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 2024-01-12.

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% 8% 4 7% 6% 4 5% 5% 6% 7% 33%	74649
	Night	16% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	75060
Transparency	Day	47% 9% 14% 19% 10% 1	74364
	Night	48% 8% 13% 20% 10% 1	74879
Seeing	Day	17% 8% 27% 29% 15% 5%	13301
	Night	18% 7% 29% 30% 13% 2	22828

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	16% 12% 5% 10% 8% 4 6% 5% 6% 7% 23%	1517
	11th to 20th	Day	17% 13% 5% 9% 9% 3 7% 5% 5% 5% 21%	1578
	21st to end	Day	12% 14% 6% 12% 8% 4 6% 5% 5% 6% 21%	1782
February	1st to 10th	Day	16% 15% 4 11% 8% 5% 6% 5% 4 5% 21%	1620
	11th to 20th	Day	12% 13% 7% 11% 7% 3 5% 5% 4 6% 27%	1573
	21st to end	Day	19% 12% 5% 10% 6% 4 5% 5% 3 5% 26%	1382
March	1st to 10th	Day	14% 10% 4 8% 8% 4 5% 6% 6% 6% 30%	1977
	11th to 20th	Day	14% 11% 5% 8% 7% 4 5% 5% 4 5% 31%	1980
	21st to end	Day	18% 11% 4 6% 6% 3 5% 4 5% 5% 33%	2178
April	1st to 10th	Day	17% 11% 4 8% 6% 4 4 5% 6% 6% 29%	2128
	11th to 20th	Day	19% 8% 3 6% 5% 3 4 4 5% 6% 36%	2199
	21st to end	Day	20% 8% 3 6% 5% 4 5% 4 5% 6% 33%	2340
May	1st to 10th	Day	16% 8% 4 7% 6% 3 4 5% 6% 7% 34%	2340
	11th to 20th	Day	14% 7% 3 5% 5% 3 5% 5% 5% 8% 39%	2466
	21st to end	Day	12% 4 3 6% 5% 3 5% 5% 5% 8% 44%	2725
June	1st to 10th	Day	14% 6% 3 5% 6% 3 5% 5% 6% 9% 37%	2520
	11th to 20th	Day	13% 4 2 6% 5% 3 4 6% 7% 10% 39%	2520
	21st to end	Day	14% 4 2 5% 6% 4 6% 5% 6% 9% 39%	2520
July	1st to 10th	Day	8% 4 2 5% 5% 3 7% 6% 7% 9% 44%	2520
	11th to 20th	Day	11% 4 2 6% 5% 4 6% 5% 7% 9% 40%	2520
	21st to end	Day	9% 5% 2 6% 7% 4 6% 7% 8% 10% 35%	2578
August	1st to 10th	Day	9% 5% 2 6% 6% 4 4 7% 7% 10% 40%	2327
	11th to 20th	Day	8% 6% 3 7% 6% 3 5% 6% 7% 10% 39%	2303
	21st to end	Day	11% 6% 3 6% 4 4 5% 6% 6% 9% 40%	2476
September	1st to 10th	Day	14% 5% 3 5% 4 4 4 6% 6% 7% 42%	2160
	11th to 20th	Day	12% 4 3 6% 5% 3 5% 5% 6% 8% 45%	2059
	21st to end	Day	12% 6% 3 6% 7% 3 6% 5% 5% 8% 37%	1940
October	1st to 10th	Day	18% 10% 3 6% 5% 3 4 5% 5% 7% 35%	1800
	11th to 20th	Day	21% 8% 4 6% 7% 4 5% 5% 5% 6% 29%	1800
	21st to end	Day	24% 11% 3 8% 7% 3 5% 5% 6% 6% 22%	1980
November	1st to 10th	Day	14% 9% 4 8% 7% 3 4 5% 5% 9% 33%	1603
	11th to 20th	Day	15% 11% 4 7% 8% 4 5% 7% 6% 6% 26%	1520
	21st to end	Day	19% 12% 4 9% 7% 4 5% 5% 6% 8% 22%	1520
December	1st to 10th	Day	18% 13% 6% 10% 7% 4 6% 5% 6% 7% 17%	1520
	11th to 20th	Day	19% 12% 5% 8% 7% 3 6% 5% 5% 7% 23%	1906
	21st to end	Day	28% 17% 6% 11% 7% 3 4 4 4 4 13%	2772
January	1st to 10th	Night	17% 14% 5% 10% 8% 4 4 5% 5% 6% 23%	2649
	11th to 20th	Night	19% 14% 6% 8% 8% 4 6% 5% 4 6% 20%	2411
	21st to end	Night	13% 12% 6% 9% 7% 4 6% 5% 6% 7% 27%	2574
February	1st to 10th	Night	15% 13% 6% 10% 7% 4 5% 5% 5% 5% 24%	2340
	11th to 20th	Night	15% 13% 5% 9% 7% 3 5% 6% 5% 5% 27%	2273
