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 2024-11-05.

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 6% 4 4 5% 4 6% 8% 42%	101209
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	97789
Transparency	Day	38% 5% 12% 22% 17% 5%	104925
	Night	39% 5% 11% 21% 19% 6%	102212
Seeing	Day	18% 6% 20% 26% 20% 11%	15186
	Night	17% 5% 20% 33% 21% 3	25241

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	16% 4 2 4 3 3 2 2 4 7% 53%	2836
	11th to 20th	Day	15% 5% 2 5% 3 3 3 2 4 6% 52%	2840
	21st to end	Day	25% 4 1 3 2 3 4 3 4 5% 46%	3124
February	1st to 10th	Day	15% 6% 2 5% 4 3 3 4 6% 8% 44%	1710
	11th to 20th	Day	17% 7% 2 5% 3 4 4 3 5% 6% 43%	1843
	21st to end	Day	18% 6% 2 6% 4 3 4 4 5% 6% 42%	1579
March	1st to 10th	Day	24% 7% 2 5% 3 3 3 3 4 6% 41%	3435
	11th to 20th	Day	12% 4 1 5% 4 3 4 3 4 6% 54%	3480
	21st to end	Day	21% 6% 2 4 3 3 4 3 4 6% 43%	3815
April	1st to 10th	Day	11% 4 1 4 3 4 5% 4 7% 8% 50%	2090
	11th to 20th	Day	16% 5% 1 4 3 4 4 3 5% 7% 48%	2248
	21st to end	Day	14% 5% 1 5% 5% 4 5% 3 5% 7% 47%	2470
May	1st to 10th	Day	13% 6% 2 7% 5% 6% 5% 5% 6% 9% 36%	4082
	11th to 20th	Day	17% 7% 3 7% 4 4 5% 4 5% 8% 36%	4081
	21st to end	Day	9% 4 2 6% 4 4 5% 4 7% 10% 45%	4476
June	1st to 10th	Day	11% 5% 2 6% 5% 6% 6% 5% 7% 9% 37%	2470
	11th to 20th	Day	7% 5% 2 7% 6% 6% 6% 5% 7% 11% 38%	2470
	21st to end	Day	8% 4 2 6% 6% 5% 7% 6% 9% 11% 37%	2470
July	1st to 10th	Day	7% 5% 3 7% 6% 6% 7% 6% 8% 10% 35%	4120
	11th to 20th	Day	8% 6% 3 8% 5% 6% 6% 5% 8% 11% 33%	4120
	21st to end	Day	12% 9% 3 9% 7% 7% 8% 6% 8% 9% 21%	4466
August	1st to 10th	Day	15% 8% 3 7% 5% 6% 6% 5% 8% 11% 25%	4104
	11th to 20th	Day	9% 5% 2 7% 6% 5% 6% 5% 7% 12% 36%	4044
	21st to end	Day	6% 3 2 5% 4 4 4 4 8% 10% 49%	4289
September	1st to 10th	Day	12% 5% 2 6% 6% 4 4 4 6% 8% 43%	2090
	11th to 20th	Day	18% 6% 2 6% 5% 4 5% 4 5% 7% 40%	2090
	21st to end	Day	21% 5% 1 6% 4 4 4 4 5% 6% 39%	2090
October	1st to 10th	Day	15% 5% 3 4 2 3 3 2 4 6% 54%	1997
	11th to 20th	Day	14% 4 2 3 3 2 3 3 4 6% 56%	1900
	21st to end	Day	15% 4 2 4 2 2 2 2 4 4 57%	2090
November	1st to 10th	Day	13% 5% 1 4 3 3 3 2 4 6% 56%	1883
	11th to 20th	Day	14% 4 2 4 3 3 3 2 5% 7% 54%	1710
	21st to end	Day	15% 4 1 4 3 3 4 3 5% 6% 52%	1710
December	1st to 10th	Day	19% 7% 3 6% 5% 4 4 4 5% 5% 41%	1710
	11th to 20th	Day	15% 4 1 4 3 2 3 3 5% 7% 52%	2153
	21st to end	Day	19% 6% 2 5% 4 4 4 4 4 7% 43%	3124
January	1st to 10th	Night	15% 5% 2 4 3 3 4 3 4 6% 51%	4109
	11th to 20th	Night	12% 6% 3 5% 4 4 4 3 5% 5% 50%	4120
	21st to end	Night	24% 6% 2 4 3 3 3 3 4 5% 43%	4532
February	1st to 10th	Night	15% 5% 2 5% 5% 4 4 4 5% 6% 46%	2470
