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 2024-11-30.

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% 7% 3 6% 4 4 4 4 6% 8% 42%	101465
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 43%	98214
Transparency	Day	21% 47% 7% 14% 10% 1	105071
	Night	22% 47% 6% 14% 10% 1	102750
Seeing	Day	14% 10% 26% 27% 18% 6%	15727
	Night	16% 9% 29% 31% 14% 1	25562

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	10% 12% 6% 12% 5% 5% 5% 3 6% 8% 29%	2527
	11th to 20th	Day	13% 9% 4 9% 8% 8% 4 5% 5% 6% 31%	2694
	21st to end	Day	20% 17% 6% 8% 4 3 3 2 3 4 28%	3135
February	1st to 10th	Day	15% 10% 5% 8% 5% 4 4 4 5% 7% 34%	1710
	11th to 20th	Day	14% 10% 3 7% 5% 4 4 4 6% 7% 36%	1828
	21st to end	Day	15% 11% 3 7% 4 4 4 3 5% 5% 39%	1570
March	1st to 10th	Day	17% 11% 4 6% 4 4 3 2 3 5% 41%	3148
	11th to 20th	Day	11% 8% 3 5% 4 3 3 4 5% 7% 47%	3480
	21st to end	Day	15% 10% 4 8% 5% 4 5% 4 6% 7% 33%	4029
April	1st to 10th	Day	11% 7% 2 8% 5% 5% 5% 3 6% 8% 41%	2280
	11th to 20th	Day	15% 8% 3 7% 4 3 4 4 5% 5% 41%	2254
	21st to end	Day	17% 8% 2 7% 5% 5% 4 3 5% 6% 38%	2280
May	1st to 10th	Day	13% 7% 3 8% 6% 5% 5% 5% 7% 10% 32%	3888
	11th to 20th	Day	17% 8% 3 6% 5% 5% 4 3 5% 7% 37%	4362
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 5% 7% 46%	4824
June	1st to 10th	Day	8% 4 3 5% 4 4 5% 4 6% 9% 48%	2660
	11th to 20th	Day	7% 4 2 5% 4 4 4 5% 7% 9% 49%	2660
	21st to end	Day	5% 4 2 5% 5% 4 6% 5% 8% 11% 46%	2660
July	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 4 7% 9% 41%	4440
	11th to 20th	Day	4 4 1 5% 4 5% 5% 5% 7% 10% 48%	4440
	21st to end	Day	6% 4 2 6% 5% 5% 6% 5% 8% 10% 41%	4624
August	1st to 10th	Day	5% 4 1 6% 4 5% 5% 4 7% 9% 50%	4086
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 6% 9% 41%	3729
	21st to end	Day	5% 4 2 5% 4 5% 5% 4 7% 9% 51%	4180
September	1st to 10th	Day	12% 5% 2 5% 3 3 4 3 5% 7% 52%	2280
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 8% 51%	2252
	21st to end	Day	7% 5% 2 5% 5% 4 4 3 5% 7% 53%	2090
October	1st to 10th	Day	11% 4 1 4 4 3 4 2 4 6% 56%	1900
	11th to 20th	Day	15% 6% 2 4 4 3 3 2 4 6% 52%	1900
	21st to end	Day	16% 8% 3 6% 3 3 3 2 4 6% 46%	2057
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 6% 53%	1762
	11th to 20th	Day	14% 9% 3 5% 4 3 3 3 5% 6% 47%	1800
	21st to end	Day	15% 12% 4 6% 4 4 3 2 4 6% 40%	1715
December	1st to 10th	Day	13% 12% 4 8% 4 4 4 3 4 6% 39%	1520
	11th to 20th	Day	14% 10% 4 6% 5% 5% 4 4 5% 6% 38%	1918
	21st to end	Day	20% 15% 4 10% 4 4 5% 4 5% 5% 24%	2783
January	1st to 10th	Night	14% 11% 6% 10% 4 4 4 3 4 5% 35%	4428
	11th to 20th	Night	14% 12% 5% 12% 5% 6% 7% 4 3 5% 28%	4271
	21st to end	Night	23% 17% 5% 9% 3 4 3 2 4 6% 24%	4532
February	1st to 10th	Night	14% 12% 5% 8% 5% 5% 3 4 5% 5% 34%	2470
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 2 6% 5% 41%	2237
