Wilkinson Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wilkinson Memorial Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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	7% 7% 3 7% 5% 5% 5% 4 6% 8% 44%	105589
	Night	5% 7% 3 7% 5% 5% 5% 4 6% 8% 44%	102166
Transparency	Day	38% 4 15% 28% 13% 2	105314
	Night	37% 4 16% 28% 13% 2	102031
Seeing	Day	11% 10% 30% 30% 16% 4	15697
	Night	9% 9% 31% 35% 14% 2	25006

By Month

Percentage Cloud Cover SW (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	4 11% 5% 11% 6% 6% 5% 5% 6% 10% 33%	2219
	11th to 20th	Day	1 6% 3 12% 8% 7% 8% 7% 7% 9% 31%	2220
	21st to end	Day	2 8% 4 8% 6% 5% 5% 4 5% 7% 47%	2747
February	1st to 10th	Day	5% 11% 5% 11% 7% 6% 6% 5% 7% 7% 30%	1710
	11th to 20th	Day	3 10% 3 11% 7% 7% 6% 6% 8% 9% 29%	1665
	21st to end	Day	4 10% 4 12% 11% 6% 4 6% 6% 9% 27%	1413
March	1st to 10th	Day	3 8% 4 10% 8% 8% 7% 4 5% 8% 36%	3278
	11th to 20th	Day	5% 6% 3 8% 6% 5% 4 4 7% 12% 40%	3490
	21st to end	Day	7% 17% 6% 20% 8% 5% 4 3 5% 6% 20%	3876
April	1st to 10th	Day	6% 10% 3 8% 7% 5% 6% 5% 8% 9% 34%	2280
	11th to 20th	Day	10% 12% 5% 10% 5% 5% 5% 4 5% 8% 31%	2391
	21st to end	Day	11% 9% 3 8% 5% 4 5% 4 7% 8% 36%	2494
May	1st to 10th	Day	16% 12% 4 8% 4 4 4 4 5% 5% 33%	4430
	11th to 20th	Day	8% 6% 3 7% 6% 6% 6% 5% 9% 8% 36%	4388
	21st to end	Day	15% 8% 3 8% 5% 6% 5% 5% 6% 7% 31%	5034
June	1st to 10th	Day	14% 7% 3 6% 5% 5% 5% 4 6% 8% 37%	2850
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 5% 6% 7% 37%	2850
	21st to end	Day	7% 4 2 5% 4 4 5% 5% 8% 9% 47%	2850
July	1st to 10th	Day	3 3 2 6% 5% 5% 6% 5% 7% 9% 50%	4740
	11th to 20th	Day	3 3 1 4 3 3 4 4 6% 9% 60%	4740
	21st to end	Day	2 2 1 3 2 3 3 3 5% 7% 69%	4996
August	1st to 10th	Day	6% 4 2 5% 3 4 4 4 6% 8% 54%	4316
	11th to 20th	Day	4 3 2 4 3 4 4 4 7% 10% 56%	4028
	21st to end	Day	5% 3 1 4 3 3 3 3 5% 7% 61%	4473
September	1st to 10th	Day	9% 5% 2 6% 4 4 4 3 5% 6% 52%	2280
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 3 5% 7% 47%	2235
	21st to end	Day	10% 5% 2 4 3 3 3 4 6% 7% 53%	2028
October	1st to 10th	Day	5% 6% 2 6% 3 3 3 3 6% 8% 54%	3170
	11th to 20th	Day	4 5% 2 4 3 4 4 4 6% 10% 55%	3150
	21st to end	Day	4 7% 3 6% 4 4 4 4 7% 7% 49%	3116
November	1st to 10th	Day	9% 12% 4 6% 5% 5% 5% 4 6% 8% 37%	1544
	11th to 20th	Day	4 7% 3 7% 6% 6% 6% 5% 7% 8% 41%	1520
	21st to end	Day	5% 10% 3 9% 5% 6% 6% 6% 6% 9% 36%	1520
December	1st to 10th	Day	5% 8% 3 11% 8% 7% 8% 5% 6% 7% 33%	1421
	11th to 20th	Day	4 8% 3 11% 7% 5% 5% 4 6% 8% 38%	1685
	21st to end	Day	4 8% 3 7% 4 4 5% 4 5% 8% 48%	2442
January	1st to 10th	Night	2 8% 4 9% 6% 7% 6% 6% 8% 9% 36%	4726
	11th to 20th	Night	2 6% 5% 13% 7% 7% 6% 7% 8% 7% 33%	4740
	21st to end	Night	2 6% 4 9% 6% 5% 5% 4 6% 10% 43%	4932
February	1st to 10th	Night	5% 8% 4 9% 6% 7% 6% 5% 7% 9% 34%	2470
