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 2024-11-02.

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% 6% 2 6% 4 4 4 3 5% 6% 52%	101380
	Night	10% 8% 2 6% 4 4 4 3 5% 6% 48%	97749
Transparency	Day	32% 8% 12% 19% 18% 12%	101124
	Night	36% 8% 12% 20% 15% 9%	97594
Seeing	Day	10% 7% 24% 32% 21% 6%	14399
	Night	13% 3 16% 40% 24% 4	23628

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	11% 17% 5% 12% 7% 6% 5% 4 5% 7% 21%	2539
	11th to 20th	Day	16% 17% 5% 8% 6% 5% 6% 4 5% 6% 23%	2540
	21st to end	Day	25% 14% 4 9% 6% 5% 5% 3 4 5% 18%	2794
February	1st to 10th	Day	12% 12% 4 9% 5% 4 4 3 5% 8% 34%	1615
	11th to 20th	Day	11% 10% 3 8% 6% 5% 6% 4 5% 7% 35%	1850
	21st to end	Day	10% 12% 4 8% 5% 5% 5% 5% 6% 7% 33%	1570
March	1st to 10th	Day	8% 12% 4 9% 5% 5% 6% 5% 5% 8% 33%	3138
	11th to 20th	Day	9% 7% 2 4 3 3 3 3 5% 6% 55%	3324
	21st to end	Day	11% 8% 3 8% 6% 5% 7% 6% 8% 9% 29%	4058
April	1st to 10th	Day	9% 8% 3 9% 5% 6% 4 4 7% 8% 37%	2280
	11th to 20th	Day	8% 7% 2 6% 5% 5% 6% 4 6% 7% 46%	2288
	21st to end	Day	8% 7% 2 6% 4 5% 5% 5% 6% 7% 45%	2470
May	1st to 10th	Day	7% 7% 3 7% 4 5% 5% 4 5% 7% 46%	4110
	11th to 20th	Day	5% 4 1 4 3 3 3 3 5% 7% 64%	4331
	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	7% 4 2 6% 4 4 4 4 5% 6% 56%	2090
	11th to 20th	Day	8% 4 1 6% 4 3 4 3 4 8% 55%	1803
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 4 5% 6% 48%	1881
November	1st to 10th	Day	15% 8% 3 5% 4 4 4 3 5% 8% 41%	1683
	11th to 20th	Day	14% 8% 2 6% 5% 4 5% 4 6% 6% 40%	1710
	21st to end	Day	11% 9% 3 7% 4 5% 6% 4 6% 6% 39%	1603
December	1st to 10th	Day	17% 12% 3 8% 5% 4 4 4 6% 6% 30%	1520
	11th to 20th	Day	19% 13% 5% 12% 6% 4 4 4 6% 6% 20%	1928
	21st to end	Day	12% 13% 4 10% 4 5% 5% 5% 6% 6% 31%	2794
January	1st to 10th	Night	13% 16% 5% 12% 6% 6% 5% 6% 6% 5% 21%	4406
	11th to 20th	Night	17% 18% 5% 10% 5% 5% 5% 3 4 5% 24%	4420
	21st to end	Night	25% 17% 4 9% 5% 3 4 3 4 5% 21%	4862
February	1st to 10th	Night	13% 13% 3 8% 5% 4 5% 4 5% 6% 33%	2559
	11th to 20th	Night	12% 12% 3 7% 4 4 4 4 6% 7% 37%	2218
	21st to end	Night	11% 13% 3 8% 6% 5% 5% 4 5% 6% 33%	1884
March	1st to 10th	Night	11% 14% 3 8% 5% 4 5% 3 5% 6% 37%	3736
	11th to 20th	Night	11% 9% 2 5% 3 3 3 3 4 5% 51%	3655
	21st to end	Night	11% 8% 2 8% 6% 6% 5% 5% 7% 8% 34%	3591
April	1st to 10th	Night	11% 7% 3 6% 4 5% 4 4 5% 7% 44%	1900
	11th to 20th	Night	8% 7% 3 6% 5% 4 4 3 7% 5% 46%	1838
	21st to end	Night	7% 5% 3 6% 3 4 5% 3 6% 8% 50%	1710
May	1st to 10th	Night	10% 5% 2 4 3 3 4 4 6% 7% 51%	2860
	11th to 20th	Night	3 3 1 4 3 2 3 3 6% 6% 64%	2561
	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	6% 4 2 5% 3 4 4 3 4 6% 60%	2090
	11th to 20th	Night	7% 4 2 5% 3 3 3 3 5% 7% 57%	2374
	21st to end	Night	8% 5% 2 7% 4 4 5% 4 6% 8% 47%	2717
November	1st to 10th	Night	14% 8% 2 6% 4 4 5% 4 5% 8% 41%	2418
	11th to 20th	Night	16% 8% 2 5% 4 4 4 3 5% 7% 42%	2470
	21st to end	Night	11% 9% 3 6% 4 4 5% 4 4 6% 44%	2574
December	1st to 10th	Night	13% 13% 3 9% 5% 5% 5% 3 5% 6% 34%	2660
	11th to 20th	Night	19% 12% 4 10% 5% 5% 5% 4 5% 5% 27%	3276
	21st to end	Night	12% 11% 4 9% 6% 5% 6% 5% 6% 6% 31%	4862

