Fort Griffin State Historic Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fort Griffin State Historic Site clear sky chart from 2005-11-06 to 2025-09-04.

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	8% 4 2 4 3 3 3 3 5% 6% 59%	126033
	Night	7% 4 1 4 3 3 3 3 5% 6% 61%	122993
Transparency	Day	26% 4 8% 22% 27% 13%	129838
	Night	24% 4 8% 22% 27% 16%	127251
Seeing	Day	9% 6% 23% 31% 23% 8%	16293
	Night	9% 6% 31% 39% 14% 1	26126

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	9% 7% 3 6% 4 3 3 2 4 5% 55%	4077
	11th to 20th	Day	5% 5% 1 3 2 2 2 2 4 6% 69%	4080
	21st to end	Day	14% 8% 2 4 3 2 3 2 4 5% 53%	4764
February	1st to 10th	Day	11% 6% 1 4 2 2 2 3 5% 5% 58%	2000
	11th to 20th	Day	10% 7% 2 6% 3 3 3 2 4 5% 55%	1949
	21st to end	Day	13% 7% 2 3 2 2 2 2 3 5% 59%	1650
March	1st to 10th	Day	12% 4 1 4 3 3 3 3 5% 7% 57%	4666
	11th to 20th	Day	11% 4 1 3 3 3 3 3 4 5% 60%	4980
	21st to end	Day	5% 5% 1 5% 4 4 4 4 5% 6% 57%	5462
April	1st to 10th	Day	10% 4 1 4 3 4 3 3 5% 6% 58%	2210
	11th to 20th	Day	11% 5% 2 4 3 2 4 3 4 5% 57%	2374
	21st to end	Day	9% 4 1 4 4 3 3 3 5% 4 59%	2539
May	1st to 10th	Day	13% 4 2 5% 3 3 3 3 5% 7% 52%	5862
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 4 6% 55%	5862
	21st to end	Day	10% 5% 2 6% 4 4 4 4 5% 7% 48%	6434
June	1st to 10th	Day	6% 3 1 5% 3 4 4 4 5% 8% 57%	2600
	11th to 20th	Day	4 2 1 4 3 4 4 4 5% 7% 62%	2600
	21st to end	Day	5% 2 1 3 3 4 3 3 5% 6% 63%	2600
July	1st to 10th	Day	10% 7% 3 7% 5% 4 5% 3 5% 8% 44%	5900
	11th to 20th	Day	3 2 1 3 3 4 4 4 6% 9% 60%	5900
	21st to end	Day	2 1 1 3 2 2 3 3 5% 9% 68%	6422
August	1st to 10th	Day	2 1 1 3 3 3 3 3 4 7% 72%	4219
	11th to 20th	Day	3 2 1 3 3 3 3 3 6% 8% 66%	3846
	21st to end	Day	4 2 1 3 3 3 4 3 5% 8% 65%	4312
September	1st to 10th	Day	11% 4 2 4 3 4 3 3 5% 5% 56%	2133
	11th to 20th	Day	11% 5% 2 4 4 3 4 3 5% 7% 51%	2090
	21st to end	Day	9% 3 2 3 2 3 3 3 4 6% 62%	2090
October	1st to 10th	Day	5% 2 1 3 3 3 3 3 4 6% 66%	2090
	11th to 20th	Day	9% 3 1 3 2 2 3 2 4 4 68%	1995
	21st to end	Day	11% 4 1 3 2 2 2 2 3 3 68%	2090
November	1st to 10th	Day	8% 4 2 4 3 2 3 3 4 5% 63%	1818
	11th to 20th	Day	6% 3 1 3 3 2 3 2 5% 6% 66%	1800
	21st to end	Day	12% 4 1 5% 3 3 3 2 4 5% 59%	1800
December	1st to 10th	Day	10% 5% 2 5% 3 3 3 3 4 6% 55%	2940
	11th to 20th	Day	7% 4 2 4 4 4 4 3 4 5% 60%	3391
	21st to end	Day	8% 6% 2 5% 3 3 2 2 3 3 63%	4488
January	1st to 10th	Night	10% 6% 2 4 3 2 2 2 4 5% 60%	5888
	11th to 20th	Night	5% 4 1 3 2 2 2 2 4 5% 71%	5900
	21st to end	Night	13% 8% 2 4 3 3 3 2 4 5% 54%	6245
February	1st to 10th	Night	10% 5% 1 5% 3 3 3 3 4 5% 59%	2400
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 5% 55%	2338
	21st to end	Night	10% 6% 2 3 2 3 3 2 4 4 62%	1980
March	1st to 10th	Night	8% 6% 2 5% 3 3 3 3 4 5% 58%	5200
