Cambridge
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cambridge clear sky chart from 2005-11-06 to 2025-05-11.

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	7% 7% 2 6% 4 4 4 3 5% 7% 51%	117606
	Night	10% 8% 2 6% 4 4 4 4 6% 6% 46%	120124
Transparency	Day	32% 8% 12% 20% 17% 11%	117350
	Night	36% 7% 14% 20% 15% 7%	119969
Seeing	Day	10% 7% 24% 32% 21% 6%	14987
	Night	13% 3 16% 40% 23% 4	24368

By Month

Percentage Cloud Cover SW (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	12% 18% 5% 12% 7% 6% 5% 4 5% 6% 21%	3639
	11th to 20th	Day	11% 12% 3 6% 5% 4 5% 4 6% 8% 36%	3640
	21st to end	Day	19% 11% 3 7% 4 4 4 3 4 6% 35%	4004
February	1st to 10th	Day	12% 12% 4 9% 5% 4 4 3 5% 7% 34%	1701
	11th to 20th	Day	12% 11% 3 8% 6% 5% 6% 4 5% 7% 34%	1950
	21st to end	Day	10% 11% 4 8% 5% 5% 5% 5% 6% 7% 34%	1650
March	1st to 10th	Day	6% 9% 4 9% 6% 5% 6% 5% 6% 8% 35%	4518
	11th to 20th	Day	7% 9% 2 6% 4 4 5% 4 6% 7% 46%	4769
	21st to end	Day	10% 10% 3 8% 6% 5% 6% 5% 7% 10% 30%	5810
April	1st to 10th	Day	8% 7% 3 8% 5% 6% 4 4 7% 8% 39%	2400
	11th to 20th	Day	7% 6% 2 6% 5% 5% 6% 4 6% 7% 46%	2410
	21st to end	Day	8% 7% 2 6% 4 5% 5% 5% 6% 8% 45%	2600
May	1st to 10th	Day	7% 6% 3 7% 4 4 5% 4 5% 7% 48%	4240
	11th to 20th	Day	5% 4 1 4 3 3 3 3 5% 7% 64%	4333
	21st to end	Day	5% 6% 2 5% 4 4 5% 5% 6% 8% 50%	4813
June	1st to 10th	Day	8% 6% 2 6% 4 5% 5% 5% 7% 9% 41%	2660
	11th to 20th	Day	7% 4 2 6% 5% 4 5% 4 6% 6% 52%	2660
	21st to end	Day	2 3 1 3 2 4 3 3 4 5% 69%	2660
July	1st to 10th	Day	1 1 2 2 2 2 1 3 5% 81%	4420
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 4 80%	4420
	21st to end	Day	1 1 1 2 2 2 2 2 4 5% 77%	4790
August	1st to 10th	Day	3 3 1 4 3 3 4 3 5% 7% 64%	4320
	11th to 20th	Day	1 1 1 3 2 2 3 2 3 5% 77%	4028
	21st to end	Day	2 2 1 3 2 2 2 2 3 5% 77%	4190
September	1st to 10th	Day	2 2 1 3 2 3 2 2 4 5% 75%	2280
	11th to 20th	Day	5% 4 1 5% 3 3 3 3 5% 6% 62%	2130
	21st to end	Day	5% 3 1 4 3 3 4 3 5% 6% 64%	2090
October	1st to 10th	Day	4 3 1 4 3 3 3 3 4 5% 66%	3490
	11th to 20th	Day	8% 4 1 5% 4 3 5% 3 4 7% 55%	2987
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 6% 6% 47%	3101
November	1st to 10th	Day	15% 8% 3 5% 4 4 4 3 5% 8% 41%	1764
	11th to 20th	Day	14% 9% 2 7% 5% 4 5% 4 6% 6% 39%	1800