	11th to 20th	Night	19% 7% 2 4 4 3 4 2 5% 5% 46%	2218
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 6% 43%	1875
March	1st to 10th	Night	24% 7% 3 6% 4 4 3 3 4 5% 37%	3418
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 4 7% 54%	3480
	21st to end	Night	19% 5% 2 5% 4 3 4 3 5% 6% 44%	3817
April	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 5% 7% 50%	2090
	11th to 20th	Night	14% 5% 2 4 4 3 3 3 5% 8% 48%	1895
	21st to end	Night	14% 4 2 5% 4 3 4 3 5% 6% 50%	1710
May	1st to 10th	Night	16% 7% 2 6% 5% 4 4 4 5% 7% 40%	2813
	11th to 20th	Night	19% 7% 3 7% 4 4 4 3 4 8% 37%	2812
	21st to end	Night	10% 5% 2 6% 4 4 4 5% 5% 10% 45%	3079
June	1st to 10th	Night	11% 5% 2 5% 4 5% 6% 6% 8% 11% 36%	1710
	11th to 20th	Night	10% 6% 1 6% 5% 5% 5% 4 7% 9% 41%	1710
	21st to end	Night	7% 5% 2 6% 6% 5% 6% 5% 8% 11% 39%	1710
July	1st to 10th	Night	9% 6% 3 7% 7% 5% 6% 5% 8% 10% 34%	2840
	11th to 20th	Night	9% 5% 2 7% 6% 5% 5% 6% 7% 12% 36%	2840
	21st to end	Night	13% 12% 4 11% 7% 6% 6% 6% 8% 9% 20%	3087
August	1st to 10th	Night	16% 8% 3 8% 7% 6% 6% 5% 7% 9% 26%	2825
	11th to 20th	Night	12% 7% 3 10% 6% 6% 6% 5% 7% 9% 29%	2785
	21st to end	Night	7% 4 2 6% 5% 5% 5% 4 7% 9% 46%	3329
September	1st to 10th	Night	12% 6% 2 7% 4 4 5% 3 5% 7% 44%	2090
	11th to 20th	Night	17% 6% 2 8% 5% 5% 4 3 6% 6% 37%	2090
	21st to end	Night	20% 6% 2 6% 4 5% 5% 3 4 6% 40%	2090
October	1st to 10th	Night	17% 5% 1 4 3 3 3 4 4 6% 49%	2181
	11th to 20th	Night	15% 4 2 4 3 3 4 3 4 6% 53%	2280
	21st to end	Night	14% 4 1 4 3 3 3 2 4 5% 58%	2508
November	1st to 10th	Night	15% 5% 2 5% 3 2 3 3 3 4 54%	2292
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 4 6% 54%	2470
	21st to end	Night	14% 4 2 4 3 4 3 4 5% 7% 50%	2470
December	1st to 10th	Night	18% 7% 3 5% 3 3 4 3 5% 5% 44%	2470
	11th to 20th	Night	16% 6% 2 4 3 3 4 3 4 5% 51%	3042
	21st to end	Night	21% 5% 2 5% 3 4 4 3 5% 6% 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	33% 3 5% 26% 26% 7%	2837
	11th to 20th	Day	36% 3 6% 18% 32% 6%	2840
	21st to end	Day	42% 3 4 21% 25% 5%	3124
February	1st to 10th	Day	39% 3 10% 23% 20% 5%	1710
	11th to 20th	Day	41% 3 9% 21% 21% 5%	1853
	21st to end	Day	43% 5% 8% 17% 19% 7%	1579
March	1st to 10th	Day	45% 4 7% 19% 16% 9%	3435
	11th to 20th	Day	32% 4 8% 21% 23% 12%	3480
	21st to end	Day	43% 4 8% 23% 20% 3	3815
April	1st to 10th	Day	31% 4 11% 24% 24% 5%	2090
	11th to 20th	Day	36% 4 10% 20% 23% 8%	2248
	21st to end	Day	37% 4 12% 26% 17% 4	2470
May	1st to 10th	Day	43% 7% 15% 20% 13% 2	4120
	11th to 20th	Day	47% 6% 12% 22% 12% 1	4085
