Cape Cod Imaging Station
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cape Cod Imaging Station clear sky chart from 2005-11-06 to 2024-11-04.

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	17% 9% 3 7% 4 4 4 3 5% 7% 37%	101131
	Night	18% 10% 3 7% 4 4 4 3 5% 6% 35%	97868
Transparency	Day	46% 6% 12% 22% 13% 2	104700
	Night	48% 5% 10% 21% 13% 2	102362
Seeing	Day	20% 10% 28% 27% 13% 3	15247
	Night	21% 11% 28% 27% 11% 1	24064

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% 10% 4 6% 4 3 4 3 4 6% 39%	2517
	11th to 20th	Day	20% 11% 3 6% 4 3 2 3 5% 6% 37%	2520
	21st to end	Day	22% 13% 3 14% 5% 4 4 3 3 4 25%	2872
February	1st to 10th	Day	20% 11% 5% 7% 5% 4 3 4 5% 6% 31%	1776
	11th to 20th	Day	14% 10% 3 8% 6% 4 4 3 4 8% 37%	1848
	21st to end	Day	19% 9% 4 8% 5% 3 4 3 5% 6% 33%	1570
March	1st to 10th	Day	24% 19% 4 7% 4 4 2 2 4 5% 27%	3116
	11th to 20th	Day	12% 8% 3 7% 4 4 4 3 5% 7% 44%	3433
	21st to end	Day	25% 10% 3 6% 4 3 3 2 4 4 35%	3892
April	1st to 10th	Day	21% 10% 3 5% 3 3 3 4 5% 6% 39%	2280
	11th to 20th	Day	17% 9% 2 6% 3 3 3 3 6% 6% 42%	2312
	21st to end	Day	18% 8% 2 6% 4 3 4 3 5% 6% 39%	2470
May	1st to 10th	Day	25% 13% 3 8% 4 3 4 3 4 6% 25%	4106
	11th to 20th	Day	24% 12% 3 6% 4 3 3 3 5% 7% 31%	4081
	21st to end	Day	14% 8% 3 6% 3 3 3 3 6% 7% 42%	4677
June	1st to 10th	Day	19% 8% 2 5% 5% 3 4 4 6% 8% 34%	2660
	11th to 20th	Day	15% 7% 2 5% 5% 5% 5% 3 6% 8% 40%	2660
	21st to end	Day	14% 8% 2 6% 4 4 4 4 6% 9% 39%	2660
July	1st to 10th	Day	13% 10% 4 7% 4 5% 4 4 5% 8% 37%	4440
	11th to 20th	Day	11% 8% 3 7% 4 5% 4 4 7% 10% 38%	4440
	21st to end	Day	17% 7% 3 7% 4 4 4 4 5% 8% 36%	4671
August	1st to 10th	Day	16% 9% 3 7% 5% 5% 4 4 5% 7% 34%	4105
	11th to 20th	Day	12% 8% 2 6% 4 5% 5% 4 7% 7% 40%	4045
	21st to end	Day	8% 4 2 4 3 3 4 4 6% 10% 52%	4230
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 47%	2218
	11th to 20th	Day	14% 6% 2 5% 3 3 3 3 5% 6% 50%	2090
	21st to end	Day	18% 8% 2 7% 4 3 3 3 5% 7% 40%	2090
October	1st to 10th	Day	21% 6% 1 5% 4 4 4 3 5% 6% 42%	2090
	11th to 20th	Day	15% 6% 2 6% 4 3 3 3 4 7% 46%	1898
	21st to end	Day	18% 7% 2 7% 4 3 4 3 5% 7% 40%	1881
November	1st to 10th	Day	19% 8% 2 7% 4 4 3 3 5% 5% 42%	1698
	11th to 20th	Day	17% 9% 3 5% 4 3 3 3 5% 8% 39%	1710
	21st to end	Day	17% 9% 2 6% 3 4 4 3 5% 7% 40%	1710
December	1st to 10th	Day	20% 10% 3 8% 4 5% 4 3 5% 6% 33%	1687
	11th to 20th	Day	21% 12% 4 7% 5% 4 3 4 4 6% 31%	1906
	21st to end	Day	21% 14% 5% 9% 4 4 5% 3 4 6% 24%	2772
January	1st to 10th	Night	18% 9% 4 6% 3 3 3 3 3 5% 42%	4429
	11th to 20th	Night	17% 12% 5% 9% 4 3 3 3 4 4 36%	4440
	21st to end	Night	22% 15% 5% 11% 6% 4 3 2 3 4 24%	4781
February	1st to 10th	Night	17% 10% 3 9% 5% 4 4 4 5% 5% 33%	2399
	11th to 20th	Night	20% 10% 3 6% 5% 4 3 3 5% 6% 35%	2218
	21st to end	Night	17% 10% 2 8% 4 3 3 4 5% 6% 37%	1884
