John Bryan State Park Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the John Bryan State Park Observatory 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	14% 9% 4 8% 5% 5% 5% 4 6% 7% 33%	132666
	Night	15% 10% 4 8% 5% 4 4 3 5% 7% 35%	130427
Transparency	Day	48% 6% 11% 21% 13% 1	136293
	Night	47% 5% 11% 22% 13% 2	134857
Seeing	Day	18% 9% 27% 23% 16% 7%	17398
	Night	18% 8% 27% 30% 14% 2	27024

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	18% 15% 8% 13% 5% 5% 4 3 4 5% 20%	4782
	11th to 20th	Day	12% 15% 6% 16% 8% 7% 5% 3 3 4 22%	5280
	21st to end	Day	18% 13% 7% 11% 7% 7% 6% 4 5% 5% 16%	5780
February	1st to 10th	Day	18% 13% 6% 11% 6% 5% 4 3 4 6% 23%	1941
	11th to 20th	Day	16% 13% 6% 12% 6% 5% 5% 3 3 5% 25%	2048
	21st to end	Day	17% 14% 4 10% 5% 4 4 4 4 6% 29%	1730
March	1st to 10th	Day	18% 13% 7% 11% 5% 4 3 3 4 5% 28%	4674
	11th to 20th	Day	12% 9% 3 9% 4 4 4 3 5% 7% 39%	5090
	21st to end	Day	18% 10% 4 9% 5% 4 4 3 5% 7% 32%	5586
April	1st to 10th	Day	15% 12% 4 8% 4 4 4 3 5% 8% 32%	2436
	11th to 20th	Day	15% 10% 3 8% 5% 5% 4 3 4 7% 37%	2596
	21st to end	Day	16% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	2730
May	1st to 10th	Day	20% 9% 3 8% 5% 5% 5% 4 6% 7% 29%	5992
	11th to 20th	Day	19% 9% 3 8% 4 4 4 3 5% 8% 33%	5991
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 5% 7% 8% 32%	6576
June	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 5% 6% 10% 37%	2600
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 5% 7% 9% 40%	2769
	21st to end	Day	10% 5% 2 6% 5% 5% 6% 5% 7% 10% 38%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	6360
	11th to 20th	Day	10% 5% 2 7% 5% 5% 6% 5% 7% 10% 37%	6049
	21st to end	Day	10% 6% 3 8% 5% 6% 7% 5% 7% 9% 35%	6424
August	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 5% 8% 10% 42%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 4 7% 5% 8% 10% 40%	4174
	21st to end	Day	6% 4 2 5% 4 4 5% 4 6% 9% 49%	4453
September	1st to 10th	Day	11% 5% 2 5% 3 4 3 4 6% 7% 51%	2252
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 6% 8% 51%	2200
	21st to end	Day	16% 7% 2 6% 4 3 4 3 5% 7% 44%	2200
October	1st to 10th	Day	11% 7% 2 6% 4 3 4 4 5% 7% 48%	2194
	11th to 20th	Day	14% 7% 2 5% 4 4 4 3 5% 7% 46%	2000
	21st to end	Day	20% 9% 3 7% 4 4 4 3 5% 7% 35%	2076
November	1st to 10th	Day	10% 7% 3 6% 4 4 4 3 5% 6% 48%	1852
	11th to 20th	Day	15% 12% 3 9% 5% 4 4 3 5% 6% 34%	1890
	21st to end	Day	18% 12% 5% 9% 6% 5% 3 2 4 6% 30%	1890
December	1st to 10th	Day	18% 10% 4 8% 5% 4 5% 4 5% 6% 32%	3020
	11th to 20th	Day	21% 15% 6% 11% 5% 5% 4 3 4 4 23%	3463
	21st to end	Day	23% 20% 6% 10% 5% 4 4 3 4 5% 15%	4534
January	1st to 10th	Night	13% 16% 6% 14% 8% 6% 5% 3 5% 5% 18%	8063
	11th to 20th	Night	10% 15% 8% 15% 7% 7% 5% 4 4 5% 19%	7680
	21st to end	Night	17% 13% 5% 9% 6% 6% 4 3 4 6% 28%	8401
February	1st to 10th	Night	19% 13% 4 9% 5% 4 5% 3 4 5% 29%	2677