	21st to end	Day	33% 6% 17% 23% 18% 4	4489
June	1st to 10th	Day	40% 7% 17% 23% 11% 1	2470
	11th to 20th	Day	37% 8% 18% 22% 14% 2	2470
	21st to end	Day	36% 9% 18% 20% 16% 2	2470
July	1st to 10th	Day	41% 10% 20% 19% 9% 2	4055
	11th to 20th	Day	41% 8% 23% 21% 6% 1	4051
	21st to end	Day	54% 9% 14% 16% 5% 1	4324
August	1st to 10th	Day	49% 8% 19% 19% 5%	4091
	11th to 20th	Day	40% 7% 23% 18% 10% 2	4083
	21st to end	Day	28% 6% 16% 36% 13% 2	4289
September	1st to 10th	Day	38% 7% 13% 22% 17% 3	2090
	11th to 20th	Day	44% 6% 11% 20% 17% 3	2090
	21st to end	Day	45% 4 9% 20% 17% 4	2090
October	1st to 10th	Day	28% 3 7% 21% 31% 10%	3322
	11th to 20th	Day	21% 2 6% 21% 26% 24%	3140
	21st to end	Day	27% 2 7% 25% 22% 16%	3428
November	1st to 10th	Day	32% 3 8% 21% 29% 7%	1883
	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	34% 4 7% 21% 27% 7%	4109
	11th to 20th	Night	35% 4 7% 18% 29% 7%	4120
	21st to end	Night	44% 3 5% 19% 22% 6%	4532
February	1st to 10th	Night	38% 5% 10% 22% 21% 5%	2470
	11th to 20th	Night	42% 3 8% 19% 22% 7%	2219
	21st to end	Night	42% 3 10% 19% 20% 6%	1875
March	1st to 10th	Night	48% 5% 7% 16% 17% 6%	3418
	11th to 20th	Night	30% 4 7% 22% 22% 15%	3480
	21st to end	Night	41% 4 8% 24% 17% 7%	3817
April	1st to 10th	Night	33% 4 11% 23% 23% 5%	2090
	11th to 20th	Night	35% 5% 8% 23% 21% 8%	1895
	21st to end	Night	35% 4 12% 26% 20% 4	1710
May	1st to 10th	Night	43% 6% 13% 25% 12% 1	2840
	11th to 20th	Night	46% 6% 12% 22% 13% 1	2813
	21st to end	Night	32% 7% 15% 28% 16% 1	3087
June	1st to 10th	Night	37% 9% 18% 22% 12% 1	1710
	11th to 20th	Night	39% 7% 19% 23% 10% 1	1710
	21st to end	Night	36% 9% 17% 20% 13% 4	1710
July	1st to 10th	Night	43% 10% 17% 19% 9% 2	2795
	11th to 20th	Night	40% 9% 25% 20% 4 2	2786
	21st to end	Night	57% 10% 15% 14% 4	2988
August	1st to 10th	Night	53% 7% 20% 15% 5%	2814
	11th to 20th	Night	48% 8% 19% 14% 9% 1	2812
	21st to end	Night	33% 7% 15% 34% 10% 2	3329
September	1st to 10th	Night	39% 7% 14% 21% 15% 4	2090
	11th to 20th	Night	46% 7% 10% 19% 16% 1	2090
	21st to end	Night	46% 6% 9% 18% 17% 3	2090
October	1st to 10th	Night	35% 5% 7% 18% 25% 10%	3636
	11th to 20th	Night	22% 3 6% 24% 28% 18%	3779
	21st to end	Night	28% 2 4 24% 28% 14%	4120
November	1st to 10th	Night	34% 4 7% 19% 26% 9%	2292
	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	27% 12% 31% 24% 7%	270
	21st to end	Day	24% 14% 35% 21% 5%	297
February	1st to 10th	Day	22% 13% 35% 22% 7%	270
	11th to 20th	Day	32% 14% 33% 18% 3	270
	21st to end	Day	32% 12% 34% 18% 4	234
March	1st to 10th	Day	22% 10% 32% 27% 8% 2	460
	11th to 20th	Day	23% 11% 35% 25% 4 2	460
