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 2024-03-08.

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	9% 7% 3 5% 5% 3 5% 5% 5% 7% 48%	78488
	Night	11% 8% 3 6% 5% 3 4 5% 5% 6% 44%	81088
Transparency	Day	36% 8% 13% 19% 16% 8%	78192
	Night	40% 8% 14% 19% 14% 6%	80905
Seeing	Day	11% 7% 24% 32% 21% 6%	13462
	Night	13% 3 16% 40% 24% 4	22887

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% 11% 9% 4 5% 6% 5% 7% 21%	2539
	11th to 20th	Day	16% 16% 5% 7% 7% 4 6% 5% 5% 6% 23%	2540
	21st to end	Day	25% 14% 4 8% 7% 4 6% 4 4 5% 19%	2794
February	1st to 10th	Day	12% 12% 4 8% 6% 3 5% 3 5% 8% 34%	1615
	11th to 20th	Day	12% 10% 3 8% 6% 3 6% 5% 5% 8% 35%	1850
	21st to end	Day	10% 10% 5% 7% 6% 3 6% 6% 6% 6% 33%	1570
March	1st to 10th	Day	11% 9% 4 7% 6% 4 7% 6% 5% 8% 32%	1871
	11th to 20th	Day	13% 9% 3 5% 5% 3 5% 6% 8% 9% 34%	1879
	21st to end	Day	12% 9% 3 8% 6% 4 6% 6% 6% 8% 31%	2308
April	1st to 10th	Day	9% 7% 4 7% 7% 4 5% 5% 7% 8% 37%	2160
	11th to 20th	Day	8% 7% 3 6% 6% 3 6% 5% 5% 7% 45%	2165
	21st to end	Day	9% 6% 3 5% 4 4 5% 6% 5% 8% 46%	2340
May	1st to 10th	Day	5% 5% 3 5% 4 3 5% 5% 6% 8% 51%	2340
	11th to 20th	Day	8% 6% 2 6% 5% 3 5% 4 6% 9% 46%	2444
	21st to end	Day	7% 7% 3 6% 6% 4 5% 7% 6% 8% 40%	2723
June	1st to 10th	Day	8% 5% 3 5% 5% 4 6% 6% 7% 9% 42%	2520
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 5% 6% 7% 51%	2520
	21st to end	Day	3 3 1 3 3 2 4 4 4 6% 69%	2520
July	1st to 10th	Day	2 2 1 2 3 2 3 3 4 7% 71%	2520
	11th to 20th	Day	1 1 1 1 1 1 2 3 3 6% 79%	2520
	21st to end	Day	1 1 1 2 3 2 2 3 4 5% 75%	2741
August	1st to 10th	Day	3 3 1 3 4 2 3 3 4 7% 67%	2466
	11th to 20th	Day	2 1 1 2 2 2 3 2 3 5% 77%	2299
	21st to end	Day	2 2 1 2 2 1 3 3 3 6% 74%	2386
September	1st to 10th	Day	2 2 2 2 2 2 3 3 5% 75%	2160
	11th to 20th	Day	5% 3 1 4 4 2 3 3 5% 6% 64%	2018
	21st to end	Day	5% 3 2 3 4 2 3 4 5% 6% 63%	1980
October	1st to 10th	Day	7% 4 2 5% 5% 3 4 5% 4 6% 54%	1980
	11th to 20th	Day	8% 4 1 5% 4 2 4 4 4 7% 56%	1709
	21st to end	Day	9% 5% 2 6% 6% 3 5% 5% 5% 6% 48%	1782
November	1st to 10th	Day	15% 7% 3 5% 5% 3 5% 4 5% 8% 41%	1674
	11th to 20th	Day	14% 8% 3 6% 5% 3 5% 5% 5% 6% 40%	1710
	21st to end	Day	11% 8% 4 6% 5% 4 6% 5% 5% 6% 39%	1603
December	1st to 10th	Day	17% 11% 4 8% 5% 3 4 5% 6% 7% 30%	1520
	11th to 20th	Day	19% 12% 5% 11% 8% 3 4 5% 6% 6% 20%	1928
	21st to end	Day	12% 12% 4 9% 6% 4 5% 6% 6% 6% 31%	2794
January	1st to 10th	Night	13% 15% 5% 10% 8% 5% 5% 7% 6% 5% 21%	4406
	11th to 20th	Night	17% 18% 5% 9% 6% 4 5% 4 4 5% 24%	4420
	21st to end	Night	25% 17% 4 8% 5% 2 5% 4 4 5% 22%	4862
February	1st to 10th	Night	13% 12% 3 7% 6% 3 5% 4 5% 7% 34%	2559
	11th to 20th	Night	12% 12% 3 6% 5% 3 5% 5% 6% 7% 37%	2218
	21st to end	Night	12% 11% 4 7% 7% 3 6% 5% 5% 6% 34%	1884
March	1st to 10th	Night	12% 11% 4 7% 6% 3 6% 5% 5% 6% 35%	2249
	11th to 20th	Night	12% 8% 3 6% 6% 3 5% 6% 6% 8% 37%	2080
	21st to end	Night	13% 9% 3 7% 6% 3 5% 6% 6% 8% 35%	2044
April	1st to 10th	Night	11% 7% 3 5% 5% 4 4 5% 6% 7% 44%	1800
	11th to 20th	Night	9% 6% 4 6% 6% 3 5% 4 6% 5% 45%	1741
