Jordan Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jordan Lake 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	13% 6% 2 6% 4 4 4 4 6% 8% 43%	125606
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	123125
Transparency	Day	39% 6% 12% 20% 18% 5%	129212
	Night	38% 6% 10% 19% 20% 6%	127588
Seeing	Day	17% 11% 25% 25% 17% 6%	16487
	Night	16% 8% 26% 31% 17% 2	26721

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	16% 5% 2 5% 3 3 2 2 3 4 56%	4056
	11th to 20th	Day	12% 6% 2 4 3 3 2 2 3 4 58%	4060
	21st to end	Day	14% 8% 3 6% 4 4 3 3 4 5% 46%	4466
February	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 5% 8% 44%	1800
	11th to 20th	Day	23% 7% 2 4 3 3 2 3 4 4 45%	1757
	21st to end	Day	18% 8% 2 6% 4 4 3 3 5% 6% 41%	1815
March	1st to 10th	Day	20% 6% 2 6% 4 4 4 3 4 6% 40%	4935
	11th to 20th	Day	11% 5% 2 5% 3 4 3 3 4 7% 53%	4980
	21st to end	Day	15% 6% 2 6% 4 4 4 3 4 5% 47%	5465
April	1st to 10th	Day	11% 4 1 4 3 4 3 4 5% 7% 54%	2315
	11th to 20th	Day	11% 5% 2 6% 3 4 4 4 6% 7% 49%	2374
	21st to end	Day	13% 5% 2 6% 4 5% 5% 4 5% 7% 46%	2400
May	1st to 10th	Day	9% 7% 3 8% 6% 5% 6% 4 7% 10% 35%	5833
	11th to 20th	Day	19% 8% 2 7% 5% 5% 6% 5% 7% 9% 29%	5861
	21st to end	Day	12% 5% 2 6% 4 5% 5% 5% 6% 9% 42%	6433
June	1st to 10th	Day	9% 5% 2 7% 4 6% 5% 5% 8% 9% 39%	2600
	11th to 20th	Day	8% 6% 2 5% 5% 5% 5% 5% 7% 10% 42%	2600
	21st to end	Day	7% 5% 2 5% 5% 5% 5% 5% 9% 11% 40%	2600
July	1st to 10th	Day	9% 5% 2 7% 5% 6% 6% 6% 8% 10% 33%	5900
	11th to 20th	Day	8% 6% 3 8% 6% 6% 7% 6% 9% 14% 28%	5900
	21st to end	Day	9% 6% 2 8% 5% 6% 6% 6% 8% 11% 33%	6424
August	1st to 10th	Day	20% 8% 3 8% 6% 6% 6% 5% 8% 9% 22%	4234
	11th to 20th	Day	9% 6% 2 8% 5% 5% 6% 5% 8% 10% 35%	4052
	21st to end	Day	7% 4 1 5% 4 4 4 4 6% 10% 51%	4312
September	1st to 10th	Day	9% 4 1 6% 4 5% 5% 5% 7% 8% 47%	2316
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 7% 42%	2135
	21st to end	Day	21% 7% 2 6% 4 3 4 3 5% 7% 39%	2057
October	1st to 10th	Day	15% 6% 2 6% 3 3 3 2 4 7% 49%	1900
	11th to 20th	Day	12% 4 1 4 3 3 3 3 5% 5% 57%	1900
	21st to end	Day	11% 4 2 6% 3 3 3 2 4 5% 57%	2090
November	1st to 10th	Day	13% 6% 2 4 3 3 3 2 5% 6% 53%	1957
	11th to 20th	Day	15% 4 2 4 3 3 3 2 5% 6% 52%	1985
	21st to end	Day	13% 5% 2 5% 3 4 4 3 5% 6% 50%	1736
December	1st to 10th	Day	13% 6% 2 6% 4 4 3 2 4 6% 52%	2600
	11th to 20th	Day	17% 6% 2 6% 4 4 3 3 5% 5% 45%	3292
	21st to end	Day	17% 6% 2 5% 4 3 4 3 4 7% 44%	4466
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 4 5% 55%	5889
	11th to 20th	Night	12% 6% 2 5% 3 3 3 2 3 5% 55%	5900
	21st to end	Night	15% 8% 2 5% 3 3 4 3 5% 5% 47%	6490
February	1st to 10th	Night	17% 6% 2 6% 3 3 4 3 5% 6% 45%	2600
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 4 5% 43%	2523
	21st to end	Night	15% 5% 2 5% 3 4 4 3 6% 6% 47%	1815
March	1st to 10th	Night	19% 8% 2 7% 4 3 4 3 4 5% 42%	4918
