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 2024-03-23.

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% 8% 3 6% 5% 3 4 4 5% 7% 39%	78647
	Night	18% 9% 4 6% 5% 3 4 4 5% 6% 36%	81229
Transparency	Day	45% 6% 11% 22% 13% 2	78357
	Night	48% 5% 10% 21% 13% 2	81053
Seeing	Day	20% 9% 27% 28% 13% 3	14366
	Night	22% 11% 28% 27% 11% 1	23349

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% 5% 6% 5% 2 4 4 4 6% 39%	2517
	11th to 20th	Day	20% 11% 3 6% 4 2 3 3 5% 6% 37%	2520
	21st to end	Day	22% 12% 4 13% 5% 3 4 3 3 4 25%	2872
February	1st to 10th	Day	20% 11% 5% 6% 6% 3 4 4 4 7% 31%	1776
	11th to 20th	Day	15% 9% 4 6% 6% 3 5% 3 4 8% 37%	1848
	21st to end	Day	20% 8% 5% 7% 5% 3 4 4 4 6% 33%	1570
March	1st to 10th	Day	18% 10% 2 5% 6% 4 4 3 5% 8% 35%	1897
	11th to 20th	Day	16% 8% 3 5% 5% 3 4 4 5% 7% 40%	2056
	21st to end	Day	18% 10% 3 5% 4 2 4 4 5% 6% 39%	2236
April	1st to 10th	Day	21% 8% 3 4 4 2 3 4 5% 6% 40%	2160
	11th to 20th	Day	16% 8% 3 5% 4 2 3 3 5% 6% 43%	2188
	21st to end	Day	19% 8% 3 6% 5% 3 4 4 5% 7% 38%	2340
May	1st to 10th	Day	24% 9% 3 6% 5% 2 4 4 4 6% 34%	2340
	11th to 20th	Day	17% 7% 3 5% 4 2 4 4 5% 8% 41%	2301
	21st to end	Day	14% 8% 3 6% 4 2 4 4 6% 8% 42%	2635
June	1st to 10th	Day	20% 7% 3 4 6% 2 5% 5% 6% 9% 33%	2520
	11th to 20th	Day	16% 7% 2 5% 6% 3 5% 4 5% 8% 39%	2520
	21st to end	Day	14% 7% 3 5% 4 3 4 5% 6% 9% 40%	2520
July	1st to 10th	Day	12% 6% 2 4 4 3 5% 5% 6% 8% 45%	2520
	11th to 20th	Day	11% 6% 2 5% 5% 3 4 5% 7% 10% 40%	2520
	21st to end	Day	13% 5% 2 6% 5% 3 5% 6% 6% 8% 42%	2668
August	1st to 10th	Day	10% 6% 2 6% 6% 3 4 5% 6% 9% 43%	2327
	11th to 20th	Day	13% 6% 2 5% 4 4 4 5% 7% 8% 42%	2304
	21st to end	Day	13% 6% 3 5% 4 2 3 4 6% 10% 45%	2412
September	1st to 10th	Day	14% 6% 2 5% 5% 3 4 4 4 6% 46%	2102
	11th to 20th	Day	14% 6% 2 5% 3 2 4 3 5% 6% 50%	1980
	21st to end	Day	18% 7% 2 5% 5% 2 3 3 5% 7% 42%	1980
October	1st to 10th	Day	22% 6% 2 4 4 3 4 4 4 6% 42%	1980
	11th to 20th	Day	16% 6% 3 6% 5% 2 3 4 4 8% 45%	1800
	21st to end	Day	19% 7% 3 6% 5% 2 4 4 5% 7% 38%	1782
November	1st to 10th	Day	20% 7% 2 6% 5% 3 3 3 5% 5% 42%	1671
	11th to 20th	Day	17% 8% 3 4 5% 2 3 4 5% 8% 39%	1710
	21st to end	Day	17% 8% 2 5% 4 3 5% 4 5% 7% 40%	1710
December	1st to 10th	Day	20% 9% 4 7% 5% 3 5% 4 4 6% 33%	1687
	11th to 20th	Day	21% 11% 4 7% 5% 4 3 4 4 6% 31%	1906
	21st to end	Day	21% 14% 6% 8% 5% 3 5% 4 4 6% 24%	2772
January	1st to 10th	Night	18% 8% 4 5% 4 2 3 4 3 6% 42%	4429
	11th to 20th	Night	17% 12% 5% 8% 5% 2 3 3 4 5% 36%	4440
	21st to end	Night	23% 14% 5% 11% 6% 3 4 3 3 4 24%	4781
February	1st to 10th	Night	18% 10% 3 8% 6% 3 5% 5% 5% 6% 33%	2399
	11th to 20th	Night	20% 10% 4 6% 6% 3 4 4 4 6% 35%	2218
	21st to end	Night	18% 8% 3 8% 5% 2 4 4 5% 6% 37%	1884
March	1st to 10th	Night	19% 9% 3 5% 6% 3 4 4 5% 6% 36%	2270
	11th to 20th	Night	16% 10% 4 6% 6% 3 4 4 5% 7% 34%	2122
	21st to end	Night	17% 11% 3 6% 5% 3 4 3 4 6% 38%	2161
April	1st to 10th	Night	20% 10% 3 5% 5% 3 5% 4 3 5% 37%	1800
	11th to 20th	Night	18% 9% 3 5% 5% 3 4 4 5% 6% 39%	1719
