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 2026-10-14.

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	14% 10% 4 9% 5% 5% 5% 4 6% 7% 32%	150311
	Night	14% 10% 4 9% 5% 5% 5% 4 5% 7% 32%	141388
Transparency	Day	50% 6% 12% 20% 11% 1	153906
	Night	50% 6% 12% 19% 12% 1	145882
Seeing	Day	20% 14% 29% 21% 11% 4	17344
	Night	19% 13% 30% 26% 10% 1	26345

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	16% 15% 8% 17% 7% 4 4 3 3 4 19%	4758
	11th to 20th	Day	9% 24% 9% 20% 10% 7% 3 2 2 4 9%	5246
	21st to end	Day	15% 15% 6% 17% 9% 7% 5% 3 4 5% 14%	5780
February	1st to 10th	Day	15% 18% 5% 12% 8% 6% 5% 3 5% 6% 18%	1890
	11th to 20th	Day	14% 14% 8% 14% 7% 4 4 3 5% 6% 20%	1843
	21st to end	Day	16% 17% 5% 11% 6% 4 5% 3 5% 6% 22%	1660
March	1st to 10th	Day	18% 12% 6% 11% 6% 4 3 3 4 6% 28%	5011
	11th to 20th	Day	12% 13% 4 10% 5% 4 5% 3 6% 8% 29%	5090
	21st to end	Day	17% 13% 5% 9% 6% 4 4 3 4 5% 29%	5586
April	1st to 10th	Day	17% 13% 5% 9% 4 4 4 3 5% 6% 28%	2436
	11th to 20th	Day	18% 11% 3 8% 5% 5% 4 3 4 6% 33%	2625
	21st to end	Day	19% 11% 3 8% 5% 4 4 3 4 5% 33%	2730
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 4 5% 6% 33%	7642
	11th to 20th	Day	14% 10% 3 8% 5% 5% 5% 4 6% 8% 33%	7923
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 7% 43%	9060
June	1st to 10th	Day	13% 6% 2 5% 5% 5% 6% 4 6% 8% 38%	2940
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 38%	3014
	21st to end	Day	13% 6% 2 5% 5% 5% 5% 4 7% 9% 39%	3150
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 9% 38%	8704
	11th to 20th	Day	7% 4 2 6% 4 4 5% 4 7% 11% 46%	8050
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 7% 8% 44%	8340
August	1st to 10th	Day	17% 8% 2 6% 4 5% 4 4 6% 7% 37%	6014
	11th to 20th	Day	9% 6% 2 8% 5% 5% 6% 5% 7% 10% 37%	5951
	21st to end	Day	10% 7% 3 8% 5% 5% 6% 4 7% 9% 37%	6414
September	1st to 10th	Day	11% 7% 2 6% 5% 4 6% 4 6% 9% 40%	2520
	11th to 20th	Day	10% 6% 3 6% 4 4 4 4 6% 9% 45%	2328
	21st to end	Day	14% 8% 2 8% 4 5% 6% 4 7% 8% 35%	2308
October	1st to 10th	Day	16% 9% 3 7% 5% 4 3 3 5% 7% 36%	2100
	11th to 20th	Day	21% 9% 3 8% 5% 5% 4 4 6% 6% 30%	2030
	21st to end	Day	23% 12% 3 9% 5% 5% 5% 4 6% 7% 22%	2200
November	1st to 10th	Day	15% 10% 4 9% 6% 4 5% 4 5% 7% 31%	1814
	11th to 20th	Day	15% 16% 3 10% 5% 5% 5% 4 5% 7% 25%	1680
	21st to end	Day	18% 19% 5% 11% 6% 4 4 3 6% 6% 19%	1680
December	1st to 10th	Day	18% 18% 6% 13% 6% 6% 6% 4 5% 5% 12%	2680
	11th to 20th	Day	21% 19% 6% 10% 5% 5% 5% 4 4 4 17%	3066
	21st to end	Day	26% 18% 6% 12% 6% 4 4 3 4 4 12%	4048
January	1st to 10th	Night	14% 14% 6% 18% 8% 5% 5% 3 5% 5% 17%	8133
	11th to 20th	Night	10% 18% 8% 17% 10% 7% 6% 4 5% 5% 12%	7639
	21st to end	Night	14% 13% 7% 14% 8% 6% 6% 3 5% 6% 19%	8401
February	1st to 10th	Night	14% 16% 5% 10% 7% 6% 6% 4 5% 6% 22%	2730
	11th to 20th	Night	14% 13% 5% 11% 6% 4 5% 4 6% 6% 24%	2663
