KW Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the KW Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	15% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	126704
	Night	15% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	122675
Transparency	Day	46% 8% 14% 20% 11% 1	130297
	Night	46% 8% 13% 20% 11% 1	127168
Seeing	Day	17% 9% 28% 28% 14% 5%	15589
	Night	18% 8% 29% 30% 13% 1	24943

By Month

Percentage Cloud Cover NE (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	13% 10% 4 13% 6% 5% 6% 5% 6% 6% 26%	3597
	11th to 20th	Day	10% 16% 6% 18% 9% 7% 6% 4 4 4 16%	3906
	21st to end	Day	17% 16% 6% 13% 6% 6% 5% 4 4 5% 19%	4466
February	1st to 10th	Day	15% 16% 4 12% 7% 6% 5% 5% 4 5% 22%	1800
	11th to 20th	Day	12% 14% 6% 12% 7% 5% 4 4 4 6% 25%	1753
	21st to end	Day	18% 13% 4 11% 6% 5% 6% 4 4 5% 25%	1542
March	1st to 10th	Day	19% 10% 4 8% 6% 5% 4 4 5% 5% 32%	4932
	11th to 20th	Day	11% 12% 4 9% 6% 5% 5% 4 6% 8% 31%	4980
	21st to end	Day	18% 11% 3 8% 5% 5% 4 3 4 5% 33%	5465
April	1st to 10th	Day	17% 13% 3 9% 5% 4 4 4 6% 6% 29%	2365
	11th to 20th	Day	19% 10% 3 7% 4 4 4 3 5% 6% 35%	2447
	21st to end	Day	19% 9% 3 8% 5% 5% 5% 3 5% 6% 33%	2600
May	1st to 10th	Day	20% 8% 3 7% 4 4 4 3 5% 7% 33%	5888
	11th to 20th	Day	16% 9% 4 7% 4 5% 4 4 5% 7% 34%	6306
	21st to end	Day	18% 6% 2 6% 5% 4 4 4 6% 7% 38%	6949
June	1st to 10th	Day	14% 7% 2 6% 6% 5% 5% 4 7% 8% 37%	2800
	11th to 20th	Day	12% 4 2 7% 5% 5% 5% 5% 7% 10% 38%	2800
	21st to end	Day	14% 5% 2 6% 6% 5% 5% 4 7% 9% 38%	2800
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 6% 5% 7% 10% 39%	6360
	11th to 20th	Day	8% 4 2 6% 4 5% 5% 4 7% 9% 47%	6360
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 7% 9% 45%	6497
August	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 9% 38%	4234
	11th to 20th	Day	10% 7% 3 8% 5% 5% 5% 5% 7% 10% 36%	4174
	21st to end	Day	9% 5% 2 6% 4 5% 5% 5% 7% 9% 42%	4487
September	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 43%	2400
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 7% 47%	2287
	21st to end	Day	13% 6% 3 7% 6% 5% 5% 4 5% 8% 37%	2155
October	1st to 10th	Day	17% 10% 3 7% 4 4 4 4 5% 7% 37%	1930
	11th to 20th	Day	20% 8% 3 7% 6% 5% 5% 4 5% 6% 31%	1900
	21st to end	Day	23% 11% 3 9% 6% 5% 5% 4 6% 6% 22%	2090
November	1st to 10th	Day	14% 9% 3 9% 6% 5% 4 3 6% 8% 32%	1688
	11th to 20th	Day	15% 12% 4 9% 7% 5% 5% 6% 6% 6% 25%	1600
	21st to end	Day	18% 13% 5% 9% 7% 5% 5% 4 6% 8% 21%	1600
December	1st to 10th	Day	19% 15% 5% 11% 7% 6% 6% 4 5% 7% 15%	2600
	11th to 20th	Day	20% 16% 4 10% 6% 4 5% 3 5% 5% 21%	2986
	21st to end	Day	26% 17% 6% 13% 5% 5% 4 3 4 4 14%	3960
January	1st to 10th	Night	14% 11% 4 11% 6% 6% 5% 5% 6% 7% 25%	6349
	11th to 20th	Night	12% 13% 5% 14% 10% 7% 6% 4 5% 6% 17%	5979
	21st to end	Night	16% 13% 5% 10% 6% 5% 5% 4 6% 6% 22%	6490
February	1st to 10th	Night	14% 14% 5% 10% 6% 5% 5% 5% 5% 5% 25%	2600