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% 12% 15% 10% 1	2539
	11th to 20th	Day	60% 12% 13% 11% 5%	2540
	21st to end	Day	65% 14% 12% 6% 2 1	2794
February	1st to 10th	Day	51% 13% 19% 13% 3 1	1615
	11th to 20th	Day	49% 11% 16% 19% 5%	1860
	21st to end	Day	49% 10% 17% 16% 7% 1	1570
March	1st to 10th	Day	48% 11% 17% 14% 10%	3138
	11th to 20th	Day	31% 8% 11% 40% 10%	3324
	21st to end	Day	44% 17% 18% 15% 5% 1	4058
April	1st to 10th	Day	44% 10% 17% 20% 9% 1	2280
	11th to 20th	Day	37% 10% 15% 22% 14% 3	2291
	21st to end	Day	37% 9% 15% 24% 15%	2470
May	1st to 10th	Day	32% 9% 12% 24% 21% 2	4110
	11th to 20th	Day	21% 5% 10% 31% 26% 7%	4334
	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	30% 7% 11% 26% 23% 3	2090
	11th to 20th	Day	30% 6% 15% 29% 19% 2	1803
	21st to end	Day	37% 9% 16% 25% 12% 1	1881
November	1st to 10th	Day	44% 8% 17% 22% 8%	1683
	11th to 20th	Day	45% 11% 19% 19% 6% 1	1710
	21st to end	Day	46% 9% 20% 17% 8% 1	1603
December	1st to 10th	Day	54% 11% 17% 14% 4	1520
	11th to 20th	Day	62% 12% 15% 8% 3	1928
	21st to end	Day	49% 12% 19% 15% 5%	2794
January	1st to 10th	Night	55% 15% 18% 9% 1	4406
	11th to 20th	Night	61% 9% 11% 15% 4	4420
	21st to end	Night	63% 10% 15% 9% 3	4862
February	1st to 10th	Night	51% 9% 18% 17% 4	2559
	11th to 20th	Night	46% 9% 18% 19% 7%	2219
	21st to end	Night	51% 7% 14% 17% 8% 2	1884
March	1st to 10th	Night	45% 8% 13% 22% 11% 1	3736
	11th to 20th	Night	35% 6% 9% 38% 12%	3655
	21st to end	Night	41% 14% 19% 19% 6%	3591
April	1st to 10th	Night	39% 8% 14% 26% 12% 1	1900
	11th to 20th	Night	39% 7% 11% 25% 15% 3	1838
	21st to end	Night	32% 8% 14% 25% 20% 1	1710
May	1st to 10th	Night	29% 7% 12% 27% 22% 2	2860
	11th to 20th	Night	19% 6% 11% 34% 28% 3	2561
	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	25% 6% 10% 26% 25% 7%	2090
	11th to 20th	Night	28% 6% 13% 27% 22% 3	2374
	21st to end	Night	35% 8% 14% 23% 18% 2	2717
November	1st to 10th	Night	42% 8% 14% 23% 12% 1	2418
	11th to 20th	Night	44% 7% 16% 25% 8%	2470
	21st to end	Night	42% 6% 18% 21% 11% 2	2574
December	1st to 10th	Night	52% 9% 15% 16% 8%	2660
	11th to 20th	Night	58% 9% 16% 14% 3	3276
	21st to end	Night	48% 12% 20% 14% 5% 1	4862

