Principia College Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Principia College Astronomical Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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% 8% 3 7% 4 4 4 4 5% 8% 42%	130962
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	130159
Transparency	Day	26% 38% 8% 16% 11% 1	134528
	Night	27% 37% 8% 16% 12% 1	134672
Seeing	Day	15% 10% 27% 26% 17% 5%	17335
	Night	16% 9% 29% 31% 14% 1	27288

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	11% 11% 6% 14% 5% 4 4 3 5% 7% 31%	4691
	11th to 20th	Day	8% 8% 3 8% 7% 7% 4 4 5% 6% 39%	5028
	21st to end	Day	16% 15% 5% 8% 4 3 3 3 3 4 35%	5812
February	1st to 10th	Day	15% 10% 5% 8% 5% 4 4 4 5% 7% 34%	1890
	11th to 20th	Day	14% 11% 3 7% 6% 4 4 4 5% 6% 36%	1976
	21st to end	Day	14% 11% 3 7% 4 4 4 3 5% 6% 40%	1650
March	1st to 10th	Day	16% 10% 3 5% 3 3 2 2 3 4 49%	4522
	11th to 20th	Day	8% 7% 2 4 4 3 4 4 6% 8% 50%	4980
	21st to end	Day	14% 9% 4 8% 5% 4 5% 4 6% 8% 34%	5775
April	1st to 10th	Day	13% 7% 2 8% 5% 5% 5% 3 5% 7% 41%	2400
	11th to 20th	Day	16% 9% 3 7% 4 3 4 3 5% 5% 41%	2374
	21st to end	Day	16% 8% 2 8% 5% 5% 4 4 5% 7% 38%	2400
May	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 4 7% 10% 31%	5584
	11th to 20th	Day	15% 8% 2 6% 4 5% 5% 4 6% 8% 37%	6268
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 4 5% 7% 41%	6935
June	1st to 10th	Day	8% 5% 3 5% 4 4 5% 4 6% 9% 48%	2800
	11th to 20th	Day	8% 4 2 5% 4 4 4 5% 7% 9% 48%	2800
	21st to end	Day	5% 4 2 5% 5% 4 6% 5% 8% 11% 47%	2800
July	1st to 10th	Day	9% 5% 2 6% 4 4 4 3 6% 9% 46%	6360
	11th to 20th	Day	6% 5% 2 7% 6% 6% 6% 5% 8% 10% 40%	6360
	21st to end	Day	6% 4 2 6% 5% 5% 6% 5% 8% 11% 40%	6651
August	1st to 10th	Day	5% 4 1 5% 4 4 5% 4 7% 10% 51%	4216
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 6% 9% 41%	3849
	21st to end	Day	5% 4 2 5% 4 5% 5% 4 7% 9% 51%	4312
September	1st to 10th	Day	12% 5% 2 5% 3 3 4 3 5% 7% 52%	2400
	11th to 20th	Day	8% 5% 2 5% 4 4 4 3 5% 8% 52%	2370
	21st to end	Day	7% 5% 2 6% 5% 4 4 3 5% 7% 53%	2200
October	1st to 10th	Day	10% 4 1 5% 4 3 4 2 4 6% 56%	2000
	11th to 20th	Day	14% 6% 2 4 4 3 3 2 4 6% 52%	2000
	21st to end	Day	17% 8% 3 6% 3 3 3 2 4 6% 46%	2165
November	1st to 10th	Day	11% 5% 2 5% 4 3 2 3 5% 6% 53%	1852
	11th to 20th	Day	14% 9% 3 5% 4 3 3 3 5% 6% 46%	1890
	21st to end	Day	15% 12% 4 7% 4 4 3 2 4 6% 39%	1804
December	1st to 10th	Day	8% 10% 3 7% 3 4 3 3 4 6% 49%	2690
	11th to 20th	Day	13% 9% 5% 9% 5% 5% 4 3 4 5% 37%	3088
	21st to end	Day	22% 17% 5% 10% 4 4 4 3 4 4 22%	4070
January	1st to 10th	Night	12% 11% 6% 10% 5% 4 4 3 4 6% 36%	8232
	11th to 20th	Night	9% 9% 4 11% 5% 5% 6% 4 4 5% 37%	7943
	21st to end	Night	16% 13% 4 9% 4 4 4 3 5% 6% 32%	8402
February	1st to 10th	Night	14% 12% 5% 7% 5% 5% 3 4 5% 5% 35%	2730
	11th to 20th	Night	16% 8% 4 7% 5% 4 4 2 5% 5% 40%	2420
