The Winter-Hawk Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Winter-Hawk Memorial Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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	14% 8% 3 6% 5% 3 5% 5% 6% 8% 37%	74895
	Night	14% 8% 3 6% 5% 3 5% 5% 6% 7% 39%	74861
Transparency	Day	44% 10% 12% 18% 14% 3	74618
	Night	43% 9% 12% 19% 14% 3	74657
Seeing	Day	18% 13% 23% 24% 17% 6%	13846
	Night	17% 10% 22% 31% 18% 3	23969

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% 10% 5% 10% 6% 3 5% 4 5% 6% 33%	1706
	11th to 20th	Day	17% 14% 5% 8% 6% 3 4 4 4 6% 30%	1644
	21st to end	Day	16% 12% 4 5% 5% 2 3 4 5% 6% 37%	1782
February	1st to 10th	Day	17% 13% 4 7% 6% 3 4 5% 4 5% 32%	1714
	11th to 20th	Day	20% 11% 5% 9% 5% 3 3 4 4 5% 31%	1748
	21st to end	Day	21% 12% 3 6% 4 3 4 4 4 5% 35%	1480
March	1st to 10th	Day	15% 11% 3 5% 4 3 4 4 4 6% 41%	1905
	11th to 20th	Day	19% 10% 4 6% 4 3 5% 4 5% 7% 34%	1980
	21st to end	Day	18% 10% 3 6% 4 3 6% 4 6% 7% 34%	2178
April	1st to 10th	Day	12% 7% 2 5% 4 3 4 4 7% 8% 43%	1980
	11th to 20th	Day	15% 7% 2 5% 5% 2 4 5% 5% 7% 43%	2180
	21st to end	Day	15% 6% 3 5% 5% 4 4 5% 5% 7% 40%	2340
May	1st to 10th	Day	14% 5% 3 5% 6% 3 5% 6% 7% 8% 38%	2340
	11th to 20th	Day	14% 6% 3 7% 7% 4 5% 6% 7% 6% 34%	2301
	21st to end	Day	11% 6% 3 6% 6% 3 6% 6% 7% 10% 36%	2531
June	1st to 10th	Day	10% 6% 2 5% 6% 4 8% 6% 7% 10% 36%	2340
	11th to 20th	Day	6% 5% 3 6% 7% 5% 8% 8% 8% 9% 35%	2340
	21st to end	Day	9% 5% 3 6% 6% 5% 7% 7% 7% 11% 35%	2340
July	1st to 10th	Day	10% 6% 4 6% 8% 5% 7% 6% 8% 10% 31%	2340
	11th to 20th	Day	11% 6% 3 6% 7% 4 8% 6% 8% 9% 32%	2340
	21st to end	Day	12% 6% 2 7% 7% 4 7% 6% 8% 10% 30%	2547
August	1st to 10th	Day	11% 5% 2 6% 8% 5% 8% 7% 7% 12% 30%	2327
	11th to 20th	Day	11% 6% 3 6% 6% 4 7% 7% 8% 9% 32%	2303
	21st to end	Day	11% 6% 3 6% 6% 4 6% 6% 7% 10% 35%	2389
September	1st to 10th	Day	11% 6% 2 5% 5% 4 5% 6% 6% 8% 42%	1980
	11th to 20th	Day	15% 5% 2 6% 5% 3 4 5% 5% 8% 41%	1980
	21st to end	Day	13% 5% 3 5% 5% 3 4 5% 6% 7% 45%	1980
October	1st to 10th	Day	13% 5% 2 5% 4 2 4 3 5% 6% 51%	1968
	11th to 20th	Day	14% 5% 2 4 4 2 4 3 5% 7% 51%	1800
	21st to end	Day	15% 7% 2 4 3 3 4 3 4 5% 49%	1980
November	1st to 10th	Day	13% 5% 1 4 4 2 4 3 3 6% 54%	1725
	11th to 20th	Day	13% 7% 3 4 4 3 4 5% 5% 7% 46%	1710
	21st to end	Day	13% 9% 4 5% 4 2 4 3 5% 7% 44%	1710
December	1st to 10th	Day	19% 12% 4 5% 4 3 4 5% 5% 6% 34%	1710
	11th to 20th	Day	15% 13% 3 7% 3 2 3 5% 7% 7% 35%	2153
	21st to end	Day	14% 16% 4 6% 6% 3 5% 4 4 6% 31%	3124
January	1st to 10th	Night	17% 10% 5% 8% 5% 3 5% 4 4 5% 34%	2459
	11th to 20th	Night	19% 12% 4 8% 6% 3 4 4 4 6% 29%	2378
	21st to end	Night	15% 13% 5% 6% 5% 2 3 4 4 5% 37%	2574
February	1st to 10th	Night	18% 10% 4 6% 4 3 4 4 5% 6% 36%	2241
	11th to 20th	Night	22% 11% 4 7% 5% 3 4 4 4 5% 32%	2098
