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 2025-03-12.

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% 4 4 5% 4 6% 8% 44%	109196
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 46%	109828
Transparency	Day	37% 6% 11% 22% 19% 6%	112986
	Night	36% 6% 9% 21% 21% 8%	114177
Seeing	Day	13% 8% 20% 24% 23% 13%	15363
	Night	14% 6% 22% 31% 22% 4	25162

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	14% 9% 5% 6% 3 2 3 2 4 6% 45%	3706
	11th to 20th	Day	12% 5% 3 6% 4 3 3 3 4 6% 51%	4167
	21st to end	Day	20% 12% 2 6% 3 3 3 2 3 5% 41%	4972
February	1st to 10th	Day	16% 10% 3 7% 4 4 3 3 5% 7% 39%	2000
	11th to 20th	Day	20% 8% 3 7% 5% 4 3 3 4 5% 40%	1948
	21st to end	Day	18% 10% 3 6% 4 4 2 2 4 5% 41%	1650
March	1st to 10th	Day	21% 10% 3 7% 4 4 4 3 5% 6% 33%	3220
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 6% 8% 41%	3316
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 4 4 6% 41%	4165
April	1st to 10th	Day	12% 5% 2 5% 4 3 4 4 5% 8% 48%	2280
	11th to 20th	Day	14% 6% 2 6% 4 3 4 3 7% 6% 46%	2254
	21st to end	Day	9% 4 2 5% 4 5% 5% 4 5% 8% 50%	2280
May	1st to 10th	Day	10% 4 2 6% 5% 5% 5% 4 6% 9% 44%	3765
	11th to 20th	Day	12% 6% 3 7% 4 4 4 4 6% 9% 40%	4081
	21st to end	Day	8% 5% 3 6% 5% 4 5% 5% 7% 10% 42%	4476
June	1st to 10th	Day	7% 6% 2 7% 5% 6% 5% 5% 7% 11% 38%	2470
	11th to 20th	Day	5% 3 2 6% 5% 4 5% 6% 8% 10% 47%	2651
	21st to end	Day	5% 4 2 6% 5% 5% 6% 4 7% 10% 45%	2660
July	1st to 10th	Day	6% 6% 3 8% 7% 7% 8% 6% 8% 11% 32%	4440
	11th to 20th	Day	8% 4 2 6% 6% 6% 7% 7% 9% 12% 32%	4286
	21st to end	Day	10% 7% 4 10% 7% 7% 7% 5% 7% 9% 26%	4118
August	1st to 10th	Day	3 2 1 4 3 4 5% 4 6% 9% 58%	3785
	11th to 20th	Day	5% 4 2 6% 5% 4 6% 4 6% 11% 47%	3729
	21st to end	Day	7% 4 1 5% 4 5% 5% 5% 6% 9% 50%	4180
September	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 7% 8% 45%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 3 6% 7% 43%	2138
	21st to end	Day	10% 4 2 5% 4 4 4 4 6% 7% 51%	2090
October	1st to 10th	Day	8% 4 2 5% 4 4 4 3 5% 6% 57%	2090
	11th to 20th	Day	9% 4 2 4 3 3 4 3 4 6% 58%	1951
	21st to end	Day	12% 4 2 5% 3 3 4 3 5% 4 56%	2076
November	1st to 10th	Day	10% 5% 2 5% 4 3 3 2 5% 5% 55%	1762
	11th to 20th	Day	12% 5% 1 5% 4 3 3 2 4 5% 54%	1800
	21st to end	Day	15% 6% 3 6% 4 3 4 3 5% 7% 45%	1800
December	1st to 10th	Day	15% 8% 3 6% 4 3 4 3 5% 5% 42%	2930
	11th to 20th	Day	15% 10% 3 7% 4 4 4 2 4 5% 42%	3373
	21st to end	Day	17% 7% 3 6% 4 4 4 3 4 5% 44%	4307
January	1st to 10th	Night	17% 8% 3 7% 4 3 3 2 3 5% 45%	6242
	11th to 20th	Night	15% 8% 4 7% 3 3 3 2 3 4 47%	5788
	21st to end	Night	21% 9% 2 5% 3 3 3 3 3 4 44%	5984
February	1st to 10th	Night	17% 8% 3 6% 4 3 4 2 5% 5% 43%	2400
	11th to 20th	Night	20% 9% 2 6% 4 4 3 2 4 4 42%	2338
	21st to end	Night	18% 6% 3 6% 4 4 4 3 5% 6% 40%	1980
