Dreher Island State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dreher Island State Park clear sky chart from 2005-11-06 to 2025-12-18.

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% 6% 2 6% 4 4 5% 4 6% 8% 42%	127141
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 44%	123904
Transparency	Day	37% 8% 12% 19% 18% 6%	130813
	Night	37% 7% 11% 18% 20% 7%	128301
Seeing	Day	15% 10% 24% 25% 19% 8%	16777
	Night	15% 9% 25% 31% 18% 2	26847

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	15% 4 1 5% 3 3 3 2 4 6% 54%	4056
	11th to 20th	Day	11% 6% 2 4 3 2 2 2 3 5% 59%	4060
	21st to end	Day	16% 8% 2 6% 3 4 3 2 4 5% 46%	4466
February	1st to 10th	Day	14% 7% 3 6% 5% 4 4 3 5% 7% 41%	1843
	11th to 20th	Day	20% 6% 2 5% 3 3 3 3 4 5% 44%	1948
	21st to end	Day	17% 7% 2 5% 4 3 4 3 5% 5% 44%	1688
March	1st to 10th	Day	20% 8% 3 7% 4 3 3 2 4 4 41%	4935
	11th to 20th	Day	7% 4 1 5% 4 4 4 3 5% 6% 55%	4980
	21st to end	Day	15% 7% 2 6% 4 3 4 3 4 4 46%	5465
April	1st to 10th	Day	10% 4 2 5% 3 4 4 4 6% 8% 50%	2200
	11th to 20th	Day	10% 6% 2 5% 4 4 4 4 6% 7% 49%	2324
	21st to end	Day	9% 4 2 4 4 4 4 4 6% 8% 50%	2557
May	1st to 10th	Day	11% 6% 3 6% 4 4 5% 4 7% 11% 40%	5862
	11th to 20th	Day	17% 7% 3 7% 5% 5% 6% 5% 7% 8% 31%	5861
	21st to end	Day	10% 6% 2 6% 4 5% 5% 4 6% 8% 44%	6433
June	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 10% 36%	2600
	11th to 20th	Day	7% 4 1 6% 5% 5% 6% 5% 8% 11% 42%	2600
	21st to end	Day	4 4 2 6% 5% 6% 6% 6% 9% 13% 40%	2600
July	1st to 10th	Day	10% 7% 2 8% 6% 7% 8% 6% 9% 10% 27%	5900
	11th to 20th	Day	4 4 2 6% 6% 7% 8% 7% 11% 15% 30%	5900
	21st to end	Day	8% 5% 3 8% 7% 7% 7% 6% 9% 13% 27%	6424
August	1st to 10th	Day	22% 6% 2 7% 5% 6% 6% 5% 7% 10% 24%	4234
	11th to 20th	Day	11% 5% 2 7% 5% 6% 6% 5% 9% 10% 34%	4174
	21st to end	Day	7% 3 2 4 4 4 5% 5% 8% 10% 47%	4299
September	1st to 10th	Day	7% 5% 2 7% 6% 5% 5% 5% 8% 9% 42%	2200
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 6% 9% 41%	2200
	21st to end	Day	16% 8% 2 5% 3 4 3 3 5% 7% 42%	2200
October	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 4 6% 51%	2110
	11th to 20th	Day	12% 4 2 4 3 2 3 2 4 5% 60%	2000
	21st to end	Day	12% 5% 2 4 3 3 3 2 3 5% 58%	2200
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 4 5% 54%	2057
	11th to 20th	Day	13% 5% 1 4 3 2 3 3 4 7% 54%	1953
	21st to end	Day	13% 6% 2 4 4 4 4 3 5% 6% 50%	1890
December	1st to 10th	Day	12% 7% 2 5% 4 3 4 3 5% 5% 49%	3020
	11th to 20th	Day	15% 6% 2 5% 3 4 4 4 5% 6% 47%	3436
	21st to end	Day	19% 6% 2 5% 3 3 2 3 5% 6% 45%	4466
January	1st to 10th	Night	12% 4 2 5% 3 3 3 2 3 5% 58%	5889
	11th to 20th	Night	11% 6% 2 6% 4 3 4 3 5% 6% 50%	5900
	21st to end	Night	14% 8% 2 6% 3 3 4 3 4 5% 47%	6490
February	1st to 10th	Night	15% 7% 2 5% 4 4 4 3 5% 6% 46%	2555
	11th to 20th	Night	19% 7% 3 5% 4 4 4 3 4 5% 44%	2338
	21st to end	Night	15% 5% 2 6% 4 4 4 3 5% 7% 46%	1940
