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 2024-02-15.

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	12% 6% 2 5% 5% 3 5% 5% 6% 8% 44%	78901
	Night	12% 6% 2 5% 4 3 4 4 5% 7% 47%	80179
Transparency	Day	38% 7% 11% 21% 19% 5%	78626
	Night	36% 6% 10% 20% 21% 7%	79974
Seeing	Day	13% 7% 19% 24% 23% 13%	13945
	Night	14% 6% 22% 31% 23% 5%	23758

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% 3 5% 5% 2 3 3 4 6% 41%	2588
	11th to 20th	Day	17% 7% 3 5% 5% 3 3 4 4 5% 46%	2911
	21st to end	Day	21% 10% 2 4 3 2 3 3 3 6% 43%	3476
February	1st to 10th	Day	17% 9% 4 6% 4 2 3 3 5% 7% 39%	1900
	11th to 20th	Day	21% 7% 3 6% 5% 3 3 3 4 5% 40%	1788
	21st to end	Day	19% 10% 3 6% 5% 3 2 3 4 5% 40%	1480
March	1st to 10th	Day	14% 7% 3 6% 5% 3 3 4 5% 6% 43%	1800
	11th to 20th	Day	15% 7% 3 6% 5% 3 5% 5% 6% 7% 38%	1876
	21st to end	Day	13% 6% 3 5% 6% 3 5% 5% 5% 7% 43%	2376
April	1st to 10th	Day	12% 4 2 4 5% 2 4 5% 5% 8% 49%	2160
	11th to 20th	Day	14% 5% 2 5% 4 2 3 4 6% 6% 48%	2134
	21st to end	Day	9% 4 2 4 5% 3 5% 5% 5% 8% 50%	2160
May	1st to 10th	Day	13% 4 2 4 4 2 5% 4 5% 8% 48%	2160
	11th to 20th	Day	9% 5% 3 5% 6% 4 5% 5% 7% 9% 43%	2301
	21st to end	Day	8% 5% 2 5% 5% 4 6% 6% 6% 10% 44%	2531
June	1st to 10th	Day	8% 5% 2 6% 6% 5% 6% 6% 6% 12% 38%	2340
	11th to 20th	Day	5% 3 2 5% 5% 3 5% 7% 8% 10% 47%	2511
	21st to end	Day	5% 4 2 6% 5% 4 6% 5% 7% 11% 46%	2520
July	1st to 10th	Day	8% 5% 3 6% 7% 4 8% 7% 7% 10% 35%	2520
	11th to 20th	Day	8% 3 3 5% 7% 5% 6% 8% 8% 11% 36%	2436
	21st to end	Day	5% 4 2 6% 7% 5% 7% 7% 9% 10% 37%	2350
August	1st to 10th	Day	5% 3 2 5% 6% 4 7% 9% 9% 13% 37%	2148
	11th to 20th	Day	8% 5% 2 6% 7% 4 7% 7% 7% 11% 35%	2125
	21st to end	Day	12% 5% 2 5% 5% 4 6% 6% 7% 9% 40%	2376
September	1st to 10th	Day	9% 5% 2 5% 4 3 6% 5% 7% 8% 45%	2160
	11th to 20th	Day	11% 5% 3 5% 5% 3 5% 5% 6% 7% 44%	2026
	21st to end	Day	10% 4 2 4 4 3 4 5% 5% 7% 52%	1980
October	1st to 10th	Day	9% 4 2 4 4 2 4 4 4 6% 56%	1980
	11th to 20th	Day	9% 4 2 4 4 2 3 4 4 7% 56%	1849
	21st to end	Day	13% 4 2 4 4 2 4 4 5% 4 54%	1967
November	1st to 10th	Day	10% 4 2 4 5% 2 4 3 5% 5% 57%	1672
	11th to 20th	Day	13% 4 2 5% 4 3 3 3 4 5% 55%	1710
	21st to end	Day	16% 5% 3 5% 4 2 4 5% 5% 7% 45%	1710
December	1st to 10th	Day	16% 8% 3 6% 6% 2 5% 4 5% 5% 39%	1710