March	1st to 10th	Night	21% 9% 3 7% 4 3 4 3 4 5% 36%	3853
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 47%	3686
	21st to end	Night	17% 8% 2 6% 4 4 4 3 5% 6% 41%	3466
April	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 6% 6% 48%	1900
	11th to 20th	Night	10% 6% 2 5% 3 3 3 4 5% 7% 50%	1875
	21st to end	Night	10% 5% 2 5% 3 4 4 4 5% 7% 51%	1900
May	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 5% 9% 45%	3112
	11th to 20th	Night	15% 7% 2 6% 4 3 4 3 5% 9% 42%	2812
	21st to end	Night	6% 5% 1 5% 5% 5% 6% 5% 8% 11% 43%	3079
June	1st to 10th	Night	7% 5% 2 7% 5% 6% 5% 4 7% 10% 44%	1710
	11th to 20th	Night	4 3 1 4 4 4 4 4 8% 11% 53%	1522
	21st to end	Night	4 5% 2 4 5% 5% 6% 5% 6% 11% 49%	1520
July	1st to 10th	Night	5% 5% 2 7% 5% 6% 5% 6% 9% 13% 37%	2530
	11th to 20th	Night	6% 4 2 5% 5% 5% 6% 5% 9% 13% 40%	2673
	21st to end	Night	8% 8% 4 10% 6% 6% 7% 6% 8% 10% 28%	3435
August	1st to 10th	Night	3 2 1 3 3 4 3 4 6% 9% 63%	3144
	11th to 20th	Night	3 3 1 4 4 4 4 4 9% 12% 53%	3100
	21st to end	Night	5% 4 2 5% 5% 4 4 5% 6% 9% 52%	3476
September	1st to 10th	Night	9% 5% 2 5% 5% 5% 4 3 6% 8% 47%	1900
	11th to 20th	Night	10% 6% 3 5% 4 4 4 4 7% 8% 46%	2023
	21st to end	Night	7% 4 2 4 3 4 4 3 5% 7% 54%	2090
October	1st to 10th	Night	6% 3 2 4 3 3 3 3 5% 6% 62%	2090
	11th to 20th	Night	10% 5% 1 4 3 3 2 4 5% 7% 55%	2226
	21st to end	Night	14% 5% 2 4 2 3 2 2 3 5% 57%	2520
November	1st to 10th	Night	11% 5% 1 5% 5% 3 3 3 4 5% 54%	2536
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 3 6% 54%	2600
	21st to end	Night	13% 6% 2 5% 4 3 4 3 4 5% 50%	2600
December	1st to 10th	Night	19% 6% 2 4 3 3 3 3 4 6% 48%	4250
	11th to 20th	Night	17% 8% 2 6% 4 3 3 3 3 5% 46%	4822
	21st to end	Night	18% 8% 3 6% 4 3 4 3 5% 6% 41%	6646

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	43% 3 4 29% 18% 3	3707
	11th to 20th	Day	35% 4 5% 26% 26% 4	4167
	21st to end	Day	47% 3 5% 17% 20% 7%	4972
February	1st to 10th	Day	45% 5% 9% 19% 18% 5%	2000
	11th to 20th	Day	49% 3 5% 18% 18% 7%	1958
	21st to end	Day	46% 3 7% 15% 21% 8%	1650
March	1st to 10th	Day	51% 6% 8% 11% 17% 7%	3220
	11th to 20th	Day	39% 6% 9% 22% 18% 7%	3316
	21st to end	Day	43% 5% 9% 18% 19% 5%	4165
April	1st to 10th	Day	34% 6% 11% 22% 19% 8%	2280
	11th to 20th	Day	37% 5% 7% 20% 23% 8%	2254
	21st to end	Day	32% 6% 10% 25% 24% 3	2280
May	1st to 10th	Day	35% 8% 11% 29% 15% 3	3800
	11th to 20th	Day	38% 8% 13% 26% 13% 1	4085
	21st to end	Day	34% 9% 15% 28% 12% 1	4489
June	1st to 10th	Day	37% 9% 14% 23% 16% 2	2470
	11th to 20th	Day	28% 10% 15% 26% 19% 3	2651
	21st to end	Day	32% 10% 17% 23% 16% 3	2660
July	1st to 10th	Day	41% 11% 19% 21% 6% 1	4371
	11th to 20th	Day	38% 10% 24% 22% 5% 1	4211
	21st to end	Day	51% 10% 13% 20% 6%	3987
August	1st to 10th	Day	20% 9% 12% 33% 25% 1	3773
	11th to 20th	Day	30% 8% 17% 29% 14% 2	3765