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 7% 54%	4980
	21st to end	Night	14% 5% 1 5% 4 4 3 3 5% 6% 49%	5467
April	1st to 10th	Night	15% 6% 1 5% 3 4 3 3 4 8% 49%	2085
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 4 6% 51%	1975
	21st to end	Night	11% 4 2 5% 4 5% 5% 4 6% 8% 46%	2000
May	1st to 10th	Night	12% 8% 2 7% 5% 5% 5% 4 6% 9% 37%	4091
	11th to 20th	Night	18% 8% 3 7% 4 4 4 4 5% 8% 34%	4032
	21st to end	Night	16% 5% 2 5% 4 3 4 3 5% 10% 43%	4421
June	1st to 10th	Night	11% 6% 2 6% 4 5% 5% 5% 6% 10% 39%	1800
	11th to 20th	Night	10% 6% 2 8% 5% 5% 5% 4 6% 9% 40%	1800
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 12% 40%	1800
July	1st to 10th	Night	12% 7% 3 7% 5% 6% 6% 5% 8% 11% 30%	4058
	11th to 20th	Night	13% 8% 3 8% 6% 6% 6% 5% 9% 11% 26%	4057
	21st to end	Night	12% 8% 3 8% 5% 6% 6% 5% 7% 10% 30%	4427
August	1st to 10th	Night	24% 6% 3 7% 6% 5% 5% 4 8% 9% 23%	2915
	11th to 20th	Night	9% 6% 2 7% 5% 5% 7% 6% 8% 11% 34%	2964
	21st to end	Night	6% 4 2 6% 4 4 4 4 7% 10% 49%	3586
September	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 6% 8% 47%	1931
	11th to 20th	Night	14% 6% 2 6% 5% 3 5% 3 5% 7% 43%	2041
	21st to end	Night	19% 7% 3 7% 5% 4 4 4 5% 7% 37%	2123
October	1st to 10th	Night	15% 6% 3 5% 4 3 3 3 4 5% 50%	2280
	11th to 20th	Night	13% 5% 1 5% 4 2 3 3 4 5% 54%	2280
	21st to end	Night	12% 5% 2 4 3 3 3 3 4 6% 55%	2508
November	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 4 5% 52%	2341
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 5% 5% 50%	2412
	21st to end	Night	13% 4 2 5% 3 3 3 3 6% 6% 51%	2653
December	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 3 4 52%	4580
	11th to 20th	Night	18% 9% 3 7% 4 4 3 3 5% 5% 40%	4893
	21st to end	Night	17% 6% 2 5% 3 3 3 3 4 6% 48%	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	35% 3 3 20% 32% 7%	4057
	11th to 20th	Day	32% 3 4 15% 37% 9%	4060
	21st to end	Day	41% 4 5% 21% 22% 8%	4466
February	1st to 10th	Day	39% 4 11% 21% 19% 5%	1800
	11th to 20th	Day	43% 4 7% 18% 22% 6%	1766
	21st to end	Day	44% 5% 9% 16% 20% 6%	1815
March	1st to 10th	Day	45% 4 6% 15% 18% 11%	4935
	11th to 20th	Day	32% 4 7% 15% 24% 16%	4980
	21st to end	Day	40% 3 9% 20% 21% 7%	5465
April	1st to 10th	Day	29% 5% 10% 24% 23% 7%	2315
	11th to 20th	Day	33% 6% 11% 22% 20% 7%	2374
	21st to end	Day	37% 8% 11% 22% 18% 4	2400
May	1st to 10th	Day	41% 8% 17% 21% 10% 1	5868
	11th to 20th	Day	50% 7% 13% 19% 9% 1	5865
	21st to end	Day	37% 7% 12% 22% 18% 3	6447
June	1st to 10th	Day	36% 11% 16% 21% 15% 1	2600
	11th to 20th	Day	34% 11% 16% 23% 15% 2	2600
	21st to end	Day	34% 11% 17% 22% 15% 1	2600
July	1st to 10th	Day	44% 7% 16% 20% 11% 2	5835
	11th to 20th	Day	45% 9% 28% 13% 3 1	5831
	21st to end	Day	44% 7% 17% 22% 10%	6240
August	1st to 10th	Day	57% 7% 18% 13% 4	4221
	11th to 20th	Day	43% 7% 20% 21% 8% 1	4091
