GreenWay Lakes Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GreenWay Lakes Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	6% 5% 2 7% 6% 6% 7% 6% 8% 11% 36%	130531
	Night	6% 5% 2 6% 5% 5% 5% 5% 7% 10% 44%	130325
Transparency	Day	38% 9% 18% 21% 11% 3	134197
	Night	35% 8% 20% 23% 12% 3	134729
Seeing	Day	8% 6% 24% 29% 20% 14%	17758
	Night	7% 3 18% 32% 28% 12%	27931

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	5% 5% 2 4 3 4 5% 4 6% 11% 52%	5258
	11th to 20th	Day	12% 7% 3 9% 6% 7% 6% 5% 7% 10% 28%	5280
	21st to end	Day	8% 7% 3 8% 5% 5% 5% 4 6% 8% 40%	5942
February	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 4 6% 10% 43%	2100
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 7% 8% 51%	2123
	21st to end	Day	6% 4 2 6% 6% 6% 5% 5% 7% 9% 45%	1815
March	1st to 10th	Day	4 4 2 5% 5% 6% 7% 6% 7% 10% 44%	4935
	11th to 20th	Day	4 2 1 4 3 3 4 4 6% 10% 59%	4980
	21st to end	Day	7% 4 2 6% 5% 5% 5% 5% 7% 9% 44%	5465
April	1st to 10th	Day	4 3 1 5% 5% 4 5% 5% 9% 12% 45%	2200
	11th to 20th	Day	5% 4 2 6% 5% 6% 7% 5% 8% 11% 41%	2258
	21st to end	Day	7% 4 2 4 5% 5% 6% 6% 8% 10% 44%	2400
May	1st to 10th	Day	2 2 1 3 4 4 4 5% 7% 12% 57%	5405
	11th to 20th	Day	7% 4 2 5% 5% 5% 6% 4 7% 9% 47%	5438
	21st to end	Day	4 4 2 7% 6% 6% 8% 7% 10% 12% 35%	6433
June	1st to 10th	Day	13% 10% 4 10% 9% 9% 7% 6% 8% 9% 15%	2600
	11th to 20th	Day	8% 8% 4 10% 8% 9% 9% 7% 9% 11% 18%	2600
	21st to end	Day	6% 7% 3 10% 8% 9% 9% 8% 10% 10% 18%	2600
July	1st to 10th	Day	5% 7% 3 9% 7% 7% 8% 6% 10% 14% 25%	5900
	11th to 20th	Day	4 5% 3 9% 7% 7% 8% 7% 10% 13% 26%	5900
	21st to end	Day	3 3 2 5% 5% 6% 7% 6% 10% 15% 38%	6424
August	1st to 10th	Day	4 5% 2 10% 9% 9% 9% 9% 11% 13% 19%	4142
	11th to 20th	Day	6% 5% 2 7% 7% 8% 8% 6% 12% 16% 24%	3850
	21st to end	Day	5% 4 2 8% 8% 8% 10% 8% 11% 16% 19%	3959
September	1st to 10th	Day	8% 5% 3 8% 9% 9% 10% 7% 10% 11% 18%	2200
	11th to 20th	Day	8% 7% 3 11% 9% 10% 9% 6% 9% 9% 20%	2200
	21st to end	Day	11% 9% 4 10% 7% 8% 8% 6% 8% 9% 20%	2200
October	1st to 10th	Day	9% 7% 4 10% 8% 7% 8% 6% 9% 11% 21%	2111
	11th to 20th	Day	10% 5% 3 9% 7% 6% 7% 6% 10% 10% 27%	2000
	21st to end	Day	11% 7% 2 7% 5% 5% 6% 5% 8% 9% 35%	2200
November	1st to 10th	Day	7% 6% 2 8% 6% 6% 6% 6% 7% 9% 37%	2057
	11th to 20th	Day	8% 6% 3 8% 7% 7% 7% 5% 6% 7% 34%	2100
	21st to end	Day	7% 6% 2 7% 6% 6% 6% 5% 5% 10% 40%	2100
December	1st to 10th	Day	5% 4 2 5% 4 4 4 4 7% 9% 53%	3328
	11th to 20th	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 11% 37%	3463
	21st to end	Day	11% 7% 3 8% 6% 7% 7% 5% 8% 9% 29%	4565
January	1st to 10th	Night	5% 3 2 5% 4 5% 5% 4 7% 9% 52%	7635
	11th to 20th	Night	13% 8% 3 8% 6% 5% 5% 5% 7% 8% 33%	7680
	21st to end	Night	10% 8% 3 8% 5% 5% 4 3 6% 7% 42%	8221
February	1st to 10th	Night	7% 4 2 6% 5% 4 4 4 6% 9% 50%	2520
	11th to 20th	Night	6% 3 2 5% 4 3 4 4 5% 7% 57%	2271
