Penitentiary Glen Reservation
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Penitentiary Glen Reservation clear sky chart from 2005-11-06 to 2026-06-01.

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	16% 12% 4 10% 5% 4 4 3 5% 6% 30%	131951
	Night	16% 14% 5% 10% 5% 4 4 3 5% 6% 28%	131189
Transparency	Day	53% 7% 11% 18% 10% 1	135583
	Night	56% 6% 10% 17% 10% 1	135627
Seeing	Day	23% 17% 28% 21% 9% 3	16538
	Night	23% 16% 27% 23% 10% 1	26366

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	14% 22% 9% 28% 5% 2 3 2 2 3 10%	5258
	11th to 20th	Day	10% 21% 10% 21% 10% 7% 4 2 3 3 9%	5280
	21st to end	Day	17% 18% 9% 17% 9% 7% 4 3 3 2 11%	5780
February	1st to 10th	Day	16% 20% 6% 14% 8% 5% 3 2 3 4 19%	1890
	11th to 20th	Day	15% 17% 8% 16% 9% 3 3 3 4 4 18%	1880
	21st to end	Day	17% 17% 6% 14% 4 4 4 4 5% 5% 21%	1730
March	1st to 10th	Day	16% 13% 8% 12% 5% 3 4 2 5% 6% 28%	4947
	11th to 20th	Day	12% 16% 5% 11% 4 4 3 3 4 5% 33%	5090
	21st to end	Day	19% 16% 6% 11% 4 3 3 3 3 5% 27%	5586
April	1st to 10th	Day	17% 14% 4 7% 4 4 4 3 5% 7% 30%	2367
	11th to 20th	Day	19% 13% 3 6% 4 4 4 4 4 7% 33%	2660
	21st to end	Day	19% 13% 3 7% 4 3 3 3 4 5% 35%	2730
May	1st to 10th	Day	22% 9% 3 9% 5% 4 5% 3 5% 5% 30%	5992
	11th to 20th	Day	18% 9% 3 7% 4 4 5% 4 5% 7% 33%	5991
	21st to end	Day	18% 8% 3 7% 4 4 4 4 5% 7% 36%	6807
June	1st to 10th	Day	17% 6% 2 6% 4 4 5% 4 7% 7% 39%	2800
	11th to 20th	Day	14% 6% 3 6% 5% 4 4 4 7% 8% 39%	2800
	21st to end	Day	13% 6% 2 5% 4 4 5% 4 6% 10% 42%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 10% 44%	6039
	11th to 20th	Day	13% 6% 2 5% 4 4 5% 4 7% 9% 41%	5900
	21st to end	Day	12% 6% 2 6% 4 4 4 4 6% 8% 44%	6424
August	1st to 10th	Day	13% 6% 2 7% 4 5% 5% 4 7% 9% 39%	4234
	11th to 20th	Day	13% 7% 2 7% 4 4 6% 5% 8% 9% 35%	4174
	21st to end	Day	11% 5% 2 6% 4 5% 5% 4 6% 9% 42%	4586
September	1st to 10th	Day	15% 7% 2 6% 4 3 4 4 6% 8% 42%	2316
	11th to 20th	Day	12% 5% 2 5% 3 4 5% 4 6% 8% 45%	2200
	21st to end	Day	19% 10% 2 7% 4 3 4 4 5% 6% 36%	2200
October	1st to 10th	Day	17% 6% 3 7% 4 4 4 4 5% 7% 40%	2054
	11th to 20th	Day	23% 10% 3 5% 5% 4 4 3 5% 6% 33%	2000
	21st to end	Day	25% 12% 3 7% 5% 4 4 4 5% 6% 26%	2200
November	1st to 10th	Day	15% 15% 4 7% 4 4 4 4 5% 7% 32%	1852
	11th to 20th	Day	18% 22% 4 8% 5% 4 4 3 4 5% 24%	1812
	21st to end	Day	20% 25% 7% 9% 5% 4 3 3 4 6% 15%	1680
December	1st to 10th	Day	17% 23% 11% 14% 6% 4 4 3 3 4 12%	2680
	11th to 20th	Day	21% 24% 8% 13% 7% 3 3 3 4 4 10%	3070
	21st to end	Day	23% 24% 7% 15% 5% 4 4 2 4 3 8%	4142
January	1st to 10th	Night	15% 23% 11% 25% 5% 3 2 2 2 3 9%	7635
	11th to 20th	Night	9% 20% 12% 21% 9% 5% 5% 3 3 3 10%	7680
	21st to end	Night	17% 19% 7% 15% 7% 5% 4 3 3 4 16%	8401
February	1st to 10th	Night	19% 17% 5% 14% 7% 4 4 2 5% 3 19%	2730
	11th to 20th	Night	16% 17% 7% 14% 6% 6% 3 3 4 4 19%	2624