	21st to end	Night	16% 10% 3 7% 5% 3 3 3 4 5% 41%	1884
March	1st to 10th	Night	21% 11% 2 5% 3 3 3 2 4 4 41%	3733
	11th to 20th	Night	14% 6% 3 5% 3 3 3 2 4 7% 51%	3480
	21st to end	Night	18% 10% 3 8% 5% 5% 4 4 5% 6% 33%	3601
April	1st to 10th	Night	12% 7% 3 6% 4 4 4 4 5% 7% 46%	1900
	11th to 20th	Night	13% 7% 2 5% 4 4 4 4 5% 7% 45%	1875
	21st to end	Night	18% 6% 4 8% 4 3 4 3 4 6% 41%	1900
May	1st to 10th	Night	17% 9% 3 6% 4 4 4 4 5% 7% 38%	3006
	11th to 20th	Night	13% 7% 3 6% 4 4 5% 3 5% 7% 44%	2538
	21st to end	Night	8% 6% 2 6% 4 5% 5% 4 7% 9% 43%	2743
June	1st to 10th	Night	8% 4 3 6% 3 3 4 3 7% 9% 50%	1520
	11th to 20th	Night	6% 4 1 5% 4 3 4 4 7% 9% 53%	1520
	21st to end	Night	4 4 2 5% 4 4 4 4 8% 10% 51%	1520
July	1st to 10th	Night	12% 6% 2 5% 4 3 4 3 6% 10% 45%	2530
	11th to 20th	Night	4 3 1 5% 4 4 5% 3 6% 10% 56%	2530
	21st to end	Night	8% 6% 3 7% 5% 5% 6% 4 7% 9% 40%	2935
August	1st to 10th	Night	5% 3 1 4 3 4 4 4 6% 8% 58%	2853
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 5% 7% 49%	3100
	21st to end	Night	5% 2 1 4 3 4 4 4 6% 10% 57%	3476
September	1st to 10th	Night	10% 5% 1 5% 4 3 4 4 4 7% 53%	1900
	11th to 20th	Night	8% 5% 2 5% 5% 3 4 3 6% 8% 50%	1909
	21st to end	Night	9% 4 2 4 4 4 4 4 5% 6% 54%	2090
October	1st to 10th	Night	8% 4 1 5% 4 3 3 3 4 7% 58%	2275
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 5% 6% 53%	2280
	21st to end	Night	17% 7% 2 5% 3 3 4 3 4 6% 46%	2540
November	1st to 10th	Night	10% 5% 2 4 4 3 4 3 5% 5% 55%	2536
	11th to 20th	Night	12% 7% 2 6% 4 3 3 3 5% 7% 48%	2600
	21st to end	Night	18% 8% 3 6% 3 3 3 3 5% 5% 43%	2679
December	1st to 10th	Night	15% 10% 4 7% 5% 5% 5% 4 6% 6% 34%	2660
	11th to 20th	Night	16% 9% 4 7% 3 4 3 2 4 6% 42%	3279
	21st to end	Night	20% 20% 5% 10% 5% 4 4 3 4 5% 21%	4884

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	34% 40% 8% 14% 4 1	2528
	11th to 20th	Day	34% 38% 11% 10% 7%	2694
	21st to end	Day	41% 39% 6% 7% 6%	3135
February	1st to 10th	Day	18% 65% 5% 7% 4	1710
	11th to 20th	Day	18% 65% 4 8% 5%	1838
	21st to end	Day	17% 65% 3 7% 8% 2	1570
March	1st to 10th	Day	31% 41% 4 11% 13% 2	3148
	11th to 20th	Day	16% 42% 8% 18% 13% 4	3480
	21st to end	Day	24% 44% 11% 16% 4	4029
April	1st to 10th	Day	18% 66% 4 7% 4 1	2280
	11th to 20th	Day	15% 66% 4 9% 5% 1	2254
	21st to end	Day	17% 65% 3 8% 6% 1	2280
May	1st to 10th	Day	25% 44% 10% 15% 5%	3923
	11th to 20th	Day	28% 42% 7% 17% 6%	4366
	21st to end	Day	19% 42% 6% 16% 16%	4837
June	1st to 10th	Day	11% 65% 5% 10% 6% 2	2660
	11th to 20th	Day	12% 66% 4 11% 7% 1	2660
	21st to end	Day	11% 66% 9% 9% 4	2660
July	1st to 10th	Day	27% 37% 6% 20% 9% 1	4371
	11th to 20th	Day	19% 38% 10% 21% 11% 1	4365
	21st to end	Day	21% 40% 14% 18% 7%	4479
August	1st to 10th	Day	17% 39% 10% 26% 7%	4072
	11th to 20th	Day	26% 38% 8% 17% 11%	3765