	11th to 20th	Day	14% 9% 4 6% 4 3 5% 3 4 5% 44%	2153
	21st to end	Day	14% 8% 3 7% 6% 4 3 5% 4 6% 39%	3017
January	1st to 10th	Night	18% 10% 3 5% 5% 2 4 3 4 5% 41%	4364
	11th to 20th	Night	16% 8% 3 6% 4 2 3 3 3 4 46%	4050
	21st to end	Night	21% 10% 3 6% 4 2 3 3 2 3 44%	4180
February	1st to 10th	Night	18% 7% 3 6% 4 3 4 3 4 6% 42%	2280
	11th to 20th	Night	20% 8% 2 5% 5% 3 3 3 4 4 44%	2153
	21st to end	Night	20% 6% 3 6% 5% 3 4 5% 5% 6% 37%	1776
March	1st to 10th	Night	15% 6% 3 5% 4 3 4 4 4 6% 46%	2160
	11th to 20th	Night	15% 6% 3 6% 5% 3 4 5% 5% 6% 42%	2097
	21st to end	Night	13% 6% 3 5% 5% 3 4 4 5% 8% 45%	1980
April	1st to 10th	Night	11% 5% 3 4 4 3 5% 5% 5% 6% 48%	1800
	11th to 20th	Night	11% 6% 2 5% 4 2 4 4 6% 7% 51%	1775
	21st to end	Night	10% 5% 2 4 4 3 4 5% 4 8% 51%	1800
May	1st to 10th	Night	10% 5% 3 4 4 2 4 4 5% 8% 53%	1796
	11th to 20th	Night	9% 5% 3 6% 5% 3 4 5% 6% 10% 45%	1592
	21st to end	Night	5% 4 1 4 5% 4 5% 6% 8% 12% 46%	1744
June	1st to 10th	Night	6% 4 2 5% 6% 4 6% 5% 7% 9% 45%	1620
	11th to 20th	Night	4 3 1 3 4 2 4 5% 8% 12% 54%	1442
	21st to end	Night	4 4 2 3 5% 3 6% 7% 6% 12% 49%	1440
July	1st to 10th	Night	4 4 2 5% 6% 3 6% 8% 9% 12% 41%	1440
	11th to 20th	Night	4 2 2 4 5% 4 6% 6% 7% 10% 49%	1515
	21st to end	Night	4 4 2 5% 6% 4 7% 8% 8% 11% 42%	1968
August	1st to 10th	Night	4 3 1 3 5% 3 6% 8% 9% 11% 47%	1790
	11th to 20th	Night	5% 4 2 5% 6% 4 5% 6% 9% 11% 43%	1776
	21st to end	Night	8% 5% 2 5% 7% 3 5% 6% 5% 9% 45%	1980
September	1st to 10th	Night	9% 5% 3 4 5% 4 4 4 6% 8% 48%	1800
	11th to 20th	Night	9% 5% 3 5% 4 3 4 6% 7% 8% 47%	1916
	21st to end	Night	7% 4 2 4 4 3 5% 4 5% 7% 55%	1980
October	1st to 10th	Night	6% 3 2 4 3 2 4 4 4 6% 62%	1980
	11th to 20th	Night	11% 5% 2 4 4 3 2 4 6% 7% 53%	2108
	21st to end	Night	15% 5% 2 4 3 2 2 3 3 5% 56%	2387
November	1st to 10th	Night	10% 4 2 4 5% 3 3 4 4 5% 57%	2406
	11th to 20th	Night	13% 5% 2 4 4 2 3 3 3 5% 54%	2470
	21st to end	Night	14% 6% 3 4 4 2 4 4 4 6% 49%	2470
December	1st to 10th	Night	17% 7% 2 5% 4 3 4 4 4 6% 44%	2470
	11th to 20th	Night	16% 6% 2 4 5% 2 4 3 4 5% 49%	3042
	21st to end	Night	16% 8% 3 6% 5% 3 4 5% 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	49% 3 5% 18% 18% 6%	1798
	21st to end	Day	48% 4 7% 14% 21% 7%	1480
March	1st to 10th	Day	42% 5% 7% 17% 18% 11%	1800
	11th to 20th	Day	44% 6% 11% 16% 19% 4	1876