	21st to end	Night	15% 11% 4 9% 7% 3 5% 5% 5% 6% 29%	1871
March	1st to 10th	Night	13% 8% 4 9% 7% 4 5% 6% 4 5% 35%	1970
	11th to 20th	Night	16% 10% 5% 9% 6% 3 4 3 4 6% 35%	1979
	21st to end	Night	16% 9% 3 7% 5% 3 4 5% 4 5% 39%	2178
April	1st to 10th	Night	18% 9% 3 7% 6% 3 4 5% 5% 6% 33%	1832
	11th to 20th	Night	15% 8% 3 7% 6% 3 5% 5% 4 6% 38%	1726
	21st to end	Night	17% 7% 3 6% 5% 3 4 5% 6% 7% 36%	1620
May	1st to 10th	Night	17% 5% 3 4 5% 4 5% 5% 5% 7% 39%	1620
	11th to 20th	Night	14% 4 3 4 5% 3 4 4 5% 8% 46%	1425
	21st to end	Night	10% 4 2 4 4 3 3 5% 5% 7% 52%	1550
June	1st to 10th	Night	12% 5% 2 5% 6% 4 5% 4 6% 9% 42%	1440
	11th to 20th	Night	10% 6% 3 5% 5% 4 5% 4 6% 10% 43%	1440
	21st to end	Night	11% 6% 2 5% 5% 3 5% 4 6% 8% 45%	1440
July	1st to 10th	Night	6% 4 2 4 4 3 4 4 5% 9% 56%	1440
	11th to 20th	Night	10% 3 3 4 5% 3 4 6% 6% 8% 49%	1440
	21st to end	Night	7% 4 2 4 4 4 4 5% 6% 8% 51%	1736
August	1st to 10th	Night	7% 4 2 5% 6% 4 5% 4 6% 9% 49%	1611
	11th to 20th	Night	7% 4 2 5% 4 2 4 6% 6% 9% 52%	1598
	21st to end	Night	10% 5% 2 4 4 3 4 5% 6% 6% 49%	1862
September	1st to 10th	Night	9% 4 3 6% 4 3 6% 6% 5% 8% 46%	1800
	11th to 20th	Night	11% 4 2 4 4 2 3 4 4 7% 54%	1898
	21st to end	Night	14% 6% 2 5% 5% 3 4 5% 6% 7% 43%	2019
October	1st to 10th	Night	16% 7% 3 6% 5% 4 4 5% 6% 6% 38%	2160
	11th to 20th	Night	19% 7% 3 6% 6% 3 5% 5% 5% 7% 33%	2160
	21st to end	Night	21% 9% 4 7% 6% 3 5% 5% 5% 7% 28%	2376
November	1st to 10th	Night	17% 7% 3 6% 5% 3 6% 4 5% 7% 37%	2470
	11th to 20th	Night	17% 8% 4 7% 6% 4 5% 6% 6% 7% 30%	2660
	21st to end	Night	20% 11% 4 6% 6% 4 5% 4 6% 7% 27%	2660
December	1st to 10th	Night	20% 13% 5% 9% 6% 4 5% 4 5% 6% 23%	2660
	11th to 20th	Night	16% 15% 5% 9% 6% 4 6% 5% 5% 6% 23%	3288
	21st to end	Night	30% 14% 5% 8% 7% 4 5% 5% 5% 5% 14%	4884

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	58% 9% 15% 12% 5%	1518
	11th to 20th	Day	63% 10% 11% 11% 5%	1578
	21st to end	Day	63% 7% 11% 12% 6% 1	1782
February	1st to 10th	Day	64% 7% 12% 14% 2 1	1620
	11th to 20th	Day	57% 7% 14% 15% 7% 1	1582
	21st to end	Day	57% 8% 12% 16% 6% 1	1382
March	1st to 10th	Day	51% 9% 14% 17% 9% 1	1980
	11th to 20th	Day	52% 8% 10% 17% 11% 2	1980
	21st to end	Day	51% 7% 10% 17% 13% 2	2178
April	1st to 10th	Day	53% 7% 12% 17% 11% 1	2128
	11th to 20th	Day	47% 7% 11% 20% 15% 1	2199
	21st to end	Day	50% 7% 12% 17% 12% 3	2340
May	1st to 10th	Day	47% 7% 11% 20% 14% 1	2340
	11th to 20th	Day	42% 8% 12% 24% 13% 2	2470
	21st to end	Day	36% 9% 16% 24% 14% 2	2725
June	1st to 10th	Day	40% 10% 13% 21% 15% 1	2520
	11th to 20th	Day	36% 9% 13% 24% 15% 2	2520
	21st to end	Day	36% 10% 14% 24% 14% 1	2520
July	1st to 10th	Day	31% 9% 14% 25% 18% 3	2450
	11th to 20th	Day	35% 9% 16% 27% 12% 1	2445
	21st to end	Day	38% 12% 16% 21% 12% 1	2434
August	1st to 10th	Day	35% 11% 17% 22% 13% 1	2314