	21st to end	Night	21% 11% 4 5% 5% 3 4 4 5% 7% 31%	1776
March	1st to 10th	Night	17% 11% 3 5% 4 3 4 4 4 5% 41%	2053
	11th to 20th	Night	19% 8% 3 6% 6% 3 5% 4 5% 7% 34%	1980
	21st to end	Night	16% 7% 3 5% 5% 3 4 5% 6% 8% 37%	2178
April	1st to 10th	Night	14% 7% 2 4 5% 2 4 4 7% 8% 43%	1980
	11th to 20th	Night	13% 8% 3 5% 5% 3 4 3 5% 8% 43%	1742
	21st to end	Night	16% 5% 2 5% 4 3 4 5% 6% 7% 43%	1620
May	1st to 10th	Night	15% 6% 2 6% 5% 3 4 5% 6% 10% 38%	1620
	11th to 20th	Night	14% 6% 3 6% 6% 4 5% 5% 7% 9% 35%	1592
	21st to end	Night	10% 5% 3 6% 6% 3 6% 5% 9% 10% 38%	1744
June	1st to 10th	Night	10% 5% 3 5% 6% 4 6% 5% 8% 11% 37%	1620
	11th to 20th	Night	5% 4 3 5% 5% 3 6% 5% 8% 12% 42%	1620
	21st to end	Night	8% 5% 2 6% 6% 4 7% 6% 6% 11% 40%	1620
July	1st to 10th	Night	11% 6% 3 5% 6% 4 6% 6% 8% 10% 34%	1620
	11th to 20th	Night	8% 4 3 6% 6% 4 6% 7% 9% 13% 34%	1620
	21st to end	Night	8% 5% 3 7% 7% 4 7% 7% 8% 11% 32%	1771
August	1st to 10th	Night	9% 4 2 7% 7% 3 5% 7% 7% 10% 39%	1611
	11th to 20th	Night	11% 6% 3 6% 7% 4 7% 6% 7% 8% 36%	1598
	21st to end	Night	10% 6% 2 5% 6% 4 6% 6% 7% 9% 38%	1944
September	1st to 10th	Night	11% 5% 3 7% 5% 3 5% 5% 6% 7% 43%	1980
	11th to 20th	Night	15% 6% 2 6% 5% 3 4 5% 6% 8% 40%	1980
	21st to end	Night	12% 5% 2 5% 4 3 4 4 5% 7% 49%	1980
October	1st to 10th	Night	14% 5% 2 4 4 2 3 3 5% 6% 51%	1990
	11th to 20th	Night	16% 6% 2 4 4 2 4 3 4 6% 50%	2160
	21st to end	Night	14% 6% 2 4 4 2 3 4 5% 7% 49%	2376
November	1st to 10th	Night	15% 6% 2 4 3 2 3 4 4 5% 51%	2352
	11th to 20th	Night	15% 7% 3 4 4 3 3 4 5% 7% 46%	2470
	21st to end	Night	14% 8% 2 5% 4 3 3 3 5% 6% 47%	2470
December	1st to 10th	Night	18% 12% 5% 7% 5% 2 4 4 5% 6% 31%	2470
	11th to 20th	Night	18% 11% 4 8% 4 2 3 4 4 6% 36%	3042
	21st to end	Night	14% 15% 4 7% 6% 4 5% 5% 6% 8% 26%	4532

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	52% 7% 9% 18% 11% 3	1707
	11th to 20th	Day	56% 6% 8% 17% 12% 2	1644
	21st to end	Day	47% 5% 10% 22% 16% 1	1782
February	1st to 10th	Day	55% 5% 9% 15% 15% 2	1714
	11th to 20th	Day	56% 6% 8% 16% 13% 2	1758
	21st to end	Day	50% 6% 10% 14% 15% 6%	1480
March	1st to 10th	Day	45% 5% 9% 19% 18% 4	1905
	11th to 20th	Day	50% 8% 10% 16% 13% 3	1980
	21st to end	Day	50% 7% 11% 20% 11% 2	2178
April	1st to 10th	Day	38% 7% 12% 22% 17% 3	1980
	11th to 20th	Day	40% 8% 9% 18% 21% 5%	2180
	21st to end	Day	43% 8% 11% 24% 12% 2	2340
May	1st to 10th	Day	41% 11% 13% 21% 11% 2	2340
	11th to 20th	Day	48% 11% 13% 17% 9% 2	2305
	21st to end	Day	41% 13% 14% 20% 11% 1	2531
June	1st to 10th	Day	41% 13% 17% 18% 10% 1	2340
	11th to 20th	Day	40% 14% 17% 15% 12% 1	2340
	21st to end	Day	41% 14% 14% 18% 11% 3	2340
July	1st to 10th	Day	45% 15% 15% 15% 9% 1	2275
	11th to 20th	Day	44% 15% 17% 11% 10% 3	2271
	21st to end	Day	45% 15% 16% 16% 8% 1	2405
August	1st to 10th	Day	45% 15% 17% 16% 7%	2314
	11th to 20th	Day	44% 15% 15% 15% 10% 1	2303