	11th to 20th	Night	9% 3 1 3 2 3 3 2 5% 7% 62%	4980
	21st to end	Night	5% 6% 2 4 3 3 3 3 5% 5% 59%	5467
April	1st to 10th	Night	7% 5% 1 4 3 3 3 2 4 8% 60%	2185
	11th to 20th	Night	10% 3 2 3 3 3 4 3 5% 5% 57%	1977
	21st to end	Night	7% 4 2 4 2 2 3 3 5% 6% 62%	1860
May	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 4 5% 54%	4053
	11th to 20th	Night	6% 3 1 5% 4 4 4 4 6% 7% 56%	4053
	21st to end	Night	9% 4 2 5% 3 3 4 4 6% 7% 52%	4443
June	1st to 10th	Night	4 2 1 4 3 3 4 5% 7% 10% 57%	1800
	11th to 20th	Night	2 2 1 3 3 3 4 4 6% 9% 64%	1800
	21st to end	Night	4 2 1 3 3 3 3 3 5% 6% 67%	1800
July	1st to 10th	Night	7% 6% 3 7% 4 4 4 3 6% 9% 48%	4078
	11th to 20th	Night	2 2 1 3 2 2 3 2 5% 10% 68%	4077
	21st to end	Night	1 1 1 2 2 3 3 3 6% 8% 70%	4449
August	1st to 10th	Night	1 1 2 2 2 2 2 5% 8% 74%	2926
	11th to 20th	Night	2 2 1 3 2 2 3 3 5% 9% 68%	3210
	21st to end	Night	3 3 1 4 3 3 4 3 5% 8% 64%	3597
September	1st to 10th	Night	9% 4 1 4 3 3 5% 3 4 6% 59%	2113
	11th to 20th	Night	10% 4 2 5% 4 3 3 3 5% 7% 54%	2090
	21st to end	Night	7% 3 1 4 2 3 2 3 5% 5% 63%	2090
October	1st to 10th	Night	6% 3 1 2 2 2 3 3 4 7% 67%	2090
	11th to 20th	Night	8% 3 1 3 2 2 2 2 3 5% 70%	2163
	21st to end	Night	9% 4 1 3 2 2 2 2 3 4 67%	2508
November	1st to 10th	Night	9% 4 1 3 2 2 3 2 4 5% 66%	2477
	11th to 20th	Night	4 3 1 4 3 3 3 2 5% 5% 67%	2600
	21st to end	Night	12% 4 2 3 3 3 3 3 4 6% 57%	2600
December	1st to 10th	Night	10% 5% 1 5% 3 3 3 2 4 5% 59%	4250
	11th to 20th	Night	7% 4 1 4 3 3 3 3 4 6% 62%	4816
	21st to end	Night	8% 5% 2 5% 3 3 3 3 4 5% 61%	6490

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 6% 22% 25% 11%	4078
	11th to 20th	Day	19% 2 6% 21% 39% 13%	4080
	21st to end	Day	36% 2 4 16% 24% 18%	4764
February	1st to 10th	Day	29% 3 6% 19% 27% 16%	2000
	11th to 20th	Day	34% 4 6% 20% 24% 13%	1959
	21st to end	Day	31% 2 4 13% 27% 22%	1650
March	1st to 10th	Day	28% 3 5% 12% 24% 28%	4666
	11th to 20th	Day	28% 3 8% 13% 23% 26%	4980
	21st to end	Day	27% 4 8% 20% 27% 13%	5464
April	1st to 10th	Day	27% 4 8% 20% 24% 16%	2210
	11th to 20th	Day	31% 4 7% 16% 25% 17%	2374
	21st to end	Day	28% 4 8% 19% 31% 10%	2539
May	1st to 10th	Day	33% 4 8% 23% 21% 11%	5900
	11th to 20th	Day	32% 4 7% 17% 28% 12%	5866
	21st to end	Day	35% 6% 10% 25% 14% 11%	6447
June	1st to 10th	Day	24% 6% 14% 30% 22% 5%	2600
	11th to 20th	Day	20% 5% 10% 30% 27% 9%	2600
	21st to end	Day	21% 5% 12% 27% 26% 9%	2600
July	1st to 10th	Day	38% 6% 10% 26% 16% 4	5836
	11th to 20th	Day	20% 5% 14% 29% 28% 4	5831
	21st to end	Day	13% 5% 13% 33% 32% 4	6240
August	1st to 10th	Day	13% 4 10% 26% 38% 9%	4205
	11th to 20th	Day	17% 5% 11% 32% 29% 6%	3882
	21st to end	Day	19% 5% 16% 29% 27% 4	4311
September	1st to 10th	Day	29% 5% 9% 21% 25% 12%	2133