March	1st to 10th	Night	31% 13% 2 4 3 3 3 2 4 5% 30%	3724
	11th to 20th	Night	11% 11% 5% 7% 5% 5% 5% 4 6% 6% 35%	3525
	21st to end	Night	26% 11% 3 7% 4 4 4 2 4 4 30%	3736
April	1st to 10th	Night	20% 11% 3 6% 4 4 4 3 4 6% 36%	1900
	11th to 20th	Night	19% 10% 3 6% 5% 4 3 3 5% 6% 37%	1815
	21st to end	Night	19% 9% 3 7% 6% 4 4 3 5% 5% 36%	1710
May	1st to 10th	Night	24% 13% 5% 10% 4 3 4 2 4 5% 26%	2834
	11th to 20th	Night	26% 11% 4 6% 4 3 5% 3 5% 7% 25%	2813
	21st to end	Night	15% 9% 4 8% 5% 5% 4 4 7% 7% 31%	2912
June	1st to 10th	Night	19% 10% 3 6% 4 4 5% 5% 6% 7% 31%	1520
	11th to 20th	Night	18% 9% 3 5% 4 3 3 3 5% 8% 39%	1520
	21st to end	Night	18% 10% 3 8% 3 3 3 4 7% 10% 32%	1520
July	1st to 10th	Night	17% 14% 3 7% 5% 4 4 3 6% 7% 29%	2520
	11th to 20th	Night	16% 8% 2 6% 4 4 5% 4 7% 9% 34%	2520
	21st to end	Night	17% 9% 3 8% 5% 5% 5% 4 6% 7% 31%	2851
August	1st to 10th	Night	19% 11% 3 7% 5% 4 5% 4 6% 7% 30%	2825
	11th to 20th	Night	10% 10% 3 8% 4 5% 5% 4 6% 8% 37%	2782
	21st to end	Night	8% 5% 2 5% 3 4 4 3 6% 8% 52%	3409
September	1st to 10th	Night	13% 6% 3 6% 4 4 4 3 5% 6% 45%	1961
	11th to 20th	Night	14% 7% 2 5% 3 3 4 3 4 6% 49%	2090
	21st to end	Night	17% 8% 3 6% 5% 4 4 4 5% 6% 39%	2090
October	1st to 10th	Night	21% 8% 3 6% 5% 3 3 4 4 5% 38%	2090
	11th to 20th	Night	17% 5% 2 5% 3 4 4 3 5% 6% 45%	2279
	21st to end	Night	19% 9% 3 6% 4 4 4 3 4 7% 36%	2717
November	1st to 10th	Night	17% 7% 3 6% 5% 4 4 3 5% 6% 41%	2453
	11th to 20th	Night	16% 9% 3 6% 5% 4 3 3 5% 5% 40%	2470
	21st to end	Night	16% 7% 2 7% 5% 3 3 3 5% 7% 41%	2470
December	1st to 10th	Night	21% 9% 3 8% 4 4 4 3 5% 6% 31%	2489
	11th to 20th	Night	18% 11% 4 8% 5% 4 3 3 5% 6% 34%	3288
	21st to end	Night	19% 14% 5% 8% 5% 4 4 4 4 5% 27%	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	45% 5% 9% 29% 10% 1	2518
	11th to 20th	Day	49% 5% 13% 22% 9% 2	2520
	21st to end	Day	62% 5% 8% 17% 7% 1	2872
February	1st to 10th	Day	54% 5% 11% 17% 12% 2	1776
	11th to 20th	Day	48% 4 12% 23% 12% 1	1857
	21st to end	Day	52% 5% 11% 18% 12% 2	1570
March	1st to 10th	Day	61% 4 10% 14% 9% 2	3119
	11th to 20th	Day	39% 6% 15% 25% 13% 2	3433
	21st to end	Day	55% 4 6% 16% 14% 6%	3892
April	1st to 10th	Day	46% 4 8% 20% 18% 3	2280
	11th to 20th	Day	43% 4 10% 25% 17% 2	2312
	21st to end	Day	46% 5% 9% 21% 15% 3	2470
May	1st to 10th	Day	60% 9% 9% 13% 8% 1	4120
	11th to 20th	Day	52% 7% 13% 16% 10% 1	4085
	21st to end	Day	40% 10% 9% 20% 19% 1	4677
June	1st to 10th	Day	48% 8% 10% 23% 11%	2660
	11th to 20th	Day	43% 6% 13% 24% 13% 1	2660
	21st to end	Day	41% 8% 16% 22% 12% 1	2660
July	1st to 10th	Day	46% 8% 12% 20% 13% 1	4370
	11th to 20th	Day	41% 7% 16% 23% 12% 1	4365
	21st to end	Day	45% 6% 14% 21% 12% 1	4522
August	1st to 10th	Day	49% 6% 16% 21% 7%	4092
	11th to 20th	Day	41% 7% 15% 29% 8%	4084