	11th to 20th	Night	14% 14% 4 10% 7% 4 5% 5% 5% 6% 26%	2533
	21st to end	Night	14% 12% 4 10% 5% 5% 5% 4 6% 6% 29%	2085
March	1st to 10th	Night	16% 10% 4 8% 6% 5% 5% 4 5% 6% 31%	4908
	11th to 20th	Night	10% 8% 3 8% 5% 5% 6% 4 7% 8% 37%	4979
	21st to end	Night	21% 8% 3 6% 4 4 4 3 4 6% 39%	5467
April	1st to 10th	Night	18% 10% 3 8% 6% 5% 4 4 6% 5% 31%	2035
	11th to 20th	Night	16% 9% 3 7% 5% 4 4 3 5% 6% 38%	1920
	21st to end	Night	16% 7% 3 7% 5% 4 4 4 6% 7% 36%	1800
May	1st to 10th	Night	22% 8% 3 6% 4 4 5% 4 4 5% 36%	4053
	11th to 20th	Night	14% 8% 3 7% 5% 4 4 3 5% 7% 39%	3585
	21st to end	Night	17% 5% 2 5% 4 4 3 3 5% 7% 44%	3926
June	1st to 10th	Night	12% 5% 2 6% 5% 5% 5% 3 7% 8% 43%	1600
	11th to 20th	Night	9% 6% 2 6% 4 5% 5% 4 6% 10% 43%	1600
	21st to end	Night	11% 6% 2 6% 4 5% 4 4 6% 8% 44%	1600
July	1st to 10th	Night	7% 5% 2 6% 4 5% 5% 4 6% 10% 47%	3600
	11th to 20th	Night	9% 4 2 5% 4 4 4 4 6% 8% 50%	3600
	21st to end	Night	8% 5% 2 5% 3 4 4 4 6% 8% 52%	4351
August	1st to 10th	Night	9% 5% 2 6% 5% 5% 5% 3 6% 8% 47%	2915
	11th to 20th	Night	11% 4 2 6% 3 3 3 4 5% 8% 50%	2875
	21st to end	Night	10% 5% 2 6% 3 4 4 4 7% 8% 47%	3378
September	1st to 10th	Night	9% 5% 2 6% 3 4 5% 5% 5% 8% 48%	2000
	11th to 20th	Night	10% 4 2 4 4 3 3 3 4 7% 56%	2110
	21st to end	Night	15% 6% 2 6% 4 5% 3 4 5% 6% 42%	2244
October	1st to 10th	Night	15% 7% 3 7% 4 5% 4 4 6% 6% 39%	2317
	11th to 20th	Night	19% 7% 3 7% 5% 4 5% 3 5% 7% 35%	2280
	21st to end	Night	20% 9% 3 7% 5% 5% 4 4 5% 7% 30%	2508
November	1st to 10th	Night	16% 8% 3 7% 5% 5% 4 3 5% 7% 37%	2604
	11th to 20th	Night	17% 9% 4 8% 5% 5% 5% 5% 6% 6% 30%	2800
	21st to end	Night	20% 11% 3 7% 6% 6% 4 3 6% 7% 26%	2800
December	1st to 10th	Night	17% 14% 5% 11% 7% 6% 6% 4 6% 6% 19%	4580
	11th to 20th	Night	16% 16% 5% 11% 5% 5% 5% 4 5% 6% 22%	5208
	21st to end	Night	25% 14% 6% 11% 5% 5% 4 3 5% 5% 16%	6996

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	51% 9% 17% 13% 9%	3598
	11th to 20th	Day	68% 7% 12% 10% 4	3940
	21st to end	Day	64% 7% 12% 10% 7%	4466
February	1st to 10th	Day	61% 8% 13% 15% 3 1	1800
	11th to 20th	Day	57% 8% 14% 14% 6% 1	1762
	21st to end	Day	56% 9% 13% 15% 6% 1	1542
March	1st to 10th	Day	50% 7% 15% 15% 13% 1	4935
	11th to 20th	Day	47% 9% 18% 17% 8% 2	4980
	21st to end	Day	49% 7% 11% 20% 12% 1	5465
April	1st to 10th	Day	53% 6% 12% 17% 11% 1	2365
	11th to 20th	Day	48% 7% 11% 20% 14% 1	2447
	21st to end	Day	49% 7% 12% 17% 12% 3	2600
May	1st to 10th	Day	48% 8% 14% 15% 12% 4	5897
	11th to 20th	Day	46% 11% 12% 15% 14% 2	6310
	21st to end	Day	42% 8% 12% 23% 13% 3	6949
June	1st to 10th	Day	41% 10% 13% 21% 15% 1	2800
	11th to 20th	Day	37% 9% 14% 24% 15% 1	2800
	21st to end	Day	37% 10% 14% 24% 14% 1	2800
July	1st to 10th	Day	36% 7% 15% 25% 15% 2	6290
	11th to 20th	Day	31% 7% 13% 32% 16% 1	6285
	21st to end	Day	32% 8% 14% 29% 17% 1	6311