March	1st to 10th	Night	18% 8% 3 6% 4 3 3 3 5% 6% 40%	4918
	11th to 20th	Night	11% 3 1 4 3 3 3 3 6% 7% 55%	4980
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 6% 48%	5467
April	1st to 10th	Night	11% 5% 2 4 4 4 4 4 5% 7% 51%	2200
	11th to 20th	Night	11% 4 2 5% 4 4 4 3 4 6% 53%	2023
	21st to end	Night	8% 4 2 6% 4 4 4 3 5% 7% 53%	1858
May	1st to 10th	Night	10% 6% 3 7% 4 4 5% 4 6% 10% 41%	4033
	11th to 20th	Night	18% 6% 2 6% 4 5% 5% 5% 7% 9% 34%	4032
	21st to end	Night	11% 5% 2 5% 4 4 4 5% 7% 10% 43%	4421
June	1st to 10th	Night	11% 5% 3 7% 6% 5% 6% 5% 8% 9% 35%	1800
	11th to 20th	Night	8% 7% 2 6% 5% 5% 4 5% 8% 11% 41%	1800
	21st to end	Night	6% 5% 3 7% 5% 5% 6% 7% 9% 12% 36%	1800
July	1st to 10th	Night	12% 8% 4 9% 6% 6% 6% 5% 8% 10% 26%	4058
	11th to 20th	Night	6% 5% 2 7% 5% 6% 7% 6% 11% 17% 27%	4057
	21st to end	Night	9% 9% 4 10% 7% 6% 7% 6% 8% 10% 23%	4427
August	1st to 10th	Night	22% 7% 3 8% 5% 5% 6% 5% 7% 10% 22%	2915
	11th to 20th	Night	11% 6% 3 7% 5% 5% 7% 4 8% 10% 35%	2875
	21st to end	Night	6% 4 2 5% 4 4 5% 5% 7% 10% 48%	3561
September	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 4 8% 9% 42%	2200
	11th to 20th	Night	12% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	2200
	21st to end	Night	16% 6% 2 6% 3 4 4 3 5% 7% 44%	2200
October	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 6% 52%	2289
	11th to 20th	Night	14% 4 2 5% 3 2 3 3 3 5% 56%	2400
	21st to end	Night	11% 4 1 4 4 3 3 3 4 5% 57%	2640
November	1st to 10th	Night	12% 6% 2 4 3 4 3 2 3 5% 55%	2461
	11th to 20th	Night	13% 6% 2 5% 3 2 3 3 4 6% 54%	2663
	21st to end	Night	13% 5% 2 5% 3 4 4 4 6% 6% 49%	2730
December	1st to 10th	Night	13% 5% 2 5% 4 3 3 3 3 5% 53%	4380
	11th to 20th	Night	16% 7% 3 5% 3 3 3 3 5% 7% 45%	4914
	21st to end	Night	19% 6% 2 6% 3 4 4 3 4 5% 43%	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	31% 5% 6% 22% 29% 7%	4057
	11th to 20th	Day	30% 3 5% 16% 37% 10%	4060
	21st to end	Day	42% 3 5% 17% 22% 11%	4466
February	1st to 10th	Day	42% 4 10% 20% 18% 6%	1843
	11th to 20th	Day	43% 4 5% 18% 24% 6%	1958
	21st to end	Day	42% 5% 9% 15% 21% 8%	1688
March	1st to 10th	Day	48% 4 5% 14% 20% 10%	4935
	11th to 20th	Day	29% 5% 8% 15% 25% 19%	4980
	21st to end	Day	41% 5% 6% 18% 22% 7%	5465
April	1st to 10th	Day	31% 6% 11% 19% 26% 6%	2200
	11th to 20th	Day	32% 6% 11% 19% 21% 10%	2324
	21st to end	Day	30% 7% 12% 25% 21% 4	2557
May	1st to 10th	Day	37% 9% 17% 24% 12% 2	5897
	11th to 20th	Day	47% 9% 16% 16% 11%	5865
	21st to end	Day	35% 8% 14% 20% 20% 3	6447
June	1st to 10th	Day	39% 11% 17% 21% 10% 2	2600
	11th to 20th	Day	32% 11% 18% 25% 13% 1	2600
	21st to end	Day	31% 13% 18% 22% 13% 2	2600
July	1st to 10th	Day	44% 13% 18% 18% 6% 1	5835
	11th to 20th	Day	33% 19% 30% 16% 3 1	5831
	21st to end	Day	41% 15% 18% 19% 6%	6240
August	1st to 10th	Day	52% 11% 18% 15% 4	4221