	21st to end	Night	19% 15% 5% 14% 5% 4 4 3 4 4 23%	2076
March	1st to 10th	Night	20% 16% 4 8% 4 3 3 2 4 4 31%	5124
	11th to 20th	Night	13% 13% 5% 8% 3 3 4 3 4 7% 37%	5090
	21st to end	Night	17% 15% 6% 10% 5% 4 3 3 3 4 31%	5588
April	1st to 10th	Night	19% 16% 5% 7% 5% 4 3 3 5% 5% 28%	2252
	11th to 20th	Night	17% 11% 3 7% 4 5% 5% 4 6% 6% 32%	1927
	21st to end	Night	16% 10% 3 7% 5% 4 4 4 6% 6% 36%	1890
May	1st to 10th	Night	24% 10% 3 8% 4 5% 5% 3 5% 6% 28%	4123
	11th to 20th	Night	15% 9% 3 7% 5% 4 4 4 6% 8% 36%	4122
	21st to end	Night	21% 7% 2 4 3 3 4 3 6% 8% 38%	4286
June	1st to 10th	Night	12% 7% 3 7% 5% 4 4 5% 5% 6% 42%	1600
	11th to 20th	Night	10% 5% 2 7% 6% 5% 5% 5% 6% 7% 42%	1600
	21st to end	Night	10% 5% 2 6% 6% 5% 5% 4 6% 9% 41%	1600
July	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 10% 45%	3912
	11th to 20th	Night	11% 5% 2 7% 4 4 4 5% 6% 8% 44%	4057
	21st to end	Night	10% 5% 2 6% 4 4 4 3 6% 7% 49%	4427
August	1st to 10th	Night	11% 6% 2 7% 5% 4 5% 4 6% 9% 40%	2915
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 5% 6% 41%	2875
	21st to end	Night	10% 6% 2 6% 5% 4 5% 4 6% 8% 43%	3279
September	1st to 10th	Night	12% 6% 2 5% 4 4 5% 4 5% 7% 45%	2080
	11th to 20th	Night	12% 4 2 5% 4 4 4 3 5% 6% 52%	2200
	21st to end	Night	18% 7% 2 6% 6% 5% 5% 3 5% 6% 37%	2200
October	1st to 10th	Night	15% 7% 3 6% 4 4 4 3 6% 7% 43%	2345
	11th to 20th	Night	21% 9% 3 7% 5% 4 4 4 5% 6% 33%	2400
	21st to end	Night	28% 12% 3 7% 4 4 3 4 4 5% 28%	2640
November	1st to 10th	Night	17% 14% 3 6% 4 3 3 3 5% 7% 35%	2664
	11th to 20th	Night	18% 22% 4 10% 4 4 4 3 4 5% 22%	2804
	21st to end	Night	21% 23% 5% 8% 4 4 4 3 5% 5% 19%	2940
December	1st to 10th	Night	16% 23% 12% 13% 5% 4 3 3 4 4 13%	4720
	11th to 20th	Night	18% 25% 8% 13% 5% 4 3 3 4 4 14%	5345
	21st to end	Night	20% 24% 7% 13% 5% 5% 4 3 5% 4 11%	7038

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	81% 4 7% 5% 2 1	5259
	11th to 20th	Day	82% 3 9% 4 2	5280
	21st to end	Day	79% 4 6% 8% 3	5780
February	1st to 10th	Day	70% 6% 7% 12% 5%	1890
	11th to 20th	Day	69% 6% 8% 12% 5%	1889
	21st to end	Day	63% 7% 12% 11% 6% 2	1730
March	1st to 10th	Day	56% 6% 13% 19% 5%	4947
	11th to 20th	Day	51% 8% 14% 20% 6% 1	5090
	21st to end	Day	58% 8% 7% 17% 9% 1	5586
April	1st to 10th	Day	52% 6% 12% 17% 12% 1	2367
	11th to 20th	Day	50% 7% 10% 18% 12% 2	2660
	21st to end	Day	50% 6% 11% 20% 10% 2	2730
May	1st to 10th	Day	53% 7% 8% 18% 10% 3	6027
	11th to 20th	Day	47% 9% 12% 19% 11% 1	5995
	21st to end	Day	45% 8% 12% 19% 13% 4	6822
June	1st to 10th	Day	41% 8% 12% 25% 13% 1	2800
	11th to 20th	Day	40% 9% 14% 21% 14% 2	2800
	21st to end	Day	37% 7% 13% 25% 16% 2	2800
July	1st to 10th	Day	34% 6% 12% 28% 17% 2	5974
	11th to 20th	Day	37% 7% 14% 26% 15% 1	5831
	21st to end	Day	35% 8% 13% 23% 21% 1	6240
August	1st to 10th	Day	39% 8% 16% 25% 12%	4221
