Colby Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Colby Observing Site clear sky chart from 2005-11-06 to 2024-01-12.

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	14% 5% 2 5% 5% 3 5% 5% 6% 8% 43%	74907
	Night	15% 5% 2 5% 5% 3 4 5% 5% 7% 45%	74810
Transparency	Day	39% 6% 12% 22% 17% 4	74630
	Night	39% 5% 11% 21% 19% 5%	74606
Seeing	Day	18% 6% 19% 26% 20% 11%	14041
	Night	17% 5% 20% 33% 21% 3	24174

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% 5% 2 4 4 3 4 3 4 7% 47%	1706
	11th to 20th	Day	17% 5% 2 5% 4 2 4 3 5% 6% 46%	1629
	21st to end	Day	16% 4 2 3 3 2 4 4 4 6% 51%	1782
February	1st to 10th	Day	15% 6% 2 4 5% 2 3 5% 6% 8% 43%	1620
	11th to 20th	Day	18% 7% 2 5% 4 3 4 4 4 6% 43%	1744
	21st to end	Day	19% 5% 2 4 5% 3 4 5% 5% 6% 42%	1488
March	1st to 10th	Day	15% 4 1 4 3 2 4 4 4 7% 51%	1980
	11th to 20th	Day	18% 5% 2 6% 5% 3 4 5% 5% 7% 39%	1980
	21st to end	Day	18% 6% 3 4 5% 3 5% 4 4 6% 42%	2178
April	1st to 10th	Day	11% 4 2 3 4 3 5% 5% 6% 8% 50%	1980
	11th to 20th	Day	17% 4 2 4 3 3 4 4 5% 7% 47%	2127
	21st to end	Day	14% 4 2 4 5% 3 5% 4 5% 7% 47%	2340
May	1st to 10th	Day	11% 5% 2 5% 4 3 6% 6% 6% 9% 43%	2340
	11th to 20th	Day	16% 6% 3 6% 5% 3 6% 6% 5% 8% 38%	2301
	21st to end	Day	13% 5% 2 5% 5% 3 6% 5% 6% 10% 40%	2531
June	1st to 10th	Day	12% 5% 3 5% 5% 5% 7% 6% 7% 9% 37%	2340
	11th to 20th	Day	8% 4 2 6% 7% 4 7% 6% 6% 11% 38%	2340
	21st to end	Day	8% 3 2 5% 7% 4 7% 8% 8% 12% 36%	2340
July	1st to 10th	Day	10% 5% 3 6% 7% 4 8% 7% 8% 11% 31%	2340
	11th to 20th	Day	9% 6% 3 7% 6% 5% 7% 7% 9% 11% 30%	2340
	21st to end	Day	9% 5% 2 5% 6% 5% 8% 7% 9% 10% 33%	2547
August	1st to 10th	Day	13% 6% 2 6% 7% 4 8% 7% 8% 10% 29%	2327
	11th to 20th	Day	13% 5% 3 6% 7% 5% 6% 7% 8% 10% 30%	2303
	21st to end	Day	11% 5% 3 6% 6% 4 5% 6% 7% 10% 36%	2443
September	1st to 10th	Day	12% 5% 2 5% 6% 3 5% 5% 6% 8% 42%	1980
	11th to 20th	Day	17% 5% 2 5% 5% 3 5% 5% 5% 7% 41%	1980
	21st to end	Day	20% 5% 1 5% 5% 3 4 5% 5% 6% 40%	1980
October	1st to 10th	Day	16% 4 3 4 3 2 3 2 4 5% 54%	1892
	11th to 20th	Day	15% 4 2 3 3 2 3 4 4 6% 54%	1800
	21st to end	Day	15% 4 2 3 3 1 3 3 4 4 56%	1980
November	1st to 10th	Day	13% 5% 1 3 3 2 3 3 3 6% 58%	1842
	11th to 20th	Day	14% 4 2 4 4 2 3 3 4 7% 54%	1710
	21st to end	Day	15% 4 1 4 3 2 4 4 4 6% 52%	1710
December	1st to 10th	Day	19% 7% 3 5% 5% 3 4 4 5% 5% 41%	1710
	11th to 20th	Day	15% 4 1 3 3 2 3 4 5% 7% 52%	2153
	21st to end	Day	20% 6% 2 4 4 3 4 4 4 7% 43%	3124
January	1st to 10th	Night	16% 5% 2 4 4 3 3 4 4 6% 50%	2459
	11th to 20th	Night	17% 6% 3 4 4 3 4 4 5% 5% 44%	2354
	21st to end	Night	16% 5% 2 3 4 3 3 4 5% 6% 49%	2574
February	1st to 10th	Night	16% 4 3 5% 5% 3 4 5% 5% 7% 45%	2340
	11th to 20th	Night	19% 7% 2 4 4 2 4 3 5% 5% 46%	2098
	21st to end	Night	18% 6% 3 5% 4 3 3 4 5% 6% 44%	1768