	21st to end	Night	20% 9% 3 6% 6% 3 5% 4 4 6% 35%	1620
May	1st to 10th	Night	22% 11% 4 6% 5% 3 4 4 5% 6% 30%	1620
	11th to 20th	Night	19% 9% 4 5% 4 2 4 5% 6% 8% 34%	1593
	21st to end	Night	17% 9% 4 7% 6% 3 5% 5% 5% 6% 33%	1654
June	1st to 10th	Night	20% 9% 4 5% 5% 3 4 6% 6% 7% 30%	1440
	11th to 20th	Night	19% 9% 3 5% 4 2 4 4 5% 7% 38%	1440
	21st to end	Night	18% 10% 3 7% 4 2 4 4 6% 10% 33%	1440
July	1st to 10th	Night	13% 9% 2 5% 6% 3 5% 5% 6% 8% 36%	1440
	11th to 20th	Night	15% 8% 3 6% 5% 2 5% 5% 6% 9% 35%	1440
	21st to end	Night	13% 7% 3 8% 5% 4 5% 5% 6% 7% 37%	1632
August	1st to 10th	Night	8% 7% 2 5% 5% 3 5% 6% 7% 9% 43%	1612
	11th to 20th	Night	11% 7% 2 5% 5% 3 6% 7% 7% 8% 38%	1596
	21st to end	Night	12% 6% 3 6% 4 3 4 4 6% 8% 46%	1941
September	1st to 10th	Night	13% 6% 3 5% 5% 3 4 4 6% 6% 44%	1857
	11th to 20th	Night	14% 6% 3 4 4 2 4 4 4 6% 48%	1980
	21st to end	Night	16% 8% 2 5% 5% 3 3 5% 4 7% 41%	1980
October	1st to 10th	Night	22% 8% 3 5% 5% 2 3 5% 4 5% 38%	1980
	11th to 20th	Night	18% 5% 2 4 4 3 4 3 6% 7% 44%	2159
	21st to end	Night	20% 9% 3 5% 5% 3 4 5% 5% 7% 34%	2574
November	1st to 10th	Night	17% 6% 3 5% 6% 3 4 3 4 6% 42%	2407
	11th to 20th	Night	17% 8% 3 5% 5% 3 4 4 4 6% 40%	2470
	21st to end	Night	16% 6% 3 5% 6% 3 4 4 5% 7% 41%	2470
December	1st to 10th	Night	21% 9% 3 7% 5% 3 4 4 5% 6% 31%	2489
	11th to 20th	Night	18% 10% 4 7% 5% 3 4 4 4 7% 34%	3288
	21st to end	Night	19% 14% 6% 7% 6% 3 5% 4 4 6% 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	49% 5% 13% 20% 12% 2	1900
	11th to 20th	Day	43% 5% 11% 24% 14% 3	2056
	21st to end	Day	47% 5% 9% 19% 16% 4	2236
April	1st to 10th	Day	45% 4 9% 21% 18% 3	2160
	11th to 20th	Day	42% 4 10% 25% 17% 2	2188
	21st to end	Day	47% 5% 9% 21% 14% 4	2340
May	1st to 10th	Day	53% 6% 7% 20% 12% 2	2340
	11th to 20th	Day	43% 7% 10% 20% 18% 2	2305
	21st to end	Day	42% 8% 13% 22% 14% 1	2635
June	1st to 10th	Day	49% 8% 10% 22% 11%	2520
	11th to 20th	Day	44% 6% 12% 23% 13% 1	2520
	21st to end	Day	40% 9% 15% 23% 12% 1	2520
July	1st to 10th	Day	38% 8% 13% 25% 14% 2	2450
	11th to 20th	Day	38% 8% 15% 26% 11% 2	2445
	21st to end	Day	37% 8% 14% 26% 14% 2	2519
August	1st to 10th	Day	38% 7% 16% 27% 11% 1	2314
	11th to 20th	Day	40% 8% 15% 24% 13% 1	2304
	21st to end	Day	37% 7% 13% 23% 18% 1	2412
September	1st to 10th	Day	39% 6% 9% 25% 19% 2	2102
	11th to 20th	Day	36% 4 10% 27% 21% 3	1980
	21st to end	Day	42% 6% 9% 23% 18% 2	1980
October	1st to 10th	Day	45% 5% 9% 23% 15% 3	1980
	11th to 20th	Day	39% 5% 11% 29% 14% 1	1800
	21st to end	Day	46% 5% 13% 20% 13% 3	1782
November	1st to 10th	Day	45% 5% 13% 21% 14% 2	1671
	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	49% 3 10% 21% 13% 3	2280
	11th to 20th	Night	49% 5% 9% 18% 16% 3	2123
	21st to end	Night	49% 5% 8% 17% 17% 5%	2161
April	1st to 10th	Night	51% 4 8% 18% 16% 3	1800
	11th to 20th	Night	47% 4 9% 21% 16% 3	1719
	21st to end	Night	51% 4 9% 19% 13% 3	1620
May	1st to 10th	Night	55% 4 9% 17% 12% 2	1620
	11th to 20th	Night	48% 6% 11% 19% 13% 4	1594
	21st to end	Night	53% 6% 11% 17% 12% 1	1655
