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 2025-11-18.

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	6% 7% 3 8% 5% 5% 5% 4 6% 8% 44%	136174
	Night	5% 7% 3 8% 5% 5% 5% 4 6% 8% 43%	131503
Transparency	Day	38% 4 15% 28% 13% 2	130776
	Night	38% 4 16% 27% 13% 2	127908
Seeing	Day	11% 10% 30% 30% 16% 4	17078
	Night	9% 9% 31% 35% 14% 2	26372

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 12% 5% 14% 8% 6% 5% 4 6% 9% 28%	3179
	11th to 20th	Day	1 5% 4 11% 7% 6% 7% 7% 8% 8% 36%	3180
	21st to end	Day	3 7% 4 9% 5% 5% 4 4 5% 7% 49%	3957
February	1st to 10th	Day	5% 11% 5% 11% 7% 6% 6% 6% 7% 7% 30%	1800
	11th to 20th	Day	3 10% 3 11% 7% 7% 6% 5% 9% 10% 30%	1755
	21st to end	Day	4 10% 4 12% 10% 6% 4 6% 7% 9% 28%	1485
March	1st to 10th	Day	3 7% 3 10% 7% 7% 6% 4 5% 8% 39%	4705
	11th to 20th	Day	4 5% 3 8% 6% 6% 5% 5% 8% 11% 39%	5000
	21st to end	Day	6% 16% 7% 18% 7% 5% 5% 3 6% 7% 21%	5563
April	1st to 10th	Day	6% 10% 3 8% 7% 5% 6% 5% 8% 9% 35%	2400
	11th to 20th	Day	10% 12% 5% 10% 5% 5% 5% 4 5% 8% 31%	2519
	21st to end	Day	11% 9% 3 8% 5% 4 5% 4 7% 8% 36%	2626
May	1st to 10th	Day	12% 9% 3 7% 4 4 4 3 5% 6% 43%	6340
	11th to 20th	Day	12% 9% 3 9% 6% 6% 6% 5% 8% 7% 28%	6298
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	7225
June	1st to 10th	Day	13% 7% 3 6% 5% 5% 5% 4 6% 8% 38%	3000
	11th to 20th	Day	13% 7% 2 7% 5% 5% 5% 5% 6% 8% 37%	3000
	21st to end	Day	8% 4 2 5% 4 4 5% 4 8% 9% 48%	3000
July	1st to 10th	Day	4 3 2 6% 4 4 5% 4 6% 9% 53%	6779
	11th to 20th	Day	4 3 2 5% 4 4 5% 5% 7% 9% 52%	6780
	21st to end	Day	4 4 2 5% 3 3 4 3 5% 7% 61%	7175
August	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 7% 9% 47%	6097
	11th to 20th	Day	3 3 1 4 3 4 5% 4 7% 10% 57%	5798
	21st to end	Day	7% 3 1 4 2 3 3 3 4 7% 64%	6407
September	1st to 10th	Day	9% 5% 2 6% 4 4 4 3 6% 6% 52%	2400
	11th to 20th	Day	11% 6% 2 6% 5% 4 4 3 5% 7% 48%	2352
	21st to end	Day	10% 5% 2 4 3 3 3 4 6% 6% 54%	2135
October	1st to 10th	Day	5% 6% 2 6% 3 3 3 3 6% 8% 55%	3270
	11th to 20th	Day	4 5% 2 5% 4 4 4 4 6% 10% 54%	3250
	21st to end	Day	4 7% 3 6% 4 4 4 4 7% 7% 50%	3215
November	1st to 10th	Day	8% 12% 4 7% 5% 5% 5% 4 6% 8% 36%	1648
	11th to 20th	Day	4 6% 3 8% 6% 6% 6% 5% 6% 8% 40%	1656
	21st to end	Day	6% 10% 3 10% 5% 7% 6% 6% 6% 8% 34%	1600
December	1st to 10th	Day	4 6% 3 9% 9% 10% 8% 5% 7% 8% 32%	2437
	11th to 20th	Day	6% 9% 4 15% 8% 5% 4 4 5% 7% 32%	2645
	21st to end	Day	3 7% 3 8% 6% 6% 6% 4 6% 8% 43%	3498
January	1st to 10th	Night	2 9% 4 10% 8% 7% 6% 5% 7% 8% 32%	6766
	11th to 20th	Night	2 6% 4 12% 7% 7% 6% 6% 7% 7% 35%	6780
	21st to end	Night	2 6% 3 11% 5% 4 5% 4 7% 10% 44%	7043
February	1st to 10th	Night	5% 8% 4 9% 6% 7% 6% 5% 7% 10% 34%	2600
	11th to 20th	Night	2 9% 4 9% 6% 6% 5% 6% 8% 10% 34%	2532
	21st to end	Night	4 8% 4 11% 8% 6% 5% 6% 6% 9% 34%	2145
March	1st to 10th	Night	3 8% 3 8% 6% 7% 7% 5% 7% 10% 36%	5164