August	1st to 10th	Day	42% 8% 14% 21% 15% 1	4221
	11th to 20th	Day	39% 8% 16% 25% 11%	4213
	21st to end	Day	34% 8% 14% 34% 10% 1	4487
September	1st to 10th	Day	36% 10% 15% 28% 11% 1	2400
	11th to 20th	Day	33% 7% 15% 29% 14%	2287
	21st to end	Day	42% 10% 14% 25% 9%	2155
October	1st to 10th	Day	35% 7% 16% 28% 13% 1	3190
	11th to 20th	Day	38% 7% 16% 25% 14%	3140
	21st to end	Day	52% 7% 14% 21% 6%	3428
November	1st to 10th	Day	46% 10% 15% 20% 8% 1	1688
	11th to 20th	Day	53% 11% 14% 14% 7% 1	1600
	21st to end	Day	58% 9% 16% 15% 3	1600
December	1st to 10th	Day	64% 8% 15% 9% 3	2600
	11th to 20th	Day	60% 8% 16% 9% 7%	2986
	21st to end	Day	71% 8% 8% 8% 5%	3960
January	1st to 10th	Night	52% 9% 13% 20% 4	6349
	11th to 20th	Night	62% 12% 11% 11% 3	6020
	21st to end	Night	57% 8% 13% 10% 10% 1	6490
February	1st to 10th	Night	57% 7% 14% 17% 4 1	2600
	11th to 20th	Night	56% 7% 13% 15% 8%	2535
	21st to end	Night	53% 7% 15% 18% 7%	2085
March	1st to 10th	Night	51% 7% 12% 17% 11% 2	4918
	11th to 20th	Night	41% 7% 20% 21% 10% 2	4980
	21st to end	Night	45% 5% 10% 21% 17% 2	5467
April	1st to 10th	Night	51% 6% 11% 18% 13% 1	2035
	11th to 20th	Night	47% 7% 9% 20% 15% 2	1920
	21st to end	Night	44% 8% 12% 19% 16% 2	1800
May	1st to 10th	Night	49% 6% 11% 14% 13% 5%	4060
	11th to 20th	Night	42% 11% 13% 18% 13% 3	3586
	21st to end	Night	38% 8% 14% 23% 16% 2	3927
June	1st to 10th	Night	37% 7% 12% 24% 17% 3	1600
	11th to 20th	Night	35% 7% 14% 22% 20% 2	1600
	21st to end	Night	36% 8% 13% 24% 18% 1	1600
July	1st to 10th	Night	33% 7% 14% 25% 20% 2	3560
	11th to 20th	Night	31% 6% 13% 30% 18% 1	3552
	21st to end	Night	29% 7% 12% 29% 22% 1	4222
August	1st to 10th	Night	35% 6% 13% 28% 15% 2	2904
	11th to 20th	Night	32% 6% 15% 33% 13% 1	2902
	21st to end	Night	33% 7% 14% 32% 13% 1	3378
September	1st to 10th	Night	32% 8% 16% 29% 14% 1	2000
	11th to 20th	Night	28% 7% 12% 36% 17% 1	2110
	21st to end	Night	41% 8% 13% 27% 11% 1	2244
October	1st to 10th	Night	33% 6% 11% 26% 23% 1	3837
	11th to 20th	Night	42% 6% 15% 26% 10%	3779
	21st to end	Night	40% 7% 17% 27% 10% 1	4120
November	1st to 10th	Night	44% 7% 14% 23% 11% 1	2604
	11th to 20th	Night	50% 9% 15% 17% 9% 1	2800
	21st to end	Night	54% 9% 18% 15% 3	2800
December	1st to 10th	Night	61% 8% 14% 13% 4	4580
	11th to 20th	Night	59% 8% 12% 13% 8%	5208
	21st to end	Night	67% 9% 11% 9% 5%	6996

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	18% 19% 33% 23% 7% 2	200
	11th to 20th	Day	21% 19% 38% 14% 7% 1	273
	21st to end	Day	22% 16% 34% 24% 4	330
February	1st to 10th	Day	22% 15% 41% 19% 3	300
	11th to 20th	Day	23% 19% 38% 16% 3	300
	21st to end	Day	27% 23% 28% 21% 2	271
March	1st to 10th	Day	20% 13% 37% 23% 7%	500
	11th to 20th	Day	18% 15% 35% 25% 6% 1	500
	21st to end	Day	25% 11% 33% 23% 7% 1	550
April	1st to 10th	Day	21% 12% 34% 26% 7%	500
	11th to 20th	Day	24% 11% 33% 20% 8% 3	492
