35% 16% 3	420
	11th to 20th	Day	21% 25% 35% 15% 3 1	420
	21st to end	Day	29% 23% 31% 13% 3 1	462
April	1st to 10th	Day	28% 24% 29% 13% 5% 1	420
	11th to 20th	Day	24% 21% 30% 14% 10% 1	412
	21st to end	Day	30% 18% 34% 14% 3 1	420
May	1st to 10th	Day	31% 19% 27% 15% 6% 2	423
	11th to 20th	Day	20% 16% 37% 19% 6% 1	447
	21st to end	Day	19% 10% 29% 27% 12% 3	485
June	1st to 10th	Day	19% 12% 28% 26% 12% 2	450
	11th to 20th	Day	19% 13% 32% 27% 9% 1	450
	21st to end	Day	14% 10% 30% 30% 14% 2	450
July	1st to 10th	Day	15% 8% 24% 31% 17% 5%	445
	11th to 20th	Day	11% 5% 27% 33% 20% 4	444
	21st to end	Day	14% 9% 27% 34% 15% 1	488
August	1st to 10th	Day	12% 9% 25% 33% 14% 6%	446
	11th to 20th	Day	15% 5% 25% 33% 16% 6%	439
	21st to end	Day	11% 8% 28% 31% 14% 8%	495
September	1st to 10th	Day	17% 9% 28% 24% 17% 5%	450
	11th to 20th	Day	15% 8% 32% 23% 13% 9%	450
	21st to end	Day	19% 12% 32% 20% 12% 4	450
October	1st to 10th	Day	28% 8% 26% 21% 13% 4	450
	11th to 20th	Day	16% 18% 32% 22% 10% 2	450
	21st to end	Day	25% 16% 30% 19% 8% 1	410
November	1st to 10th	Day	17% 15% 32% 24% 9% 3	365
	11th to 20th	Day	24% 19% 34% 17% 5%	370
	21st to end	Day	21% 19% 34% 21% 4 1	308
December	1st to 10th	Day	23% 24% 34% 13% 5%	180
	11th to 20th	Day	33% 24% 31% 10% 3	180
	21st to end	Day	28% 20% 32% 16% 4	198
January	1st to 10th	Night	22% 18% 38% 18% 3	760
	11th to 20th	Night	25% 18% 34% 21% 3	735
	21st to end	Night	20% 20% 40% 18% 3	792
February	1st to 10th	Night	23% 18% 37% 20% 2	720
	11th to 20th	Night	24% 23% 35% 16% 2	703
	21st to end	Night	26% 20% 35% 18% 3	592
March	1st to 10th	Night	23% 18% 38% 19% 3	720
	11th to 20th	Night	22% 18% 39% 17% 4	720
	21st to end	Night	22% 19% 36% 18% 4	792
April	1st to 10th	Night	23% 18% 36% 18% 4	720
	11th to 20th	Night	22% 14% 36% 22% 5%	711
	21st to end	Night	25% 13% 34% 22% 6% 1	667
May	1st to 10th	Night	25% 14% 33% 22% 7%	540
	11th to 20th	Night	24% 13% 33% 24% 6%	404
	21st to end	Night	21% 7% 21% 38% 11% 1	389
June	1st to 10th	Night	20% 5% 31% 31% 12% 1	360
	11th to 20th	Night	16% 5% 27% 39% 13%	360
	21st to end	Night	10% 4 26% 48% 13% 1	360
July	1st to 10th	Night	15% 4 21% 39% 21% 1	350
	11th to 20th	Night	14% 2 21% 42% 19% 1	348
	21st to end	Night	13% 5% 21% 42% 18% 1	394
August	1st to 10th	Night	10% 4 22% 45% 18% 1	444
	11th to 20th	Night	15% 4 19% 36% 24% 2	710
	21st to end	Night	16% 5% 20% 34% 22% 3	792
September	1st to 10th	Night	17% 5% 27% 32% 15% 3	720
	11th to 20th	Night	17% 6% 24% 32% 18% 4	720
	21st to end	Night	21% 6% 25% 33% 13% 2	720
October	1st to 10th	Night	24% 10% 27% 22% 13% 3	720
	11th to 20th	Night	18% 12% 31% 27% 11%	720
	21st to end	Night	27% 13% 32% 21% 7%	792
November	1st to 10th	Night	16% 12% 36% 25% 11% 1	740
	11th to 20th	Night	25% 15% 33% 20% 7% 1	760
	21st to end	Night	21% 17% 31% 24% 6% 1	760
December	1st to 10th	Night	26% 17% 35% 17% 5% 1	760
	11th to 20th	Night	24% 16% 35% 20% 4	760
	21st to end	Night	26% 17% 37% 17% 3	836

Page generated at 2024-01-11 04:22:40. Version ac07

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