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 2025-06-10.

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	15% 11% 4 9% 5% 5% 5% 4 5% 7% 30%	120201
	Night	15% 11% 4 9% 5% 5% 5% 4 5% 6% 32%	117993
Transparency	Day	51% 6% 12% 19% 10% 1	123836
	Night	50% 6% 12% 19% 11% 1	122519
Seeing	Day	20% 14% 29% 22% 11% 5%	15521
	Night	19% 13% 30% 26% 10% 1	24697

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	12% 15% 8% 17% 6% 4 4 3 3 5% 23%	3666
	11th to 20th	Day	10% 21% 8% 21% 9% 6% 3 3 2 4 11%	4026
	21st to end	Day	18% 17% 7% 17% 7% 5% 4 3 4 4 14%	4466
February	1st to 10th	Day	16% 18% 5% 12% 8% 6% 5% 4 5% 5% 17%	1800
	11th to 20th	Day	14% 14% 8% 14% 8% 4 4 3 5% 7% 20%	1753
	21st to end	Day	17% 16% 5% 11% 6% 4 5% 2 5% 6% 22%	1583
March	1st to 10th	Day	18% 12% 6% 11% 6% 4 3 3 4 6% 27%	4902
	11th to 20th	Day	11% 13% 4 9% 5% 4 5% 4 6% 8% 29%	4980
	21st to end	Day	17% 13% 5% 9% 5% 4 4 3 4 5% 29%	5465
April	1st to 10th	Day	17% 13% 5% 9% 4 4 5% 3 5% 7% 29%	2320
	11th to 20th	Day	18% 10% 3 7% 4 5% 4 3 4 7% 34%	2499
	21st to end	Day	19% 11% 3 8% 5% 5% 4 3 5% 5% 33%	2600
May	1st to 10th	Day	18% 10% 3 8% 5% 4 4 4 5% 6% 33%	5888
	11th to 20th	Day	16% 11% 3 8% 5% 4 4 4 5% 8% 31%	6088
	21st to end	Day	17% 7% 2 7% 5% 4 5% 4 5% 7% 38%	6949
June	1st to 10th	Day	14% 7% 2 6% 5% 6% 6% 4 6% 8% 37%	2786
	11th to 20th	Day	11% 6% 2 7% 5% 4 5% 5% 7% 9% 39%	2726
	21st to end	Day	13% 6% 2 5% 5% 5% 5% 4 7% 9% 38%	2850
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 10% 39%	4669
	11th to 20th	Day	10% 5% 2 5% 5% 4 5% 5% 8% 10% 41%	4322
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 9% 41%	4466
August	1st to 10th	Day	16% 7% 2 6% 4 4 5% 4 7% 9% 36%	4104
	11th to 20th	Day	11% 7% 2 8% 6% 5% 6% 5% 7% 9% 34%	4044
	21st to end	Day	8% 6% 3 7% 5% 5% 6% 4 7% 9% 41%	4412
September	1st to 10th	Day	11% 7% 2 6% 5% 4 6% 4 6% 8% 40%	2280
	11th to 20th	Day	10% 6% 3 6% 5% 4 4 4 6% 9% 44%	2106
	21st to end	Day	14% 8% 2 8% 4 5% 6% 4 7% 8% 34%	2088
October	1st to 10th	Day	17% 9% 3 7% 5% 5% 3 3 5% 7% 34%	1900
	11th to 20th	Day	21% 9% 3 8% 5% 5% 4 4 6% 6% 30%	1900
	21st to end	Day	23% 12% 3 9% 5% 4 4 4 6% 7% 22%	2090
November	1st to 10th	Day	15% 9% 4 9% 6% 4 5% 4 5% 7% 31%	1727
	11th to 20th	Day	15% 16% 3 10% 5% 5% 5% 4 5% 8% 24%	1600
	21st to end	Day	18% 18% 5% 11% 6% 4 4 3 6% 6% 19%	1600
December	1st to 10th	Day	19% 18% 6% 13% 7% 6% 6% 4 5% 5% 12%	2600
	11th to 20th	Day	22% 19% 6% 9% 5% 5% 4 4 4 4 17%	2986
	21st to end	Day	26% 18% 6% 12% 6% 4 4 3 4 4 12%	3960
January	1st to 10th	Night	14% 14% 5% 17% 8% 5% 5% 3 5% 5% 20%	6276
	11th to 20th	Night	12% 15% 7% 17% 10% 7% 6% 5% 5% 5% 12%	5859
	21st to end	Night	17% 14% 7% 13% 6% 5% 5% 3 5% 6% 19%	6490
February	1st to 10th	Night	15% 16% 5% 10% 7% 6% 6% 4 5% 6% 21%	2600
	11th to 20th	Night	14% 14% 5% 11% 6% 5% 5% 4 6% 6% 24%	2533