	21st to end	Day	28% 7% 15% 28% 21% 1	4230
September	1st to 10th	Day	39% 6% 8% 25% 20% 2	2218
	11th to 20th	Day	36% 4 9% 28% 20% 3	2090
	21st to end	Day	44% 6% 9% 22% 17% 2	2090
October	1st to 10th	Day	41% 5% 11% 26% 15% 2	3480
	11th to 20th	Day	33% 4 8% 29% 24% 2	3114
	21st to end	Day	33% 4 13% 34% 15% 2	3081
November	1st to 10th	Day	45% 5% 12% 21% 14% 2	1698
	11th to 20th	Day	44% 5% 12% 21% 16% 2	1710
	21st to end	Day	44% 5% 14% 25% 10% 1	1710
December	1st to 10th	Day	52% 4 13% 18% 13% 1	1687
	11th to 20th	Day	54% 6% 13% 15% 10% 2	1906
	21st to end	Day	58% 7% 12% 17% 6% 1	2772
January	1st to 10th	Night	45% 4 9% 31% 10% 2	4429
	11th to 20th	Night	51% 4 10% 22% 11% 2	4440
	21st to end	Night	65% 4 7% 16% 8% 1	4781
February	1st to 10th	Night	53% 4 11% 21% 11% 1	2399
	11th to 20th	Night	51% 4 8% 22% 13% 2	2220
	21st to end	Night	47% 5% 9% 24% 12% 3	1884
March	1st to 10th	Night	59% 2 9% 18% 10% 2	3734
	11th to 20th	Night	46% 7% 11% 17% 16% 2	3526
	21st to end	Night	58% 4 6% 11% 15% 5%	3736
April	1st to 10th	Night	51% 3 8% 19% 16% 3	1900
	11th to 20th	Night	48% 5% 9% 20% 16% 3	1815
	21st to end	Night	50% 4 8% 20% 15% 3	1710
May	1st to 10th	Night	61% 6% 10% 15% 7% 1	2840
	11th to 20th	Night	58% 6% 14% 13% 7% 2	2814
	21st to end	Night	47% 12% 10% 17% 13% 1	2913
June	1st to 10th	Night	50% 8% 11% 20% 11% 1	1520
	11th to 20th	Night	45% 6% 10% 24% 15% 1	1520
	21st to end	Night	48% 7% 12% 22% 12%	1520
July	1st to 10th	Night	54% 6% 8% 18% 14%	2480
	11th to 20th	Night	46% 7% 15% 21% 10% 1	2472
	21st to end	Night	51% 6% 12% 18% 12% 1	2760
August	1st to 10th	Night	52% 8% 13% 17% 10% 1	2813
	11th to 20th	Night	43% 7% 15% 27% 8%	2809
	21st to end	Night	29% 6% 15% 27% 21% 1	3409
September	1st to 10th	Night	39% 5% 10% 25% 19% 2	1961
	11th to 20th	Night	37% 5% 9% 25% 21% 3	2090
	21st to end	Night	45% 6% 7% 20% 20% 3	2090
October	1st to 10th	Night	44% 5% 9% 28% 13% 2	3480
	11th to 20th	Night	30% 4 8% 24% 31% 2	3776
	21st to end	Night	36% 4 10% 29% 20% 1	4467
November	1st to 10th	Night	44% 5% 12% 21% 16% 2	2453
	11th to 20th	Night	46% 5% 10% 21% 15% 3	2470
	21st to end	Night	43% 5% 14% 27% 11% 2	2470
December	1st to 10th	Night	53% 4 13% 18% 11%	2489
	11th to 20th	Night	50% 5% 13% 17% 14% 1	3288
	21st to end	Night	55% 7% 12% 17% 8%	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	23% 17% 33% 20% 7%	180
	11th to 20th	Day	23% 18% 32% 21% 5% 1	180
	21st to end	Day	20% 18% 34% 21% 6% 2	259
February	1st to 10th	Day	27% 16% 30% 23% 3	401
	11th to 20th	Day	25% 17% 39% 16% 3	456
	21st to end	Day	23% 16% 37% 19% 5% 1	382
March	1st to 10th	Day	24% 15% 34% 22% 4	460
	11th to 20th	Day	19% 16% 35% 24% 5%	460
	21st to end	Day	24% 13% 35% 18% 7% 2	506
April	1st to 10th	Day	26% 13% 32% 21% 7% 1	460
	11th to 20th	Day	22% 8% 37% 26% 6%	452
	21st to end	Day	23% 7% 33% 26% 8% 2	460