	11th to 20th	Night	2 9% 4 9% 6% 6% 5% 7% 8% 10% 33%	2402
	21st to end	Night	4 8% 4 11% 8% 6% 6% 6% 6% 8% 34%	2041
March	1st to 10th	Night	4 9% 4 10% 8% 8% 7% 5% 7% 8% 31%	3589
	11th to 20th	Night	6% 6% 2 5% 4 3 4 4 8% 11% 47%	3490
	21st to end	Night	8% 15% 7% 20% 8% 5% 4 4 5% 6% 18%	3775
April	1st to 10th	Night	6% 9% 3 8% 6% 7% 4 5% 7% 7% 37%	1900
	11th to 20th	Night	9% 11% 3 8% 5% 5% 5% 5% 7% 8% 34%	1733
	21st to end	Night	10% 8% 3 7% 6% 6% 5% 5% 7% 7% 37%	1685
May	1st to 10th	Night	16% 8% 3 7% 5% 4 4 4 6% 6% 35%	2540
	11th to 20th	Night	8% 6% 2 7% 4 4 5% 5% 8% 10% 41%	2517
	21st to end	Night	14% 6% 2 6% 6% 4 5% 5% 6% 9% 37%	2542
June	1st to 10th	Night	13% 6% 2 6% 3 4 5% 4 6% 8% 42%	1330
	11th to 20th	Night	8% 6% 2 5% 5% 4 4 4 8% 7% 47%	1330
	21st to end	Night	5% 3 2 3 4 4 4 4 5% 8% 57%	1330
July	1st to 10th	Night	3 1 1 4 3 3 4 4 7% 11% 60%	2220
	11th to 20th	Night	3 3 1 4 3 2 4 3 5% 7% 66%	2220
	21st to end	Night	2 1 3 2 3 4 4 5% 9% 67%	2560
August	1st to 10th	Night	7% 5% 1 4 4 3 3 3 5% 6% 59%	2619
	11th to 20th	Night	4 3 1 5% 5% 4 5% 4 6% 9% 54%	2811
	21st to end	Night	9% 4 2 4 3 4 4 3 5% 6% 58%	3194
September	1st to 10th	Night	8% 5% 1 4 3 4 3 3 5% 7% 56%	1900
	11th to 20th	Night	9% 5% 2 7% 4 4 4 4 6% 8% 48%	1943
	21st to end	Night	7% 5% 2 3 4 3 4 4 6% 7% 55%	2151
October	1st to 10th	Night	6% 4 2 5% 4 3 4 4 5% 8% 55%	3800
	11th to 20th	Night	5% 5% 2 4 3 3 5% 4 6% 9% 54%	3778
	21st to end	Night	4 6% 2 6% 4 4 4 4 6% 8% 52%	4408
November	1st to 10th	Night	9% 9% 4 9% 5% 5% 6% 4 6% 7% 36%	2695
	11th to 20th	Night	3 7% 2 7% 6% 5% 6% 4 7% 8% 43%	2660
	21st to end	Night	4 10% 3 9% 5% 5% 6% 5% 7% 7% 40%	2660
December	1st to 10th	Night	3 8% 3 10% 7% 6% 6% 6% 8% 10% 32%	2755
	11th to 20th	Night	3 8% 4 9% 6% 5% 6% 5% 7% 10% 37%	3506
	21st to end	Night	3 6% 2 6% 4 4 4 3 6% 8% 54%	5214

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	48% 3 20% 23% 6% 1	2219
	11th to 20th	Day	48% 4 13% 20% 14% 1	2220
	21st to end	Day	38% 4 25% 26% 7%	2747
February	1st to 10th	Day	51% 5% 14% 21% 9% 1	1710
	11th to 20th	Day	48% 4 17% 20% 10% 1	1674
	21st to end	Day	52% 5% 15% 17% 10% 1	1413
March	1st to 10th	Day	47% 4 14% 28% 7%	3278
	11th to 20th	Day	37% 4 24% 26% 8%	3490
	21st to end	Day	66% 3 13% 13% 4	3876
April	1st to 10th	Day	43% 6% 17% 25% 9% 1	2280
	11th to 20th	Day	52% 4 13% 22% 8% 1	2394
	21st to end	Day	45% 4 15% 28% 8% 1	2494
May	1st to 10th	Day	53% 2 16% 22% 6% 1	4430
	11th to 20th	Day	41% 5% 13% 31% 10%	4392
	21st to end	Day	51% 4 14% 24% 6%	5034
June	1st to 10th	Day	43% 6% 17% 23% 11%	2850
	11th to 20th	Day	44% 5% 14% 24% 12% 1	2850
	21st to end	Day	31% 5% 18% 33% 12% 1	2850
July	1st to 10th	Day	29% 5% 19% 25% 18% 4	4666
	11th to 20th	Day	20% 4 14% 37% 20% 5%	4659
	21st to end	Day	16% 3 8% 30% 32% 11%	4838
August	1st to 10th	Day	27% 5% 10% 34% 21% 3	4301
