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	15% 10% 4 8% 5% 5% 5% 4 6% 7% 31%	101845
	Night	15% 10% 4 8% 5% 4 4 4 5% 7% 33%	97117
Transparency	Day	50% 7% 12% 20% 10% 1	101611
	Night	49% 7% 12% 20% 11% 1	96971
Seeing	Day	19% 14% 29% 22% 12% 5%	14823
	Night	19% 12% 30% 27% 11% 1	23900

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% 18% 7% 12% 6% 4 4 3 3 5% 23%	2558
	11th to 20th	Day	11% 21% 8% 17% 8% 5% 4 3 3 6% 15%	2840
	21st to end	Day	22% 20% 7% 13% 7% 5% 4 3 4 3 13%	3124
February	1st to 10th	Day	15% 18% 5% 12% 8% 6% 5% 4 5% 5% 17%	1710
	11th to 20th	Day	14% 14% 8% 14% 7% 4 4 3 5% 7% 21%	1663
	21st to end	Day	17% 16% 5% 10% 6% 4 5% 2 5% 6% 23%	1506
March	1st to 10th	Day	20% 14% 6% 9% 4 4 3 3 4 6% 27%	3416
	11th to 20th	Day	12% 16% 5% 10% 5% 4 5% 4 6% 6% 28%	3480
	21st to end	Day	16% 11% 4 9% 6% 4 4 4 5% 6% 31%	3815
April	1st to 10th	Day	17% 13% 5% 8% 4 4 5% 3 5% 7% 29%	2204
	11th to 20th	Day	19% 10% 3 7% 4 5% 4 3 4 7% 34%	2372
	21st to end	Day	19% 11% 3 8% 5% 5% 4 3 4 5% 33%	2470
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 4 4 5% 7% 33%	4108
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 4 6% 9% 30%	4238
	21st to end	Day	12% 5% 2 6% 5% 4 4 4 5% 8% 46%	4837
June	1st to 10th	Day	14% 7% 2 6% 5% 6% 6% 5% 6% 8% 37%	2660
	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% 10% 4 9% 6% 4 5% 4 5% 7% 31%	1649
	11th to 20th	Day	15% 16% 3 10% 5% 6% 5% 4 5% 8% 24%	1520
	21st to end	Day	19% 18% 4 10% 6% 4 4 3 6% 6% 20%	1520
December	1st to 10th	Day	16% 17% 7% 12% 6% 6% 6% 4 5% 5% 16%	1520
	11th to 20th	Day	18% 17% 7% 11% 5% 6% 5% 4 5% 5% 16%	1906
	21st to end	Day	28% 19% 7% 13% 6% 5% 4 2 3 4 9%	2772
January	1st to 10th	Night	17% 17% 6% 13% 7% 5% 5% 4 4 5% 17%	4384
	11th to 20th	Night	15% 15% 7% 15% 9% 7% 6% 5% 5% 5% 13%	4120
	21st to end	Night	22% 15% 9% 11% 5% 4 4 3 4 5% 18%	4532
February	1st to 10th	Night	15% 16% 5% 10% 7% 5% 6% 4 5% 6% 20%	2470
	11th to 20th	Night	14% 14% 5% 11% 6% 5% 5% 4 6% 6% 24%	2403
	21st to end	Night	14% 13% 5% 12% 6% 5% 5% 4 5% 7% 24%	1946
March	1st to 10th	Night	17% 11% 4 8% 5% 5% 4 3 5% 6% 32%	3423
	11th to 20th	Night	12% 10% 4 9% 4 4 5% 4 5% 7% 37%	3479
	21st to end	Night	17% 10% 3 7% 4 4 4 3 4 5% 40%	3817
April	1st to 10th	Night	19% 10% 3 7% 6% 5% 4 3 5% 6% 33%	1976
	11th to 20th	Night	16% 10% 3 6% 5% 4 4 3 5% 7% 36%	1773
	21st to end	Night	14% 9% 4 8% 5% 5% 4 4 5% 7% 36%	1710
May	1st to 10th	Night	17% 7% 3 6% 4 4 4 3 5% 7% 40%	2833
	11th to 20th	Night	12% 9% 3 8% 5% 5% 5% 5% 5% 7% 35%	2639
	21st to end	Night	10% 5% 2 4 4 3 4 4 5% 8% 50%	2738
June	1st to 10th	Night	11% 5% 2 6% 4 5% 5% 4 6% 8% 43%	1520
	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 6% 3 5% 6% 29%	2508
November	1st to 10th	Night	14% 10% 3 7% 5% 4 4 4 6% 7% 37%	2462
	11th to 20th	Night	16% 11% 4 9% 6% 5% 4 4 6% 7% 27%	2660
	21st to end	Night	20% 11% 4 9% 6% 5% 4 4 6% 6% 24%	2660
December	1st to 10th	Night	19% 15% 5% 11% 6% 5% 5% 5% 5% 7% 18%	2660
	11th to 20th	Night	16% 17% 5% 11% 6% 4 4 4 5% 6% 21%	3288
	21st to end	Night	28% 21% 6% 11% 5% 4 4 3 4 4 9%	4884