	21st to end	Night	7% 5% 3 5% 4 3 5% 4 5% 8% 51%	1620
May	1st to 10th	Night	5% 3 2 3 4 2 3 4 6% 8% 59%	1620
	11th to 20th	Night	5% 4 2 5% 5% 3 4 5% 7% 9% 49%	1447
	21st to end	Night	7% 5% 3 7% 5% 3 4 6% 8% 10% 43%	1552
June	1st to 10th	Night	8% 5% 2 6% 5% 3 6% 5% 5% 8% 47%	1440
	11th to 20th	Night	8% 3 2 4 3 2 4 4 5% 8% 57%	1440
	21st to end	Night	3 2 1 2 3 2 4 5% 5% 6% 67%	1440
July	1st to 10th	Night	1 1 1 2 3 2 3 3 6% 7% 72%	1440
	11th to 20th	Night	1 2 1 3 3 5% 5% 80%	1440
	21st to end	Night	1 1 1 2 3 2 3 3 4 5% 76%	1576
August	1st to 10th	Night	4 2 1 3 3 2 3 4 4 7% 67%	1471
	11th to 20th	Night	1 1 2 1 2 4 4 7% 77%	1602
	21st to end	Night	3 2 1 2 2 2 2 2 4 6% 73%	1967
September	1st to 10th	Night	3 2 1 2 2 1 2 3 3 5% 75%	1800
	11th to 20th	Night	5% 2 1 3 3 2 3 4 5% 7% 65%	1941
	21st to end	Night	7% 3 1 3 2 2 3 4 4 5% 66%	1980
October	1st to 10th	Night	6% 4 2 4 3 2 4 4 4 6% 59%	1980
	11th to 20th	Night	8% 4 2 5% 4 3 3 4 5% 7% 57%	2248
	21st to end	Night	8% 5% 2 6% 6% 3 5% 5% 5% 8% 47%	2574
November	1st to 10th	Night	14% 7% 2 5% 4 3 4 5% 5% 8% 42%	2405
	11th to 20th	Night	16% 7% 2 4 5% 3 5% 4 5% 7% 42%	2470
	21st to end	Night	11% 9% 3 5% 5% 3 5% 5% 4 6% 44%	2574
December	1st to 10th	Night	13% 12% 3 8% 5% 4 5% 4 5% 6% 34%	2660
	11th to 20th	Night	19% 11% 4 9% 6% 3 5% 5% 5% 5% 27%	3276
	21st to end	Night	12% 10% 4 8% 7% 3 6% 6% 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	51% 10% 16% 16% 7%	1871
	11th to 20th	Day	45% 10% 16% 22% 6%	1879
	21st to end	Day	50% 12% 16% 16% 5%	2308
April	1st to 10th	Day	44% 10% 16% 20% 9% 1	2160
	11th to 20th	Day	38% 9% 15% 22% 13% 3	2168
	21st to end	Day	37% 8% 15% 25% 14%	2340
May	1st to 10th	Day	29% 7% 14% 31% 17% 2	2340
	11th to 20th	Day	36% 7% 14% 26% 13% 4	2447
	21st to end	Day	39% 11% 15% 21% 13% 1	2724
June	1st to 10th	Day	38% 10% 14% 23% 15%	2520
	11th to 20th	Day	31% 8% 11% 23% 20% 7%	2520
	21st to end	Day	19% 5% 8% 20% 33% 16%	2520
July	1st to 10th	Day	14% 5% 5% 17% 31% 29%	2451
	11th to 20th	Day	9% 3 6% 10% 30% 41%	2445
	21st to end	Day	12% 3 5% 12% 28% 40%	2589
August	1st to 10th	Day	19% 4 7% 18% 29% 23%	2449
	11th to 20th	Day	11% 4 5% 20% 35% 25%	2299
	21st to end	Day	14% 4 6% 22% 34% 20%	2386
September	1st to 10th	Day	13% 4 6% 22% 32% 24%	2160
	11th to 20th	Day	22% 6% 9% 22% 29% 12%	2018
	21st to end	Day	23% 5% 9% 31% 24% 7%	1980
October	1st to 10th	Day	32% 7% 12% 26% 21% 3	1980
	11th to 20th	Day	29% 6% 15% 29% 19% 2	1709
	21st to end	Day	37% 9% 16% 24% 12% 1	1782
November	1st to 10th	Day	43% 8% 17% 23% 9%	1674
	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	49% 7% 16% 20% 7% 1	2249
	11th to 20th	Night	45% 8% 15% 24% 8%	2080
	21st to end	Night	47% 9% 18% 18% 6%	2044
April	1st to 10th	Night	39% 8% 14% 26% 12% 1	1800
	11th to 20th	Night	40% 7% 10% 25% 15% 3	1741
	21st to end	Night	33% 8% 14% 25% 20% 1	1620
May	1st to 10th	Night	24% 7% 14% 32% 22% 1	1620
	11th to 20th	Night	31% 8% 15% 31% 13% 2	1447
	21st to end	Night	34% 11% 16% 26% 12% 1	1552
June	1st to 10th	Night	36% 8% 15% 27% 13%	1440
	11th to 20th	Night	26% 7% 13% 27% 21% 5%	1440
	21st to end	Night	18% 6% 9% 21% 33% 13%	1440