	11th to 20th	Day	35% 11% 17% 25% 12%	2303
	21st to end	Day	36% 10% 15% 26% 12% 1	2476
September	1st to 10th	Day	37% 10% 15% 26% 11% 1	2160
	11th to 20th	Day	34% 8% 16% 27% 14% 1	2059
	21st to end	Day	41% 10% 15% 26% 8%	1940
October	1st to 10th	Day	47% 8% 15% 18% 11% 1	1800
	11th to 20th	Day	52% 9% 13% 19% 6% 1	1800
	21st to end	Day	59% 8% 13% 14% 5% 1	1980
November	1st to 10th	Day	46% 10% 15% 20% 8% 1	1603
	11th to 20th	Day	53% 11% 15% 14% 7% 1	1520
	21st to end	Day	57% 10% 16% 14% 3	1520
December	1st to 10th	Day	63% 10% 13% 11% 3	1520
	11th to 20th	Day	58% 10% 17% 10% 5%	1906
	21st to end	Day	72% 9% 10% 8% 2	2772
January	1st to 10th	Night	59% 9% 12% 15% 4	2649
	11th to 20th	Night	64% 8% 12% 10% 5% 1	2411
	21st to end	Night	55% 8% 15% 14% 6% 1	2574
February	1st to 10th	Night	59% 7% 13% 16% 4 1	2340
	11th to 20th	Night	58% 7% 12% 15% 8%	2275
	21st to end	Night	54% 6% 15% 17% 7%	1871
March	1st to 10th	Night	49% 7% 13% 21% 8% 1	1980
	11th to 20th	Night	53% 4 10% 19% 12% 2	1980
	21st to end	Night	46% 6% 10% 21% 15% 3	2178
April	1st to 10th	Night	49% 6% 11% 19% 13% 1	1832
	11th to 20th	Night	46% 7% 9% 20% 15% 2	1726
	21st to end	Night	45% 7% 11% 20% 14% 2	1620
May	1st to 10th	Night	42% 7% 11% 21% 16% 3	1620
	11th to 20th	Night	37% 6% 10% 26% 16% 4	1426
	21st to end	Night	29% 9% 15% 24% 18% 4	1551
June	1st to 10th	Night	38% 7% 12% 24% 17% 2	1440
	11th to 20th	Night	36% 7% 13% 21% 21% 2	1440
	21st to end	Night	35% 8% 13% 24% 18% 1	1440
July	1st to 10th	Night	26% 7% 12% 27% 25% 4	1400
	11th to 20th	Night	31% 7% 14% 28% 18% 2	1392
	21st to end	Night	28% 9% 16% 28% 16% 3	1639
August	1st to 10th	Night	31% 8% 18% 28% 14% 2	1600
	11th to 20th	Night	27% 9% 16% 34% 14% 1	1598
	21st to end	Night	32% 8% 14% 29% 16% 1	1862
September	1st to 10th	Night	33% 9% 17% 26% 14% 1	1800
	11th to 20th	Night	29% 7% 12% 35% 16% 1	1898
	21st to end	Night	39% 8% 13% 28% 11%	2019
October	1st to 10th	Night	43% 9% 13% 20% 14% 1	2160
	11th to 20th	Night	49% 8% 14% 22% 7%	2160
	21st to end	Night	55% 7% 11% 17% 9% 1	2376
November	1st to 10th	Night	44% 7% 14% 23% 11% 1	2470
	11th to 20th	Night	50% 9% 15% 17% 9% 1	2660
	21st to end	Night	54% 9% 18% 15% 3	2660
December	1st to 10th	Night	59% 9% 13% 13% 5%	2660
	11th to 20th	Night	59% 9% 15% 13% 3	3288
	21st to end	Night	69% 11% 11% 7% 2	4884

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% 16% 32% 24% 7% 2	180
	11th to 20th	Day	20% 18% 41% 12% 8% 2	220
	21st to end	Day	21% 12% 36% 26% 5%	264
February	1st to 10th	Day	23% 14% 43% 17% 2	240
	11th to 20th	Day	21% 18% 41% 17% 3	240
	21st to end	Day	27% 24% 28% 20% 1	217
March	1st to 10th	Day	18% 12% 39% 24% 7%	420
	11th to 20th	Day	19% 13% 34% 26% 7% 1	420
	21st to end	Day	25% 8% 33% 24% 8% 1	462
April	1st to 10th	Day	18% 11% 35% 29% 8%	420
	11th to 20th	Day	22% 10% 33% 21% 10% 4	412