	11th to 20th	Day	40% 9% 12% 30% 7% 1	4213
	21st to end	Day	36% 8% 15% 28% 12% 1	4586
September	1st to 10th	Day	38% 8% 13% 23% 15% 2	2316
	11th to 20th	Day	35% 7% 15% 26% 16% 1	2200
	21st to end	Day	47% 7% 11% 22% 13% 1	2200
October	1st to 10th	Day	36% 6% 14% 25% 19% 1	3340
	11th to 20th	Day	47% 5% 10% 20% 16% 1	3240
	21st to end	Day	45% 5% 14% 24% 11% 1	3538
November	1st to 10th	Day	51% 7% 10% 19% 11% 1	1852
	11th to 20th	Day	63% 5% 9% 13% 7% 2	1812
	21st to end	Day	71% 5% 9% 11% 4	1680
December	1st to 10th	Day	76% 7% 10% 6% 2	2680
	11th to 20th	Day	78% 5% 7% 8% 1	3066
	21st to end	Day	81% 5% 7% 6% 1	4142
January	1st to 10th	Night	82% 4 4 7% 2	7636
	11th to 20th	Night	80% 4 7% 6% 2	7680
	21st to end	Night	72% 4 8% 12% 4	8401
February	1st to 10th	Night	69% 6% 8% 12% 4	2730
	11th to 20th	Night	68% 5% 9% 13% 5%	2626
	21st to end	Night	64% 7% 8% 13% 7% 1	2076
March	1st to 10th	Night	56% 5% 13% 17% 9%	5124
	11th to 20th	Night	46% 7% 13% 23% 10% 1	5090
	21st to end	Night	55% 7% 9% 16% 12% 1	5588
April	1st to 10th	Night	56% 6% 11% 15% 11% 2	2252
	11th to 20th	Night	50% 6% 11% 18% 12% 2	1927
	21st to end	Night	45% 8% 11% 19% 14% 3	1890
May	1st to 10th	Night	55% 7% 9% 14% 9% 6%	4150
	11th to 20th	Night	44% 10% 11% 22% 12% 2	4123
	21st to end	Night	41% 8% 13% 22% 15% 2	4293
June	1st to 10th	Night	41% 8% 10% 23% 15% 3	1600
	11th to 20th	Night	39% 8% 12% 21% 18% 2	1600
	21st to end	Night	37% 9% 12% 25% 16% 2	1600
July	1st to 10th	Night	34% 7% 13% 24% 18% 4	3869
	11th to 20th	Night	36% 7% 12% 25% 16% 3	4006
	21st to end	Night	33% 6% 12% 20% 26% 2	4297
August	1st to 10th	Night	38% 8% 12% 27% 13% 1	2904
	11th to 20th	Night	43% 6% 11% 28% 11% 1	2902
	21st to end	Night	37% 7% 12% 28% 14% 2	3279
September	1st to 10th	Night	36% 7% 14% 26% 14% 3	2080
	11th to 20th	Night	32% 6% 13% 27% 19% 2	2200
	21st to end	Night	47% 7% 10% 19% 16% 1	2200
October	1st to 10th	Night	33% 6% 9% 28% 22% 1	3839
	11th to 20th	Night	48% 5% 10% 18% 17% 2	3899
	21st to end	Night	47% 5% 12% 21% 14% 1	4252
November	1st to 10th	Night	50% 5% 11% 19% 12% 2	2664
	11th to 20th	Night	63% 6% 10% 13% 6% 3	2804
	21st to end	Night	69% 5% 9% 13% 4	2940
December	1st to 10th	Night	75% 6% 10% 7% 2	4720
	11th to 20th	Night	75% 5% 9% 10% 1	5348
	21st to end	Night	75% 7% 8% 9% 1	7038

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	30% 30% 30% 7% 3	330
	11th to 20th	Day	36% 30% 26% 6% 2	330
	21st to end	Day	31% 30% 29% 8% 1 1	363
February	1st to 10th	Day	33% 24% 34% 9% 1	330
	11th to 20th	Day	33% 32% 27% 8% 1	330
	21st to end	Day	32% 33% 29% 6%	273
March	1st to 10th	Day	28% 26% 31% 12% 3	490
	11th to 20th	Day	30% 24% 31% 12% 2 1	540
	21st to end	Day	34% 21% 29% 12% 4	594
April	1st to 10th	Day	27% 24% 29% 16% 3 1	540
	11th to 20th	Day	30% 19% 28% 16% 7% 1	532
	21st to end	Day	25% 21% 32% 16% 4 1	540