	11th to 20th	Night	16% 12% 4 10% 5% 5% 4 3 5% 5% 30%	2458
	21st to end	Night	19% 14% 5% 10% 4 4 4 3 3 5% 29%	2076
March	1st to 10th	Night	20% 12% 3 7% 4 4 3 3 4 5% 35%	5385
	11th to 20th	Night	12% 7% 3 7% 5% 5% 5% 4 5% 7% 40%	5090
	21st to end	Night	18% 9% 4 8% 5% 4 4 3 4 6% 35%	5588
April	1st to 10th	Night	16% 9% 3 6% 5% 4 4 3 6% 7% 37%	2201
	11th to 20th	Night	14% 8% 2 6% 4 4 4 4 6% 8% 39%	1972
	21st to end	Night	17% 7% 3 6% 4 4 4 4 5% 7% 38%	1890
May	1st to 10th	Night	23% 9% 3 8% 5% 4 5% 4 5% 6% 28%	4123
	11th to 20th	Night	16% 8% 2 6% 5% 4 4 3 6% 10% 36%	4122
	21st to end	Night	14% 6% 3 7% 5% 4 5% 4 6% 9% 38%	4520
June	1st to 10th	Night	9% 5% 2 5% 5% 4 5% 4 5% 8% 49%	1800
	11th to 20th	Night	8% 5% 2 6% 4 5% 4 4 6% 11% 45%	1625
	21st to end	Night	6% 5% 2 5% 5% 4 5% 5% 7% 11% 46%	1600
July	1st to 10th	Night	9% 6% 2 6% 5% 5% 4 4 6% 8% 46%	3618
	11th to 20th	Night	9% 5% 2 6% 4 5% 5% 4 7% 10% 43%	3897
	21st to end	Night	9% 6% 3 7% 5% 5% 5% 4 7% 9% 41%	4427
August	1st to 10th	Night	9% 6% 3 7% 5% 5% 5% 5% 6% 9% 42%	2915
	11th to 20th	Night	8% 8% 3 6% 4 4 4 3 5% 7% 47%	2875
	21st to end	Night	3 3 1 6% 4 5% 5% 4 7% 10% 53%	3442
September	1st to 10th	Night	12% 5% 2 4 4 3 3 3 6% 7% 52%	2126
	11th to 20th	Night	8% 5% 1 4 4 3 4 3 5% 6% 58%	2200
	21st to end	Night	15% 6% 2 6% 4 4 3 3 4 6% 48%	2200
October	1st to 10th	Night	8% 5% 2 7% 3 3 3 3 5% 7% 52%	2205
	11th to 20th	Night	15% 7% 2 6% 5% 3 4 3 4 6% 46%	2400
	21st to end	Night	18% 9% 3 6% 4 4 4 3 5% 6% 39%	2760
November	1st to 10th	Night	14% 8% 2 5% 3 4 4 3 5% 7% 47%	2666
	11th to 20th	Night	16% 10% 3 7% 3 3 4 3 5% 6% 38%	2730
	21st to end	Night	19% 14% 5% 7% 6% 4 4 3 5% 6% 27%	2730
December	1st to 10th	Night	16% 12% 4 9% 5% 4 4 3 5% 5% 32%	4380
	11th to 20th	Night	19% 16% 7% 11% 5% 4 4 3 4 5% 23%	4952
	21st to end	Night	24% 19% 7% 10% 5% 4 3 3 4 5% 16%	6633

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	68% 4 10% 13% 5%	4783
	11th to 20th	Day	68% 4 7% 10% 10% 1	5280
	21st to end	Day	69% 5% 10% 10% 6% 1	5780
February	1st to 10th	Day	63% 5% 10% 13% 9% 1	1941
	11th to 20th	Day	62% 5% 9% 15% 8% 1	2058
	21st to end	Day	57% 5% 11% 18% 8% 1	1730
March	1st to 10th	Day	60% 4 8% 20% 8% 1	4674
	11th to 20th	Day	43% 5% 10% 28% 13% 1	5090
	21st to end	Day	51% 6% 12% 19% 10% 2	5586
April	1st to 10th	Day	50% 6% 12% 20% 11% 1	2436
	11th to 20th	Day	47% 5% 12% 21% 13% 3	2596
	21st to end	Day	48% 5% 12% 22% 11% 2	2730
May	1st to 10th	Day	54% 7% 10% 18% 11% 1	6027
	11th to 20th	Day	48% 6% 12% 24% 9%	5995
	21st to end	Day	44% 9% 12% 23% 11% 2	6590
June	1st to 10th	Day	41% 7% 11% 24% 15% 2	2600
	11th to 20th	Day	38% 7% 15% 23% 16% 1	2769
	21st to end	Day	38% 8% 15% 21% 15% 3	2800
July	1st to 10th	Day	38% 7% 10% 26% 17% 2	6291
	11th to 20th	Day	38% 8% 14% 25% 14%	5976
	21st to end	Day	41% 9% 13% 19% 15% 3	6240