	21st to end	Day	25% 11% 29% 29% 6%	506
April	1st to 10th	Day	20% 12% 32% 22% 13% 2	460
	11th to 20th	Day	18% 13% 23% 27% 15% 3	452
	21st to end	Day	19% 6% 30% 24% 15% 7%	460
May	1st to 10th	Day	14% 10% 26% 30% 14% 6%	463
	11th to 20th	Day	20% 3 16% 28% 22% 10%	487
	21st to end	Day	17% 3 12% 26% 26% 16%	529
June	1st to 10th	Day	14% 3 11% 34% 27% 12%	490
	11th to 20th	Day	9% 3 16% 28% 24% 20%	490
	21st to end	Day	10% 1 12% 32% 32% 14%	490
July	1st to 10th	Day	11% 2 5% 21% 33% 28%	485
	11th to 20th	Day	7% 2 5% 25% 33% 28%	484
	21st to end	Day	11% 2 6% 28% 31% 23%	532
August	1st to 10th	Day	14% 2 12% 26% 26% 20%	486
	11th to 20th	Day	14% 2 8% 25% 30% 22%	479
	21st to end	Day	12% 2 6% 22% 32% 26%	539
September	1st to 10th	Day	14% 2 9% 24% 31% 20%	490
	11th to 20th	Day	20% 2 11% 17% 31% 18%	490
	21st to end	Day	23% 3 16% 22% 27% 8%	490
October	1st to 10th	Day	20% 3 13% 28% 26% 11%	490
	11th to 20th	Day	16% 6% 20% 29% 21% 8%	490
	21st to end	Day	19% 6% 16% 33% 22% 5%	539
November	1st to 10th	Day	22% 4 20% 32% 18% 5%	457
	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	18% 9% 33% 32% 7%	760
	11th to 20th	Night	20% 9% 32% 34% 5%	760
	21st to end	Night	19% 7% 37% 33% 4	836
February	1st to 10th	Night	18% 7% 33% 38% 4	760
	11th to 20th	Night	24% 9% 37% 27% 3	743
	21st to end	Night	20% 13% 32% 32% 3	628
March	1st to 10th	Night	18% 9% 29% 37% 6%	760
	11th to 20th	Night	18% 11% 32% 30% 9% 1	760
	21st to end	Night	19% 7% 31% 34% 8%	836
April	1st to 10th	Night	17% 9% 33% 31% 10%	760
	11th to 20th	Night	18% 10% 25% 33% 14% 1	744
	21st to end	Night	16% 5% 22% 40% 17%	570
May	1st to 10th	Night	16% 6% 23% 35% 18% 1	570
	11th to 20th	Night	17% 4 16% 37% 24% 2	561
	21st to end	Night	17% 1 9% 33% 33% 6%	617
June	1st to 10th	Night	12% 2 8% 39% 33% 6%	570
	11th to 20th	Night	14% 2 10% 29% 34% 11%	570
	21st to end	Night	8% 2 8% 32% 44% 6%	570
July	1st to 10th	Night	12% 2 4 26% 49% 8%	555
	11th to 20th	Night	11% 1 2 25% 49% 11%	552
	21st to end	Night	7% 4 3 31% 46% 10%	624
August	1st to 10th	Night	17% 2 4 26% 44% 8%	567
	11th to 20th	Night	14% 2 3 33% 40% 9%	564
	21st to end	Night	12% 2 5% 24% 46% 10%	732
September	1st to 10th	Night	14% 2 6% 30% 39% 10%	760
	11th to 20th	Night	19% 2 7% 26% 38% 7%	760
	21st to end	Night	22% 2 11% 34% 28% 3	760
October	1st to 10th	Night	19% 3 14% 30% 28% 7%	760
	11th to 20th	Night	18% 4 21% 36% 19% 2	760
	21st to end	Night	16% 5% 19% 39% 19% 2	836
November	1st to 10th	Night	18% 5% 18% 40% 16% 3	760
	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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