	11th to 20th	Day	23% 4 17% 38% 17% 2	4067
	21st to end	Day	23% 2 12% 41% 19% 3	4472
September	1st to 10th	Day	32% 4 11% 27% 20% 6%	2280
	11th to 20th	Day	37% 3 15% 28% 14% 3	2235
	21st to end	Day	31% 3 13% 33% 18% 2	2028
October	1st to 10th	Day	28% 3 14% 38% 14% 4	3170
	11th to 20th	Day	26% 2 15% 41% 13% 3	3150
	21st to end	Day	33% 4 13% 38% 12%	3116
November	1st to 10th	Day	46% 4 15% 26% 8% 1	1544
	11th to 20th	Day	39% 5% 18% 27% 10% 1	1520
	21st to end	Day	44% 6% 18% 23% 8% 1	1520
December	1st to 10th	Day	49% 5% 18% 20% 8% 1	1421
	11th to 20th	Day	44% 4 23% 23% 5%	1684
	21st to end	Day	36% 3 13% 27% 12% 9%	2442
January	1st to 10th	Night	43% 5% 21% 23% 7% 1	4726
	11th to 20th	Night	46% 5% 13% 20% 13% 2	4740
	21st to end	Night	38% 5% 20% 31% 7%	4932
February	1st to 10th	Night	45% 4 14% 23% 12% 2	2470
	11th to 20th	Night	43% 5% 18% 21% 11% 1	2403
	21st to end	Night	46% 4 12% 25% 11% 2	2041
March	1st to 10th	Night	49% 5% 15% 19% 11%	3589
	11th to 20th	Night	32% 3 27% 29% 10%	3490
	21st to end	Night	68% 3 11% 13% 5%	3775
April	1st to 10th	Night	44% 4 15% 25% 11% 1	1900
	11th to 20th	Night	46% 5% 10% 24% 12% 2	1733
	21st to end	Night	44% 4 15% 25% 12% 1	1685
May	1st to 10th	Night	48% 3 15% 25% 9% 1	2540
	11th to 20th	Night	36% 4 16% 30% 13%	2518
	21st to end	Night	43% 4 17% 29% 7%	2542
June	1st to 10th	Night	39% 5% 17% 26% 12% 1	1330
	11th to 20th	Night	34% 3 18% 26% 17% 2	1330
	21st to end	Night	26% 4 18% 36% 14% 2	1330
July	1st to 10th	Night	17% 3 20% 37% 19% 3	2185
	11th to 20th	Night	19% 4 13% 33% 23% 8%	2178
	21st to end	Night	14% 2 9% 33% 32% 9%	2480
August	1st to 10th	Night	26% 3 9% 36% 22% 3	2611
	11th to 20th	Night	26% 4 14% 38% 16% 3	2838
	21st to end	Night	28% 3 10% 36% 19% 4	3194
September	1st to 10th	Night	29% 3 11% 29% 19% 9%	1900
	11th to 20th	Night	35% 4 13% 28% 16% 4	1943
	21st to end	Night	27% 4 13% 34% 19% 3	2151
October	1st to 10th	Night	27% 3 12% 38% 19% 1	3800
	11th to 20th	Night	27% 3 15% 38% 17% 1	3778
	21st to end	Night	32% 4 12% 39% 13%	4408
November	1st to 10th	Night	46% 5% 14% 24% 11% 1	2695
	11th to 20th	Night	36% 6% 22% 25% 11%	2660
	21st to end	Night	41% 5% 23% 22% 8% 1	2660
December	1st to 10th	Night	44% 4 19% 21% 10% 1	2755
	11th to 20th	Night	42% 4 24% 24% 5% 1	3507
	21st to end	Night	29% 3 14% 28% 19% 7%	5214

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	8% 9% 34% 28% 16% 4	180
	11th to 20th	Day	8% 7% 36% 27% 17% 4	180
	21st to end	Day	6% 13% 39% 30% 11% 2	254
February	1st to 10th	Day	13% 12% 42% 23% 9%	270
	11th to 20th	Day	10% 9% 37% 33% 8% 2	270
	21st to end	Day	6% 8% 39% 28% 16% 4	225
March	1st to 10th	Day	9% 12% 34% 32% 12% 1	370
	11th to 20th	Day	6% 12% 37% 27% 13% 5%	460
	21st to end	Day	12% 8% 34% 27% 14% 4	506
April	1st to 10th	Day	8% 13% 39% 29% 10% 1	460
	11th to 20th	Day	14% 13% 35% 27% 8% 2	452