	21st to end	Day	18% 39% 11% 24% 7% 1	4179
September	1st to 10th	Day	10% 60% 6% 13% 9% 2	2280
	11th to 20th	Day	11% 60% 4 10% 12% 2	2252
	21st to end	Day	14% 60% 4 13% 8% 1	2090
October	1st to 10th	Day	10% 37% 3 9% 36% 6%	3160
	11th to 20th	Day	11% 36% 4 12% 31% 6%	3140
	21st to end	Day	20% 36% 4 19% 20% 1	3371
November	1st to 10th	Day	17% 55% 5% 12% 8% 2	1762
	11th to 20th	Day	16% 57% 3 10% 10% 3	1800
	21st to end	Day	23% 57% 4 12% 4	1715
December	1st to 10th	Day	20% 60% 4 9% 7%	1520
	11th to 20th	Day	21% 50% 6% 13% 8% 2	1915
	21st to end	Day	43% 39% 6% 7% 4 1	2783
January	1st to 10th	Night	35% 37% 7% 15% 4 1	4428
	11th to 20th	Night	40% 37% 6% 12% 5%	4271
	21st to end	Night	42% 40% 5% 10% 3	4532
February	1st to 10th	Night	18% 65% 4 7% 5% 1	2470
	11th to 20th	Night	16% 64% 4 8% 8%	2238
	21st to end	Night	17% 65% 3 5% 8% 1	1884
March	1st to 10th	Night	30% 41% 4 15% 9% 1	3733
	11th to 20th	Night	16% 41% 7% 16% 18% 3	3480
	21st to end	Night	28% 43% 6% 19% 4	3601
April	1st to 10th	Night	16% 66% 4 8% 5% 1	1900
	11th to 20th	Night	12% 65% 5% 9% 8% 1	1875
	21st to end	Night	16% 66% 2 8% 7% 1	1900
May	1st to 10th	Night	25% 44% 12% 12% 7%	3036
	11th to 20th	Night	24% 43% 8% 19% 6%	2539
	21st to end	Night	19% 43% 9% 15% 12% 1	2751
June	1st to 10th	Night	11% 66% 6% 7% 9% 2	1520
	11th to 20th	Night	11% 65% 3 10% 9% 1	1520
	21st to end	Night	9% 65% 7% 13% 5% 1	1520
July	1st to 10th	Night	25% 37% 7% 20% 9% 1	2490
	11th to 20th	Night	15% 37% 7% 27% 13% 1	2482
	21st to end	Night	26% 38% 13% 15% 7% 1	2842
August	1st to 10th	Night	15% 39% 11% 25% 10%	2843
	11th to 20th	Night	23% 38% 8% 16% 14% 1	3130
	21st to end	Night	15% 39% 11% 26% 9%	3476
September	1st to 10th	Night	9% 60% 5% 12% 11% 2	1900
	11th to 20th	Night	11% 61% 4 11% 10% 3	1909
	21st to end	Night	14% 61% 5% 10% 9%	2090
October	1st to 10th	Night	9% 36% 3 10% 36% 4	3795
	11th to 20th	Night	10% 36% 5% 13% 31% 5%	3779
	21st to end	Night	17% 36% 6% 18% 18% 4	4176
November	1st to 10th	Night	17% 56% 5% 11% 8% 3	2536
	11th to 20th	Night	15% 56% 4 10% 12% 3	2600
	21st to end	Night	20% 57% 4 11% 7%	2679
December	1st to 10th	Night	20% 60% 5% 7% 7% 1	2660
	11th to 20th	Night	20% 49% 6% 15% 10% 1	3281
	21st to end	Night	46% 38% 6% 6% 4 1	4884

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	21% 17% 36% 19% 6%	270
	11th to 20th	Day	25% 18% 37% 17% 3	270
	21st to end	Day	19% 18% 43% 17% 3	297
February	1st to 10th	Day	23% 16% 41% 19% 3	270
	11th to 20th	Day	21% 22% 33% 19% 4 1	270
	21st to end	Day	24% 17% 39% 18% 2	225
March	1st to 10th	Day	17% 16% 45% 18% 3	270
	11th to 20th	Day	26% 19% 34% 18% 4	270
	21st to end	Day	28% 15% 35% 16% 6%	422
April	1st to 10th	Day	15% 13% 41% 20% 10% 1	460
	11th to 20th	Day	20% 16% 29% 22% 11% 1	452
	21st to end	Day	20% 16% 33% 22% 7% 2	460