	11th to 20th	Day	34% 6% 8% 19% 26% 9%	2090
	21st to end	Day	25% 4 8% 20% 28% 16%	2090
October	1st to 10th	Day	13% 3 5% 15% 37% 27%	3480
	11th to 20th	Day	18% 3 7% 20% 31% 20%	3300
	21st to end	Day	16% 2 3 27% 39% 14%	3428
November	1st to 10th	Day	25% 3 7% 23% 26% 16%	1818
	11th to 20th	Day	21% 3 6% 23% 30% 17%	1800
	21st to end	Day	30% 2 5% 20% 28% 14%	1800
December	1st to 10th	Day	31% 4 5% 25% 27% 8%	2940
	11th to 20th	Day	28% 3 8% 16% 29% 16%	3389
	21st to end	Day	29% 2 4 25% 26% 14%	4488
January	1st to 10th	Night	29% 2 6% 25% 29% 9%	5888
	11th to 20th	Night	19% 2 5% 19% 36% 20%	5900
	21st to end	Night	34% 2 7% 14% 27% 16%	6245
February	1st to 10th	Night	29% 3 6% 17% 26% 18%	2400
	11th to 20th	Night	31% 4 8% 17% 23% 17%	2338
	21st to end	Night	28% 3 5% 16% 24% 25%	1980
March	1st to 10th	Night	29% 3 3 12% 22% 30%	5200
	11th to 20th	Night	24% 3 8% 18% 16% 32%	4980
	21st to end	Night	27% 4 6% 19% 31% 14%	5467
April	1st to 10th	Night	25% 4 9% 19% 22% 21%	2185
	11th to 20th	Night	28% 5% 10% 17% 22% 19%	1977
	21st to end	Night	23% 4 11% 24% 29% 10%	1860
May	1st to 10th	Night	34% 5% 8% 21% 21% 12%	4080
	11th to 20th	Night	27% 5% 9% 18% 22% 19%	4054
	21st to end	Night	30% 6% 11% 22% 19% 12%	4452
June	1st to 10th	Night	20% 7% 16% 30% 23% 5%	1800
	11th to 20th	Night	16% 6% 12% 29% 27% 9%	1800
	21st to end	Night	19% 4 11% 27% 27% 12%	1800
July	1st to 10th	Night	32% 6% 11% 29% 18% 3	4035
	11th to 20th	Night	14% 4 9% 34% 30% 8%	4026
	21st to end	Night	12% 4 14% 35% 30% 4	4320
August	1st to 10th	Night	10% 3 6% 26% 44% 11%	2916
	11th to 20th	Night	15% 3 12% 31% 30% 9%	3240
	21st to end	Night	19% 5% 18% 28% 25% 5%	3597
September	1st to 10th	Night	27% 5% 9% 22% 24% 13%	2113
	11th to 20th	Night	30% 5% 11% 20% 23% 10%	2090
	21st to end	Night	23% 4 6% 20% 28% 19%	2090
October	1st to 10th	Night	11% 2 7% 13% 34% 34%	3480
	11th to 20th	Night	15% 3 7% 20% 31% 24%	3584
	21st to end	Night	17% 3 4 21% 42% 15%	4120
November	1st to 10th	Night	23% 4 7% 20% 26% 20%	2477
	11th to 20th	Night	20% 4 6% 23% 29% 19%	2600
	21st to end	Night	29% 3 5% 17% 30% 15%	2600
December	1st to 10th	Night	29% 3 5% 25% 26% 12%	4250
	11th to 20th	Night	26% 3 7% 18% 30% 17%	4817
	21st to end	Night	28% 3 5% 23% 27% 15%	6490

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	10% 14% 37% 29% 9%	300
	11th to 20th	Day	15% 12% 32% 30% 10% 1	300
	21st to end	Day	13% 10% 41% 27% 9% 1	330
February	1st to 10th	Day	15% 11% 38% 27% 9% 1	300
	11th to 20th	Day	17% 12% 41% 23% 7%	300
	21st to end	Day	20% 12% 37% 23% 6%	249
March	1st to 10th	Day	17% 11% 35% 29% 7%	300
	11th to 20th	Day	19% 11% 38% 25% 5% 1	300
	21st to end	Day	11% 14% 43% 23% 8% 1	330
April	1st to 10th	Day	13% 14% 40% 22% 10% 2	310
	11th to 20th	Day	14% 14% 47% 19% 6%	492