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% 16% 34% 16% 6%	180
	11th to 20th	Day	21% 7% 19% 30% 16% 8%	180
	21st to end	Day	20% 2 21% 34% 18% 5%	198
February	1st to 10th	Day	20% 5% 28% 30% 15% 2	253
	11th to 20th	Day	15% 8% 31% 28% 16% 2	455
	21st to end	Day	15% 15% 26% 28% 13% 2	382
March	1st to 10th	Day	17% 10% 30% 32% 11% 1	460
	11th to 20th	Day	16% 8% 25% 28% 20% 3	460
	21st to end	Day	17% 11% 27% 28% 15% 3	506
April	1st to 10th	Day	13% 12% 29% 25% 15% 6%	460
	11th to 20th	Day	12% 12% 32% 27% 13% 4	452
	21st to end	Day	10% 13% 32% 28% 12% 5%	460
May	1st to 10th	Day	7% 13% 32% 29% 17% 3	463
	11th to 20th	Day	15% 10% 25% 28% 15% 7%	488
	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% 22% 33% 18% 3	273
	11th to 20th	Day	16% 4 20% 34% 20% 6%	270
	21st to end	Day	13% 4 19% 37% 22% 4	219
December	1st to 10th	Day	22% 3 17% 33% 16% 9%	180
	11th to 20th	Day	26% 4 19% 34% 14% 2	180
	21st to end	Day	19% 4 24% 29% 18% 7%	198
January	1st to 10th	Night	22% 4 12% 36% 22% 4	760
	11th to 20th	Night	22% 4 12% 34% 22% 6%	760
	21st to end	Night	19% 3 11% 37% 25% 6%	836
February	1st to 10th	Night	19% 4 18% 41% 15% 3	760
	11th to 20th	Night	16% 3 19% 37% 21% 3	744
	21st to end	Night	16% 4 17% 41% 19% 3	628
March	1st to 10th	Night	17% 4 19% 43% 15% 3	760
	11th to 20th	Night	16% 4 16% 41% 21% 3	760
	21st to end	Night	18% 4 17% 43% 17% 2	836
April	1st to 10th	Night	15% 6% 15% 40% 21% 3	760
	11th to 20th	Night	13% 6% 23% 35% 19% 3	710
	21st to end	Night	11% 3 26% 39% 21% 1	570
May	1st to 10th	Night	7% 6% 23% 42% 20% 2	570
	11th to 20th	Night	7% 5% 19% 35% 29% 5%	406
	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	18% 3 13% 41% 23% 2	744
	11th to 20th	Night	21% 2 13% 42% 19% 3	760
	21st to end	Night	15% 3 15% 38% 24% 5%	760
December	1st to 10th	Night	19% 4 13% 40% 20% 4	760
	11th to 20th	Night	24% 3 11% 40% 18% 3	760
	21st to end	Night	24% 3 15% 40% 15% 2	836

Page generated at 2024-10-29 23:22:57. Version ac07

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