March	1st to 10th	Night	13% 5% 2 5% 4 2 4 4 4 6% 50%	1980
	11th to 20th	Night	17% 6% 3 5% 5% 2 5% 4 4 7% 42%	1980
	21st to end	Night	16% 6% 3 5% 5% 3 5% 4 5% 6% 42%	2178
April	1st to 10th	Night	12% 5% 2 4 4 3 4 5% 5% 7% 51%	1980
	11th to 20th	Night	15% 5% 2 4 4 3 3 3 5% 8% 49%	1795
	21st to end	Night	14% 4 2 4 4 2 4 4 5% 7% 50%	1620
May	1st to 10th	Night	13% 7% 2 4 5% 3 5% 5% 5% 7% 45%	1620
	11th to 20th	Night	15% 6% 3 6% 6% 3 6% 5% 5% 7% 36%	1592
	21st to end	Night	14% 6% 2 5% 6% 3 5% 5% 6% 9% 40%	1744
June	1st to 10th	Night	10% 4 3 5% 5% 3 6% 7% 8% 12% 36%	1620
	11th to 20th	Night	10% 5% 2 6% 5% 3 5% 6% 6% 10% 42%	1620
	21st to end	Night	7% 4 2 6% 7% 4 6% 7% 8% 12% 38%	1620
July	1st to 10th	Night	10% 6% 3 6% 6% 5% 5% 5% 7% 11% 35%	1620
	11th to 20th	Night	10% 5% 2 6% 7% 3 6% 7% 7% 12% 34%	1620
	21st to end	Night	7% 5% 2 6% 7% 4 7% 8% 9% 12% 34%	1771
August	1st to 10th	Night	15% 6% 2 6% 7% 4 5% 6% 8% 9% 32%	1611
	11th to 20th	Night	13% 6% 2 7% 7% 4 6% 6% 7% 9% 33%	1598
	21st to end	Night	11% 5% 2 6% 6% 4 6% 6% 8% 10% 36%	1889
September	1st to 10th	Night	12% 5% 2 6% 5% 3 5% 5% 5% 7% 44%	1980
	11th to 20th	Night	16% 6% 2 6% 7% 3 4 5% 6% 6% 39%	1980
	21st to end	Night	19% 6% 2 5% 4 4 5% 4 4 6% 42%	1980
October	1st to 10th	Night	18% 5% 1 3 4 2 3 5% 4 6% 49%	2066
	11th to 20th	Night	16% 4 2 4 4 2 3 4 4 6% 52%	2160
	21st to end	Night	14% 4 1 3 3 2 3 3 4 5% 57%	2376
November	1st to 10th	Night	15% 5% 2 5% 4 2 3 3 3 3 55%	2233
	11th to 20th	Night	14% 5% 2 4 3 2 3 3 4 6% 54%	2470
	21st to end	Night	14% 4 2 4 3 3 3 4 5% 7% 50%	2470
December	1st to 10th	Night	18% 6% 3 5% 4 3 3 4 5% 5% 44%	2470
	11th to 20th	Night	16% 6% 2 4 3 2 4 4 4 5% 51%	3042
	21st to end	Night	21% 5% 2 4 4 3 4 4 5% 7% 41%	4532

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	38% 4 8% 24% 21% 5%	1707
	11th to 20th	Day	40% 3 8% 21% 22% 5%	1629
	21st to end	Day	36% 3 6% 26% 22% 7%	1782
February	1st to 10th	Day	40% 4 11% 22% 19% 5%	1620
	11th to 20th	Day	42% 4 9% 22% 20% 4	1754
	21st to end	Day	43% 5% 9% 18% 18% 8%	1488
March	1st to 10th	Day	35% 3 8% 22% 25% 7%	1980
	11th to 20th	Day	43% 5% 10% 18% 17% 6%	1980
	21st to end	Day	44% 5% 10% 22% 16% 3	2178
April	1st to 10th	Day	31% 4 11% 25% 23% 6%	1980
	11th to 20th	Day	36% 4 9% 20% 23% 8%	2127
	21st to end	Day	37% 4 12% 26% 17% 4	2340
May	1st to 10th	Day	36% 6% 16% 23% 15% 3	2340
	11th to 20th	Day	44% 7% 15% 20% 11% 2	2305
	21st to end	Day	39% 6% 15% 26% 12% 1	2531
June	1st to 10th	Day	40% 7% 17% 23% 12% 1	2340
	11th to 20th	Day	38% 8% 18% 20% 14% 2	2340
	21st to end	Day	36% 9% 18% 20% 15% 1	2340
July	1st to 10th	Day	43% 9% 18% 16% 11% 3	2275
	11th to 20th	Day	42% 10% 19% 17% 10% 1	2271
	21st to end	Day	40% 9% 20% 21% 9% 2	2405
August	1st to 10th	Day	45% 10% 17% 19% 7% 1	2314
	11th to 20th	Day	46% 9% 17% 18% 8% 1	2303
