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 2005-11-05 to 2024-10-14.

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% 43%	100757
	Night	12% 6% 2 6% 4 4 4 4 5% 7% 46%	97759
Transparency	Day	37% 7% 11% 22% 18% 4	100565
	Night	36% 6% 10% 20% 20% 6%	97626
Seeing	Day	13% 7% 19% 24% 23% 13%	14916
	Night	14% 6% 22% 31% 23% 5%	24545

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	15% 12% 2 6% 4 3 3 3 4 6% 41%	2588
	11th to 20th	Day	16% 7% 3 5% 4 3 3 3 4 5% 46%	2911
	21st to end	Day	21% 11% 2 5% 3 3 3 2 3 6% 43%	3476
February	1st to 10th	Day	16% 10% 3 7% 4 3 3 3 5% 7% 39%	1900
	11th to 20th	Day	20% 8% 2 7% 5% 4 3 3 4 5% 41%	1848
	21st to end	Day	19% 10% 3 6% 4 4 2 3 4 5% 40%	1570
March	1st to 10th	Day	21% 10% 3 7% 4 4 4 3 5% 6% 32%	3120
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 6% 8% 41%	3306
	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 5% 3 3 4 3 4 6% 56%	1881
	21st to end	Day	13% 4 2 5% 3 3 4 3 5% 4 54%	1967
November	1st to 10th	Day	10% 4 1 5% 4 3 3 2 5% 5% 57%	1672
	11th to 20th	Day	12% 5% 1 5% 4 3 3 2 4 5% 54%	1710
	21st to end	Day	16% 6% 3 5% 3 3 4 4 5% 7% 45%	1710
December	1st to 10th	Day	16% 9% 3 6% 5% 4 4 4 5% 5% 39%	1710
	11th to 20th	Day	14% 10% 3 6% 4 4 4 2 4 5% 44%	2153
	21st to end	Day	14% 8% 3 8% 5% 4 4 3 5% 6% 39%	3017
January	1st to 10th	Night	18% 10% 2 6% 4 3 4 2 4 5% 41%	4364
	11th to 20th	Night	16% 9% 3 7% 3 3 3 2 3 4 46%	4050
	21st to end	Night	21% 10% 3 6% 3 3 2 2 2 3 44%	4180
February	1st to 10th	Night	18% 8% 3 6% 4 3 4 2 5% 5% 42%	2280
	11th to 20th	Night	20% 9% 2 5% 4 3 3 3 4 4 44%	2218
	21st to end	Night	19% 7% 3 6% 4 4 4 3 6% 6% 38%	1884
March	1st to 10th	Night	21% 10% 3 7% 4 3 3 3 4 5% 36%	3733
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 47%	3667
	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	11% 5% 1 5% 3 3 2 4 6% 7% 54%	2149
	21st to end	Night	15% 5% 2 5% 2 3 2 2 3 5% 56%	2387
November	1st to 10th	Night	10% 5% 1 5% 5% 3 3 3 4 5% 57%	2406
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 3 6% 54%	2470
	21st to end	Night	14% 6% 2 5% 4 3 4 3 4 6% 49%	2470
December	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 4 6% 44%	2470
	11th to 20th	Night	16% 6% 2 5% 4 4 3 3 4 5% 48%	3042
	21st to end	Night	16% 8% 3 7% 4 4 4 4 6% 7% 36%	4632

