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 2026-09-14.

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	6% 6% 2 5% 4 4 4 3 5% 7% 54%	157889
	Night	8% 7% 2 6% 4 4 4 4 6% 7% 49%	150548
Transparency	Day	29% 7% 11% 20% 20% 13%	152639
	Night	33% 7% 14% 21% 16% 9%	146800
Seeing	Day	10% 7% 24% 32% 21% 6%	16760
	Night	13% 3 17% 40% 23% 3	25927

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	14% 18% 5% 11% 6% 5% 4 4 5% 6% 21%	4722
	11th to 20th	Day	9% 9% 3 6% 5% 5% 5% 4 7% 9% 38%	4740
	21st to end	Day	15% 10% 3 7% 5% 5% 5% 3 5% 6% 35%	5187
February	1st to 10th	Day	12% 12% 4 9% 5% 4 4 3 5% 8% 34%	1787
	11th to 20th	Day	11% 11% 3 9% 6% 5% 6% 4 5% 8% 34%	2050
	21st to end	Day	10% 11% 4 8% 5% 5% 5% 5% 6% 7% 35%	1730
March	1st to 10th	Day	6% 9% 4 9% 6% 5% 6% 5% 6% 8% 34%	4618
	11th to 20th	Day	7% 9% 3 6% 4 4 5% 4 6% 7% 46%	4873
	21st to end	Day	10% 10% 3 8% 6% 5% 6% 5% 7% 10% 30%	5938
April	1st to 10th	Day	8% 7% 3 8% 5% 6% 4 4 7% 8% 39%	2520
	11th to 20th	Day	8% 7% 2 6% 5% 4 5% 4 6% 7% 46%	2532
	21st to end	Day	8% 7% 2 5% 4 5% 5% 5% 6% 8% 46%	2730
May	1st to 10th	Day	4 4 2 4 3 3 4 3 5% 6% 61%	7626
	11th to 20th	Day	5% 3 1 4 3 3 3 3 6% 8% 61%	8064
	21st to end	Day	4 4 2 4 4 4 5% 4 6% 7% 57%	8992
June	1st to 10th	Day	8% 6% 2 6% 4 5% 5% 5% 7% 8% 44%	2940
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 6% 53%	2940
	21st to end	Day	3 3 1 4 2 4 3 3 4 6% 68%	2940
July	1st to 10th	Day	2 1 2 1 2 2 2 3 5% 80%	8219
	11th to 20th	Day	2 2 1 3 2 3 2 2 4 4 75%	8220
	21st to end	Day	2 2 1 2 2 2 3 2 5% 7% 74%	8922
August	1st to 10th	Day	2 2 1 3 3 3 3 3 4 7% 69%	7968
	11th to 20th	Day	1 2 1 3 2 2 3 2 4 6% 75%	7565
	21st to end	Day	3 2 1 4 3 4 4 3 5% 6% 64%	7772
September	1st to 10th	Day	3 2 1 3 2 3 2 2 4 6% 73%	2520
	11th to 20th	Day	4 4 1 5% 3 3 3 3 5% 6% 62%	2278
	21st to end	Day	5% 3 2 4 3 3 4 3 4 5% 65%	2200
October	1st to 10th	Day	4 3 1 4 4 3 3 3 4 5% 66%	3600
	11th to 20th	Day	7% 4 1 5% 4 4 5% 3 4 7% 55%	3081
	21st to end	Day	9% 6% 2 6% 5% 4 5% 4 6% 6% 48%	3200
November	1st to 10th	Day	14% 8% 3 5% 4 4 4 3 5% 8% 42%	1854
	11th to 20th	Day	14% 8% 2 7% 5% 4 6% 4 6% 6% 38%	1890
	21st to end	Day	11% 9% 3 7% 5% 5% 6% 4 6% 6% 38%	1771
December	1st to 10th	Day	10% 8% 3 9% 5% 4 5% 4 7% 7% 39%	2700
	11th to 20th	Day	20% 15% 4 10% 6% 5% 5% 4 7% 6% 19%	3108
	21st to end	Day	15% 14% 4 11% 5% 5% 5% 4 6% 6% 25%	4092
January	1st to 10th	Night	15% 17% 5% 11% 6% 6% 6% 5% 5% 5% 18%	8172
	11th to 20th	Night	9% 10% 3 6% 4 5% 5% 5% 11% 9% 33%	8220
	21st to end	Night	14% 10% 2 7% 4 4 5% 4 6% 7% 36%	8994
February	1st to 10th	Night	14% 13% 3 8% 5% 4 4 4 6% 6% 33%	2827
	11th to 20th	Night	11% 13% 3 7% 4 4 4 5% 6% 7% 35%	2458
	21st to end	Night	11% 12% 3 8% 6% 5% 5% 4 6% 6% 34%	2076
March	1st to 10th	Night	8% 11% 3 8% 4 4 5% 4 6% 6% 39%	5496