	21st to end	Day	42% 13% 15% 17% 10% 3	2389
September	1st to 10th	Day	39% 11% 14% 18% 13% 4	1980
	11th to 20th	Day	41% 10% 12% 19% 16% 2	1980
	21st to end	Day	39% 8% 14% 20% 15% 4	1980
October	1st to 10th	Day	35% 7% 11% 20% 23% 4	1968
	11th to 20th	Day	34% 7% 9% 23% 21% 6%	1800
	21st to end	Day	37% 5% 8% 24% 22% 5%	1980
November	1st to 10th	Day	32% 5% 9% 21% 27% 6%	1725
	11th to 20th	Day	36% 6% 9% 21% 23% 5%	1710
	21st to end	Day	40% 6% 8% 22% 20% 4	1710
December	1st to 10th	Day	51% 8% 8% 20% 12% 2	1710
	11th to 20th	Day	44% 8% 11% 17% 17% 3	2150
	21st to end	Day	51% 9% 8% 22% 9% 1	3124
January	1st to 10th	Night	54% 6% 9% 16% 14% 3	2459
	11th to 20th	Night	57% 6% 10% 12% 13% 2	2378
	21st to end	Night	51% 5% 9% 19% 15% 2	2574
February	1st to 10th	Night	49% 6% 11% 18% 15% 2	2241
	11th to 20th	Night	55% 5% 8% 17% 12% 3	2099
	21st to end	Night	52% 7% 9% 16% 13% 4	1776
March	1st to 10th	Night	47% 5% 9% 16% 20% 4	2053
	11th to 20th	Night	48% 9% 11% 18% 12% 2	1980
	21st to end	Night	46% 8% 13% 21% 12% 2	2178
April	1st to 10th	Night	38% 8% 13% 22% 16% 3	1980
	11th to 20th	Night	40% 7% 10% 20% 19% 4	1742
	21st to end	Night	40% 7% 12% 26% 13% 1	1620
May	1st to 10th	Night	40% 10% 17% 21% 10% 2	1620
	11th to 20th	Night	42% 13% 14% 22% 8% 1	1593
	21st to end	Night	38% 13% 15% 23% 9% 1	1745
June	1st to 10th	Night	37% 13% 19% 19% 11% 1	1620
	11th to 20th	Night	32% 13% 21% 21% 11% 1	1620
	21st to end	Night	37% 12% 17% 21% 10% 4	1620
July	1st to 10th	Night	41% 13% 17% 17% 11% 1	1575
	11th to 20th	Night	36% 16% 21% 15% 9% 3	1566
	21st to end	Night	40% 16% 18% 19% 7%	1672
August	1st to 10th	Night	36% 15% 21% 19% 8% 1	1600
	11th to 20th	Night	41% 14% 16% 17% 11% 1	1598
	21st to end	Night	39% 13% 18% 20% 9% 1	1944
September	1st to 10th	Night	39% 9% 14% 20% 13% 4	1980
	11th to 20th	Night	40% 11% 14% 17% 17% 1	1980
	21st to end	Night	35% 9% 11% 22% 19% 4	1980
October	1st to 10th	Night	35% 6% 11% 18% 25% 5%	1990
	11th to 20th	Night	37% 6% 11% 23% 19% 4	2160
	21st to end	Night	35% 6% 8% 21% 24% 6%	2376
November	1st to 10th	Night	36% 6% 9% 19% 23% 7%	2352
	11th to 20th	Night	37% 6% 8% 20% 23% 6%	2470
	21st to end	Night	39% 5% 7% 26% 18% 4	2470
December	1st to 10th	Night	54% 8% 10% 16% 10% 2	2470
	11th to 20th	Night	50% 7% 8% 15% 18% 3	3044
	21st to end	Night	50% 11% 12% 16% 10% 1	4532

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	23% 29% 30% 15% 3	270
	11th to 20th	Day	34% 23% 26% 15% 2	248
	21st to end	Day	26% 30% 30% 10% 4	264
February	1st to 10th	Day	24% 29% 35% 11% 1	240
	11th to 20th	Day	39% 30% 24% 7%	240
	21st to end	Day	40% 24% 28% 8% 1	198
March	1st to 10th	Day	24% 21% 29% 20% 6%	345
	11th to 20th	Day	26% 20% 32% 18% 4 1	420
	21st to end	Day	26% 20% 26% 21% 7%	462
April	1st to 10th	Day	20% 20% 25% 24% 10% 1	420
	11th to 20th	Day	18% 19% 25% 24% 12% 2	412