July	1st to 10th	Night	12% 4 7% 20% 30% 27%	1400
	11th to 20th	Night	8% 4 6% 17% 28% 38%	1392
	21st to end	Night	11% 4 3 12% 29% 41%	1488
August	1st to 10th	Night	18% 5% 7% 17% 30% 23%	1463
	11th to 20th	Night	9% 4 7% 18% 32% 30%	1602
	21st to end	Night	15% 4 5% 19% 34% 23%	1967
September	1st to 10th	Night	14% 3 5% 20% 31% 26%	1800
	11th to 20th	Night	20% 5% 8% 22% 27% 18%	1941
	21st to end	Night	21% 5% 7% 26% 28% 13%	1980
October	1st to 10th	Night	26% 7% 11% 27% 24% 6%	1980
	11th to 20th	Night	29% 6% 13% 28% 22% 3	2248
	21st to end	Night	35% 8% 14% 22% 18% 2	2574
November	1st to 10th	Night	42% 8% 14% 23% 12% 1	2405
	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% 10% 1	449
	11th to 20th	Day	17% 8% 25% 29% 19% 1	420
	21st to end	Day	17% 11% 27% 27% 15% 3	462
April	1st to 10th	Day	13% 13% 28% 25% 14% 6%	420
	11th to 20th	Day	13% 12% 33% 25% 13% 4	412
	21st to end	Day	11% 13% 31% 29% 11% 5%	420
May	1st to 10th	Day	6% 13% 31% 30% 18% 3	423
	11th to 20th	Day	16% 10% 25% 27% 15% 7%	448
	21st to end	Day	12% 6% 20% 29% 24% 9%	485
June	1st to 10th	Day	8% 6% 27% 30% 22% 7%	450
	11th to 20th	Day	11% 11% 27% 29% 17% 4	450
	21st to end	Day	2 7% 26% 33% 24% 7%	450
July	1st to 10th	Day	4 3 26% 39% 24% 4	446
	11th to 20th	Day	2 2 25% 41% 25% 3	444
	21st to end	Day	2 2 22% 39% 28% 8%	487
August	1st to 10th	Day	4 3 16% 40% 27% 10%	447
	11th to 20th	Day	1 3 16% 35% 35% 11%	437
	21st to end	Day	4 3 17% 36% 28% 12%	495
September	1st to 10th	Day	4 2 19% 31% 33% 10%	450
	11th to 20th	Day	6% 5% 19% 32% 29% 9%	450
	21st to end	Day	6% 3 21% 36% 28% 6%	450
October	1st to 10th	Day	10% 6% 19% 29% 29% 7%	450
	11th to 20th	Day	10% 5% 25% 28% 24% 8%	355
	21st to end	Day	15% 6% 25% 31% 17% 6%	297
November	1st to 10th	Day	19% 5% 22% 33% 18% 3	270
	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% 41% 16% 3	750
	11th to 20th	Night	16% 4 15% 42% 21% 3	720
	21st to end	Night	18% 4 16% 43% 17% 2	792
April	1st to 10th	Night	14% 6% 14% 41% 21% 3	720
	11th to 20th	Night	14% 6% 22% 36% 20% 3	673
	21st to end	Night	11% 3 25% 39% 21% 1	540
May	1st to 10th	Night	7% 6% 23% 41% 21% 2	540
	11th to 20th	Night	7% 5% 19% 35% 29% 5%	385
	21st to end	Night	8% 5% 15% 37% 32% 4	389
June	1st to 10th	Night	9% 4 19% 38% 27% 3	360
	11th to 20th	Night	9% 6% 23% 36% 24% 1	360
	21st to end	Night	3 2 14% 47% 33% 1	360
July	1st to 10th	Night	2 2 19% 45% 30% 2	350
	11th to 20th	Night	1 1 22% 56% 19% 2	348
	21st to end	Night	2 1 15% 48% 32% 2	394
August	1st to 10th	Night	6% 1 13% 44% 33% 4	397
	11th to 20th	Night	1 1 12% 44% 37% 4	534
	21st to end	Night	4 2 15% 43% 31% 5%	779
September	1st to 10th	Night	3 2 13% 43% 35% 3	720
	11th to 20th	Night	6% 2 12% 39% 35% 8%	720
	21st to end	Night	9% 3 13% 36% 32% 7%	720
October	1st to 10th	Night	7% 2 17% 41% 28% 4	720
	11th to 20th	Night	11% 4 13% 39% 26% 8%	720
	21st to end	Night	11% 3 17% 41% 23% 5%	792
November	1st to 10th	Night	18% 3 12% 41% 23% 2	740
	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 2024-03-05 05:24:04. Version ac07

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