	21st to end	Day	28% 8% 16% 23% 21% 4	4179
September	1st to 10th	Day	35% 9% 14% 20% 18% 5%	2280
	11th to 20th	Day	39% 8% 11% 15% 22% 6%	2138
	21st to end	Day	32% 6% 10% 23% 19% 10%	2090
October	1st to 10th	Day	20% 4 5% 15% 28% 29%	3480
	11th to 20th	Day	17% 3 4 15% 37% 23%	3217
	21st to end	Day	23% 3 5% 20% 37% 12%	3402
November	1st to 10th	Day	31% 4 7% 21% 27% 9%	1762
	11th to 20th	Day	33% 3 6% 18% 27% 12%	1800
	21st to end	Day	39% 6% 6% 18% 25% 7%	1800
December	1st to 10th	Day	43% 5% 6% 22% 21% 3	2930
	11th to 20th	Day	46% 3 8% 19% 18% 6%	3370
	21st to end	Day	42% 4 8% 23% 22% 1	4307
January	1st to 10th	Night	43% 3 5% 24% 23% 2	6242
	11th to 20th	Night	42% 3 3 20% 25% 7%	5788
	21st to end	Night	45% 2 4 17% 24% 7%	5984
February	1st to 10th	Night	44% 4 7% 17% 21% 8%	2400
	11th to 20th	Night	47% 4 5% 16% 18% 9%	2339
	21st to end	Night	44% 6% 6% 15% 19% 9%	1980
March	1st to 10th	Night	50% 5% 8% 13% 15% 9%	3853
	11th to 20th	Night	36% 6% 8% 20% 20% 10%	3686
	21st to end	Night	44% 5% 7% 19% 21% 4	3466
April	1st to 10th	Night	36% 5% 9% 21% 20% 9%	1900
	11th to 20th	Night	32% 5% 9% 21% 22% 11%	1875
	21st to end	Night	32% 6% 9% 24% 24% 5%	1900
May	1st to 10th	Night	35% 8% 13% 22% 19% 4	3142
	11th to 20th	Night	38% 8% 11% 26% 14% 2	2813
	21st to end	Night	30% 11% 13% 25% 18% 3	3087
June	1st to 10th	Night	34% 10% 15% 21% 18% 3	1710
	11th to 20th	Night	23% 8% 16% 29% 19% 6%	1522
	21st to end	Night	27% 10% 18% 21% 20% 4	1520
July	1st to 10th	Night	35% 10% 26% 20% 7% 2	2490
	11th to 20th	Night	32% 10% 24% 26% 7% 1	2625
	21st to end	Night	49% 10% 18% 15% 6% 1	3325
August	1st to 10th	Night	16% 8% 13% 35% 26% 3	3132
	11th to 20th	Night	22% 9% 20% 30% 16% 2	3130
	21st to end	Night	27% 7% 15% 21% 22% 7%	3476
September	1st to 10th	Night	33% 7% 15% 21% 17% 6%	1900
	11th to 20th	Night	34% 9% 12% 17% 21% 8%	2023
	21st to end	Night	28% 6% 10% 23% 23% 10%	2090
October	1st to 10th	Night	17% 4 6% 15% 27% 31%	3480
	11th to 20th	Night	19% 3 4 16% 30% 28%	3699
	21st to end	Night	22% 2 5% 21% 31% 20%	4144
November	1st to 10th	Night	32% 5% 7% 17% 26% 13%	2536
	11th to 20th	Night	33% 4 7% 18% 26% 13%	2600
	21st to end	Night	36% 4 6% 21% 23% 10%	2600
December	1st to 10th	Night	39% 3 6% 22% 26% 4	4250
	11th to 20th	Night	43% 3 8% 19% 24% 4	4824
	21st to end	Night	42% 5% 8% 22% 22% 2	6646

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% 21% 31% 20% 4	300
	11th to 20th	Day	23% 20% 34% 15% 7% 2	300
	21st to end	Day	21% 21% 35% 18% 4 2	330
February	1st to 10th	Day	20% 17% 35% 21% 7% 1	300
	11th to 20th	Day	34% 15% 31% 16% 4 1	300
	21st to end	Day	27% 22% 30% 18% 4	249
March	1st to 10th	Day	22% 17% 35% 19% 8%	300
	11th to 20th	Day	20% 17% 37% 21% 5%	278
	21st to end	Day	20% 11% 32% 26% 10% 1	506
April	1st to 10th	Day	15% 16% 28% 27% 12% 2	460
	11th to 20th	Day	18% 11% 30% 25% 12% 4	452