	21st to end	Day	40% 7% 18% 23% 8% 3	2443
September	1st to 10th	Day	39% 7% 13% 22% 16% 3	1980
	11th to 20th	Day	42% 6% 11% 20% 18% 3	1980
	21st to end	Day	44% 4 9% 21% 17% 4	1980
October	1st to 10th	Day	33% 3 10% 22% 28% 4	1892
	11th to 20th	Day	31% 4 9% 27% 22% 8%	1800
	21st to end	Day	32% 3 7% 24% 25% 9%	1980
November	1st to 10th	Day	31% 3 8% 21% 29% 7%	1842
	11th to 20th	Day	33% 3 7% 23% 27% 8%	1710
	21st to end	Day	34% 3 8% 24% 26% 6%	1710
December	1st to 10th	Day	45% 5% 8% 21% 17% 5%	1710
	11th to 20th	Day	31% 4 8% 23% 24% 11%	2150
	21st to end	Day	40% 5% 7% 26% 18% 3	3124
January	1st to 10th	Night	36% 3 10% 20% 23% 7%	2459
	11th to 20th	Night	42% 4 8% 18% 22% 5%	2354
	21st to end	Night	36% 3 6% 23% 26% 5%	2574
February	1st to 10th	Night	38% 5% 10% 22% 20% 5%	2340
	11th to 20th	Night	42% 3 8% 19% 21% 6%	2099
	21st to end	Night	42% 3 10% 19% 20% 6%	1768
March	1st to 10th	Night	36% 4 8% 19% 24% 9%	1980
	11th to 20th	Night	43% 4 8% 22% 18% 4	1980
	21st to end	Night	43% 5% 9% 22% 17% 4	2178
April	1st to 10th	Night	33% 4 11% 23% 23% 5%	1980
	11th to 20th	Night	35% 4 9% 23% 21% 8%	1795
	21st to end	Night	35% 4 11% 26% 20% 4	1620
May	1st to 10th	Night	38% 6% 13% 26% 15% 2	1620
	11th to 20th	Night	47% 6% 13% 20% 13% 1	1593
	21st to end	Night	39% 8% 15% 24% 13% 1	1745
June	1st to 10th	Night	37% 9% 18% 23% 11% 1	1620
	11th to 20th	Night	38% 7% 20% 23% 10% 1	1620
	21st to end	Night	36% 9% 17% 20% 12% 4	1620
July	1st to 10th	Night	42% 9% 19% 18% 10% 3	1575
	11th to 20th	Night	41% 11% 20% 19% 7% 3	1566
	21st to end	Night	36% 12% 22% 22% 7% 1	1672
August	1st to 10th	Night	45% 10% 20% 18% 8%	1600
	11th to 20th	Night	44% 9% 17% 18% 10% 1	1598
	21st to end	Night	41% 9% 18% 22% 9% 2	1889
September	1st to 10th	Night	39% 7% 15% 21% 14% 4	1980
	11th to 20th	Night	44% 7% 10% 20% 17% 2	1980
	21st to end	Night	44% 6% 9% 19% 18% 4	1980
October	1st to 10th	Night	36% 5% 9% 23% 23% 4	2066
	11th to 20th	Night	34% 4 8% 27% 21% 7%	2160
	21st to end	Night	32% 3 5% 22% 28% 10%	2376
November	1st to 10th	Night	34% 4 7% 19% 26% 10%	2233
	11th to 20th	Night	33% 3 7% 19% 26% 11%	2470
	21st to end	Night	34% 3 7% 24% 26% 6%	2470
December	1st to 10th	Night	43% 3 10% 20% 19% 5%	2470
	11th to 20th	Night	36% 3 8% 18% 27% 9%	3044
	21st to end	Night	42% 5% 9% 21% 21% 2	4532

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% 14% 31% 24% 7%	270
	11th to 20th	Day	26% 11% 29% 27% 7%	243
	21st to end	Day	22% 16% 35% 21% 6%	264
February	1st to 10th	Day	24% 14% 35% 21% 7%	240
	11th to 20th	Day	33% 13% 34% 17% 3	240
	21st to end	Day	31% 14% 34% 17% 4	206
March	1st to 10th	Day	20% 11% 33% 27% 8% 1	420
	11th to 20th	Day	24% 11% 34% 26% 4 2	420
	21st to end	Day	24% 11% 29% 29% 6%	462
April	1st to 10th	Day	20% 10% 31% 23% 14% 2	420
	11th to 20th	Day	18% 11% 23% 27% 16% 4	412