	11th to 20th	Day	38% 13% 17% 18% 13% 1	4213
	21st to end	Day	28% 9% 16% 28% 17% 2	4299
September	1st to 10th	Day	34% 10% 16% 17% 18% 4	2200
	11th to 20th	Day	40% 8% 12% 18% 19% 3	2200
	21st to end	Day	41% 6% 10% 21% 15% 7%	2200
October	1st to 10th	Day	31% 5% 7% 19% 29% 10%	3435
	11th to 20th	Day	18% 3 5% 17% 27% 30%	3240
	21st to end	Day	26% 4 4 21% 31% 14%	3538
November	1st to 10th	Day	34% 4 7% 18% 25% 12%	2057
	11th to 20th	Day	31% 4 7% 21% 26% 10%	1953
	21st to end	Day	35% 5% 9% 20% 23% 8%	1890
December	1st to 10th	Day	37% 4 6% 17% 27% 9%	3020
	11th to 20th	Day	37% 5% 11% 21% 19% 7%	3433
	21st to end	Day	40% 3 8% 23% 24% 2	4466
January	1st to 10th	Night	30% 4 5% 19% 35% 7%	5889
	11th to 20th	Night	34% 4 6% 16% 32% 8%	5900
	21st to end	Night	40% 4 4 15% 28% 9%	6490
February	1st to 10th	Night	39% 4 8% 20% 21% 8%	2555
	11th to 20th	Night	43% 4 6% 18% 21% 8%	2339
	21st to end	Night	38% 5% 8% 17% 20% 12%	1940
March	1st to 10th	Night	43% 5% 9% 12% 24% 8%	4918
	11th to 20th	Night	27% 5% 8% 12% 29% 19%	4980
	21st to end	Night	36% 5% 7% 20% 21% 11%	5467
April	1st to 10th	Night	33% 5% 11% 19% 23% 8%	2200
	11th to 20th	Night	32% 5% 9% 21% 22% 11%	2023
	21st to end	Night	29% 6% 11% 27% 23% 5%	1858
May	1st to 10th	Night	37% 8% 15% 25% 13% 2	4060
	11th to 20th	Night	44% 9% 20% 16% 11%	4033
	21st to end	Night	33% 10% 13% 23% 19% 3	4429
June	1st to 10th	Night	41% 10% 16% 19% 11% 2	1800
	11th to 20th	Night	35% 11% 18% 23% 12% 2	1800
	21st to end	Night	35% 15% 18% 19% 12% 2	1800
July	1st to 10th	Night	48% 13% 19% 14% 5% 1	4015
	11th to 20th	Night	37% 19% 29% 11% 3 1	4006
	21st to end	Night	52% 14% 16% 14% 4	4297
August	1st to 10th	Night	55% 11% 17% 13% 3	2904
	11th to 20th	Night	40% 12% 21% 19% 7% 1	2902
	21st to end	Night	28% 9% 15% 29% 17% 3	3561
September	1st to 10th	Night	35% 10% 16% 20% 15% 5%	2200
	11th to 20th	Night	41% 8% 13% 18% 17% 3	2200
	21st to end	Night	40% 7% 13% 19% 17% 5%	2200
October	1st to 10th	Night	31% 5% 10% 18% 30% 6%	3744
	11th to 20th	Night	20% 3 5% 16% 26% 30%	3899
	21st to end	Night	25% 4 6% 18% 28% 18%	4252
November	1st to 10th	Night	32% 4 8% 16% 27% 13%	2461
	11th to 20th	Night	33% 4 7% 17% 26% 13%	2663
	21st to end	Night	34% 5% 8% 18% 25% 10%	2730
December	1st to 10th	Night	35% 4 7% 17% 30% 8%	4380
	11th to 20th	Night	39% 5% 11% 17% 20% 8%	4916
	21st to end	Night	43% 4 7% 24% 19% 3	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	19% 22% 43% 13% 3	300
	11th to 20th	Day	20% 25% 33% 19% 3	300
	21st to end	Day	21% 25% 39% 13% 2	330
February	1st to 10th	Day	16% 23% 38% 17% 5%	300
	11th to 20th	Day	34% 18% 34% 14% 1	300
	21st to end	Day	24% 20% 33% 18% 5%	287
March	1st to 10th	Day	18% 20% 34% 20% 8% 1	500
	11th to 20th	Day	18% 18% 40% 18% 6%	500
	21st to end	Day	19% 19% 36% 19% 8%	550
April	1st to 10th	Day	18% 16% 34% 21% 11%	500
	11th to 20th	Day	13% 14% 32% 24% 14% 2	492