May	1st to 10th	Day	25% 21% 31% 18% 4 1	543
	11th to 20th	Day	19% 13% 34% 26% 8% 2	567
	21st to end	Day	17% 10% 29% 26% 14% 3	617
June	1st to 10th	Day	18% 9% 35% 26% 10% 2	530
	11th to 20th	Day	17% 11% 29% 29% 12% 1	530
	21st to end	Day	14% 9% 33% 30% 11% 2	530
July	1st to 10th	Day	12% 6% 26% 30% 21% 6%	525
	11th to 20th	Day	15% 8% 26% 28% 18% 5%	524
	21st to end	Day	10% 6% 30% 32% 16% 6%	576
August	1st to 10th	Day	10% 6% 24% 35% 18% 7%	526
	11th to 20th	Day	14% 7% 24% 31% 17% 6%	519
	21st to end	Day	12% 8% 24% 30% 18% 7%	583
September	1st to 10th	Day	17% 10% 20% 28% 20% 5%	530
	11th to 20th	Day	12% 4 21% 33% 22% 9%	530
	21st to end	Day	22% 13% 27% 22% 12% 5%	530
October	1st to 10th	Day	15% 14% 25% 26% 16% 4	530
	11th to 20th	Day	26% 17% 32% 18% 5% 1	530
	21st to end	Day	33% 19% 28% 17% 2	583
November	1st to 10th	Day	24% 28% 26% 14% 6% 1	330
	11th to 20th	Day	37% 21% 25% 12% 4	289
	21st to end	Day	37% 25% 30% 7%	220
December	1st to 10th	Day	28% 30% 29% 10% 2 1	220
	11th to 20th	Day	28% 34% 29% 8% 1	220
	21st to end	Day	34% 28% 30% 8%	264
January	1st to 10th	Night	28% 28% 30% 10% 3	840
	11th to 20th	Night	33% 25% 29% 12% 2	840
	21st to end	Night	26% 25% 32% 14% 3	924
February	1st to 10th	Night	29% 23% 28% 17% 3	840
	11th to 20th	Night	29% 23% 31% 15% 2	823
	21st to end	Night	29% 25% 31% 15% 1	692
March	1st to 10th	Night	25% 21% 31% 19% 3	840
	11th to 20th	Night	25% 22% 29% 18% 5% 1	840
	21st to end	Night	26% 18% 29% 20% 6%	924
April	1st to 10th	Night	26% 19% 27% 22% 5%	840
	11th to 20th	Night	24% 15% 35% 19% 7% 1	683
	21st to end	Night	20% 12% 34% 26% 7%	630
May	1st to 10th	Night	23% 11% 26% 30% 10% 1	630
	11th to 20th	Night	18% 11% 27% 34% 10% 2	621
	21st to end	Night	16% 3 24% 35% 18% 2	578
June	1st to 10th	Night	12% 5% 33% 34% 15% 1	400
	11th to 20th	Night	14% 5% 28% 35% 19% 1	400
	21st to end	Night	11% 5% 28% 39% 17%	400
July	1st to 10th	Night	8% 4 17% 39% 27% 5%	517
	11th to 20th	Night	10% 5% 22% 36% 23% 4	582
	21st to end	Night	10% 6% 17% 38% 26% 2	657
August	1st to 10th	Night	10% 3 18% 38% 26% 4	597
	11th to 20th	Night	12% 3 14% 38% 28% 4	594
	21st to end	Night	13% 5% 20% 32% 27% 3	690
September	1st to 10th	Night	15% 7% 19% 32% 24% 4	800
	11th to 20th	Night	13% 4 16% 36% 26% 5%	800
	21st to end	Night	20% 9% 24% 26% 18% 4	800
October	1st to 10th	Night	18% 10% 25% 28% 17% 2	800
	11th to 20th	Night	25% 17% 26% 23% 8%	800
	21st to end	Night	36% 16% 27% 18% 3	880
November	1st to 10th	Night	26% 17% 29% 20% 7% 1	820
	11th to 20th	Night	31% 21% 29% 13% 4 1	840
	21st to end	Night	35% 23% 27% 11% 3	840
December	1st to 10th	Night	33% 24% 27% 12% 3	840
	11th to 20th	Night	30% 28% 29% 11% 1	840
	21st to end	Night	33% 22% 32% 12% 1	924

Page generated at 2026-05-29 01:53:26. Version ac07

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