	21st to end	Day	19% 6% 30% 22% 15% 7%	420
May	1st to 10th	Day	14% 10% 28% 29% 13% 6%	423
	11th to 20th	Day	20% 3 16% 28% 22% 11%	447
	21st to end	Day	19% 2 10% 26% 26% 17%	485
June	1st to 10th	Day	14% 3 10% 33% 28% 13%	450
	11th to 20th	Day	10% 3 16% 26% 24% 21%	450
	21st to end	Day	10% 1 11% 32% 31% 15%	450
July	1st to 10th	Day	11% 2 5% 21% 32% 28%	445
	11th to 20th	Day	8% 2 5% 25% 33% 28%	444
	21st to end	Day	9% 2 4 28% 32% 25%	488
August	1st to 10th	Day	14% 2 11% 26% 26% 22%	446
	11th to 20th	Day	14% 2 6% 24% 31% 23%	439
	21st to end	Day	13% 2 7% 21% 31% 26%	495
September	1st to 10th	Day	15% 2 7% 24% 31% 21%	450
	11th to 20th	Day	20% 2 10% 18% 32% 19%	450
	21st to end	Day	22% 3 15% 23% 28% 9%	450
October	1st to 10th	Day	21% 3 14% 25% 26% 11%	450
	11th to 20th	Day	17% 6% 19% 29% 21% 8%	450
	21st to end	Day	20% 6% 16% 34% 19% 5%	495
November	1st to 10th	Day	21% 3 21% 31% 18% 5%	440
	11th to 20th	Day	16% 10% 33% 26% 11% 3	270
	21st to end	Day	14% 9% 31% 28% 15% 3	270
December	1st to 10th	Day	29% 9% 26% 29% 6% 1	270
	11th to 20th	Day	18% 9% 30% 31% 11% 1	270
	21st to end	Day	25% 8% 30% 27% 8% 2	297
January	1st to 10th	Night	19% 9% 33% 32% 7%	760
	11th to 20th	Night	21% 9% 31% 34% 6%	725
	21st to end	Night	18% 7% 37% 34% 4	792
February	1st to 10th	Night	19% 7% 33% 37% 4	720
	11th to 20th	Night	24% 9% 37% 27% 3	703
	21st to end	Night	20% 13% 32% 32% 4	592
March	1st to 10th	Night	17% 10% 30% 38% 5%	720
	11th to 20th	Night	18% 10% 31% 30% 9% 1	720
	21st to end	Night	19% 8% 30% 35% 8%	792
April	1st to 10th	Night	17% 8% 33% 31% 11%	720
	11th to 20th	Night	19% 10% 25% 31% 15% 1	705
	21st to end	Night	16% 5% 22% 40% 17%	540
May	1st to 10th	Night	16% 6% 24% 35% 17% 1	540
	11th to 20th	Night	16% 4 17% 37% 24% 2	531
	21st to end	Night	17% 1 8% 32% 34% 7%	584
June	1st to 10th	Night	12% 2 7% 39% 34% 6%	540
	11th to 20th	Night	14% 2 10% 29% 35% 11%	540
	21st to end	Night	8% 2 8% 32% 43% 7%	540
July	1st to 10th	Night	12% 2 4 25% 49% 8%	525
	11th to 20th	Night	11% 1 2 25% 48% 12%	522
	21st to end	Night	6% 4 2 30% 48% 10%	591
August	1st to 10th	Night	16% 1 3 26% 45% 9%	537
	11th to 20th	Night	14% 1 3 31% 41% 10%	534
	21st to end	Night	13% 2 5% 24% 46% 10%	693
September	1st to 10th	Night	13% 2 6% 30% 39% 10%	720
	11th to 20th	Night	18% 2 6% 27% 39% 8%	720
	21st to end	Night	21% 2 11% 34% 28% 3	720
October	1st to 10th	Night	20% 3 15% 28% 28% 7%	720
	11th to 20th	Night	18% 4 21% 35% 19% 2	720
	21st to end	Night	17% 5% 19% 39% 18% 2	792
November	1st to 10th	Night	18% 5% 19% 40% 16% 3	740
	11th to 20th	Night	16% 6% 27% 38% 12% 1	760
	21st to end	Night	15% 5% 26% 37% 15% 1	760
December	1st to 10th	Night	22% 7% 24% 39% 8%	760
	11th to 20th	Night	19% 4 28% 39% 9% 1	760
	21st to end	Night	24% 8% 24% 37% 7%	836

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