	21st to end	Day	11% 11% 31% 26% 17% 5%	500
May	1st to 10th	Day	10% 12% 35% 27% 13% 4	503
	11th to 20th	Day	16% 5% 23% 31% 17% 8%	527
	21st to end	Day	12% 6% 20% 28% 23% 11%	573
June	1st to 10th	Day	11% 5% 17% 33% 25% 9%	530
	11th to 20th	Day	8% 4 18% 26% 30% 14%	530
	21st to end	Day	6% 3 15% 31% 31% 15%	530
July	1st to 10th	Day	9% 3 9% 25% 32% 23%	525
	11th to 20th	Day	6% 3 10% 24% 34% 22%	524
	21st to end	Day	6% 2 10% 32% 35% 15%	576
August	1st to 10th	Day	16% 5% 11% 26% 29% 14%	526
	11th to 20th	Day	13% 4 11% 24% 30% 17%	519
	21st to end	Day	13% 3 10% 30% 28% 17%	583
September	1st to 10th	Day	8% 3 12% 32% 33% 12%	530
	11th to 20th	Day	16% 4 13% 27% 29% 11%	530
	21st to end	Day	18% 3 18% 28% 25% 8%	530
October	1st to 10th	Day	15% 5% 22% 28% 22% 8%	530
	11th to 20th	Day	12% 8% 22% 30% 22% 5%	530
	21st to end	Day	14% 8% 27% 28% 19% 5%	583
November	1st to 10th	Day	18% 9% 26% 27% 16% 5%	535
	11th to 20th	Day	17% 11% 38% 25% 7% 3	393
	21st to end	Day	9% 15% 39% 25% 11% 1	330
December	1st to 10th	Day	25% 17% 36% 18% 4	330
	11th to 20th	Day	20% 15% 36% 21% 7% 2	321
	21st to end	Day	27% 17% 33% 17% 5% 3	330
January	1st to 10th	Night	20% 18% 32% 24% 6%	800
	11th to 20th	Night	19% 16% 33% 26% 6%	800
	21st to end	Night	19% 18% 35% 24% 3	880
February	1st to 10th	Night	20% 14% 32% 30% 5%	800
	11th to 20th	Night	22% 16% 36% 23% 2	783
	21st to end	Night	17% 17% 36% 26% 4	660
March	1st to 10th	Night	17% 13% 35% 29% 5%	800
	11th to 20th	Night	17% 13% 40% 25% 6%	800
	21st to end	Night	16% 15% 38% 25% 7%	880
April	1st to 10th	Night	14% 16% 32% 29% 9%	800
	11th to 20th	Night	13% 12% 30% 33% 12%	791
	21st to end	Night	9% 9% 32% 37% 13%	658
May	1st to 10th	Night	10% 9% 31% 37% 13%	600
	11th to 20th	Night	12% 4 30% 34% 18% 2	591
	21st to end	Night	10% 3 19% 38% 28% 4	650
June	1st to 10th	Night	12% 3 21% 38% 24% 4	600
	11th to 20th	Night	9% 3 16% 35% 29% 8%	600
	21st to end	Night	7% 3 14% 39% 30% 7%	600
July	1st to 10th	Night	8% 2 9% 36% 37% 7%	585
	11th to 20th	Night	10% 2 6% 34% 39% 9%	582
	21st to end	Night	8% 2 11% 34% 39% 6%	657
August	1st to 10th	Night	14% 2 10% 32% 36% 6%	597
	11th to 20th	Night	15% 3 8% 33% 36% 6%	594
	21st to end	Night	12% 2 11% 31% 39% 6%	810
September	1st to 10th	Night	9% 3 13% 35% 35% 5%	800
	11th to 20th	Night	14% 3 14% 28% 37% 6%	800
	21st to end	Night	18% 2 15% 35% 27% 4	800
October	1st to 10th	Night	15% 5% 18% 32% 26% 4	800
	11th to 20th	Night	15% 7% 23% 31% 22% 3	800
	21st to end	Night	12% 8% 24% 32% 22% 2	880
November	1st to 10th	Night	15% 7% 24% 33% 18% 3	820
	11th to 20th	Night	15% 11% 29% 32% 13% 1	840
	21st to end	Night	13% 10% 30% 33% 14% 1	840
December	1st to 10th	Night	18% 14% 26% 32% 8% 1	840
	11th to 20th	Night	19% 10% 26% 33% 10% 1	829
	21st to end	Night	25% 13% 25% 29% 7%	880

Page generated at 2025-12-15 01:13:52. Version ac07

Copyright © 2025 Attilla Danko.