	21st to end	Night	5% 3 1 4 3 4 4 3 6% 9% 58%	1815
March	1st to 10th	Night	3 4 2 6% 4 5% 5% 4 6% 9% 51%	4918
	11th to 20th	Night	3 3 1 4 3 3 3 3 5% 8% 62%	4980
	21st to end	Night	5% 3 1 5% 4 5% 6% 4 6% 8% 52%	5467
April	1st to 10th	Night	4 2 1 4 3 3 4 4 7% 10% 58%	2200
	11th to 20th	Night	5% 3 1 4 3 4 5% 4 7% 8% 56%	2090
	21st to end	Night	6% 4 2 4 4 4 4 4 7% 10% 52%	2000
May	1st to 10th	Night	2 1 1 2 2 3 4 4 7% 10% 64%	4490
	11th to 20th	Night	5% 3 2 5% 4 3 4 3 6% 8% 56%	4490
	21st to end	Night	4 4 1 5% 4 5% 6% 6% 9% 12% 45%	4427
June	1st to 10th	Night	12% 7% 3 9% 6% 7% 6% 6% 9% 10% 25%	1800
	11th to 20th	Night	7% 5% 2 6% 6% 6% 6% 7% 9% 14% 33%	1800
	21st to end	Night	4 5% 2 6% 6% 6% 8% 8% 11% 14% 31%	1800
July	1st to 10th	Night	3 5% 2 8% 6% 6% 6% 6% 10% 15% 34%	4058
	11th to 20th	Night	3 4 2 7% 5% 6% 6% 5% 10% 14% 38%	4057
	21st to end	Night	2 3 2 4 4 4 5% 5% 8% 14% 49%	4427
August	1st to 10th	Night	3 4 2 7% 6% 7% 7% 7% 9% 13% 35%	2973
	11th to 20th	Night	3 4 2 6% 5% 6% 6% 6% 10% 15% 38%	3197
	21st to end	Night	4 4 2 7% 6% 6% 8% 8% 12% 16% 27%	3931
September	1st to 10th	Night	5% 4 2 8% 7% 6% 8% 5% 10% 12% 33%	2200
	11th to 20th	Night	8% 7% 3 9% 7% 7% 7% 6% 9% 11% 28%	2200
	21st to end	Night	8% 6% 2 9% 7% 7% 8% 6% 9% 11% 27%	2200
October	1st to 10th	Night	9% 5% 3 9% 6% 8% 7% 7% 9% 11% 27%	2286
	11th to 20th	Night	8% 6% 3 7% 5% 6% 5% 5% 7% 11% 37%	2400
	21st to end	Night	10% 6% 3 8% 5% 5% 5% 4 6% 8% 42%	2640
November	1st to 10th	Night	8% 5% 2 6% 5% 5% 5% 4 7% 9% 46%	2461
	11th to 20th	Night	8% 6% 3 7% 5% 6% 6% 5% 7% 8% 40%	2520
	21st to end	Night	7% 4 2 6% 5% 5% 4 4 6% 8% 48%	2520
December	1st to 10th	Night	5% 4 2 4 4 4 3 3 6% 10% 54%	4066
	11th to 20th	Night	11% 8% 3 7% 5% 4 4 4 6% 9% 40%	4952
	21st to end	Night	9% 8% 4 9% 6% 6% 6% 6% 7% 8% 32%	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	26% 5% 9% 35% 19% 7%	5259
	11th to 20th	Day	50% 7% 14% 19% 7% 3	5280
	21st to end	Day	41% 5% 8% 27% 15% 4	5942
February	1st to 10th	Day	36% 6% 10% 25% 18% 6%	2100
	11th to 20th	Day	28% 6% 11% 27% 22% 6%	2133
	21st to end	Day	31% 6% 10% 22% 24% 7%	1815
March	1st to 10th	Day	31% 9% 13% 21% 22% 4	4935
	11th to 20th	Day	18% 6% 15% 30% 19% 11%	4980
	21st to end	Day	32% 7% 12% 28% 17% 5%	5465
April	1st to 10th	Day	27% 8% 15% 31% 16% 3	2200
	11th to 20th	Day	32% 9% 14% 21% 18% 5%	2258
	21st to end	Day	29% 8% 12% 25% 22% 4	2400
May	1st to 10th	Day	17% 7% 18% 36% 19% 3	5437
	11th to 20th	Day	30% 9% 18% 34% 8% 1	5442
	21st to end	Day	33% 10% 20% 30% 7%	6447
June	1st to 10th	Day	60% 11% 16% 11% 2	2600
	11th to 20th	Day	53% 13% 19% 13% 2	2600
	21st to end	Day	52% 13% 23% 10% 1	2600
July	1st to 10th	Day	43% 12% 32% 12%	5835
	11th to 20th	Day	43% 11% 33% 12%	5831
	21st to end	Day	27% 10% 38% 23% 2	6240
August	1st to 10th	Day	48% 14% 26% 10% 1	4129