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% 7% 11% 10% 8%	2559
	11th to 20th	Day	73% 4 10% 9% 4	2840
	21st to end	Day	75% 6% 8% 7% 4	3124
February	1st to 10th	Day	67% 7% 10% 12% 4 1	1710
	11th to 20th	Day	63% 5% 10% 14% 7% 1	1672
	21st to end	Day	62% 5% 11% 14% 7% 1	1506
March	1st to 10th	Day	58% 5% 11% 19% 6% 1	3419
	11th to 20th	Day	54% 6% 14% 17% 7% 2	3480
	21st to end	Day	52% 6% 11% 19% 11% 2	3815
April	1st to 10th	Day	55% 6% 12% 16% 11%	2204
	11th to 20th	Day	50% 5% 10% 21% 12% 1	2372
	21st to end	Day	53% 6% 9% 18% 11% 2	2470
May	1st to 10th	Day	48% 6% 13% 19% 12% 3	4120
	11th to 20th	Day	49% 9% 14% 21% 6% 1	4242
	21st to end	Day	36% 6% 13% 21% 19% 4	4837
June	1st to 10th	Day	42% 8% 12% 22% 14% 2	2660
	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	49% 6% 13% 18% 12% 1	1900
	11th to 20th	Day	53% 7% 13% 18% 9% 1	1900
	21st to end	Day	59% 6% 13% 15% 6%	2090
November	1st to 10th	Day	51% 7% 13% 21% 7% 1	1649
	11th to 20th	Day	57% 7% 14% 13% 8% 1	1520
	21st to end	Day	63% 6% 14% 12% 4	1520
December	1st to 10th	Day	68% 8% 11% 9% 4	1520
	11th to 20th	Day	67% 9% 11% 8% 5%	1906
	21st to end	Day	79% 6% 7% 7% 1	2772
January	1st to 10th	Night	65% 8% 10% 11% 4 1	4384
	11th to 20th	Night	69% 9% 11% 8% 3	4120
	21st to end	Night	68% 7% 10% 11% 4 1	4532
February	1st to 10th	Night	62% 6% 11% 16% 4	2470
	11th to 20th	Night	57% 7% 12% 15% 9% 1	2405
	21st to end	Night	58% 5% 13% 16% 8%	1946
March	1st to 10th	Night	52% 6% 14% 21% 6% 1	3433
	11th to 20th	Night	44% 6% 17% 20% 12% 2	3480
	21st to end	Night	45% 6% 9% 22% 16% 2	3817
April	1st to 10th	Night	52% 6% 9% 18% 14% 1	1976
	11th to 20th	Night	47% 6% 9% 20% 16% 2	1773
	21st to end	Night	47% 5% 10% 19% 15% 3	1710
May	1st to 10th	Night	43% 7% 11% 19% 16% 5%	2840
	11th to 20th	Night	45% 8% 15% 22% 8% 2	2640
	21st to end	Night	31% 7% 13% 23% 23% 3	2739
June	1st to 10th	Night	37% 7% 11% 26% 17% 2	1520
	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	43% 6% 10% 24% 16% 1	2280
	11th to 20th	Night	47% 7% 13% 23% 10%	2280
	21st to end	Night	54% 6% 10% 19% 10% 1	2508
November	1st to 10th	Night	44% 7% 13% 22% 13% 1	2462
	11th to 20th	Night	54% 7% 14% 18% 7% 1	2660
	21st to end	Night	58% 6% 17% 13% 5%	2660
December	1st to 10th	Night	65% 8% 12% 11% 4	2660
	11th to 20th	Night	63% 6% 13% 13% 4	3288
	21st to end	Night	78% 7% 8% 5% 2	4884