	21st to end	Day	22% 10% 32% 24% 9% 2	500
May	1st to 10th	Day	20% 9% 29% 26% 12% 5%	503
	11th to 20th	Day	15% 6% 23% 33% 18% 5%	527
	21st to end	Day	15% 5% 22% 32% 20% 6%	573
June	1st to 10th	Day	14% 6% 25% 37% 15% 2	530
	11th to 20th	Day	13% 7% 26% 32% 19% 3	530
	21st to end	Day	15% 6% 22% 33% 19% 5%	530
July	1st to 10th	Day	9% 4 20% 33% 26% 8%	525
	11th to 20th	Day	14% 3 19% 34% 21% 9%	524
	21st to end	Day	8% 3 21% 39% 22% 7%	576
August	1st to 10th	Day	11% 4 18% 30% 25% 11%	526
	11th to 20th	Day	9% 4 18% 30% 27% 11%	519
	21st to end	Day	11% 5% 20% 32% 22% 10%	583
September	1st to 10th	Day	13% 5% 20% 29% 21% 11%	530
	11th to 20th	Day	8% 4 16% 33% 24% 15%	530
	21st to end	Day	14% 5% 27% 31% 15% 7%	530
October	1st to 10th	Day	19% 8% 26% 26% 15% 6%	502
	11th to 20th	Day	19% 9% 36% 24% 10% 2	490
	21st to end	Day	26% 10% 32% 25% 6% 1	539
November	1st to 10th	Day	18% 16% 28% 26% 11% 1	286
	11th to 20th	Day	20% 20% 33% 22% 6% 1	200
	21st to end	Day	23% 16% 38% 20% 3 1	200
December	1st to 10th	Day	24% 15% 36% 19% 7% 1	200
	11th to 20th	Day	26% 19% 33% 18% 5%	200
	21st to end	Day	28% 17% 37% 16% 2	220
January	1st to 10th	Night	23% 14% 37% 21% 6% 1	800
	11th to 20th	Night	25% 13% 33% 25% 4	800
	21st to end	Night	19% 9% 38% 26% 7%	880
February	1st to 10th	Night	21% 13% 40% 23% 3	800
	11th to 20th	Night	21% 14% 36% 24% 5%	783
	21st to end	Night	19% 12% 41% 23% 5%	660
March	1st to 10th	Night	18% 11% 37% 26% 7%	800
	11th to 20th	Night	20% 13% 36% 24% 7% 1	800
	21st to end	Night	21% 9% 31% 29% 9%	880
April	1st to 10th	Night	24% 9% 33% 28% 6%	800
	11th to 20th	Night	18% 10% 33% 29% 9% 2	736
	21st to end	Night	20% 7% 29% 32% 12%	600
May	1st to 10th	Night	18% 5% 27% 33% 16% 2	600
	11th to 20th	Night	15% 3 28% 37% 17%	404
	21st to end	Night	12% 3 23% 39% 21% 1	433
June	1st to 10th	Night	13% 3 24% 41% 17% 2	400
	11th to 20th	Night	12% 3 26% 37% 21% 1	400
	21st to end	Night	12% 5% 19% 43% 20% 1	400
July	1st to 10th	Night	7% 3 17% 44% 28% 2	390
	11th to 20th	Night	11% 3 19% 40% 24% 3	388
	21st to end	Night	8% 2 16% 46% 26% 2	619
August	1st to 10th	Night	9% 2 14% 39% 32% 3	597
	11th to 20th	Night	8% 2 15% 38% 32% 4	594
	21st to end	Night	10% 3 21% 35% 28% 3	750
September	1st to 10th	Night	11% 4 21% 35% 23% 7%	800
	11th to 20th	Night	12% 3 17% 32% 31% 6%	800
	21st to end	Night	17% 5% 20% 33% 21% 5%	800
October	1st to 10th	Night	18% 7% 24% 28% 21% 2	773
	11th to 20th	Night	20% 8% 30% 30% 11% 1	760
	21st to end	Night	25% 7% 31% 30% 6% 1	836
November	1st to 10th	Night	21% 9% 27% 30% 12% 1	780
	11th to 20th	Night	21% 11% 34% 25% 8% 2	800
	21st to end	Night	25% 10% 33% 26% 6%	800
December	1st to 10th	Night	27% 11% 36% 21% 6%	800
	11th to 20th	Night	22% 12% 40% 21% 4	800
	21st to end	Night	30% 9% 35% 23% 3	880

Page generated at 2025-10-01 01:39:04. Version ac07

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