	21st to end	Day	17% 11% 28% 30% 12% 2	484
May	1st to 10th	Day	16% 9% 21% 29% 21% 4	653
	11th to 20th	Day	14% 6% 28% 28% 18% 6%	675
	21st to end	Day	19% 8% 22% 26% 20% 5%	734
June	1st to 10th	Day	15% 12% 22% 32% 16% 2	680
	11th to 20th	Day	16% 13% 34% 24% 12% 2	680
	21st to end	Day	11% 6% 28% 29% 21% 4	680
July	1st to 10th	Day	6% 5% 23% 38% 23% 5%	671
	11th to 20th	Day	5% 6% 26% 38% 19% 5%	668
	21st to end	Day	5% 6% 25% 34% 24% 6%	739
August	1st to 10th	Day	9% 8% 21% 36% 21% 5%	620
	11th to 20th	Day	7% 6% 23% 36% 22% 6%	477
	21st to end	Day	6% 8% 27% 35% 18% 6%	539
September	1st to 10th	Day	11% 11% 30% 28% 16% 4	490
	11th to 20th	Day	13% 9% 30% 27% 18% 3	490
	21st to end	Day	16% 10% 33% 24% 12% 5%	428
October	1st to 10th	Day	17% 8% 26% 24% 19% 4	300
	11th to 20th	Day	7% 11% 36% 29% 14% 3	300
	21st to end	Day	14% 13% 39% 25% 8% 2	330
November	1st to 10th	Day	15% 13% 34% 28% 9%	291
	11th to 20th	Day	9% 20% 40% 23% 7% 1	270
	21st to end	Day	8% 14% 41% 24% 9% 4	270
December	1st to 10th	Day	8% 13% 41% 30% 6% 1	223
	11th to 20th	Day	11% 16% 40% 28% 6%	180
	21st to end	Day	9% 6% 32% 37% 14% 3	198
January	1st to 10th	Night	6% 13% 38% 31% 10% 2	950
	11th to 20th	Night	7% 15% 38% 30% 9% 1	950
	21st to end	Night	7% 15% 36% 31% 10% 2	992
February	1st to 10th	Night	9% 10% 29% 34% 15% 3	760
	11th to 20th	Night	7% 10% 37% 35% 9% 2	744
	21st to end	Night	6% 8% 29% 37% 16% 3	628
March	1st to 10th	Night	7% 9% 37% 32% 13% 2	760
	11th to 20th	Night	8% 8% 34% 33% 13% 3	760
	21st to end	Night	12% 7% 30% 34% 14% 3	808
April	1st to 10th	Night	10% 8% 34% 36% 11% 2	570
	11th to 20th	Night	15% 7% 30% 37% 9% 1	563
	21st to end	Night	15% 8% 28% 37% 12% 1	570
May	1st to 10th	Night	13% 7% 21% 40% 18% 2	570
	11th to 20th	Night	11% 7% 25% 37% 19% 2	561
	21st to end	Night	18% 7% 24% 30% 17% 3	488
June	1st to 10th	Night	16% 9% 22% 33% 18% 2	380
	11th to 20th	Night	11% 9% 32% 36% 13%	380
	21st to end	Night	7% 6% 26% 42% 18% 2	380
July	1st to 10th	Night	4 5% 23% 43% 23% 2	370
	11th to 20th	Night	4 7% 26% 45% 17% 2	368
	21st to end	Night	4 4 21% 46% 24% 1	501
August	1st to 10th	Night	7% 6% 20% 46% 20% 2	567
	11th to 20th	Night	5% 3 21% 42% 26% 2	564
	21st to end	Night	11% 5% 21% 40% 22% 1	627
September	1st to 10th	Night	11% 5% 24% 39% 19% 2	570
	11th to 20th	Night	9% 7% 26% 38% 18% 2	613
	21st to end	Night	8% 7% 29% 39% 16% 2	760
October	1st to 10th	Night	14% 7% 25% 35% 18% 2	760
	11th to 20th	Night	9% 10% 32% 32% 15% 2	760
	21st to end	Night	11% 10% 36% 34% 8% 1	924
November	1st to 10th	Night	14% 12% 35% 31% 8% 1	963
	11th to 20th	Night	8% 14% 40% 29% 9% 1	950
	21st to end	Night	7% 13% 36% 35% 9%	950
December	1st to 10th	Night	7% 10% 37% 30% 14% 1	950
	11th to 20th	Night	7% 11% 39% 31% 10% 1	950
	21st to end	Night	10% 11% 33% 34% 10% 2	1045

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