	21st to end	Night	14% 14% 5% 12% 6% 5% 6% 4 5% 6% 24%	2144
March	1st to 10th	Night	14% 11% 4 8% 6% 5% 4 4 5% 7% 31%	5048
	11th to 20th	Night	10% 9% 3 9% 5% 4 5% 4 6% 8% 36%	5089
	21st to end	Night	19% 10% 3 7% 4 4 4 3 4 5% 36%	5588
April	1st to 10th	Night	19% 10% 4 8% 6% 5% 4 3 5% 6% 32%	2184
	11th to 20th	Night	16% 10% 3 7% 5% 4 4 4 5% 7% 36%	1962
	21st to end	Night	14% 8% 4 7% 5% 5% 4 4 5% 7% 37%	1890
May	1st to 10th	Night	18% 8% 3 6% 4 4 4 3 5% 7% 37%	5257
	11th to 20th	Night	13% 9% 3 8% 6% 5% 5% 4 5% 6% 36%	4921
	21st to end	Night	18% 5% 2 5% 3 3 3 3 5% 7% 45%	5114
June	1st to 10th	Night	11% 5% 2 5% 4 5% 5% 4 6% 8% 45%	1680
	11th to 20th	Night	10% 6% 2 6% 5% 5% 4 4 7% 9% 43%	1597
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 6% 9% 44%	1470
July	1st to 10th	Night	7% 5% 2 7% 4 4 4 3 6% 9% 49%	4257
	11th to 20th	Night	8% 5% 2 5% 3 3 4 3 5% 10% 50%	4893
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 7% 50%	5737
August	1st to 10th	Night	13% 7% 3 6% 4 4 4 4 6% 6% 42%	4135
	11th to 20th	Night	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	4095
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 8% 45%	4664
September	1st to 10th	Night	10% 5% 2 5% 4 5% 4 4 5% 8% 47%	2100
	11th to 20th	Night	9% 5% 2 5% 4 3 4 3 4 8% 54%	2289
	21st to end	Night	15% 7% 2 7% 5% 4 4 4 5% 7% 40%	2311
October	1st to 10th	Night	14% 7% 2 7% 4 3 3 3 5% 6% 44%	2520
	11th to 20th	Night	17% 9% 3 7% 4 4 5% 4 5% 7% 35%	2437
	21st to end	Night	19% 9% 3 9% 5% 4 5% 3 5% 6% 30%	2640
November	1st to 10th	Night	14% 9% 3 7% 5% 4 4 4 6% 7% 37%	2702
	11th to 20th	Night	16% 12% 4 9% 6% 5% 5% 4 6% 7% 27%	2940
	21st to end	Night	20% 12% 5% 9% 6% 5% 4 4 5% 6% 23%	2940
December	1st to 10th	Night	14% 16% 6% 14% 7% 5% 6% 5% 6% 6% 16%	4720
	11th to 20th	Night	16% 19% 5% 11% 6% 5% 4 4 4 5% 21%	5348
	21st to end	Night	24% 20% 6% 12% 5% 4 4 3 4 5% 13%	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	69% 5% 8% 10% 8% 1	4759
	11th to 20th	Day	81% 3 7% 6% 3	5280
	21st to end	Day	72% 5% 9% 8% 5%	5780
February	1st to 10th	Day	66% 6% 10% 13% 5% 1	1890
	11th to 20th	Day	64% 5% 10% 14% 6% 1	1852
	21st to end	Day	63% 5% 12% 14% 6% 1	1660
March	1st to 10th	Day	58% 4 14% 15% 8%	5014
	11th to 20th	Day	51% 5% 17% 18% 6% 2	5090
	21st to end	Day	56% 5% 9% 18% 10% 1	5586
April	1st to 10th	Day	55% 6% 11% 16% 11% 1	2436
	11th to 20th	Day	51% 5% 10% 21% 11% 1	2625
	21st to end	Day	52% 6% 9% 20% 11% 2	2730
May	1st to 10th	Day	49% 5% 12% 19% 10% 4	7651
	11th to 20th	Day	48% 7% 11% 21% 12% 1	7927
	21st to end	Day	41% 5% 10% 25% 17% 2	9061
June	1st to 10th	Day	41% 7% 11% 23% 16% 1	2940
	11th to 20th	Day	40% 7% 14% 22% 15% 1	3014
	21st to end	Day	40% 7% 14% 24% 15%	3150
July	1st to 10th	Day	40% 7% 13% 25% 14% 1	8633
	11th to 20th	Day	30% 6% 13% 34% 16% 1	7975
	21st to end	Day	34% 7% 12% 27% 19% 1	8156
