First Colony Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the First Colony Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	11% 6% 2 6% 5% 4 5% 4 6% 8% 42%	137723
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 45%	134993
Transparency	Day	38% 6% 11% 21% 18% 5%	141477
	Night	37% 5% 10% 20% 20% 7%	139316
Seeing	Day	13% 8% 20% 25% 22% 12%	17233
	Night	14% 6% 22% 31% 22% 4	26843

By Month

Percentage Cloud Cover SE (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	17% 11% 5% 7% 3 2 3 2 4 5% 40%	4808
	11th to 20th	Day	10% 5% 3 5% 3 3 3 3 4 6% 56%	5423
	21st to end	Day	22% 11% 2 7% 5% 3 3 2 3 4 38%	6436
February	1st to 10th	Day	16% 10% 3 7% 4 4 3 3 5% 7% 40%	2100
	11th to 20th	Day	20% 8% 3 7% 4 4 3 3 4 5% 39%	2048
	21st to end	Day	18% 10% 2 6% 4 4 2 3 4 5% 42%	1730
March	1st to 10th	Day	19% 9% 3 8% 5% 4 4 3 5% 6% 35%	4580
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 7% 45%	4826
	21st to end	Day	13% 7% 3 7% 5% 5% 4 3 4 6% 42%	6099
April	1st to 10th	Day	11% 5% 2 5% 5% 4 4 4 6% 8% 47%	2520
	11th to 20th	Day	12% 6% 2 5% 4 3 3 3 6% 6% 48%	2494
	21st to end	Day	10% 5% 2 5% 4 5% 5% 4 5% 8% 47%	2520
May	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 7% 35%	7021
	11th to 20th	Day	10% 6% 3 8% 5% 5% 5% 4 7% 9% 38%	7626
	21st to end	Day	13% 8% 4 8% 6% 5% 6% 5% 6% 8% 32%	8391
June	1st to 10th	Day	8% 6% 2 7% 5% 6% 5% 5% 7% 11% 38%	2730
	11th to 20th	Day	6% 4 2 6% 5% 4 5% 6% 8% 10% 44%	2931
	21st to end	Day	5% 4 2 6% 5% 5% 6% 5% 7% 11% 44%	2940
July	1st to 10th	Day	5% 5% 2 7% 6% 6% 7% 6% 8% 10% 37%	6500
	11th to 20th	Day	6% 3 2 5% 5% 5% 6% 7% 9% 14% 36%	6207
	21st to end	Day	7% 6% 3 8% 6% 6% 7% 5% 9% 13% 30%	5922
August	1st to 10th	Day	3 2 1 4 3 4 5% 4 6% 9% 58%	3905
	11th to 20th	Day	5% 4 2 6% 5% 4 6% 4 7% 11% 47%	3849
	21st to end	Day	7% 4 1 5% 4 5% 5% 5% 6% 9% 50%	4312
September	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 7% 8% 45%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 3 6% 7% 45%	2250
	21st to end	Day	9% 4 2 5% 4 4 4 4 6% 7% 51%	2200
October	1st to 10th	Day	8% 4 2 5% 4 3 4 3 5% 6% 57%	2200
	11th to 20th	Day	8% 4 2 4 3 3 4 3 4 6% 59%	2053
	21st to end	Day	12% 5% 2 5% 3 3 4 3 5% 4 55%	2185
November	1st to 10th	Day	10% 5% 2 5% 4 3 3 2 5% 5% 56%	1852
	11th to 20th	Day	12% 5% 1 5% 4 3 3 2 4 6% 55%	1890
	21st to end	Day	15% 6% 3 6% 4 3 4 3 5% 7% 45%	1890
December	1st to 10th	Day	15% 9% 3 6% 4 3 4 3 5% 5% 41%	3020
	11th to 20th	Day	14% 10% 3 7% 4 4 4 2 4 5% 43%	3463
	21st to end	Day	17% 7% 3 6% 4 4 4 3 4 5% 44%	4402
January	1st to 10th	Night	18% 8% 3 7% 4 3 3 2 4 5% 42%	8098
	11th to 20th	Night	13% 8% 3 7% 3 3 3 2 4 5% 50%	7526
	21st to end	Night	22% 8% 2 7% 4 3 3 3 3 4 41%	7745
February	1st to 10th	Night	17% 7% 3 6% 3 3 4 2 5% 5% 44%	2520
	11th to 20th	Night	20% 9% 2 6% 4 4 3 2 4 4 42%	2458
	21st to end	Night	18% 7% 3 6% 4 4 4 3 5% 6% 40%	2076