May	1st to 10th	Day	29% 9% 27% 27% 7% 1	463
	11th to 20th	Day	20% 7% 30% 29% 11% 2	487
	21st to end	Day	20% 3 24% 33% 16% 3	529
June	1st to 10th	Day	22% 5% 21% 33% 18% 1	490
	11th to 20th	Day	15% 5% 28% 37% 13% 3	490
	21st to end	Day	15% 7% 23% 34% 19% 2	490
July	1st to 10th	Day	16% 5% 18% 32% 24% 5%	485
	11th to 20th	Day	12% 5% 18% 35% 25% 6%	484
	21st to end	Day	12% 5% 20% 39% 21% 2	532
August	1st to 10th	Day	13% 4 19% 38% 21% 4	486
	11th to 20th	Day	15% 6% 15% 36% 23% 5%	479
	21st to end	Day	12% 4 19% 35% 22% 9%	539
September	1st to 10th	Day	17% 4 23% 28% 20% 7%	490
	11th to 20th	Day	17% 6% 22% 27% 19% 9%	490
	21st to end	Day	22% 8% 27% 27% 13% 4	490
October	1st to 10th	Day	24% 9% 19% 26% 17% 5%	490
	11th to 20th	Day	18% 10% 28% 25% 15% 4	409
	21st to end	Day	21% 10% 34% 21% 11% 3	429
November	1st to 10th	Day	18% 14% 28% 27% 11% 3	374
	11th to 20th	Day	22% 15% 31% 25% 7%	370
	21st to end	Day	19% 16% 30% 24% 9% 1	370
December	1st to 10th	Day	23% 14% 34% 22% 7%	347
	11th to 20th	Day	28% 18% 34% 17% 3 1	180
	21st to end	Day	24% 14% 37% 18% 6% 2	198
January	1st to 10th	Night	23% 16% 36% 19% 4	760
	11th to 20th	Night	28% 20% 33% 16% 4	760
	21st to end	Night	22% 17% 36% 21% 3 1	836
February	1st to 10th	Night	21% 16% 36% 24% 3	760
	11th to 20th	Night	25% 19% 36% 17% 2	743
	21st to end	Night	21% 19% 32% 23% 4	628
March	1st to 10th	Night	24% 16% 33% 22% 5%	760
	11th to 20th	Night	21% 15% 36% 22% 6% 1	760
	21st to end	Night	23% 14% 31% 22% 8% 1	836
April	1st to 10th	Night	26% 11% 31% 27% 5%	760
	11th to 20th	Night	23% 8% 32% 30% 6%	691
	21st to end	Night	23% 7% 29% 29% 12% 1	570
May	1st to 10th	Night	26% 8% 27% 28% 9% 2	570
	11th to 20th	Night	23% 6% 31% 30% 9% 1	562
	21st to end	Night	21% 5% 20% 38% 14% 1	520
June	1st to 10th	Night	23% 6% 21% 34% 14% 2	380
	11th to 20th	Night	23% 4 17% 39% 15% 2	380
	21st to end	Night	20% 7% 15% 38% 19% 1	380
July	1st to 10th	Night	18% 4 17% 38% 21% 1	370
	11th to 20th	Night	18% 4 15% 38% 24% 1	368
	21st to end	Night	17% 4 15% 41% 23% 1	478
August	1st to 10th	Night	11% 3 19% 42% 24% 2	568
	11th to 20th	Night	15% 4 14% 41% 24% 2	562
	21st to end	Night	13% 4 14% 35% 28% 5%	795
September	1st to 10th	Night	16% 3 19% 36% 24% 3	760
	11th to 20th	Night	16% 5% 18% 32% 23% 6%	760
	21st to end	Night	22% 6% 21% 30% 18% 3	760
October	1st to 10th	Night	25% 7% 24% 24% 17% 3	760
	11th to 20th	Night	19% 10% 28% 24% 15% 3	760
	21st to end	Night	23% 11% 30% 26% 8% 1	836
November	1st to 10th	Night	19% 11% 28% 27% 13% 1	755
	11th to 20th	Night	22% 17% 31% 19% 10%	760
	21st to end	Night	20% 17% 30% 25% 7% 1	760
December	1st to 10th	Night	25% 15% 33% 21% 5% 1	760
	11th to 20th	Night	23% 21% 33% 18% 4	760
	21st to end	Night	23% 16% 37% 20% 4	836

Page generated at 2024-11-01 22:47:47. Version ac07

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