	21st to end	Day	30% 6% 14% 32% 16% 2	4312
September	1st to 10th	Day	32% 10% 15% 19% 19% 5%	2316
	11th to 20th	Day	41% 7% 10% 18% 20% 4	2135
	21st to end	Day	45% 6% 9% 19% 15% 5%	2057
October	1st to 10th	Day	35% 3 7% 20% 26% 9%	3160
	11th to 20th	Day	19% 3 5% 20% 26% 26%	3140
	21st to end	Day	26% 2 6% 25% 24% 17%	3428
November	1st to 10th	Day	33% 4 8% 21% 26% 8%	1957
	11th to 20th	Day	33% 4 9% 20% 27% 7%	1985
	21st to end	Day	34% 4 9% 22% 24% 6%	1736
December	1st to 10th	Day	36% 3 6% 21% 26% 7%	2600
	11th to 20th	Day	42% 3 10% 18% 21% 6%	3289
	21st to end	Day	39% 5% 11% 25% 19% 2	4466
January	1st to 10th	Night	32% 4 5% 17% 33% 9%	5889
	11th to 20th	Night	33% 4 6% 15% 35% 8%	5900
	21st to end	Night	39% 4 6% 18% 28% 6%	6490
February	1st to 10th	Night	41% 4 9% 22% 19% 5%	2600
	11th to 20th	Night	44% 4 8% 15% 21% 7%	2525
	21st to end	Night	36% 6% 9% 19% 22% 9%	1815
March	1st to 10th	Night	45% 5% 5% 12% 22% 10%	4918
	11th to 20th	Night	31% 4 6% 14% 26% 18%	4980
	21st to end	Night	37% 4 8% 22% 23% 6%	5467
April	1st to 10th	Night	36% 4 10% 18% 24% 8%	2085
	11th to 20th	Night	34% 5% 10% 22% 21% 8%	1975
	21st to end	Night	34% 7% 13% 24% 17% 5%	2000
May	1st to 10th	Night	42% 7% 17% 19% 13% 2	4118
	11th to 20th	Night	47% 7% 14% 18% 13% 1	4033
	21st to end	Night	36% 7% 14% 23% 17% 2	4429
June	1st to 10th	Night	38% 10% 15% 20% 15% 2	1800
	11th to 20th	Night	38% 9% 17% 19% 15% 2	1800
	21st to end	Night	35% 12% 16% 21% 13% 3	1800
July	1st to 10th	Night	46% 9% 18% 17% 9% 1	4015
	11th to 20th	Night	49% 13% 22% 12% 3 1	4006
	21st to end	Night	48% 7% 16% 20% 9%	4297
August	1st to 10th	Night	56% 8% 18% 13% 5%	2904
	11th to 20th	Night	42% 8% 20% 22% 7% 1	2991
	21st to end	Night	30% 7% 12% 28% 21% 3	3586
September	1st to 10th	Night	34% 9% 14% 21% 16% 5%	1931
	11th to 20th	Night	39% 8% 11% 17% 21% 3	2041
	21st to end	Night	46% 8% 12% 15% 15% 4	2123
October	1st to 10th	Night	38% 3 7% 21% 25% 6%	3800
	11th to 20th	Night	25% 3 6% 18% 28% 21%	3779
	21st to end	Night	25% 2 7% 25% 24% 16%	4120
November	1st to 10th	Night	34% 5% 7% 19% 26% 9%	2341
	11th to 20th	Night	36% 4 8% 15% 25% 11%	2412
	21st to end	Night	32% 5% 8% 22% 26% 7%	2653
December	1st to 10th	Night	37% 3 5% 19% 28% 8%	4580
	11th to 20th	Night	48% 3 8% 17% 16% 8%	4895
	21st to end	Night	38% 3 12% 24% 21% 2	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	27% 24% 32% 14% 3	300
	11th to 20th	Day	25% 24% 34% 13% 4	300
	21st to end	Day	19% 24% 41% 14% 2	330
February	1st to 10th	Day	19% 18% 42% 16% 4	300
	11th to 20th	Day	39% 20% 32% 9% 1	304
	21st to end	Day	24% 19% 34% 20% 3	414
March	1st to 10th	Day	24% 16% 34% 20% 6%	500
	11th to 20th	Day	20% 21% 38% 16% 5% 1	500
	21st to end	Day	21% 23% 33% 17% 5% 1	550
April	1st to 10th	Day	21% 18% 33% 20% 6% 1	500
	11th to 20th	Day	14% 19% 30% 24% 10% 4	492
	21st to end	Day	17% 12% 30% 28% 12% 1	500