August	1st to 10th	Day	45% 6% 13% 18% 15% 3	6001
	11th to 20th	Day	40% 7% 17% 28% 8%	5990
	21st to end	Day	40% 7% 14% 30% 8%	6414
September	1st to 10th	Day	40% 7% 14% 26% 11% 1	2520
	11th to 20th	Day	35% 5% 16% 29% 14%	2328
	21st to end	Day	44% 7% 13% 24% 10% 1	2308
October	1st to 10th	Day	40% 6% 13% 25% 15% 1	3360
	11th to 20th	Day	41% 6% 14% 21% 17%	3270
	21st to end	Day	52% 6% 15% 20% 7%	3538
November	1st to 10th	Day	52% 7% 12% 21% 7% 1	1814
	11th to 20th	Day	57% 7% 13% 14% 9% 1	1680
	21st to end	Day	65% 6% 13% 12% 4	1680
December	1st to 10th	Day	72% 7% 10% 8% 3	2680
	11th to 20th	Day	69% 7% 9% 9% 6%	3066
	21st to end	Day	75% 6% 6% 7% 5%	4048
January	1st to 10th	Night	66% 7% 9% 13% 5%	8133
	11th to 20th	Night	71% 7% 8% 9% 5%	7680
	21st to end	Night	65% 6% 9% 12% 8%	8401
February	1st to 10th	Night	61% 6% 11% 16% 5% 1	2730
	11th to 20th	Night	57% 6% 12% 16% 8% 1	2665
	21st to end	Night	59% 5% 12% 16% 7%	2144
March	1st to 10th	Night	51% 5% 14% 17% 10% 2	5058
	11th to 20th	Night	43% 5% 18% 21% 11% 1	5090
	21st to end	Night	49% 5% 9% 19% 16% 2	5588
April	1st to 10th	Night	53% 6% 10% 17% 13% 1	2184
	11th to 20th	Night	48% 6% 8% 20% 16% 2	1962
	21st to end	Night	46% 5% 11% 21% 15% 3	1890
May	1st to 10th	Night	46% 6% 11% 16% 15% 6%	5264
	11th to 20th	Night	47% 7% 12% 21% 11% 2	4922
	21st to end	Night	39% 5% 10% 26% 18% 2	5115
June	1st to 10th	Night	36% 6% 11% 26% 19% 2	1680
	11th to 20th	Night	35% 7% 13% 26% 17% 2	1597
	21st to end	Night	35% 7% 14% 25% 18% 1	1470
July	1st to 10th	Night	32% 6% 15% 27% 18% 3	4220
	11th to 20th	Night	31% 4 11% 31% 22% 2	4845
	21st to end	Night	32% 6% 11% 25% 22% 2	5606
August	1st to 10th	Night	41% 6% 11% 23% 14% 5%	4124
	11th to 20th	Night	34% 6% 18% 31% 10% 1	4122
	21st to end	Night	35% 6% 15% 29% 13% 1	4664
September	1st to 10th	Night	35% 6% 13% 29% 15% 1	2100
	11th to 20th	Night	30% 6% 12% 32% 20% 1	2289
	21st to end	Night	43% 7% 11% 26% 13% 1	2311
October	1st to 10th	Night	32% 5% 9% 29% 24% 1	4040
	11th to 20th	Night	44% 5% 12% 24% 15%	3936
	21st to end	Night	41% 6% 15% 27% 10%	4252
November	1st to 10th	Night	45% 7% 13% 22% 13% 1	2702
	11th to 20th	Night	54% 7% 14% 17% 7% 1	2940
	21st to end	Night	60% 6% 16% 13% 5%	2940
December	1st to 10th	Night	67% 7% 12% 11% 3	4720
	11th to 20th	Night	64% 6% 10% 12% 8%	5348
	21st to end	Night	73% 6% 9% 7% 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% 23% 30% 15% 5%	237
	11th to 20th	Day	27% 27% 32% 9% 4 1	330
	21st to end	Day	29% 21% 33% 14% 4	363
February	1st to 10th	Day	30% 21% 33% 12% 4	330
	11th to 20th	Day	28% 24% 36% 11% 1	330
	21st to end	Day	33% 24% 33% 10%	273
March	1st to 10th	Day	25% 20% 32% 18% 5%	522
	11th to 20th	Day	29% 18% 30% 19% 4 1	540
	21st to end	Day	29% 18% 31% 17% 5% 1	594
April	1st to 10th	Day	24% 21% 31% 17% 6% 1	540
	11th to 20th	Day	29% 19% 25% 18% 8% 2	532