	21st to end	Night	14% 13% 5% 12% 6% 5% 6% 4 5% 6% 24%	2045
March	1st to 10th	Night	14% 11% 4 8% 6% 5% 5% 4 5% 7% 31%	4937
	11th to 20th	Night	10% 9% 3 9% 5% 4 5% 4 6% 8% 37%	4979
	21st to end	Night	20% 10% 3 7% 4 4 4 3 4 5% 37%	5467
April	1st to 10th	Night	19% 10% 4 8% 6% 5% 4 3 5% 6% 32%	2080
	11th to 20th	Night	16% 10% 3 7% 5% 4 4 3 5% 7% 36%	1867
	21st to end	Night	14% 9% 4 7% 5% 5% 4 4 5% 7% 36%	1800
May	1st to 10th	Night	19% 8% 3 6% 4 4 4 3 5% 7% 36%	4053
	11th to 20th	Night	12% 9% 3 8% 6% 5% 5% 4 5% 6% 36%	3789
	21st to end	Night	17% 5% 2 5% 3 3 4 3 5% 8% 44%	3926
June	1st to 10th	Night	11% 5% 2 6% 5% 5% 5% 4 6% 8% 44%	1598
	11th to 20th	Night	10% 5% 2 6% 5% 5% 4 4 8% 9% 43%	1445
	21st to end	Night	10% 5% 2 6% 5% 4 4 4 6% 8% 44%	1330
July	1st to 10th	Night	6% 4 2 6% 4 4 4 3 5% 8% 53%	2292
	11th to 20th	Night	10% 5% 1 5% 3 3 3 3 5% 9% 52%	2635
	21st to end	Night	6% 5% 2 6% 4 4 4 4 7% 8% 50%	3087
August	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 7% 45%	2825
	11th to 20th	Night	11% 6% 2 5% 3 3 4 4 6% 8% 46%	2785
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 8% 48%	3211
September	1st to 10th	Night	10% 5% 2 5% 4 5% 4 4 5% 8% 47%	1900
	11th to 20th	Night	10% 5% 2 5% 3 3 4 3 4 8% 53%	2071
	21st to end	Night	15% 7% 2 6% 5% 4 4 4 5% 7% 40%	2091
October	1st to 10th	Night	15% 8% 2 8% 5% 3 3 3 5% 6% 42%	2280
	11th to 20th	Night	17% 9% 3 7% 4 4 5% 4 5% 7% 35%	2280
	21st to end	Night	20% 9% 3 9% 5% 4 5% 3 5% 6% 29%	2508
November	1st to 10th	Night	14% 9% 3 7% 5% 4 4 4 6% 7% 37%	2570
	11th to 20th	Night	16% 12% 4 9% 6% 5% 4 4 6% 7% 27%	2800
	21st to end	Night	20% 11% 4 9% 6% 5% 5% 4 6% 6% 24%	2800
December	1st to 10th	Night	15% 16% 6% 14% 7% 5% 6% 5% 6% 6% 16%	4580
	11th to 20th	Night	16% 19% 5% 11% 6% 5% 4 4 4 5% 21%	5208
	21st to end	Night	24% 20% 6% 12% 5% 4 4 3 4 5% 13%	6996

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	64% 6% 9% 10% 10% 1	3667
	11th to 20th	Day	79% 3 9% 7% 3	4060
	21st to end	Day	73% 5% 9% 7% 6%	4466
February	1st to 10th	Day	67% 6% 10% 12% 4 1	1800
	11th to 20th	Day	64% 5% 10% 14% 6% 1	1762
	21st to end	Day	63% 5% 11% 14% 6% 1	1583
March	1st to 10th	Day	58% 4 14% 15% 8% 1	4905
	11th to 20th	Day	51% 5% 17% 18% 6% 2	4980
	21st to end	Day	56% 5% 9% 19% 10% 1	5465
April	1st to 10th	Day	55% 6% 11% 16% 11% 1	2320
	11th to 20th	Day	51% 5% 10% 21% 12% 1	2499
	21st to end	Day	53% 6% 9% 19% 11% 3	2600
May	1st to 10th	Day	51% 6% 11% 17% 10% 6%	5897
	11th to 20th	Day	50% 8% 10% 18% 12% 2	6092
	21st to end	Day	45% 6% 11% 19% 16% 3	6949
June	1st to 10th	Day	42% 8% 11% 22% 15% 1	2786
	11th to 20th	Day	39% 7% 15% 22% 16% 1	2726
	21st to end	Day	40% 8% 14% 24% 15%	2850
July	1st to 10th	Day	37% 7% 15% 24% 15% 2	4597
	11th to 20th	Day	34% 7% 13% 33% 12% 1	4247
	21st to end	Day	36% 9% 13% 22% 18% 1	4324
August	1st to 10th	Day	43% 7% 13% 22% 13% 1	4091