	21st to end	Day	11% 8% 3 7% 4 5% 6% 4 5% 6% 39%	1687
December	1st to 10th	Day	10% 8% 3 8% 5% 4 5% 4 7% 7% 40%	2620
	11th to 20th	Day	20% 15% 4 10% 6% 5% 5% 4 7% 6% 18%	3028
	21st to end	Day	15% 14% 4 11% 5% 5% 5% 4 6% 6% 25%	4004
January	1st to 10th	Night	15% 16% 5% 12% 6% 6% 6% 5% 6% 4 18%	6306
	11th to 20th	Night	12% 13% 3 8% 5% 5% 4 3 8% 8% 32%	6320
	21st to end	Night	18% 12% 3 7% 4 3 4 3 5% 6% 36%	6952
February	1st to 10th	Night	14% 13% 3 8% 5% 4 5% 4 5% 6% 32%	2693
	11th to 20th	Night	12% 13% 3 7% 4 4 3 4 6% 7% 36%	2338
	21st to end	Night	11% 13% 3 8% 6% 5% 5% 4 6% 6% 34%	1980
March	1st to 10th	Night	8% 11% 3 8% 5% 4 5% 4 6% 6% 39%	5376
	11th to 20th	Night	11% 14% 3 6% 4 3 3 3 4 5% 43%	5230
	21st to end	Night	9% 8% 2 9% 6% 6% 6% 5% 7% 8% 32%	5148
April	1st to 10th	Night	10% 7% 3 6% 4 5% 4 4 6% 7% 45%	2000
	11th to 20th	Night	8% 7% 3 6% 5% 4 4 3 7% 5% 48%	1936
	21st to end	Night	7% 5% 3 6% 3 4 5% 3 6% 8% 50%	1800
May	1st to 10th	Night	10% 5% 2 4 3 3 4 4 6% 7% 52%	2950
	11th to 20th	Night	3 3 1 4 3 2 3 3 6% 6% 64%	2565
	21st to end	Night	7% 7% 2 7% 4 3 3 4 6% 7% 51%	2762
June	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 4 5% 8% 47%	1520
	11th to 20th	Night	8% 3 2 5% 3 3 4 3 5% 8% 58%	1520
	21st to end	Night	3 2 1 3 3 2 4 4 5% 6% 67%	1520
July	1st to 10th	Night	1 1 1 1 2 2 4 5% 82%	2540
	11th to 20th	Night	1 1 2 2 2 5% 5% 82%	2540
	21st to end	Night	2 2 1 3 2 2 3 2 4 5% 73%	2761
August	1st to 10th	Night	4 3 1 4 3 3 4 4 5% 7% 63%	2610
	11th to 20th	Night	1 1 2 2 2 2 3 4 6% 78%	2811
	21st to end	Night	3 2 1 3 2 2 1 2 3 4 77%	3485
September	1st to 10th	Night	3 2 1 2 2 2 3 2 3 5% 75%	1900
	11th to 20th	Night	5% 3 1 4 3 3 4 3 5% 7% 63%	2049
	21st to end	Night	7% 3 1 3 2 2 3 3 4 5% 67%	2090
October	1st to 10th	Night	3 3 1 4 2 3 3 2 3 4 71%	3490
	11th to 20th	Night	5% 4 2 5% 3 3 4 3 5% 7% 60%	3942
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	4457
November	1st to 10th	Night	14% 7% 2 6% 4 4 5% 4 5% 8% 41%	2535
	11th to 20th	Night	16% 8% 2 5% 4 4 4 4 5% 7% 41%	2600
	21st to end	Night	11% 9% 3 6% 4 4 5% 4 5% 6% 43%	2710
December	1st to 10th	Night	8% 9% 3 7% 3 4 5% 6% 12% 7% 37%	4560
	11th to 20th	Night	19% 15% 4 10% 5% 5% 6% 5% 6% 5% 20%	5176
	21st to end	Night	16% 12% 4 9% 6% 5% 6% 5% 8% 6% 24%	6952