August	1st to 10th	Day	33% 9% 16% 28% 12% 1	4221
	11th to 20th	Day	34% 8% 13% 31% 15%	4213
	21st to end	Day	30% 7% 13% 29% 18% 3	4452
September	1st to 10th	Day	33% 5% 12% 25% 22% 4	2252
	11th to 20th	Day	31% 5% 12% 25% 24% 3	2200
	21st to end	Day	41% 4 9% 22% 22% 2	2200
October	1st to 10th	Day	24% 4 10% 32% 28% 2	3571
	11th to 20th	Day	31% 4 10% 29% 24% 2	3240
	21st to end	Day	38% 4 11% 24% 21% 2	3326
November	1st to 10th	Day	37% 4 11% 27% 18% 3	1852
	11th to 20th	Day	51% 4 10% 20% 13% 2	1890
	21st to end	Day	57% 5% 10% 19% 9% 1	1890
December	1st to 10th	Day	53% 4 13% 23% 7%	3020
	11th to 20th	Day	65% 4 9% 16% 6%	3460
	21st to end	Day	73% 5% 8% 10% 4	4534
January	1st to 10th	Night	67% 5% 9% 13% 5%	8064
	11th to 20th	Night	66% 5% 8% 12% 8% 1	7680
	21st to end	Night	58% 4 11% 18% 7% 1	8401
February	1st to 10th	Night	58% 4 10% 17% 9% 1	2677
	11th to 20th	Night	55% 4 9% 19% 11% 1	2459
	21st to end	Night	59% 4 11% 16% 9% 2	2076
March	1st to 10th	Night	51% 4 7% 26% 10% 2	5385
	11th to 20th	Night	40% 6% 11% 29% 12% 1	5090
	21st to end	Night	49% 5% 8% 24% 12% 1	5588
April	1st to 10th	Night	45% 6% 12% 23% 12% 2	2201
	11th to 20th	Night	42% 6% 13% 18% 17% 4	1972
	21st to end	Night	44% 6% 11% 23% 13% 2	1890
May	1st to 10th	Night	55% 7% 9% 18% 8% 3	4150
	11th to 20th	Night	43% 7% 12% 24% 12% 1	4123
	21st to end	Night	39% 7% 14% 26% 10% 3	4528
June	1st to 10th	Night	34% 7% 12% 24% 22% 2	1800
	11th to 20th	Night	32% 8% 16% 24% 17% 4	1625
	21st to end	Night	30% 8% 18% 24% 17% 3	1600
July	1st to 10th	Night	35% 7% 12% 25% 18% 4	3580
	11th to 20th	Night	35% 8% 15% 27% 14% 1	3850
	21st to end	Night	36% 7% 15% 23% 17% 3	4297
August	1st to 10th	Night	36% 7% 16% 27% 13% 1	2904
	11th to 20th	Night	35% 7% 10% 28% 19%	2902
	21st to end	Night	26% 7% 16% 29% 20% 3	3442
September	1st to 10th	Night	31% 6% 12% 25% 20% 6%	2126
	11th to 20th	Night	26% 5% 10% 26% 29% 3	2200
	21st to end	Night	37% 6% 9% 21% 25% 2	2200
October	1st to 10th	Night	23% 4 7% 29% 34% 2	3608
	11th to 20th	Night	30% 4 10% 26% 28% 3	3899
	21st to end	Night	34% 4 7% 25% 25% 4	4447
November	1st to 10th	Night	38% 4 12% 24% 21% 2	2666
	11th to 20th	Night	46% 4 8% 23% 15% 3	2730
	21st to end	Night	57% 4 10% 19% 9% 1	2730
December	1st to 10th	Night	53% 5% 11% 23% 7% 1	4380
	11th to 20th	Night	62% 5% 10% 16% 6%	4954
	21st to end	Night	71% 4 8% 13% 3	6633

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	25% 15% 35% 19% 5% 1	330
	11th to 20th	Day	24% 21% 38% 13% 3	330
	21st to end	Day	28% 16% 34% 17% 4	363
February	1st to 10th	Day	25% 14% 38% 18% 4	330
	11th to 20th	Day	25% 23% 35% 14% 4	330
	21st to end	Day	23% 14% 44% 15% 3	273
March	1st to 10th	Day	26% 15% 37% 18% 4	372
	11th to 20th	Day	24% 15% 35% 18% 7% 1	540
	21st to end	Day	28% 14% 35% 19% 5%	594
April	1st to 10th	Day	20% 14% 36% 20% 8% 2	540
	11th to 20th	Day	20% 16% 32% 20% 9% 3	532
