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 2024-01-12.

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	8% 7% 3 6% 6% 4 5% 5% 6% 8% 42%	74714
	Night	6% 6% 3 6% 6% 4 5% 5% 6% 9% 43%	75012
Transparency	Day	40% 4 16% 26% 12% 2	74399
	Night	38% 4 16% 26% 13% 2	74849
Seeing	Day	11% 10% 30% 30% 16% 4	14502
	Night	9% 9% 31% 35% 14% 2	24010

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 9% 4 8% 7% 5% 6% 6% 6% 9% 36%	1329
	11th to 20th	Day	2 9% 4 9% 8% 4 9% 7% 8% 8% 32%	1270
	21st to end	Day	1 8% 5% 9% 10% 6% 7% 8% 8% 9% 30%	1565
February	1st to 10th	Day	5% 9% 6% 10% 8% 5% 6% 6% 7% 8% 30%	1620
	11th to 20th	Day	3 10% 4 9% 9% 5% 7% 6% 8% 10% 28%	1575
	21st to end	Day	5% 9% 4 11% 12% 4 5% 7% 6% 8% 28%	1332
March	1st to 10th	Day	4 7% 4 10% 9% 5% 7% 5% 6% 10% 32%	1875
	11th to 20th	Day	5% 6% 5% 10% 8% 4 6% 5% 6% 9% 36%	1980
	21st to end	Day	7% 9% 5% 11% 8% 4 6% 6% 6% 8% 30%	2204
April	1st to 10th	Day	6% 9% 4 6% 7% 4 6% 6% 8% 10% 34%	2160
	11th to 20th	Day	11% 12% 5% 9% 6% 4 6% 4 5% 8% 31%	2263
	21st to end	Day	12% 9% 3 6% 6% 3 5% 5% 7% 8% 35%	2362
May	1st to 10th	Day	13% 9% 3 7% 6% 4 5% 5% 5% 6% 35%	2520
	11th to 20th	Day	9% 6% 3 6% 6% 4 5% 6% 7% 8% 39%	2478
	21st to end	Day	20% 9% 3 7% 6% 3 5% 5% 4 6% 32%	2842
June	1st to 10th	Day	14% 7% 3 5% 6% 3 5% 5% 6% 8% 37%	2700
	11th to 20th	Day	13% 6% 3 7% 5% 3 5% 6% 6% 8% 37%	2700
	21st to end	Day	8% 4 2 4 5% 3 5% 6% 7% 9% 48%	2700
July	1st to 10th	Day	4 3 2 5% 5% 3 5% 6% 7% 9% 54%	2700
	11th to 20th	Day	4 3 1 4 4 3 4 5% 7% 9% 55%	2700
	21st to end	Day	4 3 1 3 3 2 4 4 6% 7% 64%	2858
August	1st to 10th	Day	7% 3 2 4 4 3 5% 5% 6% 7% 54%	2454
	11th to 20th	Day	6% 4 2 4 4 3 4 5% 5% 10% 53%	2299
	21st to end	Day	6% 3 2 4 4 3 4 4 5% 8% 57%	2552
September	1st to 10th	Day	10% 5% 2 5% 5% 3 4 4 5% 6% 51%	2160
	11th to 20th	Day	11% 6% 2 5% 5% 3 4 4 5% 7% 48%	2118
	21st to end	Day	11% 5% 2 4 4 2 3 5% 5% 7% 52%	1922
October	1st to 10th	Day	9% 10% 4 7% 4 2 2 4 5% 8% 44%	1800
	11th to 20th	Day	7% 6% 3 6% 6% 4 5% 6% 6% 8% 43%	1800
	21st to end	Day	6% 10% 4 7% 6% 4 4 4 7% 8% 41%	1800
November	1st to 10th	Day	9% 11% 4 6% 6% 4 5% 5% 6% 8% 36%	1488
	11th to 20th	Day	4 6% 4 7% 6% 4 7% 6% 6% 9% 42%	1520
	21st to end	Day	5% 8% 4 8% 6% 5% 6% 7% 6% 9% 36%	1520
December	1st to 10th	Day	5% 8% 3 9% 9% 5% 8% 7% 6% 7% 33%	1421
	11th to 20th	Day	4 7% 3 8% 10% 4 5% 5% 6% 8% 38%	1685
	21st to end	Day	4 8% 3 6% 5% 3 5% 5% 5% 8% 48%	2442
January	1st to 10th	Night	3 7% 5% 8% 7% 5% 6% 7% 8% 8% 36%	2836
	11th to 20th	Night	4 7% 3 8% 7% 4 6% 7% 9% 9% 36%	2730
	21st to end	Night	3 6% 3 8% 8% 6% 6% 7% 7% 12% 33%	2808