	21st to end	Day	22% 8% 31% 27% 10% 3	420
May	1st to 10th	Day	19% 8% 28% 26% 13% 6%	423
	11th to 20th	Day	15% 5% 23% 33% 18% 5%	447
	21st to end	Day	14% 4 19% 35% 21% 7%	485
June	1st to 10th	Day	14% 3 23% 41% 16% 3	450
	11th to 20th	Day	14% 6% 22% 33% 22% 3	450
	21st to end	Day	14% 4 19% 36% 22% 5%	450
July	1st to 10th	Day	8% 2 15% 36% 29% 9%	445
	11th to 20th	Day	14% 2 15% 36% 24% 10%	444
	21st to end	Day	8% 3 18% 39% 23% 8%	488
August	1st to 10th	Day	9% 3 16% 33% 26% 13%	446
	11th to 20th	Day	8% 3 16% 31% 29% 12%	439
	21st to end	Day	12% 4 18% 31% 24% 11%	495
September	1st to 10th	Day	13% 3 19% 31% 22% 12%	450
	11th to 20th	Day	9% 4 17% 32% 22% 15%	450
	21st to end	Day	12% 6% 26% 31% 16% 8%	450
October	1st to 10th	Day	20% 8% 25% 24% 16% 7%	450
	11th to 20th	Day	19% 9% 36% 25% 9% 2	450
	21st to end	Day	27% 9% 32% 25% 7% 1	495
November	1st to 10th	Day	17% 16% 27% 27% 12% 1	261
	11th to 20th	Day	19% 17% 33% 23% 7% 1	180
	21st to end	Day	22% 12% 41% 21% 3 1	180
December	1st to 10th	Day	23% 13% 38% 18% 7% 1	180
	11th to 20th	Day	23% 18% 35% 19% 5%	180
	21st to end	Day	27% 16% 39% 17% 1	198
January	1st to 10th	Night	23% 13% 36% 22% 6% 1	760
	11th to 20th	Night	26% 11% 32% 25% 4	727
	21st to end	Night	18% 9% 38% 28% 7%	792
February	1st to 10th	Night	21% 13% 41% 22% 3	720
	11th to 20th	Night	20% 13% 37% 25% 5%	703
	21st to end	Night	20% 11% 41% 23% 5%	592
March	1st to 10th	Night	18% 10% 38% 27% 8%	720
	11th to 20th	Night	21% 12% 36% 24% 7% 1	720
	21st to end	Night	20% 7% 31% 31% 10% 1	792
April	1st to 10th	Night	24% 8% 32% 30% 6%	720
	11th to 20th	Night	17% 9% 33% 30% 10% 2	662
	21st to end	Night	20% 6% 29% 33% 12%	540
May	1st to 10th	Night	17% 4 27% 32% 17% 3	540
	11th to 20th	Night	15% 2 28% 37% 17%	364
	21st to end	Night	10% 2 22% 40% 23% 2	389
June	1st to 10th	Night	13% 2 23% 42% 19% 2	360
	11th to 20th	Night	13% 2 25% 38% 22% 1	360
	21st to end	Night	12% 4 18% 43% 22% 1	360
July	1st to 10th	Night	6% 1 16% 44% 30% 2	350
	11th to 20th	Night	11% 3 18% 40% 25% 3	348
	21st to end	Night	8% 2 15% 47% 26% 2	556
August	1st to 10th	Night	9% 1 13% 41% 32% 4	537
	11th to 20th	Night	7% 2 13% 40% 34% 4	534
	21st to end	Night	11% 2 20% 36% 28% 3	674
September	1st to 10th	Night	11% 3 20% 36% 23% 7%	720
	11th to 20th	Night	13% 3 18% 31% 29% 6%	720
	21st to end	Night	16% 4 20% 33% 22% 5%	720
October	1st to 10th	Night	18% 7% 24% 28% 22% 2	720
	11th to 20th	Night	21% 8% 31% 30% 10% 2	720
	21st to end	Night	27% 7% 31% 30% 6% 1	792
November	1st to 10th	Night	20% 9% 26% 31% 12% 1	740
	11th to 20th	Night	20% 10% 34% 26% 9% 2	760
	21st to end	Night	24% 9% 33% 27% 7%	760
December	1st to 10th	Night	27% 10% 36% 21% 6%	760
	11th to 20th	Night	21% 12% 41% 22% 4	760
	21st to end	Night	31% 9% 35% 23% 3	836

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