	11th to 20th	Night	5% 7% 2 7% 5% 5% 5% 5% 7% 10% 43%	5000
	21st to end	Night	8% 14% 6% 18% 8% 5% 5% 4 5% 6% 20%	5408
April	1st to 10th	Night	6% 9% 3 8% 6% 7% 4 5% 7% 8% 38%	2000
	11th to 20th	Night	9% 11% 3 9% 5% 5% 6% 5% 7% 8% 34%	1825
	21st to end	Night	10% 8% 3 7% 6% 6% 5% 5% 7% 7% 37%	1773
May	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 5% 7% 48%	3640
	11th to 20th	Night	14% 7% 3 9% 5% 6% 6% 5% 7% 9% 31%	3617
	21st to end	Night	11% 5% 2 5% 4 4 4 4 6% 8% 45%	3654
June	1st to 10th	Night	13% 6% 2 6% 4 4 5% 4 6% 8% 43%	1400
	11th to 20th	Night	8% 6% 2 5% 5% 4 4 4 8% 7% 47%	1400
	21st to end	Night	6% 3 2 4 4 4 4 4 5% 9% 57%	1400
July	1st to 10th	Night	3 2 1 4 3 3 3 3 6% 9% 61%	3180
	11th to 20th	Night	6% 4 2 5% 3 4 4 4 5% 8% 57%	3180
	21st to end	Night	5% 3 1 4 3 3 4 4 6% 9% 60%	3685
August	1st to 10th	Night	8% 6% 2 6% 4 4 3 4 6% 7% 50%	3711
	11th to 20th	Night	3 2 1 5% 4 4 5% 4 7% 9% 56%	4051
	21st to end	Night	10% 4 1 3 2 3 3 2 4 5% 62%	4570
September	1st to 10th	Night	8% 4 1 4 3 4 3 3 5% 7% 57%	2000
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 6% 8% 49%	2045
	21st to end	Night	6% 4 2 3 4 3 4 4 6% 7% 57%	2264
October	1st to 10th	Night	6% 4 2 5% 4 3 4 4 5% 8% 56%	3920
	11th to 20th	Night	5% 5% 2 4 3 3 5% 4 6% 9% 53%	3898
	21st to end	Night	4 6% 2 6% 4 4 4 4 5% 8% 52%	4549
November	1st to 10th	Night	9% 9% 4 9% 5% 5% 6% 4 6% 8% 36%	2870
	11th to 20th	Night	3 7% 2 8% 6% 5% 6% 4 7% 8% 42%	2899
	21st to end	Night	4 10% 3 10% 5% 5% 6% 5% 6% 7% 39%	2800
December	1st to 10th	Night	3 7% 4 11% 7% 6% 6% 5% 7% 10% 34%	4730
	11th to 20th	Night	3 10% 5% 13% 9% 6% 5% 4 6% 8% 30%	5546
	21st to end	Night	3 7% 3 7% 4 4 5% 3 7% 8% 49%	7458

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	55% 3 17% 19% 5%	3179
	11th to 20th	Day	43% 4 13% 22% 15% 3	3180
	21st to end	Day	36% 3 25% 29% 6%	3957
February	1st to 10th	Day	51% 5% 13% 20% 11% 1	1800
	11th to 20th	Day	47% 4 17% 20% 11% 1	1764
	21st to end	Day	50% 5% 16% 18% 10% 1	1485
March	1st to 10th	Day	42% 4 17% 27% 10%	4705
	11th to 20th	Day	38% 4 19% 29% 10%	5000
	21st to end	Day	62% 4 15% 14% 5%	5563
April	1st to 10th	Day	42% 6% 16% 26% 9% 1	2400
	11th to 20th	Day	52% 4 13% 22% 8% 1	2522
	21st to end	Day	45% 3 15% 28% 8% 1	2626
May	1st to 10th	Day	43% 2 12% 28% 12% 2	6340
	11th to 20th	Day	51% 4 11% 26% 7%	6302
	21st to end	Day	44% 4 13% 28% 10% 1	7226
June	1st to 10th	Day	42% 6% 17% 25% 11%	3000
	11th to 20th	Day	43% 5% 14% 25% 12% 1	3000
	21st to end	Day	31% 5% 17% 34% 12% 1	3000
July	1st to 10th	Day	27% 5% 18% 24% 21% 6%	6706
	11th to 20th	Day	26% 6% 17% 31% 16% 4	6699
	21st to end	Day	23% 4 11% 31% 23% 8%	6968
August	1st to 10th	Day	27% 5% 11% 33% 21% 3	4438
	11th to 20th	Day	23% 4 17% 38% 17% 2	4197
	21st to end	Day	23% 2 12% 41% 19% 3	4614
September	1st to 10th	Day	31% 4 11% 29% 19% 6%	2400
	11th to 20th	Day	36% 4 15% 29% 14% 3	2352
	21st to end	Day	30% 3 12% 34% 19% 2	2135