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	62% 10% 13% 12% 3	3639
	11th to 20th	Day	44% 10% 19% 21% 6% 1	3640
	21st to end	Day	48% 11% 12% 18% 7% 3	4004
February	1st to 10th	Day	52% 12% 18% 13% 4 1	1701
	11th to 20th	Day	49% 11% 16% 18% 5%	1960
	21st to end	Day	49% 10% 18% 16% 7% 1	1650
March	1st to 10th	Day	45% 9% 18% 16% 12%	4518
	11th to 20th	Day	35% 8% 14% 33% 10%	4769
	21st to end	Day	45% 14% 18% 18% 4 1	5810
April	1st to 10th	Day	42% 10% 17% 21% 9% 1	2400
	11th to 20th	Day	36% 9% 15% 22% 15% 3	2413
	21st to end	Day	37% 9% 15% 25% 15% 1	2600
May	1st to 10th	Day	31% 9% 12% 24% 22% 2	4240
	11th to 20th	Day	21% 5% 10% 31% 26% 7%	4336
	21st to end	Day	28% 11% 16% 30% 13% 1	4814
June	1st to 10th	Day	37% 10% 14% 23% 15%	2660
	11th to 20th	Day	30% 8% 11% 23% 21% 8%	2660
	21st to end	Day	18% 5% 8% 20% 33% 16%	2660
July	1st to 10th	Day	9% 3 3 10% 23% 53%	4351
	11th to 20th	Day	10% 2 4 7% 21% 56%	4345
	21st to end	Day	10% 2 4 11% 31% 42%	4639
August	1st to 10th	Day	20% 3 5% 13% 29% 29%	4304
	11th to 20th	Day	11% 3 4 20% 36% 26%	4067
	21st to end	Day	13% 4 5% 24% 33% 21%	4189
September	1st to 10th	Day	13% 4 6% 22% 32% 23%	2280
	11th to 20th	Day	23% 5% 10% 23% 28% 12%	2130
	21st to end	Day	22% 5% 9% 31% 25% 7%	2090
October	1st to 10th	Day	22% 4 7% 23% 36% 7%	3490
	11th to 20th	Day	30% 5% 15% 31% 17% 3	2987
	21st to end	Day	37% 7% 13% 29% 13% 1	3101
November	1st to 10th	Day	43% 8% 17% 23% 8%	1764
	11th to 20th	Day	46% 11% 19% 18% 6% 1	1800
	21st to end	Day	46% 8% 21% 16% 8% 1	1687
December	1st to 10th	Day	39% 10% 28% 20% 2	2620
	11th to 20th	Day	63% 12% 17% 6% 2	3028
	21st to end	Day	56% 10% 18% 12% 4	4004
January	1st to 10th	Night	57% 14% 17% 9% 2	6306
	11th to 20th	Night	46% 8% 19% 19% 8%	6320
	21st to end	Night	47% 8% 16% 19% 7% 3	6952
February	1st to 10th	Night	53% 9% 18% 16% 4	2693
	11th to 20th	Night	47% 9% 18% 18% 7% 1	2339
	21st to end	Night	50% 7% 15% 17% 9% 2	1980
March	1st to 10th	Night	40% 7% 14% 27% 11% 1	5376
	11th to 20th	Night	43% 5% 8% 32% 11%	5230
	21st to end	Night	43% 12% 19% 20% 5%	5148
April	1st to 10th	Night	38% 7% 15% 27% 12% 1	2000
	11th to 20th	Night	37% 6% 11% 27% 16% 3	1936
	21st to end	Night	32% 8% 14% 26% 20% 1	1800
May	1st to 10th	Night	29% 7% 12% 27% 23% 2	2950
	11th to 20th	Night	19% 6% 11% 34% 28% 3	2565
	21st to end	Night	31% 9% 12% 35% 12% 1	2762
June	1st to 10th	Night	36% 8% 15% 27% 14% 1	1520
	11th to 20th	Night	26% 6% 13% 27% 23% 6%	1520
	21st to end	Night	18% 6% 8% 21% 34% 14%	1520
July	1st to 10th	Night	7% 2 4 12% 23% 52%	2500
	11th to 20th	Night	6% 2 4 10% 23% 55%	2492
	21st to end	Night	13% 3 3 11% 31% 39%	2674
August	1st to 10th	Night	20% 4 5% 12% 35% 25%	2602
	11th to 20th	Night	9% 3 5% 17% 34% 32%	2838
	21st to end	Night	14% 3 4 21% 35% 24%	3485
September	1st to 10th	Night	14% 3 5% 20% 32% 26%	1900
	11th to 20th	Night	22% 5% 9% 22% 25% 17%	2049
	21st to end	Night	20% 5% 7% 27% 28% 13%	2090
October	1st to 10th	Night	19% 4 6% 19% 37% 15%	3490
	11th to 20th	Night	25% 5% 15% 27% 25% 3	3942
	21st to end	Night	33% 7% 15% 28% 16% 1	4457
November	1st to 10th	Night	41% 8% 15% 23% 12% 1	2535
	11th to 20th	Night	45% 7% 16% 24% 8%	2600
	21st to end	Night	42% 6% 19% 21% 10% 2	2710
December	1st to 10th	Night	35% 8% 36% 17% 4	4560
	11th to 20th	Night	62% 9% 17% 9% 2	5176
	21st to end	Night	53% 11% 19% 12% 4	6952