	21st to end	Night	15% 9% 3 7% 4 3 3 3 4 5% 43%	1980
March	1st to 10th	Night	19% 12% 3 5% 3 3 3 2 4 4 42%	5373
	11th to 20th	Night	12% 7% 3 5% 3 3 3 3 4 7% 50%	4980
	21st to end	Night	17% 9% 3 8% 5% 5% 4 4 5% 6% 34%	5153
April	1st to 10th	Night	12% 7% 3 6% 4 4 4 4 5% 7% 45%	2000
	11th to 20th	Night	13% 7% 2 5% 4 4 4 4 5% 7% 46%	1975
	21st to end	Night	17% 6% 4 8% 4 3 4 3 4 6% 40%	2000
May	1st to 10th	Night	17% 9% 3 7% 4 4 4 4 6% 8% 34%	4310
	11th to 20th	Night	13% 7% 2 5% 4 4 5% 3 6% 8% 43%	3641
	21st to end	Night	12% 6% 2 7% 4 5% 5% 4 6% 8% 40%	3942
June	1st to 10th	Night	8% 5% 3 6% 3 4 4 4 7% 9% 49%	1600
	11th to 20th	Night	6% 5% 1 5% 4 3 4 4 7% 9% 52%	1600
	21st to end	Night	4 4 2 5% 4 4 4 4 8% 10% 51%	1600
July	1st to 10th	Night	9% 6% 2 6% 4 4 4 3 5% 9% 47%	3618
	11th to 20th	Night	5% 4 2 6% 5% 5% 6% 4 7% 10% 46%	3617
	21st to end	Night	7% 5% 3 6% 5% 5% 6% 5% 8% 10% 41%	4216
August	1st to 10th	Night	5% 3 1 4 3 4 4 4 6% 8% 58%	2943
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 5% 7% 50%	3200
	21st to end	Night	5% 2 1 4 3 4 4 4 6% 10% 58%	3586
September	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 4 7% 53%	2000
	11th to 20th	Night	8% 5% 2 5% 4 3 4 4 6% 8% 51%	2010
	21st to end	Night	9% 5% 2 5% 4 4 4 4 5% 6% 54%	2200
October	1st to 10th	Night	8% 4 2 5% 4 3 3 3 5% 7% 58%	2395
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 5% 6% 53%	2400
	21st to end	Night	18% 8% 2 5% 3 3 4 3 4 5% 46%	2674
November	1st to 10th	Night	10% 5% 2 4 4 3 4 3 5% 5% 54%	2666
	11th to 20th	Night	12% 7% 2 6% 4 3 3 3 5% 7% 48%	2730
	21st to end	Night	18% 9% 3 7% 3 3 3 3 4 5% 42%	2814
December	1st to 10th	Night	12% 8% 3 7% 5% 4 4 3 4 5% 45%	4720
	11th to 20th	Night	15% 8% 4 7% 3 4 4 3 5% 7% 41%	5339
	21st to end	Night	24% 19% 4 9% 5% 4 4 3 3 4 21%	7150

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% 23% 9% 17% 6% 1	4692
	11th to 20th	Day	35% 21% 11% 17% 13% 3	5028
	21st to end	Day	43% 22% 8% 13% 11% 3	5812
February	1st to 10th	Day	21% 59% 7% 9% 4	1890
	11th to 20th	Day	22% 60% 3 9% 5% 1	1986
	21st to end	Day	17% 62% 3 8% 9% 2	1650
March	1st to 10th	Day	30% 28% 3 16% 21% 2	4522
	11th to 20th	Day	18% 30% 6% 22% 20% 4	4980
	21st to end	Day	30% 32% 11% 21% 6%	5777
April	1st to 10th	Day	20% 63% 4 7% 5% 1	2400
	11th to 20th	Day	17% 63% 4 10% 5% 1	2374
	21st to end	Day	18% 62% 4 9% 6% 1	2400
May	1st to 10th	Day	34% 32% 10% 15% 9%	5619
	11th to 20th	Day	32% 31% 8% 19% 9% 1	6272
	21st to end	Day	29% 31% 7% 17% 15%	6949
June	1st to 10th	Day	13% 62% 6% 11% 7% 2	2800
	11th to 20th	Day	14% 63% 4 11% 7% 1	2800
	21st to end	Day	12% 63% 9% 10% 5% 1	2800
July	1st to 10th	Day	26% 27% 7% 24% 15% 1	6291
	11th to 20th	Day	30% 30% 12% 18% 9%	6285
	21st to end	Day	25% 30% 15% 24% 7%	6463
August	1st to 10th	Day	17% 38% 11% 27% 7%	4202