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	44% 4 6% 21% 21% 4	2589
	11th to 20th	Day	41% 4 7% 25% 21% 3	2911
	21st to end	Day	45% 4 7% 19% 20% 6%	3476
February	1st to 10th	Day	45% 5% 9% 19% 17% 5%	1900
	11th to 20th	Day	48% 3 5% 18% 19% 7%	1858
	21st to end	Day	47% 3 7% 14% 21% 7%	1570
March	1st to 10th	Day	51% 6% 8% 11% 16% 7%	3120
	11th to 20th	Day	39% 6% 9% 22% 18% 7%	3306
	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	29% 4 8% 21% 25% 13%	2090
	11th to 20th	Day	29% 4 7% 21% 29% 10%	1881
	21st to end	Day	32% 4 6% 18% 30% 9%	1967
November	1st to 10th	Day	29% 4 7% 21% 28% 10%	1672
	11th to 20th	Day	33% 3 6% 19% 26% 12%	1710
	21st to end	Day	39% 6% 6% 17% 24% 7%	1710
December	1st to 10th	Day	46% 6% 8% 16% 18% 6%	1710
	11th to 20th	Day	45% 4 6% 18% 23% 4	2150
	21st to end	Day	43% 6% 8% 20% 21% 2	3017
January	1st to 10th	Night	44% 4 6% 16% 26% 3	4364
	11th to 20th	Night	43% 4 4 22% 20% 8%	4050
	21st to end	Night	48% 3 4 19% 22% 5%	4180
February	1st to 10th	Night	44% 4 7% 17% 21% 7%	2280
	11th to 20th	Night	45% 4 5% 16% 19% 10%	2219
	21st to end	Night	46% 6% 6% 14% 19% 9%	1884
March	1st to 10th	Night	49% 5% 8% 13% 15% 9%	3733
	11th to 20th	Night	36% 6% 8% 20% 20% 10%	3667
	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	23% 4 9% 22% 24% 18%	2090
	11th to 20th	Night	30% 5% 7% 19% 28% 12%	2154
	21st to end	Night	33% 3 5% 16% 28% 14%	2387
November	1st to 10th	Night	30% 5% 7% 17% 28% 13%	2406
	11th to 20th	Night	33% 4 7% 18% 26% 12%	2470
	21st to end	Night	36% 4 6% 21% 23% 10%	2470
December	1st to 10th	Night	42% 5% 9% 19% 21% 5%	2470
	11th to 20th	Night	39% 3 7% 18% 26% 6%	3044
	21st to end	Night	43% 6% 9% 17% 22% 3	4632

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% 21% 30% 20% 5%	270
	11th to 20th	Day	23% 20% 33% 13% 8% 2	270
	21st to end	Day	21% 21% 35% 18% 4 2	297
February	1st to 10th	Day	20% 17% 34% 20% 7% 1	270
	11th to 20th	Day	33% 14% 31% 17% 4 1	270
	21st to end	Day	29% 22% 30% 16% 4	225
March	1st to 10th	Day	20% 16% 34% 21% 9%	270
	11th to 20th	Day	20% 17% 37% 20% 5%	275
	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	10% 6% 27% 30% 22% 5%	330
	21st to end	Day	18% 10% 24% 27% 16% 5%	297
November	1st to 10th	Day	16% 6% 24% 32% 19% 4	270
	11th to 20th	Day	13% 10% 33% 31% 11% 2	270
	21st to end	Day	15% 17% 27% 27% 10% 4	270
December	1st to 10th	Day	22% 12% 34% 21% 8% 1	270
	11th to 20th	Day	18% 15% 34% 20% 13%	270
	21st to end	Day	23% 13% 34% 22% 7% 1	297
January	1st to 10th	Night	22% 10% 33% 28% 6%	950
	11th to 20th	Night	22% 12% 31% 27% 9%	906
	21st to end	Night	20% 12% 36% 29% 3	836
February	1st to 10th	Night	21% 11% 32% 30% 6%	760
	11th to 20th	Night	24% 13% 31% 27% 5%	743
	21st to end	Night	21% 15% 39% 23% 3	628
March	1st to 10th	Night	19% 11% 31% 33% 6%	760
	11th to 20th	Night	17% 9% 34% 31% 9%	698
	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	13% 5% 21% 36% 23% 2	735
	21st to end	Night	16% 6% 21% 33% 21% 2	792
November	1st to 10th	Night	12% 5% 20% 38% 22% 3	740
	11th to 20th	Night	14% 7% 29% 35% 14% 1	760
	21st to end	Night	17% 8% 26% 34% 14% 1	760
December	1st to 10th	Night	20% 8% 30% 33% 9%	760
	11th to 20th	Night	22% 7% 30% 30% 10%	760
	21st to end	Night	24% 10% 30% 30% 7%	897

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