	11th to 20th	Night	11% 13% 3 6% 4 3 3 3 4 5% 43%	5346
	21st to end	Night	9% 8% 3 9% 6% 6% 6% 5% 7% 8% 33%	5262
April	1st to 10th	Night	10% 7% 3 5% 4 4 4 4 6% 8% 45%	2100
	11th to 20th	Night	9% 7% 3 6% 5% 4 4 3 7% 5% 47%	2034
	21st to end	Night	7% 5% 3 6% 3 4 5% 3 6% 8% 51%	1890
May	1st to 10th	Night	7% 3 1 3 2 2 3 3 5% 7% 64%	5321
	11th to 20th	Night	4 4 2 4 3 3 4 3 5% 7% 60%	4785
	21st to end	Night	5% 5% 2 6% 3 3 3 4 5% 7% 57%	5181
June	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 4 5% 8% 49%	1680
	11th to 20th	Night	8% 3 2 5% 3 3 4 3 5% 7% 58%	1680
	21st to end	Night	3 3 2 3 3 2 4 4 5% 6% 66%	1680
July	1st to 10th	Night	1 1 1 2 2 2 3 2 5% 6% 76%	4740
	11th to 20th	Night	1 1 1 1 2 2 2 2 5% 5% 79%	4740
	21st to end	Night	2 1 1 2 2 2 4 2 5% 6% 72%	5153
August	1st to 10th	Night	2 2 1 2 2 2 3 3 5% 6% 72%	4830
	11th to 20th	Night	1 1 1 2 2 2 2 3 4 7% 75%	5291
	21st to end	Night	4 3 1 5% 3 3 3 3 6% 7% 62%	6473
September	1st to 10th	Night	3 2 1 3 2 2 3 2 4 6% 73%	2100
	11th to 20th	Night	5% 3 1 4 3 3 4 3 5% 7% 63%	2188
	21st to end	Night	7% 3 1 3 2 2 3 3 4 5% 68%	2200
October	1st to 10th	Night	4 3 1 4 2 3 3 2 3 4 71%	3600
	11th to 20th	Night	5% 4 2 5% 3 3 4 3 5% 7% 60%	4068
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 48%	4600
November	1st to 10th	Night	14% 7% 2 6% 4 4 5% 3 6% 8% 41%	2665
	11th to 20th	Night	16% 8% 2 5% 4 5% 4 4 5% 7% 39%	2730
	21st to end	Night	11% 9% 3 6% 4 4 5% 4 4 7% 43%	2846
December	1st to 10th	Night	8% 9% 3 7% 3 4 5% 6% 12% 6% 37%	4700
	11th to 20th	Night	19% 15% 4 10% 5% 5% 6% 5% 7% 5% 20%	5316
	21st to end	Night	16% 12% 4 9% 6% 5% 6% 5% 7% 6% 24%	7106

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% 11% 14% 11% 2	4723
	11th to 20th	Day	37% 12% 25% 21% 6% 1	4740
	21st to end	Day	45% 12% 14% 19% 7% 2	5187
February	1st to 10th	Day	51% 12% 20% 13% 4 1	1787
	11th to 20th	Day	49% 11% 16% 18% 6%	2060
	21st to end	Day	48% 9% 17% 17% 7% 1	1730
March	1st to 10th	Day	45% 9% 18% 16% 12%	4618
	11th to 20th	Day	35% 8% 14% 32% 10%	4873
	21st to end	Day	45% 14% 18% 18% 5% 1	5938
April	1st to 10th	Day	41% 9% 17% 22% 9% 1	2520
	11th to 20th	Day	37% 9% 15% 22% 14% 3	2535
	21st to end	Day	36% 9% 14% 25% 15% 1	2730
May	1st to 10th	Day	21% 6% 9% 27% 34% 3	7626
	11th to 20th	Day	20% 5% 10% 31% 27% 6%	8067
	21st to end	Day	23% 8% 11% 28% 26% 4	8994
June	1st to 10th	Day	35% 10% 13% 23% 18% 1	2940
	11th to 20th	Day	29% 7% 10% 22% 23% 8%	2940
	21st to end	Day	19% 5% 8% 20% 33% 16%	2940
July	1st to 10th	Day	9% 3 3 13% 32% 41%	8151
	11th to 20th	Day	14% 2 3 7% 24% 50%	8145
	21st to end	Day	11% 2 4 15% 33% 35%	8723
August	1st to 10th	Day	15% 2 4 10% 24% 45%	6315
	11th to 20th	Day	13% 3 6% 22% 36% 20%	5964
	21st to end	Day	18% 4 5% 25% 33% 17%	6099
September	1st to 10th	Day	15% 4 6% 23% 32% 22%	2520