February	1st to 10th	Night	4 6% 5% 8% 6% 5% 7% 6% 7% 10% 35%	2340
	11th to 20th	Night	3 9% 4 8% 7% 5% 6% 8% 8% 11% 33%	2272
	21st to end	Night	4 7% 5% 9% 10% 4 6% 6% 6% 9% 33%	1924
March	1st to 10th	Night	4 8% 4 8% 8% 5% 7% 7% 7% 9% 33%	2082
	11th to 20th	Night	5% 8% 3 7% 6% 3 6% 6% 8% 10% 38%	1980
	21st to end	Night	7% 8% 6% 10% 7% 4 6% 6% 7% 9% 30%	2152
April	1st to 10th	Night	6% 8% 4 7% 7% 6% 5% 5% 7% 8% 38%	1800
	11th to 20th	Night	10% 10% 4 7% 6% 4 5% 7% 7% 8% 33%	1641
	21st to end	Night	10% 8% 3 6% 7% 4 6% 5% 7% 8% 37%	1597
May	1st to 10th	Night	10% 7% 2 6% 6% 4 5% 5% 6% 8% 41%	1440
	11th to 20th	Night	9% 6% 3 6% 5% 3 4 5% 6% 9% 45%	1417
	21st to end	Night	17% 6% 3 5% 7% 3 5% 5% 6% 8% 35%	1430
June	1st to 10th	Night	14% 5% 2 5% 4 3 5% 5% 5% 9% 43%	1260
	11th to 20th	Night	8% 5% 3 5% 5% 3 5% 5% 8% 7% 47%	1260
	21st to end	Night	6% 3 2 3 4 3 5% 4 5% 8% 56%	1260
July	1st to 10th	Night	4 2 1 4 4 3 4 4 5% 8% 60%	1260
	11th to 20th	Night	3 2 2 4 4 2 3 4 5% 8% 61%	1260
	21st to end	Night	4 2 1 3 2 2 3 4 5% 10% 64%	1459
August	1st to 10th	Night	6% 4 1 3 5% 2 2 4 5% 7% 60%	1481
	11th to 20th	Night	6% 3 2 3 5% 2 4 4 5% 9% 56%	1602
	21st to end	Night	9% 3 2 3 3 3 4 4 5% 7% 57%	1804
September	1st to 10th	Night	9% 5% 2 4 5% 3 4 4 5% 7% 54%	1800
	11th to 20th	Night	9% 5% 2 5% 5% 3 4 5% 5% 7% 49%	1840
	21st to end	Night	7% 4 2 3 4 3 4 5% 6% 8% 54%	2037
October	1st to 10th	Night	9% 7% 3 5% 5% 3 4 4 5% 8% 47%	2160
	11th to 20th	Night	6% 7% 3 5% 5% 3 5% 6% 5% 9% 46%	2160
	21st to end	Night	7% 8% 3 7% 6% 4 4 5% 6% 8% 42%	2535
November	1st to 10th	Night	10% 8% 4 8% 6% 4 6% 5% 6% 7% 37%	2590
	11th to 20th	Night	4 6% 2 6% 7% 4 7% 5% 6% 9% 43%	2660
	21st to end	Night	4 8% 4 8% 6% 4 5% 6% 6% 7% 41%	2660
December	1st to 10th	Night	3 7% 4 9% 7% 5% 7% 7% 8% 10% 33%	2755
	11th to 20th	Night	3 8% 4 8% 8% 3 6% 6% 7% 10% 37%	3506
	21st to end	Night	3 6% 3 5% 5% 3 5% 4 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	45% 5% 18% 24% 8% 1	1329
	11th to 20th	Day	47% 6% 16% 22% 7% 1	1270
	21st to end	Day	48% 7% 17% 20% 7%	1565
February	1st to 10th	Day	50% 5% 14% 21% 9% 1	1620
	11th to 20th	Day	49% 4 18% 20% 8%	1584
	21st to end	Day	53% 5% 14% 16% 11% 1	1332
March	1st to 10th	Day	47% 6% 18% 22% 7%	1875
	11th to 20th	Day	44% 5% 19% 24% 8% 1	1980
	21st to end	Day	50% 5% 18% 19% 7%	2204
April	1st to 10th	Day	43% 6% 17% 25% 9% 1	2160
	11th to 20th	Day	53% 4 13% 21% 8% 1	2266
	21st to end	Day	45% 4 16% 28% 7% 1	2362
May	1st to 10th	Day	48% 4 15% 23% 10% 1	2520
	11th to 20th	Day	41% 5% 15% 29% 9%	2482