	11th to 20th	Day	26% 37% 9% 18% 11%	3885
	21st to end	Day	18% 38% 11% 24% 7% 1	4311
September	1st to 10th	Day	11% 57% 6% 14% 10% 2	2400
	11th to 20th	Day	11% 58% 5% 11% 13% 2	2370
	21st to end	Day	15% 58% 5% 12% 10% 1	2200
October	1st to 10th	Day	10% 36% 3 9% 36% 6%	3260
	11th to 20th	Day	11% 35% 4 13% 31% 6%	3240
	21st to end	Day	21% 35% 4 18% 20% 1	3479
November	1st to 10th	Day	17% 53% 5% 14% 9% 2	1852
	11th to 20th	Day	18% 54% 3 11% 10% 3	1890
	21st to end	Day	25% 54% 4 12% 5%	1804
December	1st to 10th	Day	22% 35% 10% 20% 12%	2690
	11th to 20th	Day	34% 31% 8% 15% 10% 1	3085
	21st to end	Day	54% 27% 8% 7% 3 1	4070
January	1st to 10th	Night	41% 22% 12% 19% 5%	8233
	11th to 20th	Night	39% 20% 6% 18% 14% 3	7943
	21st to end	Night	42% 23% 9% 14% 11% 1	8402
February	1st to 10th	Night	19% 60% 5% 10% 5% 1	2730
	11th to 20th	Night	20% 59% 4 9% 7%	2421
	21st to end	Night	17% 62% 3 8% 9% 1	1980
March	1st to 10th	Night	35% 29% 3 15% 16% 1	5373
	11th to 20th	Night	22% 29% 6% 19% 19% 5%	4980
	21st to end	Night	34% 31% 6% 19% 8% 1	5153
April	1st to 10th	Night	18% 62% 5% 9% 5% 1	2000
	11th to 20th	Night	14% 62% 5% 11% 8% 1	1975
	21st to end	Night	17% 63% 3 9% 6% 1	2000
May	1st to 10th	Night	33% 34% 11% 14% 8%	4340
	11th to 20th	Night	28% 31% 9% 20% 10% 2	3642
	21st to end	Night	29% 31% 10% 17% 13%	3950
June	1st to 10th	Night	13% 63% 6% 9% 9% 2	1600
	11th to 20th	Night	13% 63% 3 11% 9% 1	1600
	21st to end	Night	9% 62% 8% 14% 6% 2	1600
July	1st to 10th	Night	27% 27% 7% 23% 15% 1	3580
	11th to 20th	Night	24% 30% 13% 23% 10% 1	3572
	21st to end	Night	27% 29% 16% 20% 7%	4094
August	1st to 10th	Night	15% 38% 12% 25% 10%	2933
	11th to 20th	Night	23% 37% 8% 17% 13% 1	3230
	21st to end	Night	15% 38% 11% 26% 10%	3586
September	1st to 10th	Night	11% 57% 5% 13% 12% 2	2000
	11th to 20th	Night	11% 58% 4 12% 12% 3	2010
	21st to end	Night	15% 58% 6% 10% 10% 1	2200
October	1st to 10th	Night	10% 35% 3 10% 37% 5%	3915
	11th to 20th	Night	11% 35% 5% 14% 30% 5%	3899
	21st to end	Night	18% 35% 6% 18% 19% 4	4310
November	1st to 10th	Night	18% 53% 5% 11% 9% 3	2666
	11th to 20th	Night	16% 54% 4 12% 12% 2	2730
	21st to end	Night	23% 54% 4 12% 6%	2814
December	1st to 10th	Night	25% 35% 7% 22% 11% 1	4720
	11th to 20th	Night	29% 31% 10% 17% 13%	5341
	21st to end	Night	54% 27% 9% 7% 3	7150

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	22% 19% 35% 19% 6%	330
	11th to 20th	Day	25% 21% 37% 14% 3	330
	21st to end	Day	19% 19% 43% 15% 3	363
February	1st to 10th	Day	23% 15% 42% 18% 3	330
	11th to 20th	Day	24% 23% 32% 17% 3 1	315
	21st to end	Day	23% 18% 39% 18% 2	249
March	1st to 10th	Day	18% 16% 45% 18% 3	300
	11th to 20th	Day	27% 19% 33% 17% 4	300
	21st to end	Day	29% 15% 35% 15% 5%	462
April	1st to 10th	Day	19% 14% 38% 19% 10% 1	500
	11th to 20th	Day	22% 16% 29% 21% 11% 1	492
	21st to end	Day	19% 16% 34% 22% 7% 1	500