	11th to 20th	Day	23% 5% 10% 23% 28% 11%	2278
	21st to end	Day	22% 5% 9% 31% 26% 7%	2200
October	1st to 10th	Day	22% 4 7% 23% 36% 7%	3600
	11th to 20th	Day	29% 5% 15% 31% 17% 3	3081
	21st to end	Day	36% 7% 14% 29% 13% 1	3200
November	1st to 10th	Day	42% 8% 18% 23% 8%	1854
	11th to 20th	Day	46% 11% 19% 18% 5% 1	1890
	21st to end	Day	46% 9% 21% 17% 7% 1	1771
December	1st to 10th	Day	40% 11% 27% 20% 2	2700
	11th to 20th	Day	62% 12% 17% 7% 2	3108
	21st to end	Day	56% 10% 18% 13% 4	4092
January	1st to 10th	Night	58% 14% 18% 8% 2	8172
	11th to 20th	Night	37% 9% 27% 20% 7%	8220
	21st to end	Night	41% 11% 19% 19% 7% 4	8994
February	1st to 10th	Night	52% 8% 19% 16% 4	2827
	11th to 20th	Night	47% 8% 18% 18% 7% 1	2459
	21st to end	Night	50% 7% 15% 17% 8% 2	2076
March	1st to 10th	Night	40% 7% 14% 27% 11% 1	5496
	11th to 20th	Night	43% 5% 8% 32% 11%	5346
	21st to end	Night	43% 12% 19% 20% 5%	5262
April	1st to 10th	Night	37% 7% 15% 29% 12% 1	2100
	11th to 20th	Night	38% 6% 11% 27% 15% 3	2034
	21st to end	Night	32% 8% 14% 25% 20% 1	1890
May	1st to 10th	Night	20% 4 8% 27% 36% 4	5319
	11th to 20th	Night	22% 5% 10% 33% 27% 2	4785
	21st to end	Night	25% 6% 11% 35% 21% 2	5182
June	1st to 10th	Night	34% 7% 14% 27% 17% 1	1680
	11th to 20th	Night	25% 6% 12% 26% 24% 7%	1680
	21st to end	Night	19% 5% 8% 21% 33% 14%	1680
July	1st to 10th	Night	11% 3 4 16% 25% 41%	4700
	11th to 20th	Night	9% 2 3 9% 28% 49%	4692
	21st to end	Night	13% 3 3 19% 29% 33%	5040
August	1st to 10th	Night	14% 3 4 9% 28% 44%	3813
	11th to 20th	Night	10% 4 7% 20% 32% 27%	4168
	21st to end	Night	18% 3 4 24% 31% 19%	5066
September	1st to 10th	Night	15% 3 5% 21% 32% 24%	2100
	11th to 20th	Night	22% 5% 9% 22% 26% 16%	2188
	21st to end	Night	20% 5% 6% 26% 29% 14%	2200
October	1st to 10th	Night	19% 4 7% 19% 36% 15%	3600
	11th to 20th	Night	25% 5% 15% 27% 25% 3	4068
	21st to end	Night	33% 7% 15% 29% 16% 1	4600
November	1st to 10th	Night	41% 8% 15% 23% 11% 1	2665
	11th to 20th	Night	45% 8% 17% 23% 7%	2730
	21st to end	Night	42% 6% 20% 20% 10% 2	2846
December	1st to 10th	Night	36% 8% 35% 16% 4	4700
	11th to 20th	Night	61% 9% 18% 9% 2	5316
	21st to end	Night	53% 11% 19% 12% 4	7106

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	20% 7% 20% 33% 15% 5%	220
	11th to 20th	Day	17% 6% 20% 30% 20% 7%	220
	21st to end	Day	17% 2 21% 35% 20% 6%	242
February	1st to 10th	Day	18% 6% 28% 30% 14% 4	305
	11th to 20th	Day	15% 7% 30% 30% 15% 2	535
	21st to end	Day	15% 13% 26% 30% 13% 2	446
March	1st to 10th	Day	15% 10% 30% 33% 12% 1	540
	11th to 20th	Day	17% 9% 25% 29% 18% 2	540
	21st to end	Day	16% 12% 26% 28% 14% 4	594
April	1st to 10th	Day	12% 11% 29% 25% 16% 7%	540
	11th to 20th	Day	13% 11% 30% 26% 14% 5%	532
	21st to end	Day	10% 12% 30% 30% 13% 5%	540
May	1st to 10th	Day	6% 12% 31% 29% 18% 4	543