	21st to end	Day	41% 5% 10% 24% 18% 3	2376
April	1st to 10th	Day	33% 6% 12% 23% 20% 7%	2160
	11th to 20th	Day	36% 5% 7% 21% 24% 7%	2134
	21st to end	Day	32% 6% 10% 25% 24% 3	2160
May	1st to 10th	Day	36% 5% 10% 24% 21% 4	2160
	11th to 20th	Day	36% 9% 13% 23% 17% 1	2305
	21st to end	Day	34% 9% 14% 26% 15% 2	2531
June	1st to 10th	Day	37% 9% 14% 23% 16% 2	2340
	11th to 20th	Day	27% 10% 16% 26% 18% 2	2511
	21st to end	Day	32% 10% 16% 23% 16% 4	2520
July	1st to 10th	Day	39% 13% 17% 19% 9% 3	2451
	11th to 20th	Day	36% 13% 21% 19% 9% 2	2361
	21st to end	Day	36% 13% 19% 22% 9% 1	2219
August	1st to 10th	Day	32% 14% 19% 25% 9% 1	2136
	11th to 20th	Day	40% 12% 16% 19% 11% 2	2125
	21st to end	Day	37% 11% 17% 21% 10% 5%	2376
September	1st to 10th	Day	34% 9% 14% 20% 18% 5%	2160
	11th to 20th	Day	38% 8% 11% 15% 22% 7%	2026
	21st to end	Day	31% 6% 10% 23% 20% 10%	1980
October	1st to 10th	Day	30% 4 8% 22% 25% 11%	1980
	11th to 20th	Day	29% 4 7% 21% 29% 9%	1849
	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% 9%	2154
	21st to end	Night	46% 6% 6% 14% 18% 9%	1776
March	1st to 10th	Night	39% 4 8% 16% 19% 13%	2160
	11th to 20th	Night	41% 7% 10% 17% 18% 7%	2097
	21st to end	Night	38% 6% 8% 22% 20% 5%	1980
April	1st to 10th	Night	36% 5% 9% 22% 19% 8%	1800
	11th to 20th	Night	32% 5% 9% 22% 22% 11%	1775
	21st to end	Night	32% 6% 9% 24% 24% 5%	1800
May	1st to 10th	Night	30% 5% 11% 24% 24% 6%	1796
	11th to 20th	Night	34% 9% 14% 22% 18% 3	1593
	21st to end	Night	27% 10% 15% 24% 21% 3	1745
June	1st to 10th	Night	33% 10% 15% 22% 19% 2	1620
	11th to 20th	Night	22% 8% 17% 29% 18% 5%	1442
	21st to end	Night	27% 10% 18% 22% 19% 4	1440
July	1st to 10th	Night	30% 13% 20% 23% 11% 3	1400
	11th to 20th	Night	27% 13% 21% 26% 12% 2	1467
	21st to end	Night	29% 13% 25% 22% 9% 2	1858
August	1st to 10th	Night	25% 12% 21% 30% 11% 1	1778
	11th to 20th	Night	30% 14% 20% 21% 11% 4	1776
	21st to end	Night	35% 10% 16% 21% 12% 6%	1980
September	1st to 10th	Night	33% 7% 15% 22% 18% 5%	1800
	11th to 20th	Night	32% 9% 12% 17% 21% 8%	1916
	21st to end	Night	28% 6% 10% 22% 24% 10%	1980
October	1st to 10th	Night	24% 4 9% 23% 24% 16%	1980
	11th to 20th	Night	30% 5% 8% 19% 28% 11%	2108
	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% 15% 31% 17% 4	252
	21st to end	Day	27% 20% 31% 18% 4	198