March	1st to 10th	Night	19% 9% 3 7% 4 4 4 4 5% 5% 36%	5493
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 5% 6% 52%	5368
	21st to end	Night	17% 7% 3 6% 4 3 4 3 4 6% 44%	5072
April	1st to 10th	Night	12% 5% 2 5% 4 4 5% 4 6% 6% 47%	2100
	11th to 20th	Night	10% 6% 2 5% 3 3 3 4 5% 7% 52%	2075
	21st to end	Night	11% 5% 2 6% 3 4 4 4 5% 8% 49%	2100
May	1st to 10th	Night	18% 8% 3 6% 4 4 4 3 5% 8% 37%	5814
	11th to 20th	Night	13% 6% 3 8% 5% 5% 5% 4 6% 8% 37%	5234
	21st to end	Night	11% 8% 2 8% 6% 6% 6% 5% 8% 9% 31%	5761
June	1st to 10th	Night	6% 4 2 6% 5% 6% 5% 4 7% 10% 44%	1890
	11th to 20th	Night	5% 4 1 4 4 4 4 4 8% 11% 50%	1682
	21st to end	Night	3 5% 1 4 5% 4 6% 5% 7% 11% 48%	1680
July	1st to 10th	Night	4 5% 2 6% 5% 6% 5% 6% 9% 13% 40%	3698
	11th to 20th	Night	5% 4 2 5% 4 5% 5% 5% 9% 14% 41%	3868
	21st to end	Night	6% 7% 3 9% 5% 6% 7% 6% 10% 13% 29%	4929
August	1st to 10th	Night	3 2 1 3 3 4 4 4 6% 9% 63%	3244
	11th to 20th	Night	3 3 1 4 4 4 4 4 9% 12% 52%	3200
	21st to end	Night	4 4 2 5% 5% 4 5% 5% 6% 9% 51%	3586
September	1st to 10th	Night	9% 5% 2 5% 5% 5% 4 4 6% 8% 48%	2000
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 48%	2130
	21st to end	Night	7% 4 2 5% 3 4 4 4 5% 7% 54%	2200
October	1st to 10th	Night	6% 4 2 5% 3 3 3 3 5% 6% 61%	2200
	11th to 20th	Night	10% 4 1 4 3 3 2 3 5% 7% 56%	2343
	21st to end	Night	15% 5% 2 4 2 3 2 2 3 5% 57%	2653
November	1st to 10th	Night	10% 5% 1 5% 5% 3 3 3 4 5% 55%	2666
	11th to 20th	Night	12% 5% 2 4 3 3 3 2 3 6% 55%	2730
	21st to end	Night	13% 6% 2 5% 4 3 4 3 4 5% 49%	2730
December	1st to 10th	Night	19% 6% 2 4 3 3 3 3 4 6% 47%	4380
	11th to 20th	Night	17% 8% 2 6% 4 3 3 3 3 5% 46%	4952
	21st to end	Night	18% 8% 3 6% 3 3 3 3 5% 6% 42%	6792

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	48% 3 7% 27% 14% 2	4809
	11th to 20th	Day	30% 3 5% 25% 33% 4	5423
	21st to end	Day	52% 3 5% 15% 19% 6%	6436
February	1st to 10th	Day	44% 4 8% 19% 19% 5%	2100
	11th to 20th	Day	48% 3 5% 19% 18% 6%	2058
	21st to end	Day	46% 3 7% 15% 22% 8%	1730
March	1st to 10th	Day	50% 5% 6% 14% 18% 6%	4580
	11th to 20th	Day	37% 5% 6% 18% 22% 11%	4826
	21st to end	Day	43% 4 9% 21% 18% 5%	6101
April	1st to 10th	Day	34% 6% 11% 23% 19% 7%	2520
	11th to 20th	Day	35% 5% 7% 23% 23% 8%	2494
	21st to end	Day	34% 6% 10% 24% 22% 3	2520
May	1st to 10th	Day	47% 6% 11% 22% 13% 2	7056
	11th to 20th	Day	41% 6% 16% 21% 14% 1	7630
	21st to end	Day	48% 7% 14% 19% 11% 1	8405
June	1st to 10th	Day	36% 8% 14% 23% 16% 2	2730
	11th to 20th	Day	30% 10% 16% 24% 17% 2	2931
	21st to end	Day	31% 10% 17% 24% 15% 3	2940
July	1st to 10th	Day	37% 9% 18% 25% 8% 1	6431
	11th to 20th	Day	33% 10% 30% 23% 4 1	6132
	21st to end	Day	42% 9% 22% 22% 4	5752
August	1st to 10th	Day	20% 9% 12% 33% 25% 1	3893
	11th to 20th	Day	30% 8% 17% 29% 14% 2	3885
	21st to end	Day	28% 8% 15% 24% 21% 4	4311