	11th to 20th	Day	39% 13% 33% 15% 1	3886
	21st to end	Day	45% 13% 35% 7% 1	3959
September	1st to 10th	Day	51% 16% 21% 11% 1	2200
	11th to 20th	Day	53% 13% 21% 10% 2	2200
	21st to end	Day	55% 13% 15% 12% 5%	2200
October	1st to 10th	Day	48% 11% 21% 15% 5% 1	3436
	11th to 20th	Day	49% 10% 13% 18% 9% 1	3240
	21st to end	Day	38% 5% 10% 28% 15% 3	3538
November	1st to 10th	Day	40% 7% 12% 18% 18% 6%	2057
	11th to 20th	Day	45% 7% 8% 18% 17% 5%	2100
	21st to end	Day	38% 7% 12% 17% 22% 4	2100
December	1st to 10th	Day	28% 5% 10% 32% 22% 4	3328
	11th to 20th	Day	39% 6% 12% 20% 19% 4	3460
	21st to end	Day	47% 7% 14% 15% 16% 1	4565
January	1st to 10th	Night	27% 5% 12% 31% 19% 5%	7636
	11th to 20th	Night	49% 5% 18% 17% 8% 3	7680
	21st to end	Night	40% 6% 9% 30% 13% 3	8221
February	1st to 10th	Night	31% 4 13% 26% 20% 6%	2520
	11th to 20th	Night	26% 5% 12% 27% 23% 7%	2272
	21st to end	Night	23% 6% 12% 23% 29% 7%	1815
March	1st to 10th	Night	28% 8% 12% 23% 24% 5%	4918
	11th to 20th	Night	19% 5% 19% 28% 19% 10%	4980
	21st to end	Night	30% 6% 13% 26% 21% 3	5467
April	1st to 10th	Night	20% 7% 18% 35% 18% 3	2200
	11th to 20th	Night	25% 7% 17% 24% 22% 5%	2090
	21st to end	Night	27% 6% 15% 28% 21% 3	2000
May	1st to 10th	Night	14% 6% 24% 40% 14% 2	4520
	11th to 20th	Night	25% 7% 17% 38% 12% 1	4491
	21st to end	Night	27% 9% 28% 28% 8%	4435
June	1st to 10th	Night	50% 12% 22% 14% 2	1800
	11th to 20th	Night	39% 12% 27% 20% 2	1800
	21st to end	Night	38% 15% 36% 10%	1800
July	1st to 10th	Night	36% 12% 40% 12%	4015
	11th to 20th	Night	34% 12% 41% 12%	4006
	21st to end	Night	26% 9% 41% 21% 3	4297
August	1st to 10th	Night	39% 13% 31% 15% 2	2962
	11th to 20th	Night	31% 13% 38% 18% 1	3227
	21st to end	Night	40% 15% 37% 7% 1	3931
September	1st to 10th	Night	40% 17% 28% 14% 2	2200
	11th to 20th	Night	47% 16% 23% 13% 2	2200
	21st to end	Night	47% 14% 20% 13% 6%	2200
October	1st to 10th	Night	49% 12% 19% 17% 4 1	3741
	11th to 20th	Night	49% 8% 18% 14% 11% 1	3899
	21st to end	Night	40% 6% 10% 27% 15% 3	4252
November	1st to 10th	Night	35% 6% 13% 22% 19% 5%	2461
	11th to 20th	Night	41% 8% 8% 23% 16% 4	2520
	21st to end	Night	33% 6% 14% 22% 20% 5%	2520
December	1st to 10th	Night	27% 6% 13% 33% 17% 4	4066
	11th to 20th	Night	41% 5% 11% 22% 17% 5%	4954
	21st to end	Night	47% 7% 14% 16% 14% 2	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	6% 15% 42% 29% 7% 1	330
	11th to 20th	Day	16% 16% 40% 23% 4 1	330
	21st to end	Day	10% 17% 49% 19% 4	363
February	1st to 10th	Day	8% 12% 42% 30% 7% 1	330
	11th to 20th	Day	6% 9% 37% 34% 13% 2	441
	21st to end	Day	6% 9% 31% 36% 17% 1	414
March	1st to 10th	Day	5% 12% 40% 32% 9% 2	500
	11th to 20th	Day	6% 13% 36% 28% 15% 2	500
	21st to end	Day	5% 10% 40% 29% 13% 3	550
April	1st to 10th	Day	5% 8% 41% 27% 14% 5%	500
	11th to 20th	Day	6% 8% 40% 26% 15% 5%	492