	11th to 20th	Day	43% 8% 16% 24% 9%	4083
	21st to end	Day	36% 7% 12% 33% 11% 1	4412
September	1st to 10th	Day	41% 7% 14% 26% 12% 1	2280
	11th to 20th	Day	35% 5% 15% 29% 15%	2106
	21st to end	Day	45% 8% 14% 24% 9% 1	2088
October	1st to 10th	Day	41% 6% 14% 24% 14% 1	3160
	11th to 20th	Day	41% 6% 14% 21% 17%	3140
	21st to end	Day	52% 6% 15% 20% 7%	3428
November	1st to 10th	Day	51% 7% 13% 21% 8% 1	1727
	11th to 20th	Day	57% 7% 13% 14% 8% 1	1600
	21st to end	Day	64% 6% 13% 12% 4	1600
December	1st to 10th	Day	72% 7% 11% 7% 3	2600
	11th to 20th	Day	69% 7% 9% 9% 6%	2986
	21st to end	Day	75% 6% 6% 7% 6%	3960
January	1st to 10th	Night	62% 7% 9% 15% 6% 1	6276
	11th to 20th	Night	70% 7% 10% 9% 3	5900
	21st to end	Night	64% 6% 10% 11% 8% 1	6490
February	1st to 10th	Night	62% 6% 11% 16% 4	2600
	11th to 20th	Night	57% 7% 12% 15% 8% 1	2535
	21st to end	Night	58% 6% 12% 16% 7%	2045
March	1st to 10th	Night	52% 5% 14% 17% 10% 2	4947
	11th to 20th	Night	43% 5% 19% 21% 11% 1	4980
	21st to end	Night	49% 5% 9% 19% 16% 2	5467
April	1st to 10th	Night	53% 6% 9% 17% 13% 1	2080
	11th to 20th	Night	48% 6% 8% 20% 16% 2	1867
	21st to end	Night	46% 5% 11% 20% 16% 3	1800
May	1st to 10th	Night	47% 7% 11% 15% 13% 8%	4060
	11th to 20th	Night	46% 7% 12% 21% 11% 2	3790
	21st to end	Night	39% 6% 13% 24% 17% 2	3927
June	1st to 10th	Night	37% 6% 11% 26% 17% 2	1598
	11th to 20th	Night	35% 7% 13% 26% 17% 2	1445
	21st to end	Night	36% 7% 14% 25% 18% 1	1330
July	1st to 10th	Night	29% 5% 12% 27% 22% 5%	2255
	11th to 20th	Night	30% 5% 13% 33% 17% 1	2587
	21st to end	Night	31% 8% 15% 27% 17% 2	2988
August	1st to 10th	Night	37% 7% 12% 27% 15% 2	2814
	11th to 20th	Night	35% 6% 16% 30% 11% 1	2812
	21st to end	Night	32% 6% 13% 31% 16% 1	3211
September	1st to 10th	Night	35% 7% 13% 28% 16% 2	1900
	11th to 20th	Night	30% 6% 12% 32% 20% 1	2071
	21st to end	Night	43% 7% 12% 26% 12%	2091
October	1st to 10th	Night	33% 5% 9% 28% 24% 1	3800
	11th to 20th	Night	43% 5% 12% 25% 15%	3779
	21st to end	Night	42% 5% 16% 27% 10%	4120
November	1st to 10th	Night	44% 7% 13% 22% 13% 1	2570
	11th to 20th	Night	54% 7% 14% 17% 7% 1	2800
	21st to end	Night	59% 6% 17% 13% 5%	2800
December	1st to 10th	Night	67% 7% 12% 11% 3	4580
	11th to 20th	Night	65% 6% 10% 12% 8%	5208
	21st to end	Night	73% 6% 9% 7% 5%	6996

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	24% 23% 30% 16% 6%	215
	11th to 20th	Day	25% 26% 34% 10% 4 1	300
	21st to end	Day	28% 21% 33% 14% 4	330
February	1st to 10th	Day	30% 23% 33% 11% 4	300
	11th to 20th	Day	29% 24% 35% 11% 1	300
	21st to end	Day	34% 23% 33% 9%	249
March	1st to 10th	Day	25% 19% 33% 18% 5%	483
	11th to 20th	Day	26% 18% 32% 20% 4 1	500
	21st to end	Day	29% 17% 31% 18% 5% 1	550
April	1st to 10th	Day	23% 21% 31% 18% 6% 1	500
	11th to 20th	Day	28% 18% 25% 18% 8% 2	492
	21st to end	Day	25% 13% 34% 18% 8% 2	500