May	1st to 10th	Day	13% 18% 31% 23% 10% 4	503
	11th to 20th	Day	17% 7% 24% 30% 18% 5%	527
	21st to end	Day	19% 8% 20% 25% 22% 7%	573
June	1st to 10th	Day	12% 5% 25% 31% 22% 6%	530
	11th to 20th	Day	10% 5% 21% 30% 25% 9%	530
	21st to end	Day	9% 4 17% 35% 27% 9%	530
July	1st to 10th	Day	11% 1 10% 31% 35% 12%	525
	11th to 20th	Day	6% 2 9% 32% 37% 14%	524
	21st to end	Day	7% 2 15% 33% 32% 12%	576
August	1st to 10th	Day	16% 2 17% 31% 24% 10%	526
	11th to 20th	Day	11% 2 14% 29% 29% 14%	519
	21st to end	Day	11% 3 13% 31% 25% 17%	583
September	1st to 10th	Day	11% 3 17% 26% 27% 16%	502
	11th to 20th	Day	18% 4 12% 28% 27% 11%	490
	21st to end	Day	26% 6% 20% 30% 15% 4	490
October	1st to 10th	Day	17% 7% 22% 23% 22% 9%	490
	11th to 20th	Day	12% 9% 26% 29% 19% 5%	490
	21st to end	Day	13% 10% 27% 28% 17% 6%	539
November	1st to 10th	Day	18% 9% 30% 28% 13% 2	495
	11th to 20th	Day	17% 15% 28% 28% 11% 1	485
	21st to end	Day	13% 19% 29% 24% 13% 1	267
December	1st to 10th	Day	22% 17% 39% 15% 7%	200
	11th to 20th	Day	22% 14% 35% 21% 8% 1	273
	21st to end	Day	25% 15% 32% 18% 8% 1	330
January	1st to 10th	Night	21% 15% 34% 25% 5% 1	800
	11th to 20th	Night	20% 13% 34% 28% 6%	800
	21st to end	Night	20% 11% 39% 27% 3	880
February	1st to 10th	Night	20% 12% 38% 28% 3	800
	11th to 20th	Night	25% 16% 38% 21% 1	783
	21st to end	Night	18% 18% 35% 25% 5%	660
March	1st to 10th	Night	17% 15% 35% 29% 5%	800
	11th to 20th	Night	20% 14% 35% 23% 8%	800
	21st to end	Night	17% 13% 40% 25% 4	880
April	1st to 10th	Night	19% 15% 35% 25% 6%	800
	11th to 20th	Night	15% 13% 34% 27% 11% 1	791
	21st to end	Night	14% 6% 36% 34% 10%	800
May	1st to 10th	Night	12% 11% 34% 35% 8%	632
	11th to 20th	Night	18% 7% 23% 36% 16% 1	591
	21st to end	Night	15% 2 16% 40% 26% 2	650
June	1st to 10th	Night	13% 4 17% 42% 24% 1	600
	11th to 20th	Night	13% 2 19% 37% 26% 5%	600
	21st to end	Night	8% 2 16% 40% 32% 2	600
July	1st to 10th	Night	9% 2 12% 36% 39% 3	585
	11th to 20th	Night	9% 2 8% 38% 36% 7%	582
	21st to end	Night	7% 1 12% 40% 35% 4	657
August	1st to 10th	Night	16% 2 10% 36% 33% 4	597
	11th to 20th	Night	12% 2 10% 35% 34% 5%	644
	21st to end	Night	12% 2 12% 31% 38% 5%	880
September	1st to 10th	Night	12% 2 13% 34% 34% 6%	773
	11th to 20th	Night	16% 3 13% 30% 33% 4	760
	21st to end	Night	21% 5% 17% 33% 23% 1	760
October	1st to 10th	Night	17% 3 19% 29% 27% 5%	760
	11th to 20th	Night	16% 7% 26% 29% 20% 2	760
	21st to end	Night	14% 7% 26% 31% 21% 1	836
November	1st to 10th	Night	16% 7% 26% 33% 15% 3	780
	11th to 20th	Night	15% 11% 28% 34% 11%	800
	21st to end	Night	15% 10% 30% 31% 13% 1	800
December	1st to 10th	Night	21% 10% 32% 30% 6%	800
	11th to 20th	Night	19% 9% 30% 34% 7%	800
	21st to end	Night	21% 12% 28% 31% 8%	880

Page generated at 2025-09-01 01:51:08. Version ac07

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