	21st to end	Day	11% 12% 26% 30% 19% 2	460
May	1st to 10th	Day	15% 8% 22% 26% 23% 6%	463
	11th to 20th	Day	10% 4 18% 31% 23% 14%	487
	21st to end	Day	9% 4 18% 32% 20% 17%	529
June	1st to 10th	Day	9% 4 13% 26% 29% 19%	490
	11th to 20th	Day	5% 2 9% 29% 34% 21%	671
	21st to end	Day	6% 2 11% 25% 35% 20%	680
July	1st to 10th	Day	5% 3 9% 21% 38% 24%	671
	11th to 20th	Day	7% 2 7% 21% 33% 31%	630
	21st to end	Day	5% 1 6% 21% 36% 31%	532
August	1st to 10th	Day	6% 2 7% 20% 36% 28%	486
	11th to 20th	Day	8% 1 7% 22% 37% 25%	479
	21st to end	Day	12% 2 6% 19% 34% 28%	539
September	1st to 10th	Day	9% 1 9% 23% 39% 17%	490
	11th to 20th	Day	12% 2 10% 21% 34% 21%	490
	21st to end	Day	11% 3 12% 26% 33% 15%	490
October	1st to 10th	Day	9% 3 15% 33% 29% 11%	490
	11th to 20th	Day	9% 5% 28% 30% 22% 5%	351
	21st to end	Day	16% 9% 22% 28% 18% 7%	330
November	1st to 10th	Day	16% 7% 24% 32% 17% 4	300
	11th to 20th	Day	14% 10% 32% 30% 12% 2	300
	21st to end	Day	14% 18% 27% 26% 11% 4	300
December	1st to 10th	Day	22% 13% 35% 21% 8% 1	300
	11th to 20th	Day	19% 15% 33% 20% 12%	300
	21st to end	Day	24% 13% 33% 22% 8% 1	330
January	1st to 10th	Night	22% 10% 34% 28% 6%	1000
	11th to 20th	Night	21% 13% 31% 27% 8%	953
	21st to end	Night	20% 12% 36% 29% 3	880
February	1st to 10th	Night	21% 12% 32% 31% 6%	800
	11th to 20th	Night	25% 13% 30% 27% 5%	783
	21st to end	Night	21% 14% 38% 24% 2	660
March	1st to 10th	Night	19% 11% 31% 33% 6%	800
	11th to 20th	Night	17% 9% 33% 32% 9%	705
	21st to end	Night	16% 8% 37% 27% 12%	627
April	1st to 10th	Night	12% 12% 31% 32% 12% 1	570
	11th to 20th	Night	12% 6% 32% 34% 14% 2	563
	21st to end	Night	11% 6% 31% 35% 14% 2	570
May	1st to 10th	Night	11% 4 25% 36% 22% 3	570
	11th to 20th	Night	11% 3 18% 37% 26% 5%	561
	21st to end	Night	7% 1 14% 35% 35% 8%	617
June	1st to 10th	Night	7% 2 9% 36% 38% 8%	570
	11th to 20th	Night	4 2 8% 26% 49% 10%	570
	21st to end	Night	5% 2 8% 29% 44% 12%	570
July	1st to 10th	Night	4 1 5% 24% 50% 15%	555
	11th to 20th	Night	5% 1 5% 26% 44% 19%	552
	21st to end	Night	5% 2 7% 20% 50% 15%	624
August	1st to 10th	Night	4 1 4 23% 49% 20%	567
	11th to 20th	Night	5% 1 4 27% 48% 15%	564
	21st to end	Night	8% 1 6% 26% 44% 15%	627
September	1st to 10th	Night	9% 1 7% 33% 43% 6%	570
	11th to 20th	Night	11% 2 12% 26% 43% 6%	693
	21st to end	Night	9% 2 11% 33% 41% 5%	760
October	1st to 10th	Night	6% 3 13% 37% 35% 6%	760
	11th to 20th	Night	12% 5% 21% 37% 23% 2	760
	21st to end	Night	15% 6% 21% 34% 22% 2	836
November	1st to 10th	Night	12% 5% 21% 38% 21% 3	780
	11th to 20th	Night	14% 7% 29% 35% 14% 1	800
	21st to end	Night	17% 9% 26% 34% 14% 1	800
December	1st to 10th	Night	20% 8% 30% 33% 9%	800
	11th to 20th	Night	22% 7% 29% 31% 10%	800
	21st to end	Night	24% 10% 30% 30% 7%	945

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