September	1st to 10th	Day	34% 8% 14% 21% 17% 5%	2400
	11th to 20th	Day	37% 7% 11% 17% 22% 6%	2250
	21st to end	Day	31% 6% 11% 23% 19% 9%	2200
October	1st to 10th	Day	20% 4 5% 15% 28% 29%	3590
	11th to 20th	Day	17% 3 4 15% 37% 24%	3319
	21st to end	Day	24% 3 5% 20% 36% 12%	3511
November	1st to 10th	Day	30% 4 7% 21% 27% 9%	1852
	11th to 20th	Day	32% 3 7% 20% 27% 11%	1890
	21st to end	Day	39% 6% 6% 18% 25% 7%	1890
December	1st to 10th	Day	44% 4 6% 21% 21% 3	3020
	11th to 20th	Day	46% 3 8% 19% 19% 6%	3460
	21st to end	Day	41% 4 8% 23% 22% 1	4402
January	1st to 10th	Night	45% 3 7% 24% 19% 2	8099
	11th to 20th	Night	39% 3 4 17% 28% 8%	7526
	21st to end	Night	48% 2 4 17% 21% 7%	7745
February	1st to 10th	Night	43% 3 7% 18% 22% 7%	2520
	11th to 20th	Night	47% 4 6% 17% 18% 9%	2459
	21st to end	Night	44% 6% 6% 15% 19% 10%	2076
March	1st to 10th	Night	49% 4 7% 14% 16% 10%	5493
	11th to 20th	Night	33% 5% 6% 18% 21% 17%	5368
	21st to end	Night	43% 4 7% 22% 19% 7%	5072
April	1st to 10th	Night	37% 6% 9% 21% 19% 8%	2100
	11th to 20th	Night	31% 5% 8% 23% 23% 11%	2075
	21st to end	Night	34% 5% 11% 23% 22% 5%	2100
May	1st to 10th	Night	45% 6% 13% 17% 15% 3	5844
	11th to 20th	Night	44% 6% 12% 23% 13% 2	5235
	21st to end	Night	46% 10% 12% 16% 14% 2	5771
June	1st to 10th	Night	32% 9% 15% 22% 19% 2	1890
	11th to 20th	Night	25% 8% 17% 27% 18% 5%	1682
	21st to end	Night	27% 10% 19% 22% 19% 4	1680
July	1st to 10th	Night	32% 10% 24% 24% 9% 2	3660
	11th to 20th	Night	31% 11% 29% 24% 5% 1	3823
	21st to end	Night	42% 11% 22% 19% 5% 1	4785
August	1st to 10th	Night	17% 8% 13% 35% 25% 3	3232
	11th to 20th	Night	23% 9% 20% 31% 16% 2	3230
	21st to end	Night	28% 7% 15% 21% 22% 7%	3586
September	1st to 10th	Night	33% 7% 15% 22% 17% 6%	2000
	11th to 20th	Night	33% 8% 11% 19% 22% 7%	2130
	21st to end	Night	28% 6% 10% 23% 24% 9%	2200
October	1st to 10th	Night	18% 4 6% 15% 27% 30%	3590
	11th to 20th	Night	18% 3 4 16% 30% 28%	3816
	21st to end	Night	23% 2 4 21% 31% 19%	4277
November	1st to 10th	Night	31% 5% 7% 17% 27% 13%	2666
	11th to 20th	Night	31% 3 7% 19% 27% 12%	2730
	21st to end	Night	37% 4 6% 21% 23% 9%	2730
December	1st to 10th	Night	40% 3 6% 22% 25% 3	4380
	11th to 20th	Night	42% 3 8% 19% 24% 4	4954
	21st to end	Night	42% 5% 8% 22% 22% 2	6792

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	25% 20% 30% 20% 5%	330
	11th to 20th	Day	21% 21% 34% 15% 7% 2	330
	21st to end	Day	23% 20% 35% 17% 4 1	363
February	1st to 10th	Day	19% 17% 38% 19% 6% 1	330
	11th to 20th	Day	33% 16% 30% 16% 4 1	330
	21st to end	Day	27% 22% 29% 18% 4	273
March	1st to 10th	Day	20% 15% 38% 19% 7%	330
	11th to 20th	Day	21% 17% 39% 19% 4	335
	21st to end	Day	18% 10% 33% 28% 10% 1	594
April	1st to 10th	Day	15% 16% 29% 27% 12% 1	540
	11th to 20th	Day	16% 11% 33% 25% 12% 4	532
	21st to end	Day	12% 11% 25% 30% 19% 2	540