May	1st to 10th	Day	17% 12% 31% 28% 10% 1	503
	11th to 20th	Day	17% 9% 28% 25% 17% 4	700
	21st to end	Day	13% 7% 30% 29% 14% 6%	789
June	1st to 10th	Day	9% 6% 22% 31% 23% 8%	730
	11th to 20th	Day	9% 3 23% 35% 23% 6%	730
	21st to end	Day	5% 5% 28% 34% 21% 7%	730
July	1st to 10th	Day	10% 3 14% 28% 29% 15%	721
	11th to 20th	Day	6% 3 17% 36% 28% 9%	718
	21st to end	Day	8% 3 14% 34% 31% 10%	794
August	1st to 10th	Day	7% 2 16% 37% 29% 9%	721
	11th to 20th	Day	12% 4 14% 28% 31% 11%	519
	21st to end	Day	7% 2 15% 30% 35% 12%	583
September	1st to 10th	Day	11% 4 16% 32% 26% 10%	530
	11th to 20th	Day	8% 2 13% 32% 31% 14%	530
	21st to end	Day	10% 7% 21% 28% 25% 9%	530
October	1st to 10th	Day	8% 10% 23% 37% 17% 5%	330
	11th to 20th	Day	14% 16% 30% 25% 14% 1	330
	21st to end	Day	27% 20% 28% 20% 6%	363
November	1st to 10th	Day	14% 11% 36% 23% 12% 3	330
	11th to 20th	Day	19% 20% 32% 17% 10% 2	330
	21st to end	Day	26% 14% 28% 24% 7% 1	330
December	1st to 10th	Day	13% 17% 43% 18% 6% 3	330
	11th to 20th	Day	19% 19% 35% 23% 4	330
	21st to end	Day	25% 18% 38% 16% 3	363
January	1st to 10th	Night	17% 14% 38% 26% 5%	1050
	11th to 20th	Night	20% 17% 36% 22% 4	1050
	21st to end	Night	19% 13% 39% 27% 2	1155
February	1st to 10th	Night	20% 14% 38% 24% 3	1050
	11th to 20th	Night	21% 15% 38% 24% 2	825
	21st to end	Night	20% 15% 37% 26% 2	660
March	1st to 10th	Night	22% 14% 36% 25% 4	800
	11th to 20th	Night	22% 11% 33% 28% 6%	600
	21st to end	Night	24% 12% 33% 27% 4	660
April	1st to 10th	Night	16% 12% 40% 25% 7%	600
	11th to 20th	Night	16% 10% 35% 29% 9% 1	593
	21st to end	Night	20% 9% 35% 29% 7% 1	600
May	1st to 10th	Night	17% 9% 29% 35% 9% 1	600
	11th to 20th	Night	15% 6% 26% 35% 17% 1	591
	21st to end	Night	8% 6% 25% 40% 18% 3	650
June	1st to 10th	Night	9% 2 21% 39% 28% 1	600
	11th to 20th	Night	8% 2 19% 47% 21% 2	600
	21st to end	Night	5% 2 23% 43% 26% 2	600
July	1st to 10th	Night	11% 2 10% 34% 35% 8%	585
	11th to 20th	Night	4 2 15% 42% 34% 2	582
	21st to end	Night	7% 2 13% 41% 35% 3	657
August	1st to 10th	Night	8% 2 11% 43% 33% 3	597
	11th to 20th	Night	7% 1 14% 41% 34% 4	594
	21st to end	Night	5% 1 12% 38% 40% 5%	660
September	1st to 10th	Night	13% 2 14% 36% 30% 5%	600
	11th to 20th	Night	8% 3 14% 35% 36% 5%	610
	21st to end	Night	10% 6% 22% 32% 27% 4	800
October	1st to 10th	Night	9% 4 26% 34% 24% 4	800
	11th to 20th	Night	15% 10% 31% 30% 14% 1	800
	21st to end	Night	23% 11% 32% 28% 6%	880
November	1st to 10th	Night	13% 8% 38% 31% 9% 1	820
	11th to 20th	Night	15% 15% 35% 29% 7%	840
	21st to end	Night	23% 13% 32% 26% 6%	924
December	1st to 10th	Night	20% 12% 35% 27% 5% 1	1050
	11th to 20th	Night	23% 10% 32% 30% 5%	1050
	21st to end	Night	28% 14% 33% 23% 3	1155

Page generated at 2026-02-13 00:42:14. Version ac07

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