	21st to end	Day	12% 12% 39% 28% 9%	639
May	1st to 10th	Day	10% 9% 36% 34% 9% 1	703
	11th to 20th	Day	13% 6% 29% 33% 14% 4	725
	21st to end	Day	9% 5% 31% 34% 16% 4	789
June	1st to 10th	Day	7% 2 15% 44% 26% 6%	730
	11th to 20th	Day	4 2 18% 47% 25% 5%	730
	21st to end	Day	4 1 10% 37% 37% 11%	730
July	1st to 10th	Day	7% 1 6% 33% 37% 16%	721
	11th to 20th	Day	3 1 6% 33% 42% 15%	718
	21st to end	Day	2 6% 26% 46% 20%	794
August	1st to 10th	Day	2 5% 30% 46% 17%	724
	11th to 20th	Day	5% 1 5% 26% 45% 18%	517
	21st to end	Day	4 1 5% 28% 43% 19%	583
September	1st to 10th	Day	12% 1 4 21% 39% 24%	319
	11th to 20th	Day	5% 1 6% 26% 40% 22%	300
	21st to end	Day	12% 2 7% 26% 35% 17%	300
October	1st to 10th	Day	3 2 10% 38% 34% 13%	300
	11th to 20th	Day	9% 5% 29% 37% 17% 4	300
	21st to end	Day	15% 7% 26% 31% 17% 3	330
November	1st to 10th	Day	15% 7% 27% 32% 17% 3	300
	11th to 20th	Day	12% 9% 30% 27% 17% 5%	300
	21st to end	Day	16% 12% 32% 27% 10% 3	300
December	1st to 10th	Day	14% 8% 34% 31% 10% 2	300
	11th to 20th	Day	8% 8% 39% 31% 13%	300
	21st to end	Day	17% 10% 40% 24% 9%	330
January	1st to 10th	Night	10% 8% 37% 36% 9%	1000
	11th to 20th	Night	11% 10% 35% 38% 7%	1000
	21st to end	Night	11% 10% 37% 36% 6%	998
February	1st to 10th	Night	13% 8% 41% 33% 5%	800
	11th to 20th	Night	14% 10% 41% 31% 4	784
	21st to end	Night	13% 14% 47% 23% 3	660
March	1st to 10th	Night	12% 12% 39% 32% 4	730
	11th to 20th	Night	13% 11% 41% 30% 5%	600
	21st to end	Night	7% 13% 46% 30% 4	660
April	1st to 10th	Night	8% 11% 53% 25% 3	600
	11th to 20th	Night	11% 12% 48% 27% 2	593
	21st to end	Night	7% 11% 47% 30% 5%	600
May	1st to 10th	Night	9% 8% 48% 31% 5%	600
	11th to 20th	Night	7% 6% 39% 38% 10% 1	591
	21st to end	Night	7% 5% 40% 35% 12% 1	650
June	1st to 10th	Night	4 2 28% 46% 19% 1	600
	11th to 20th	Night	2 2 30% 57% 9%	600
	21st to end	Night	4 2 18% 54% 20% 2	600
July	1st to 10th	Night	5% 2 8% 48% 35% 2	585
	11th to 20th	Night	3 7% 53% 35% 2	582
	21st to end	Night	2 6% 53% 37% 2	657
August	1st to 10th	Night	2 6% 54% 37% 1	597
	11th to 20th	Night	5% 1 8% 46% 38% 2	594
	21st to end	Night	4 1 6% 42% 43% 4	660
September	1st to 10th	Night	10% 2 10% 43% 33% 3	579
	11th to 20th	Night	11% 1 11% 48% 28% 2	570
	21st to end	Night	9% 2 14% 48% 26% 2	570
October	1st to 10th	Night	7% 2 24% 43% 24% 1	570
	11th to 20th	Night	10% 4 29% 42% 14% 1	643
	21st to end	Night	11% 7% 31% 36% 14% 1	836
November	1st to 10th	Night	10% 4 30% 41% 13% 1	917
	11th to 20th	Night	4 9% 35% 41% 11% 1	1000
	21st to end	Night	14% 7% 36% 30% 12% 1	1000
December	1st to 10th	Night	12% 6% 34% 41% 7%	1000
	11th to 20th	Night	14% 8% 35% 37% 7%	1000
	21st to end	Night	11% 7% 39% 35% 8%	1100

Page generated at 2025-09-01 00:51:09. Version ac07

Copyright © 2025 Attilla Danko.