	21st to end	Day	25% 14% 34% 18% 7% 2	540
May	1st to 10th	Day	24% 14% 30% 20% 7% 4	543
	11th to 20th	Day	18% 14% 28% 23% 13% 4	567
	21st to end	Day	15% 10% 30% 24% 13% 6%	617
June	1st to 10th	Day	15% 16% 29% 27% 11% 3	570
	11th to 20th	Day	15% 12% 32% 25% 13% 4	644
	21st to end	Day	15% 9% 32% 31% 10% 3	780
July	1st to 10th	Day	10% 7% 26% 31% 19% 7%	715
	11th to 20th	Day	13% 8% 26% 27% 18% 9%	564
	21st to end	Day	9% 5% 30% 32% 18% 6%	620
August	1st to 10th	Day	12% 7% 22% 28% 22% 9%	566
	11th to 20th	Day	12% 8% 25% 24% 21% 11%	559
	21st to end	Day	14% 7% 25% 26% 19% 9%	627
September	1st to 10th	Day	13% 11% 22% 26% 17% 11%	570
	11th to 20th	Day	9% 6% 21% 27% 24% 14%	570
	21st to end	Day	17% 10% 26% 25% 14% 7%	570
October	1st to 10th	Day	21% 14% 25% 23% 11% 6%	570
	11th to 20th	Day	21% 18% 33% 19% 7% 2	542
	21st to end	Day	28% 17% 32% 18% 5%	583
November	1st to 10th	Day	27% 20% 32% 13% 6% 1	314
	11th to 20th	Day	27% 20% 31% 15% 5% 2	220
	21st to end	Day	29% 22% 34% 12% 3	220
December	1st to 10th	Day	30% 25% 30% 10% 4	220
	11th to 20th	Day	32% 28% 28% 10% 3	220
	21st to end	Day	32% 25% 32% 9% 2	242
January	1st to 10th	Night	23% 22% 35% 16% 5%	840
	11th to 20th	Night	28% 20% 32% 18% 2	840
	21st to end	Night	22% 15% 37% 21% 5%	924
February	1st to 10th	Night	23% 19% 35% 21% 2	840
	11th to 20th	Night	23% 19% 34% 20% 4	823
	21st to end	Night	23% 21% 34% 20% 3	692
March	1st to 10th	Night	20% 16% 36% 23% 5%	840
	11th to 20th	Night	23% 16% 35% 20% 5%	840
	21st to end	Night	22% 15% 32% 24% 7% 1	924
April	1st to 10th	Night	25% 15% 32% 25% 3	840
	11th to 20th	Night	22% 15% 31% 22% 8% 1	718
	21st to end	Night	17% 11% 35% 29% 8%	630
May	1st to 10th	Night	17% 10% 32% 28% 12% 1	630
	11th to 20th	Night	16% 6% 32% 34% 11% 1	516
	21st to end	Night	13% 7% 29% 33% 18%	455
June	1st to 10th	Night	13% 6% 31% 38% 11%	420
	11th to 20th	Night	12% 7% 34% 34% 11% 1	420
	21st to end	Night	12% 6% 27% 39% 15%	420
July	1st to 10th	Night	4 4 27% 44% 20% 2	410
	11th to 20th	Night	9% 5% 27% 38% 20% 2	478
	21st to end	Night	10% 3 23% 45% 18% 2	690
August	1st to 10th	Night	9% 4 21% 41% 24% 2	627
	11th to 20th	Night	8% 2 18% 40% 28% 3	624
	21st to end	Night	11% 5% 25% 33% 24% 2	747
September	1st to 10th	Night	12% 7% 21% 35% 21% 4	840
	11th to 20th	Night	11% 5% 20% 32% 28% 4	840
	21st to end	Night	18% 8% 22% 28% 18% 5%	840
October	1st to 10th	Night	17% 10% 28% 26% 17% 2	840
	11th to 20th	Night	21% 13% 29% 28% 8% 1	813
	21st to end	Night	24% 14% 31% 26% 5%	880
November	1st to 10th	Night	20% 14% 30% 26% 9% 1	820
	11th to 20th	Night	22% 19% 31% 20% 6% 1	840
	21st to end	Night	28% 16% 33% 18% 5%	840
December	1st to 10th	Night	26% 21% 30% 18% 5%	840
	11th to 20th	Night	24% 22% 34% 17% 3	840
	21st to end	Night	32% 15% 34% 17% 3	924

Page generated at 2026-10-11 05:06:33. Version ac07

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