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	22% 6% 18% 35% 16% 6%	200
	11th to 20th	Day	19% 7% 19% 33% 16% 8%	200
	21st to end	Day	18% 2 22% 33% 20% 5%	220
February	1st to 10th	Day	19% 6% 29% 30% 14% 2	279
	11th to 20th	Day	16% 8% 31% 29% 15% 2	495
	21st to end	Day	15% 14% 27% 28% 14% 2	414
March	1st to 10th	Day	15% 10% 30% 32% 12% 1	500
	11th to 20th	Day	16% 9% 25% 28% 19% 2	500
	21st to end	Day	16% 11% 27% 27% 15% 4	550
April	1st to 10th	Day	12% 12% 29% 25% 15% 7%	500
	11th to 20th	Day	12% 12% 30% 26% 14% 5%	492
	21st to end	Day	10% 13% 30% 29% 13% 5%	500
May	1st to 10th	Day	7% 13% 31% 29% 17% 4	503
	11th to 20th	Day	15% 10% 25% 28% 15% 7%	489
	21st to end	Day	11% 7% 20% 30% 24% 9%	529
June	1st to 10th	Day	8% 7% 27% 30% 21% 7%	490
	11th to 20th	Day	10% 11% 27% 30% 17% 4	490
	21st to end	Day	2 7% 26% 33% 24% 8%	490
July	1st to 10th	Day	4 3 26% 39% 24% 4	486
	11th to 20th	Day	3 2 26% 39% 26% 4	484
	21st to end	Day	2 2 22% 39% 27% 8%	531
August	1st to 10th	Day	4 2 17% 39% 26% 10%	487
	11th to 20th	Day	1 3 17% 36% 33% 10%	477
	21st to end	Day	4 3 18% 35% 29% 11%	539
September	1st to 10th	Day	4 2 19% 30% 33% 11%	490
	11th to 20th	Day	7% 4 19% 32% 28% 10%	490
	21st to end	Day	6% 3 21% 36% 28% 6%	490
October	1st to 10th	Day	9% 6% 18% 28% 30% 8%	490
	11th to 20th	Day	10% 4 23% 29% 25% 8%	389
	21st to end	Day	14% 5% 25% 34% 16% 5%	330
November	1st to 10th	Day	19% 5% 23% 31% 18% 4	300
	11th to 20th	Day	15% 6% 20% 35% 18% 5%	300
	21st to end	Day	15% 4 19% 36% 21% 5%	243
December	1st to 10th	Day	20% 3 18% 31% 18% 13%	200
	11th to 20th	Day	26% 5% 20% 33% 14% 2	200
	21st to end	Day	19% 5% 24% 30% 16% 6%	220
January	1st to 10th	Night	22% 4 13% 36% 22% 4	800
	11th to 20th	Night	21% 4 14% 35% 22% 5%	800
	21st to end	Night	18% 2 12% 37% 24% 6%	880
February	1st to 10th	Night	20% 4 19% 40% 14% 2	800
	11th to 20th	Night	16% 3 20% 38% 20% 3	784
	21st to end	Night	16% 3 18% 40% 19% 3	660
March	1st to 10th	Night	16% 4 20% 43% 15% 3	800
	11th to 20th	Night	16% 5% 16% 40% 20% 3	800
	21st to end	Night	18% 5% 17% 43% 16% 2	880
April	1st to 10th	Night	14% 6% 16% 40% 22% 3	800
	11th to 20th	Night	13% 7% 24% 35% 19% 2	748
	21st to end	Night	10% 3 26% 40% 20% 1	600
May	1st to 10th	Night	7% 6% 23% 42% 20% 2	600
	11th to 20th	Night	7% 5% 20% 35% 29% 5%	408
	21st to end	Night	8% 5% 16% 37% 31% 3	411
June	1st to 10th	Night	9% 4 20% 38% 26% 3	380
	11th to 20th	Night	9% 6% 24% 36% 24% 1	380
	21st to end	Night	3 2 16% 46% 32% 1	380
July	1st to 10th	Night	2 2 20% 46% 28% 2	370
	11th to 20th	Night	1 1 22% 56% 19% 2	368
	21st to end	Night	2 1 15% 49% 31% 2	416
August	1st to 10th	Night	6% 1 14% 43% 33% 4	420
	11th to 20th	Night	1 1 14% 44% 36% 3	564
	21st to end	Night	4 2 15% 43% 31% 5%	823
September	1st to 10th	Night	3 2 13% 44% 34% 3	760
	11th to 20th	Night	6% 2 12% 39% 34% 7%	760
	21st to end	Night	9% 3 14% 36% 31% 7%	760
October	1st to 10th	Night	7% 2 16% 42% 29% 4	760
	11th to 20th	Night	11% 4 13% 39% 26% 7%	760
	21st to end	Night	11% 4 18% 41% 22% 5%	836
November	1st to 10th	Night	17% 3 13% 41% 23% 2	780
	11th to 20th	Night	21% 2 15% 41% 18% 3	800
	21st to end	Night	16% 3 15% 38% 23% 5%	800
December	1st to 10th	Night	18% 3 13% 39% 22% 5%	800
	11th to 20th	Night	25% 3 12% 40% 18% 3	800
	21st to end	Night	24% 3 16% 40% 15% 2	880

Page generated at 2025-05-08 07:33:58. Version ac07

Copyright © 2025 Attilla Danko.