March	1st to 10th	Day	19% 16% 34% 22% 9%	240
	11th to 20th	Day	21% 18% 36% 19% 5%	244
	21st to end	Day	19% 11% 31% 26% 11% 1	462
April	1st to 10th	Day	13% 15% 28% 29% 14% 2	420
	11th to 20th	Day	18% 10% 29% 25% 14% 4	412
	21st to end	Day	11% 11% 26% 30% 20% 2	420
May	1st to 10th	Day	16% 8% 21% 26% 24% 6%	423
	11th to 20th	Day	10% 4 18% 30% 24% 15%	447
	21st to end	Day	9% 4 15% 32% 21% 19%	485
June	1st to 10th	Day	10% 4 11% 24% 31% 21%	450
	11th to 20th	Day	5% 1 9% 28% 35% 21%	621
	21st to end	Day	6% 2 12% 26% 34% 20%	630
July	1st to 10th	Day	5% 3 8% 20% 38% 25%	621
	11th to 20th	Day	7% 2 7% 21% 31% 32%	582
	21st to end	Day	3 1 6% 20% 37% 33%	488
August	1st to 10th	Day	7% 2 7% 19% 35% 31%	446
	11th to 20th	Day	8% 1 6% 20% 38% 27%	439
	21st to end	Day	13% 2 6% 19% 33% 28%	495
September	1st to 10th	Day	9% 2 8% 22% 41% 18%	450
	11th to 20th	Day	10% 2 10% 20% 35% 23%	450
	21st to end	Day	10% 3 11% 26% 36% 15%	450
October	1st to 10th	Day	10% 3 15% 32% 28% 12%	450
	11th to 20th	Day	10% 6% 28% 29% 21% 5%	319
	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% 30% 28% 5%	722
	21st to end	Night	21% 13% 38% 24% 3	592
March	1st to 10th	Night	18% 10% 32% 34% 6%	720
	11th to 20th	Night	18% 9% 32% 31% 10%	662
	21st to end	Night	16% 7% 37% 28% 12%	594
April	1st to 10th	Night	12% 11% 31% 33% 13% 1	540
	11th to 20th	Night	12% 6% 32% 34% 15% 2	533
	21st to end	Night	11% 6% 31% 36% 15% 2	540
May	1st to 10th	Night	11% 4 25% 36% 22% 3	540
	11th to 20th	Night	11% 3 18% 37% 26% 5%	531
	21st to end	Night	7% 1 12% 35% 37% 8%	584
June	1st to 10th	Night	6% 2 9% 36% 39% 8%	540
	11th to 20th	Night	4 2 8% 26% 50% 10%	540
	21st to end	Night	6% 1 8% 29% 43% 13%	540
July	1st to 10th	Night	4 1 5% 24% 50% 16%	525
	11th to 20th	Night	4 5% 27% 43% 20%	522
	21st to end	Night	5% 1 7% 20% 52% 16%	591
August	1st to 10th	Night	5% 1 4 21% 49% 20%	537
	11th to 20th	Night	6% 1 3 26% 48% 15%	534
	21st to end	Night	9% 1 6% 25% 44% 15%	594
September	1st to 10th	Night	9% 1 6% 32% 44% 6%	540
	11th to 20th	Night	10% 1 12% 26% 44% 6%	656
	21st to end	Night	9% 1 11% 33% 42% 4	720
October	1st to 10th	Night	7% 3 13% 36% 35% 6%	720
	11th to 20th	Night	13% 5% 21% 36% 23% 2	720
	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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