	21st to end	Day	21% 10% 36% 20% 11% 3	540
May	1st to 10th	Day	26% 13% 29% 20% 8% 3	543
	11th to 20th	Day	21% 8% 26% 25% 14% 6%	567
	21st to end	Day	15% 8% 25% 26% 15% 10%	617
June	1st to 10th	Day	15% 6% 21% 30% 22% 8%	530
	11th to 20th	Day	12% 5% 22% 31% 22% 8%	699
	21st to end	Day	11% 6% 25% 29% 20% 9%	730
July	1st to 10th	Day	13% 3 15% 26% 24% 19%	721
	11th to 20th	Day	11% 3 21% 24% 27% 14%	591
	21st to end	Day	10% 2 14% 30% 27% 17%	576
August	1st to 10th	Day	11% 2 13% 24% 29% 20%	526
	11th to 20th	Day	12% 3 15% 29% 26% 14%	519
	21st to end	Day	9% 2 12% 30% 31% 17%	583
September	1st to 10th	Day	11% 6% 12% 24% 32% 14%	530
	11th to 20th	Day	9% 2 12% 26% 31% 20%	530
	21st to end	Day	20% 7% 16% 24% 24% 9%	530
October	1st to 10th	Day	12% 6% 21% 29% 23% 9%	530
	11th to 20th	Day	16% 10% 30% 25% 14% 4	530
	21st to end	Day	23% 9% 35% 26% 6% 1	459
November	1st to 10th	Day	16% 12% 33% 25% 9% 4	330
	11th to 20th	Day	20% 16% 35% 17% 10% 2	330
	21st to end	Day	27% 12% 37% 20% 4	330
December	1st to 10th	Day	20% 18% 37% 18% 7%	330
	11th to 20th	Day	25% 14% 42% 16% 4	330
	21st to end	Day	34% 14% 33% 15% 4	363
January	1st to 10th	Night	23% 15% 35% 22% 5% 1	998
	11th to 20th	Night	24% 16% 36% 21% 3	840
	21st to end	Night	23% 13% 36% 24% 4	924
February	1st to 10th	Night	25% 10% 35% 27% 3	840
	11th to 20th	Night	24% 14% 36% 24% 2	823
	21st to end	Night	25% 11% 40% 22% 2	692
March	1st to 10th	Night	22% 12% 35% 26% 5%	840
	11th to 20th	Night	22% 12% 35% 24% 6% 1	840
	21st to end	Night	24% 11% 32% 25% 7%	924
April	1st to 10th	Night	20% 12% 34% 28% 6%	731
	11th to 20th	Night	18% 11% 33% 27% 11% 1	623
	21st to end	Night	19% 7% 40% 25% 8% 1	630
May	1st to 10th	Night	22% 7% 32% 27% 11% 1	630
	11th to 20th	Night	19% 5% 26% 33% 15% 2	621
	21st to end	Night	10% 3 23% 38% 22% 4	683
June	1st to 10th	Night	11% 2 22% 43% 21% 1	600
	11th to 20th	Night	9% 5% 17% 43% 24% 2	600
	21st to end	Night	8% 3 22% 38% 28% 2	600
July	1st to 10th	Night	11% 1 11% 36% 33% 8%	585
	11th to 20th	Night	9% 2 14% 40% 32% 5%	582
	21st to end	Night	9% 2 11% 39% 36% 3	657
August	1st to 10th	Night	7% 2 11% 41% 36% 4	597
	11th to 20th	Night	9% 2 11% 39% 34% 5%	594
	21st to end	Night	7% 2 12% 37% 36% 7%	660
September	1st to 10th	Night	14% 3 13% 34% 31% 5%	726
	11th to 20th	Night	10% 1 11% 36% 37% 5%	800
	21st to end	Night	17% 5% 18% 36% 22% 3	800
October	1st to 10th	Night	12% 4 24% 34% 22% 5%	800
	11th to 20th	Night	19% 7% 27% 33% 12% 1	800
	21st to end	Night	21% 9% 33% 28% 9%	880
November	1st to 10th	Night	17% 8% 31% 28% 15% 1	820
	11th to 20th	Night	20% 14% 32% 25% 9% 1	840
	21st to end	Night	25% 11% 29% 27% 7%	840
December	1st to 10th	Night	25% 12% 35% 23% 5%	840
	11th to 20th	Night	25% 10% 35% 24% 5% 1	840
	21st to end	Night	29% 14% 30% 23% 3	924

Page generated at 2026-05-29 01:53:28. Version ac07

Copyright © 2026 Attilla Danko.