May	1st to 10th	Day	23% 14% 29% 21% 8% 5%	503
	11th to 20th	Day	18% 13% 27% 24% 14% 5%	527
	21st to end	Day	15% 9% 30% 26% 14% 7%	573
June	1st to 10th	Day	16% 16% 29% 27% 10% 3	526
	11th to 20th	Day	13% 10% 30% 28% 15% 4	556
	21st to end	Day	16% 9% 30% 32% 11% 3	680
July	1st to 10th	Day	9% 6% 25% 31% 21% 8%	621
	11th to 20th	Day	14% 7% 24% 26% 18% 10%	484
	21st to end	Day	8% 5% 29% 32% 19% 7%	532
August	1st to 10th	Day	12% 7% 21% 27% 22% 10%	486
	11th to 20th	Day	12% 7% 21% 24% 23% 13%	479
	21st to end	Day	13% 6% 25% 25% 21% 9%	539
September	1st to 10th	Day	13% 10% 22% 26% 16% 12%	490
	11th to 20th	Day	8% 6% 19% 27% 25% 15%	490
	21st to end	Day	18% 11% 26% 24% 15% 7%	490
October	1st to 10th	Day	21% 13% 26% 23% 11% 7%	490
	11th to 20th	Day	20% 19% 32% 19% 8% 2	490
	21st to end	Day	28% 16% 32% 18% 6%	539
November	1st to 10th	Day	25% 22% 31% 14% 7% 1	287
	11th to 20th	Day	26% 22% 31% 16% 5% 2	200
	21st to end	Day	29% 21% 34% 13% 4	200
December	1st to 10th	Day	30% 26% 30% 10% 5% 1	200
	11th to 20th	Day	31% 28% 29% 10% 3	200
	21st to end	Day	31% 24% 34% 9% 2	220
January	1st to 10th	Night	22% 21% 35% 16% 5%	800
	11th to 20th	Night	27% 19% 33% 19% 2	800
	21st to end	Night	21% 14% 37% 22% 5%	880
February	1st to 10th	Night	24% 20% 35% 20% 2	800
	11th to 20th	Night	23% 18% 35% 20% 4	783
	21st to end	Night	22% 21% 35% 20% 3	660
March	1st to 10th	Night	19% 15% 37% 23% 6%	800
	11th to 20th	Night	22% 16% 37% 20% 6%	800
	21st to end	Night	22% 14% 31% 25% 7% 1	880
April	1st to 10th	Night	24% 15% 32% 25% 4	800
	11th to 20th	Night	22% 15% 30% 22% 8% 1	683
	21st to end	Night	17% 11% 36% 28% 8%	600
May	1st to 10th	Night	17% 10% 31% 28% 12% 1	600
	11th to 20th	Night	16% 5% 31% 34% 12% 1	491
	21st to end	Night	12% 6% 28% 34% 18%	433
June	1st to 10th	Night	13% 6% 32% 38% 11% 1	400
	11th to 20th	Night	12% 6% 33% 36% 12% 1	380
	21st to end	Night	12% 7% 28% 38% 16%	380
July	1st to 10th	Night	4 4 26% 44% 20% 2	370
	11th to 20th	Night	10% 4 25% 39% 21% 2	431
	21st to end	Night	9% 3 23% 45% 19% 2	624
August	1st to 10th	Night	8% 3 21% 42% 23% 2	567
	11th to 20th	Night	9% 2 18% 39% 29% 3	564
	21st to end	Night	11% 4 25% 32% 26% 2	675
September	1st to 10th	Night	12% 7% 21% 35% 21% 4	760
	11th to 20th	Night	11% 5% 19% 32% 29% 4	760
	21st to end	Night	19% 9% 21% 27% 19% 5%	760
October	1st to 10th	Night	17% 10% 28% 26% 17% 3	760
	11th to 20th	Night	21% 13% 29% 28% 8% 1	760
	21st to end	Night	25% 14% 31% 26% 5%	836
November	1st to 10th	Night	19% 14% 30% 26% 9% 2	780
	11th to 20th	Night	23% 18% 31% 20% 7% 1	800
	21st to end	Night	27% 16% 33% 19% 5%	800
December	1st to 10th	Night	27% 21% 30% 17% 5%	800
	11th to 20th	Night	24% 21% 34% 18% 4	800
	21st to end	Night	31% 15% 34% 17% 3	880

Page generated at 2025-06-07 10:28:18. Version ac07

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