	11th to 20th	Day	14% 12% 25% 29% 15% 6%	568
	21st to end	Day	10% 7% 21% 28% 24% 9%	617
June	1st to 10th	Day	7% 7% 28% 31% 20% 8%	570
	11th to 20th	Day	9% 11% 27% 32% 17% 4	570
	21st to end	Day	3 7% 26% 34% 23% 8%	570
July	1st to 10th	Day	4 3 25% 39% 25% 5%	566
	11th to 20th	Day	3 2 26% 39% 26% 4	564
	21st to end	Day	2 2 21% 39% 27% 8%	619
August	1st to 10th	Day	4 3 17% 39% 26% 11%	567
	11th to 20th	Day	2 2 18% 36% 32% 10%	557
	21st to end	Day	4 4 19% 34% 29% 11%	627
September	1st to 10th	Day	4 3 18% 30% 34% 11%	570
	11th to 20th	Day	6% 4 19% 32% 28% 11%	543
	21st to end	Day	6% 3 20% 34% 29% 8%	530
October	1st to 10th	Day	9% 6% 19% 29% 30% 8%	530
	11th to 20th	Day	10% 5% 24% 29% 24% 8%	423
	21st to end	Day	14% 5% 25% 34% 18% 5%	363
November	1st to 10th	Day	18% 5% 25% 31% 18% 4	330
	11th to 20th	Day	16% 6% 19% 35% 18% 5%	330
	21st to end	Day	15% 4 19% 37% 20% 4	267
December	1st to 10th	Day	20% 3 18% 31% 16% 11%	220
	11th to 20th	Day	25% 6% 20% 33% 14% 3	220
	21st to end	Day	19% 6% 23% 29% 17% 6%	242
January	1st to 10th	Night	22% 5% 13% 36% 21% 3	840
	11th to 20th	Night	20% 4 13% 36% 22% 5%	840
	21st to end	Night	18% 2 13% 38% 24% 6%	924
February	1st to 10th	Night	20% 4 18% 40% 15% 2	840
	11th to 20th	Night	16% 3 20% 38% 20% 3	824
	21st to end	Night	16% 3 20% 40% 18% 3	692
March	1st to 10th	Night	16% 3 21% 43% 15% 3	840
	11th to 20th	Night	16% 5% 18% 40% 19% 2	840
	21st to end	Night	17% 5% 19% 42% 15% 2	924
April	1st to 10th	Night	14% 6% 16% 40% 22% 3	840
	11th to 20th	Night	13% 7% 24% 35% 18% 2	786
	21st to end	Night	10% 3 26% 40% 20% 1	630
May	1st to 10th	Night	7% 6% 23% 42% 20% 2	630
	11th to 20th	Night	7% 5% 21% 35% 27% 5%	449
	21st to end	Night	7% 5% 19% 36% 29% 3	455
June	1st to 10th	Night	9% 4 21% 39% 25% 3	420
	11th to 20th	Night	9% 6% 26% 35% 22% 1	420
	21st to end	Night	4 2 18% 45% 31%	420
July	1st to 10th	Night	2 2 20% 47% 28% 2	410
	11th to 20th	Night	1 1 23% 56% 18% 2	408
	21st to end	Night	2 1 15% 50% 30% 2	460
August	1st to 10th	Night	5% 1 14% 44% 31% 4	465
	11th to 20th	Night	2 1 16% 45% 33% 3	624
	21st to end	Night	5% 2 16% 44% 29% 4	910
September	1st to 10th	Night	4 2 13% 44% 33% 3	840
	11th to 20th	Night	6% 2 12% 40% 33% 7%	812
	21st to end	Night	9% 3 14% 36% 32% 7%	800
October	1st to 10th	Night	7% 2 17% 42% 28% 4	800
	11th to 20th	Night	10% 4 15% 39% 25% 7%	800
	21st to end	Night	11% 4 18% 42% 21% 4	880
November	1st to 10th	Night	17% 3 14% 41% 23% 2	820
	11th to 20th	Night	21% 2 15% 42% 18% 3	840
	21st to end	Night	16% 3 16% 38% 23% 5%	840
December	1st to 10th	Night	19% 3 14% 39% 21% 4	840
	11th to 20th	Night	24% 3 13% 40% 17% 3	840
	21st to end	Night	24% 3 17% 39% 15% 2	924

Page generated at 2026-09-11 02:11:55. Version ac07

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