May	1st to 10th	Day	17% 12% 30% 29% 11% 1	463
	11th to 20th	Day	17% 8% 28% 26% 17% 4	651
	21st to end	Day	12% 6% 30% 31% 15% 7%	734
June	1st to 10th	Day	8% 6% 22% 32% 24% 8%	680
	11th to 20th	Day	8% 3 23% 36% 24% 6%	680
	21st to end	Day	5% 5% 29% 34% 20% 7%	680
July	1st to 10th	Day	11% 3 14% 27% 30% 15%	671
	11th to 20th	Day	6% 3 16% 36% 29% 10%	668
	21st to end	Day	8% 3 14% 33% 31% 10%	739
August	1st to 10th	Day	7% 2 15% 36% 30% 10%	671
	11th to 20th	Day	13% 4 15% 27% 30% 11%	479
	21st to end	Day	8% 1 14% 29% 35% 12%	539
September	1st to 10th	Day	11% 4 16% 31% 27% 11%	490
	11th to 20th	Day	9% 3 13% 31% 30% 14%	490
	21st to end	Day	10% 8% 22% 29% 23% 8%	490
October	1st to 10th	Day	8% 10% 24% 35% 16% 6%	300
	11th to 20th	Day	14% 17% 31% 25% 12% 1	300
	21st to end	Day	25% 20% 28% 21% 7%	330
November	1st to 10th	Day	14% 9% 35% 24% 13% 4	300
	11th to 20th	Day	19% 20% 31% 17% 11% 2	300
	21st to end	Day	23% 15% 30% 24% 7% 1	299
December	1st to 10th	Day	11% 16% 42% 19% 7% 4	270
	11th to 20th	Day	18% 17% 34% 26% 4	270
	21st to end	Day	24% 19% 39% 15% 4	297
January	1st to 10th	Night	17% 14% 38% 25% 5%	950
	11th to 20th	Night	21% 15% 36% 23% 5%	950
	21st to end	Night	20% 13% 38% 27% 2	1045
February	1st to 10th	Night	20% 13% 38% 25% 4	950
	11th to 20th	Night	19% 14% 40% 25% 2	762
	21st to end	Night	21% 15% 36% 25% 2	628
March	1st to 10th	Night	21% 13% 37% 25% 4	760
	11th to 20th	Night	22% 11% 33% 28% 6%	570
	21st to end	Night	24% 11% 33% 27% 4	627
April	1st to 10th	Night	15% 13% 40% 25% 7%	570
	11th to 20th	Night	16% 10% 36% 29% 9% 1	563
	21st to end	Night	20% 9% 35% 29% 7% 1	570
May	1st to 10th	Night	16% 9% 29% 35% 9% 1	570
	11th to 20th	Night	16% 6% 25% 36% 17% 1	561
	21st to end	Night	8% 5% 24% 41% 19% 3	617
June	1st to 10th	Night	9% 2 21% 39% 29% 1	570
	11th to 20th	Night	8% 2 19% 48% 21% 2	570
	21st to end	Night	5% 2 23% 44% 25% 2	570
July	1st to 10th	Night	11% 2 10% 35% 34% 8%	555
	11th to 20th	Night	4 2 15% 43% 34% 3	552
	21st to end	Night	7% 1 12% 41% 35% 2	624
August	1st to 10th	Night	8% 2 10% 43% 34% 3	567
	11th to 20th	Night	7% 1 14% 40% 34% 4	564
	21st to end	Night	5% 1 12% 38% 40% 5%	627
September	1st to 10th	Night	13% 2 13% 36% 30% 6%	570
	11th to 20th	Night	8% 3 14% 34% 36% 5%	579
	21st to end	Night	10% 6% 22% 32% 26% 4	760
October	1st to 10th	Night	9% 4 26% 32% 24% 4	760
	11th to 20th	Night	16% 10% 30% 30% 13% 1	760
	21st to end	Night	22% 11% 31% 29% 6%	836
November	1st to 10th	Night	13% 8% 37% 32% 9% 1	780
	11th to 20th	Night	16% 14% 34% 29% 7%	800
	21st to end	Night	22% 12% 34% 26% 6%	880
December	1st to 10th	Night	20% 12% 35% 27% 5% 1	950
	11th to 20th	Night	23% 8% 31% 32% 5%	950
	21st to end	Night	27% 15% 32% 23% 3	1045

Page generated at 2024-11-27 16:30:53. Version ac07

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