	21st to end	Day	5% 5% 36% 31% 18% 6%	500
May	1st to 10th	Day	4 3 35% 29% 19% 10%	503
	11th to 20th	Day	10% 7% 25% 30% 18% 10%	527
	21st to end	Day	9% 4 15% 32% 25% 14%	573
June	1st to 10th	Day	13% 3 16% 30% 22% 15%	530
	11th to 20th	Day	9% 2 12% 28% 25% 24%	530
	21st to end	Day	6% 2 6% 26% 27% 33%	530
July	1st to 10th	Day	4 2 7% 22% 30% 35%	525
	11th to 20th	Day	5% 1 6% 26% 31% 31%	524
	21st to end	Day	4 1 8% 24% 29% 35%	576
August	1st to 10th	Day	6% 2 9% 26% 26% 32%	526
	11th to 20th	Day	6% 1 6% 25% 29% 34%	519
	21st to end	Day	8% 1 12% 25% 27% 26%	583
September	1st to 10th	Day	7% 2 7% 26% 26% 32%	530
	11th to 20th	Day	6% 2 8% 22% 32% 30%	530
	21st to end	Day	9% 1 14% 26% 26% 24%	530
October	1st to 10th	Day	9% 4 16% 38% 24% 9%	530
	11th to 20th	Day	12% 3 21% 34% 21% 10%	530
	21st to end	Day	11% 6% 23% 37% 18% 5%	583
November	1st to 10th	Day	9% 6% 30% 33% 18% 5%	535
	11th to 20th	Day	9% 7% 29% 33% 19% 3	540
	21st to end	Day	8% 8% 28% 37% 16% 3	540
December	1st to 10th	Day	9% 7% 29% 37% 17% 2	521
	11th to 20th	Day	9% 9% 35% 35% 9% 2	330
	21st to end	Day	10% 11% 36% 30% 12% 2	363
January	1st to 10th	Night	7% 5% 25% 40% 21% 3	840
	11th to 20th	Night	9% 7% 31% 36% 16% 1	840
	21st to end	Night	9% 9% 29% 40% 12% 1	924
February	1st to 10th	Night	8% 6% 24% 39% 21% 1	840
	11th to 20th	Night	6% 5% 28% 41% 19% 1	804
	21st to end	Night	7% 4 24% 46% 18% 1	660
March	1st to 10th	Night	5% 6% 31% 40% 16% 2	800
	11th to 20th	Night	6% 5% 27% 42% 18% 2	800
	21st to end	Night	6% 5% 27% 43% 18% 1	880
April	1st to 10th	Night	5% 4 25% 44% 20% 2	800
	11th to 20th	Night	5% 4 28% 41% 21% 1	791
	21st to end	Night	5% 3 24% 43% 23% 2	800
May	1st to 10th	Night	4 2 20% 42% 29% 3	800
	11th to 20th	Night	8% 2 19% 40% 28% 2	789
	21st to end	Night	8% 1 8% 28% 43% 11%	656
June	1st to 10th	Night	14% 1 8% 26% 37% 14%	600
	11th to 20th	Night	8% 2 4 15% 43% 29%	600
	21st to end	Night	4 5% 15% 39% 37%	600
July	1st to 10th	Night	4 1 3 9% 46% 36%	585
	11th to 20th	Night	4 3 13% 43% 38%	582
	21st to end	Night	3 1 2 12% 40% 42%	657
August	1st to 10th	Night	6% 4 13% 44% 33%	629
	11th to 20th	Night	3 1 3 16% 42% 35%	790
	21st to end	Night	7% 1 9% 18% 38% 27%	880
September	1st to 10th	Night	7% 2 7% 15% 37% 33%	800
	11th to 20th	Night	9% 2 6% 18% 35% 31%	800
	21st to end	Night	10% 1 10% 19% 36% 25%	800
October	1st to 10th	Night	10% 2 12% 32% 34% 10%	800
	11th to 20th	Night	9% 3 13% 32% 33% 10%	800
	21st to end	Night	12% 3 22% 34% 24% 5%	880
November	1st to 10th	Night	8% 5% 22% 38% 23% 3	820
	11th to 20th	Night	10% 4 20% 32% 29% 5%	840
	21st to end	Night	9% 3 21% 41% 23% 3	840
December	1st to 10th	Night	7% 4 19% 40% 27% 2	840
	11th to 20th	Night	10% 5% 21% 37% 23% 3	840
	21st to end	Night	9% 6% 24% 34% 24% 3	924

Page generated at 2026-02-13 01:42:17. Version ac07

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