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% 29% 17% 7% 1	193
	11th to 20th	Day	25% 23% 36% 11% 4 1	270
	21st to end	Day	28% 19% 34% 16% 4	297
February	1st to 10th	Day	27% 24% 34% 11% 4	270
	11th to 20th	Day	25% 24% 38% 12% 1	270
	21st to end	Day	35% 24% 32% 10%	225
March	1st to 10th	Day	25% 18% 33% 19% 6%	444
	11th to 20th	Day	24% 17% 33% 20% 4 1	460
	21st to end	Day	27% 17% 31% 18% 5% 1	506
April	1st to 10th	Day	20% 21% 32% 18% 7% 1	460
	11th to 20th	Day	28% 18% 25% 18% 9% 2	452
	21st to end	Day	25% 12% 34% 19% 8% 2	460
May	1st to 10th	Day	22% 14% 28% 22% 9% 5%	463
	11th to 20th	Day	18% 11% 27% 25% 14% 5%	487
	21st to end	Day	13% 8% 30% 27% 15% 7%	529
June	1st to 10th	Day	15% 16% 27% 28% 11% 3	490
	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% 17% 31% 18% 6%	539
November	1st to 10th	Day	25% 21% 30% 15% 8% 1	263
	11th to 20th	Day	27% 19% 31% 16% 5% 2	180
	21st to end	Day	28% 19% 35% 14% 4	180
December	1st to 10th	Day	29% 26% 31% 9% 4 1	180
	11th to 20th	Day	29% 26% 31% 11% 3	180
	21st to end	Day	30% 25% 35% 10% 1	198
January	1st to 10th	Night	23% 21% 34% 16% 5%	760
	11th to 20th	Night	27% 18% 33% 19% 3	760
	21st to end	Night	21% 13% 38% 23% 5%	836
February	1st to 10th	Night	23% 19% 35% 21% 2	760
	11th to 20th	Night	22% 17% 35% 21% 4	743
	21st to end	Night	22% 21% 34% 20% 3	628
March	1st to 10th	Night	18% 15% 37% 24% 6%	760
	11th to 20th	Night	22% 16% 35% 21% 6%	760
	21st to end	Night	21% 14% 32% 25% 8% 1	836
April	1st to 10th	Night	23% 15% 32% 26% 4	760
	11th to 20th	Night	22% 15% 31% 22% 9% 1	649
	21st to end	Night	16% 11% 35% 29% 8%	570
May	1st to 10th	Night	17% 9% 31% 29% 13% 1	570
	11th to 20th	Night	15% 5% 32% 35% 12% 2	466
	21st to end	Night	11% 6% 27% 36% 19%	411
June	1st to 10th	Night	13% 6% 31% 38% 11% 1	380
	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% 13% 30% 26% 10% 2	748
	11th to 20th	Night	22% 18% 32% 20% 7% 1	760
	21st to end	Night	26% 16% 33% 19% 6%	760
December	1st to 10th	Night	27% 21% 30% 17% 5%	760
	11th to 20th	Night	24% 20% 35% 18% 4	760
	21st to end	Night	32% 15% 34% 17% 3	836

Page generated at 2024-10-30 13:19:22. Version ac07

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