	21st to end	Day	52% 4 13% 22% 8% 1	2842
June	1st to 10th	Day	43% 6% 17% 24% 10%	2700
	11th to 20th	Day	44% 5% 15% 24% 12% 1	2700
	21st to end	Day	30% 6% 18% 32% 13% 1	2700
July	1st to 10th	Day	26% 5% 18% 31% 16% 4	2626
	11th to 20th	Day	25% 4 15% 29% 22% 4	2619
	21st to end	Day	19% 3 11% 36% 25% 6%	2699
August	1st to 10th	Day	28% 4 13% 31% 19% 4	2438
	11th to 20th	Day	27% 5% 16% 32% 18% 3	2299
	21st to end	Day	26% 3 12% 31% 23% 4	2552
September	1st to 10th	Day	33% 4 11% 28% 19% 6%	2160
	11th to 20th	Day	36% 3 14% 29% 15% 3	2118
	21st to end	Day	32% 3 12% 33% 17% 2	1922
October	1st to 10th	Day	39% 3 18% 28% 11% 1	1800
	11th to 20th	Day	39% 3 18% 28% 11% 1	1800
	21st to end	Day	41% 4 19% 25% 10%	1800
November	1st to 10th	Day	46% 4 16% 25% 8% 1	1488
	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% 7% 19% 22% 8% 2	2836
	11th to 20th	Night	42% 7% 18% 25% 7% 2	2730
	21st to end	Night	44% 6% 17% 21% 11% 1	2808
February	1st to 10th	Night	43% 5% 15% 24% 11% 2	2340
	11th to 20th	Night	44% 5% 19% 21% 10% 1	2273
	21st to end	Night	47% 4 12% 24% 11% 2	1924
March	1st to 10th	Night	45% 6% 18% 19% 11%	2082
	11th to 20th	Night	41% 4 18% 26% 10% 1	1980
	21st to end	Night	48% 6% 16% 21% 9% 1	2152
April	1st to 10th	Night	43% 5% 14% 25% 12% 1	1800
	11th to 20th	Night	47% 6% 10% 24% 12% 2	1641
	21st to end	Night	44% 3 15% 25% 11% 1	1597
May	1st to 10th	Night	39% 4 13% 28% 14% 1	1440
	11th to 20th	Night	35% 4 15% 29% 17% 1	1418
	21st to end	Night	47% 5% 14% 25% 9%	1430
June	1st to 10th	Night	39% 5% 18% 27% 11%	1260
	11th to 20th	Night	34% 3 19% 26% 17% 1	1260
	21st to end	Night	26% 4 18% 36% 15% 2	1260
July	1st to 10th	Night	21% 3 16% 35% 21% 5%	1225
	11th to 20th	Night	22% 4 15% 28% 23% 8%	1218
	21st to end	Night	16% 2 12% 35% 29% 5%	1378
August	1st to 10th	Night	24% 3 12% 35% 21% 5%	1473
	11th to 20th	Night	25% 4 12% 35% 19% 5%	1602
	21st to end	Night	26% 3 9% 28% 26% 7%	1804
September	1st to 10th	Night	30% 3 12% 28% 19% 9%	1800
	11th to 20th	Night	34% 4 12% 28% 17% 5%	1840
	21st to end	Night	27% 4 13% 34% 18% 3	2037
October	1st to 10th	Night	36% 3 15% 28% 16% 1	2160
	11th to 20th	Night	34% 4 15% 31% 15% 1	2160
	21st to end	Night	41% 5% 16% 27% 11% 1	2535
November	1st to 10th	Night	46% 5% 14% 23% 11% 1	2590
	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	9% 7% 36% 25% 18% 5%	163
	21st to end	Day	6% 15% 38% 30% 10% 2	226
February	1st to 10th	Day	13% 12% 45% 21% 9%	240
	11th to 20th	Day	10% 10% 36% 33% 8% 2	240
	21st to end	Day	7% 7% 38% 28% 16% 4	198
March	1st to 10th	Day	10% 11% 35% 31% 12% 1	334
	11th to 20th	Day	5% 12% 36% 27% 13% 6%	420