June	1st to 10th	Night	51% 8% 11% 19% 11% 1	1440
	11th to 20th	Night	46% 6% 10% 22% 15% 1	1440
	21st to end	Night	47% 7% 12% 22% 12%	1440
July	1st to 10th	Night	44% 7% 12% 22% 13% 1	1400
	11th to 20th	Night	45% 6% 11% 25% 10% 1	1392
	21st to end	Night	44% 7% 11% 26% 10% 2	1541
August	1st to 10th	Night	36% 8% 15% 25% 15% 1	1600
	11th to 20th	Night	40% 9% 16% 22% 11% 1	1596
	21st to end	Night	37% 7% 11% 25% 18% 2	1941
September	1st to 10th	Night	40% 5% 11% 25% 18% 2	1857
	11th to 20th	Night	37% 5% 10% 24% 21% 2	1980
	21st to end	Night	44% 5% 8% 20% 21% 3	1980
October	1st to 10th	Night	50% 5% 9% 21% 13% 2	1980
	11th to 20th	Night	41% 5% 11% 28% 14% 1	2159
	21st to end	Night	50% 6% 10% 20% 13% 1	2574
November	1st to 10th	Night	45% 5% 11% 21% 15% 2	2407
	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	22% 13% 36% 19% 8% 3	470
April	1st to 10th	Day	27% 12% 32% 21% 8% 1	420
	11th to 20th	Day	22% 7% 37% 27% 7%	412
	21st to end	Day	24% 8% 32% 25% 9% 2	420
May	1st to 10th	Day	30% 7% 26% 27% 8% 1	423
	11th to 20th	Day	18% 8% 31% 30% 11% 2	447
	21st to end	Day	21% 3 23% 33% 17% 4	485
June	1st to 10th	Day	22% 4 21% 32% 19% 1	450
	11th to 20th	Day	16% 4 26% 37% 13% 3	450
	21st to end	Day	14% 5% 21% 36% 21% 2	450
July	1st to 10th	Day	16% 4 16% 34% 25% 5%	445
	11th to 20th	Day	12% 4 15% 36% 27% 6%	444
	21st to end	Day	11% 4 19% 41% 22% 3	488
August	1st to 10th	Day	13% 3 17% 40% 22% 5%	446
	11th to 20th	Day	15% 6% 13% 38% 23% 5%	439
	21st to end	Day	13% 4 17% 34% 23% 9%	495
September	1st to 10th	Day	18% 3 20% 30% 21% 8%	450
	11th to 20th	Day	17% 6% 24% 28% 17% 8%	450
	21st to end	Day	21% 8% 26% 26% 14% 5%	450
October	1st to 10th	Day	26% 8% 19% 25% 17% 5%	450
	11th to 20th	Day	18% 8% 28% 26% 16% 3	378
	21st to end	Day	23% 9% 33% 21% 11% 3	396
November	1st to 10th	Day	18% 13% 27% 27% 11% 3	365
	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	22% 14% 31% 23% 9% 1	801
April	1st to 10th	Night	25% 11% 31% 27% 5%	720
	11th to 20th	Night	23% 7% 33% 31% 6%	655
	21st to end	Night	24% 7% 28% 28% 12% 1	540
May	1st to 10th	Night	26% 7% 27% 28% 9% 2	540
	11th to 20th	Night	22% 6% 32% 31% 8% 1	532
	21st to end	Night	22% 5% 19% 39% 14% 1	493
June	1st to 10th	Night	24% 6% 20% 33% 15% 2	360
	11th to 20th	Night	24% 4 15% 39% 16% 2	360
	21st to end	Night	20% 6% 15% 39% 20% 1	360
July	1st to 10th	Night	17% 3 17% 39% 22% 1	350
	11th to 20th	Night	18% 3 14% 39% 25% 1	348
	21st to end	Night	16% 3 15% 41% 23% 2	452
August	1st to 10th	Night	10% 3 18% 43% 25% 2	538
	11th to 20th	Night	16% 3 14% 42% 24% 2	532
	21st to end	Night	14% 4 14% 35% 29% 5%	753
September	1st to 10th	Night	16% 3 18% 36% 25% 3	720
	11th to 20th	Night	16% 5% 19% 32% 22% 5%	720
	21st to end	Night	21% 6% 22% 30% 18% 4	720
October	1st to 10th	Night	26% 7% 24% 23% 17% 3	720
	11th to 20th	Night	20% 10% 28% 25% 16% 3	720
	21st to end	Night	24% 11% 31% 26% 8% 1	792
November	1st to 10th	Night	20% 11% 28% 27% 14% 1	740
	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 2024-03-19 23:31:02. Version ac07

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