May	1st to 10th	Day	15% 10% 25% 25% 20% 5%	543
	11th to 20th	Day	10% 5% 22% 30% 21% 12%	567
	21st to end	Day	11% 5% 18% 33% 18% 15%	617
June	1st to 10th	Day	10% 4 13% 27% 29% 18%	570
	11th to 20th	Day	7% 2 10% 30% 33% 19%	771
	21st to end	Day	6% 2 11% 24% 36% 22%	780
July	1st to 10th	Day	5% 2 8% 22% 39% 24%	771
	11th to 20th	Day	7% 2 6% 21% 33% 31%	704
	21st to end	Day	5% 1 6% 20% 38% 30%	576
August	1st to 10th	Day	6% 2 7% 22% 36% 28%	526
	11th to 20th	Day	7% 1 6% 21% 37% 28%	519
	21st to end	Day	11% 2 7% 21% 33% 27%	583
September	1st to 10th	Day	9% 2 10% 24% 39% 16%	530
	11th to 20th	Day	11% 2 9% 21% 34% 22%	530
	21st to end	Day	10% 3 13% 27% 32% 15%	530
October	1st to 10th	Day	9% 3 16% 34% 27% 10%	530
	11th to 20th	Day	9% 5% 28% 32% 21% 5%	383
	21st to end	Day	17% 9% 23% 28% 17% 6%	363
November	1st to 10th	Day	16% 7% 23% 32% 17% 4	330
	11th to 20th	Day	13% 9% 33% 32% 12% 2	330
	21st to end	Day	15% 19% 28% 25% 10% 4	330
December	1st to 10th	Day	23% 13% 35% 21% 7% 1	330
	11th to 20th	Day	18% 15% 33% 21% 12% 1	330
	21st to end	Day	24% 13% 32% 22% 8% 1	363
January	1st to 10th	Night	22% 10% 34% 28% 5%	1050
	11th to 20th	Night	21% 13% 32% 27% 8%	1000
	21st to end	Night	21% 12% 36% 29% 3	924
February	1st to 10th	Night	20% 12% 32% 31% 5%	840
	11th to 20th	Night	25% 13% 30% 27% 5%	823
	21st to end	Night	21% 15% 38% 24% 3	692
March	1st to 10th	Night	19% 11% 31% 33% 6%	840
	11th to 20th	Night	17% 10% 33% 31% 9%	772
	21st to end	Night	16% 7% 37% 29% 12%	693
April	1st to 10th	Night	13% 11% 31% 32% 12% 1	630
	11th to 20th	Night	12% 6% 33% 34% 14% 2	623
	21st to end	Night	12% 7% 29% 36% 15% 1	630
May	1st to 10th	Night	13% 4 25% 36% 20% 3	630
	11th to 20th	Night	12% 3 19% 38% 24% 5%	621
	21st to end	Night	8% 1 14% 36% 33% 7%	683
June	1st to 10th	Night	6% 2 10% 37% 37% 7%	630
	11th to 20th	Night	5% 3 8% 28% 47% 9%	630
	21st to end	Night	5% 2 7% 29% 45% 13%	630
July	1st to 10th	Night	4 1 4 24% 51% 15%	615
	11th to 20th	Night	5% 1 5% 25% 45% 19%	597
	21st to end	Night	5% 2 7% 20% 51% 15%	657
August	1st to 10th	Night	5% 1 4 22% 49% 19%	597
	11th to 20th	Night	6% 1 3 27% 47% 16%	594
	21st to end	Night	8% 1 6% 26% 43% 15%	660
September	1st to 10th	Night	10% 1 7% 34% 42% 6%	600
	11th to 20th	Night	11% 2 11% 26% 44% 6%	730
	21st to end	Night	9% 2 11% 33% 40% 5%	800
October	1st to 10th	Night	7% 3 14% 38% 34% 5%	800
	11th to 20th	Night	12% 5% 21% 37% 24% 2	800
	21st to end	Night	16% 6% 21% 34% 21% 2	880
November	1st to 10th	Night	12% 5% 20% 38% 21% 3	820
	11th to 20th	Night	14% 6% 29% 36% 14% 1	840
	21st to end	Night	17% 9% 26% 34% 13% 1	840
December	1st to 10th	Night	20% 8% 30% 33% 8%	840
	11th to 20th	Night	21% 8% 29% 31% 10%	840
	21st to end	Night	24% 10% 29% 30% 7%	992

Page generated at 2026-07-13 02:14:50. Version ac07

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