	21st to end	Day	11% 9% 35% 27% 14% 4	462
April	1st to 10th	Day	7% 13% 39% 29% 11% 1	420
	11th to 20th	Day	15% 12% 35% 27% 8% 1	412
	21st to end	Day	17% 12% 28% 29% 12% 1	442
May	1st to 10th	Day	15% 9% 21% 30% 21% 4	603
	11th to 20th	Day	14% 6% 27% 28% 18% 6%	625
	21st to end	Day	19% 9% 22% 25% 20% 6%	679
June	1st to 10th	Day	14% 11% 22% 32% 17% 3	630
	11th to 20th	Day	16% 12% 34% 24% 12% 2	630
	21st to end	Day	11% 6% 26% 30% 22% 5%	630
July	1st to 10th	Day	6% 6% 24% 36% 23% 5%	621
	11th to 20th	Day	5% 7% 27% 37% 19% 5%	618
	21st to end	Day	5% 6% 26% 34% 23% 6%	684
August	1st to 10th	Day	9% 7% 21% 38% 20% 5%	573
	11th to 20th	Day	8% 6% 22% 35% 23% 7%	437
	21st to end	Day	5% 7% 26% 35% 20% 6%	495
September	1st to 10th	Day	12% 11% 31% 28% 14% 3	450
	11th to 20th	Day	13% 9% 28% 28% 19% 3	450
	21st to end	Day	16% 10% 32% 24% 13% 6%	392
October	1st to 10th	Day	19% 6% 26% 24% 20% 5%	270
	11th to 20th	Day	8% 11% 36% 28% 14% 3	270
	21st to end	Day	15% 13% 36% 25% 8% 2	297
November	1st to 10th	Day	15% 14% 34% 27% 10%	270
	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% 37% 29% 9% 2	910
	21st to end	Night	7% 15% 37% 29% 10% 2	940
February	1st to 10th	Night	8% 10% 30% 34% 15% 3	720
	11th to 20th	Night	7% 10% 37% 35% 9% 2	704
	21st to end	Night	6% 8% 29% 37% 17% 4	592
March	1st to 10th	Night	7% 9% 38% 32% 13% 2	720
	11th to 20th	Night	8% 9% 33% 33% 13% 3	720
	21st to end	Night	11% 7% 30% 34% 14% 3	766
April	1st to 10th	Night	10% 8% 32% 36% 12% 2	540
	11th to 20th	Night	15% 8% 30% 37% 9% 1	533
	21st to end	Night	14% 8% 28% 37% 12% 1	540
May	1st to 10th	Night	12% 7% 21% 39% 19% 2	540
	11th to 20th	Night	11% 7% 24% 37% 19% 2	531
	21st to end	Night	19% 6% 23% 31% 17% 3	462
June	1st to 10th	Night	15% 9% 22% 33% 19% 2	360
	11th to 20th	Night	11% 9% 31% 36% 13%	360
	21st to end	Night	7% 5% 26% 41% 19% 2	360
July	1st to 10th	Night	4 5% 24% 43% 21% 2	350
	11th to 20th	Night	4 7% 27% 46% 14% 1	348
	21st to end	Night	5% 4 21% 46% 24% 1	474
August	1st to 10th	Night	7% 6% 20% 47% 19% 2	537
	11th to 20th	Night	6% 3 22% 41% 26% 2	534
	21st to end	Night	10% 5% 20% 40% 23% 1	594
September	1st to 10th	Night	11% 5% 24% 39% 18% 2	540
	11th to 20th	Night	9% 6% 25% 38% 19% 2	580
	21st to end	Night	8% 7% 29% 39% 16% 2	720
October	1st to 10th	Night	14% 7% 25% 35% 18% 2	720
	11th to 20th	Night	9% 9% 34% 30% 15% 2	720
	21st to end	Night	11% 11% 36% 33% 9% 1	875
November	1st to 10th	Night	14% 12% 35% 30% 8% 1	925
	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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