	21st to end	Day	19% 16% 25% 22% 14% 3	420
May	1st to 10th	Day	20% 14% 28% 22% 11% 4	423
	11th to 20th	Day	16% 11% 22% 29% 15% 7%	447
	21st to end	Day	15% 6% 21% 26% 21% 11%	485
June	1st to 10th	Day	11% 6% 23% 29% 24% 8%	450
	11th to 20th	Day	6% 6% 19% 28% 28% 12%	450
	21st to end	Day	10% 6% 26% 29% 17% 11%	450
July	1st to 10th	Day	12% 3 15% 25% 31% 14%	445
	11th to 20th	Day	11% 5% 14% 27% 27% 16%	444
	21st to end	Day	12% 3 15% 27% 30% 13%	488
August	1st to 10th	Day	13% 6% 16% 26% 25% 14%	446
	11th to 20th	Day	12% 3 15% 29% 28% 12%	439
	21st to end	Day	12% 4 11% 25% 32% 16%	495
September	1st to 10th	Day	11% 4 12% 27% 32% 13%	450
	11th to 20th	Day	14% 5% 15% 25% 32% 9%	450
	21st to end	Day	15% 6% 17% 31% 26% 5%	450
October	1st to 10th	Day	17% 7% 21% 28% 21% 6%	450
	11th to 20th	Day	15% 14% 23% 28% 18% 2	450
	21st to end	Day	22% 12% 22% 30% 12% 2	495
November	1st to 10th	Day	15% 13% 30% 29% 12% 2	323
	11th to 20th	Day	18% 18% 33% 23% 7%	270
	21st to end	Day	19% 15% 37% 20% 8% 1	270
December	1st to 10th	Day	29% 21% 31% 16% 3	270
	11th to 20th	Day	24% 18% 32% 22% 4	270
	21st to end	Day	22% 27% 33% 13% 4 1	297
January	1st to 10th	Night	23% 21% 28% 23% 5%	760
	11th to 20th	Night	25% 16% 30% 24% 5%	732
	21st to end	Night	22% 18% 32% 25% 3	792
February	1st to 10th	Night	24% 15% 28% 29% 4	720
	11th to 20th	Night	27% 20% 32% 20% 1	703
	21st to end	Night	27% 22% 28% 22% 2	592
March	1st to 10th	Night	23% 15% 29% 28% 4	720
	11th to 20th	Night	23% 16% 31% 23% 7% 1	720
	21st to end	Night	21% 14% 29% 28% 8%	792
April	1st to 10th	Night	18% 16% 28% 29% 8% 1	720
	11th to 20th	Night	17% 13% 26% 29% 13% 1	574
	21st to end	Night	16% 9% 25% 38% 11% 1	540
May	1st to 10th	Night	16% 11% 24% 31% 17% 1	540
	11th to 20th	Night	14% 7% 20% 37% 20% 3	531
	21st to end	Night	10% 2 15% 35% 34% 4	584
June	1st to 10th	Night	9% 2 14% 42% 28% 4	540
	11th to 20th	Night	6% 2 16% 35% 33% 8%	540
	21st to end	Night	9% 2 14% 38% 33% 5%	540
July	1st to 10th	Night	12% 3 7% 34% 38% 7%	525
	11th to 20th	Night	9% 3 8% 32% 40% 8%	522
	21st to end	Night	8% 2 9% 32% 41% 9%	591
August	1st to 10th	Night	7% 3 7% 34% 41% 7%	537
	11th to 20th	Night	10% 4 9% 32% 42% 4	534
	21st to end	Night	10% 2 10% 33% 38% 7%	612
September	1st to 10th	Night	13% 3 11% 31% 37% 6%	720
	11th to 20th	Night	16% 4 12% 32% 31% 5%	720
	21st to end	Night	13% 4 18% 35% 27% 3	720
October	1st to 10th	Night	14% 8% 17% 29% 26% 6%	720
	11th to 20th	Night	17% 7% 26% 31% 16% 3	720
	21st to end	Night	17% 11% 23% 34% 13% 2	792
November	1st to 10th	Night	18% 8% 20% 36% 16% 3	740
	11th to 20th	Night	18% 13% 23% 35% 10% 1	760
	21st to end	Night	19% 11% 25% 31% 13% 1	760
December	1st to 10th	Night	25% 14% 28% 27% 6%	760
	11th to 20th	Night	23% 13% 25% 31% 7% 1	760
	21st to end	Night	23% 15% 30% 28% 4	836

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