October	1st to 10th	Day	28% 3 13% 38% 14% 4	3270
	11th to 20th	Day	28% 2 15% 40% 13% 3	3250
	21st to end	Day	32% 4 13% 37% 13%	3215
November	1st to 10th	Day	45% 4 16% 25% 8% 1	1648
	11th to 20th	Day	40% 5% 18% 27% 9% 1	1656
	21st to end	Day	46% 5% 19% 22% 8% 1	1600
December	1st to 10th	Day	50% 5% 22% 18% 5%	2437
	11th to 20th	Day	52% 3 22% 19% 3	2644
	21st to end	Day	38% 4 12% 27% 13% 6%	3498
January	1st to 10th	Night	49% 4 17% 20% 8% 1	6766
	11th to 20th	Night	45% 5% 15% 16% 16% 3	6780
	21st to end	Night	36% 4 21% 32% 7%	7043
February	1st to 10th	Night	45% 4 14% 23% 13% 2	2600
	11th to 20th	Night	43% 5% 18% 21% 13% 2	2533
	21st to end	Night	45% 4 13% 25% 11% 1	2145
March	1st to 10th	Night	42% 4 18% 21% 14%	5164
	11th to 20th	Night	36% 3 20% 29% 11%	5000
	21st to end	Night	64% 4 13% 12% 6%	5408
April	1st to 10th	Night	43% 4 14% 25% 12% 1	2000
	11th to 20th	Night	46% 5% 10% 25% 12% 2	1825
	21st to end	Night	44% 4 15% 25% 12% 1	1773
May	1st to 10th	Night	37% 2 12% 30% 17% 2	3640
	11th to 20th	Night	48% 4 16% 24% 9%	3618
	21st to end	Night	37% 4 16% 32% 11%	3654
June	1st to 10th	Night	38% 5% 16% 26% 13% 1	1400
	11th to 20th	Night	34% 3 17% 27% 17% 2	1400
	21st to end	Night	26% 4 17% 38% 14% 2	1400
July	1st to 10th	Night	20% 3 18% 32% 23% 5%	3145
	11th to 20th	Night	26% 4 17% 30% 18% 5%	3138
	21st to end	Night	21% 4 13% 33% 22% 6%	3580
August	1st to 10th	Night	26% 3 10% 36% 22% 3	2694
	11th to 20th	Night	26% 4 15% 37% 15% 3	2928
	21st to end	Night	28% 3 10% 36% 20% 4	3294
September	1st to 10th	Night	27% 3 11% 31% 19% 9%	2000
	11th to 20th	Night	33% 4 14% 29% 16% 4	2045
	21st to end	Night	26% 4 13% 35% 20% 3	2264
October	1st to 10th	Night	27% 3 12% 38% 19% 1	3920
	11th to 20th	Night	27% 3 14% 37% 17% 1	3898
	21st to end	Night	31% 3 12% 39% 14% 1	4549
November	1st to 10th	Night	45% 5% 14% 24% 11% 1	2870
	11th to 20th	Night	37% 5% 22% 24% 11%	2899
	21st to end	Night	43% 5% 22% 21% 8% 1	2800
December	1st to 10th	Night	45% 5% 23% 20% 7% 1	4730
	11th to 20th	Night	52% 3 22% 18% 3	5547
	21st to end	Night	34% 3 13% 28% 18% 5%	7458

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	11% 9% 34% 27% 17% 4	200
	11th to 20th	Day	9% 7% 41% 25% 16% 4	200
	21st to end	Day	6% 14% 42% 27% 10% 2	283
February	1st to 10th	Day	13% 12% 43% 23% 9%	300
	11th to 20th	Day	9% 9% 37% 33% 10% 2	300
	21st to end	Day	6% 9% 40% 27% 14% 3	249
March	1st to 10th	Day	9% 12% 34% 32% 13% 1	405
	11th to 20th	Day	6% 11% 38% 27% 13% 5%	500
	21st to end	Day	13% 9% 34% 27% 14% 4	550
April	1st to 10th	Day	8% 13% 38% 29% 11% 1	500
	11th to 20th	Day	15% 15% 35% 26% 8% 2	492
	21st to end	Day	17% 10% 29% 29% 12% 2	526
May	1st to 10th	Day	16% 9% 22% 30% 20% 4	703
	11th to 20th	Day	15% 7% 27% 27% 17% 6%	725
	21st to end	Day	18% 8% 22% 27% 19% 5%	789
June	1st to 10th	Day	16% 12% 22% 31% 16% 2	730
	11th to 20th	Day	16% 13% 34% 24% 12% 2	730
	21st to end	Day	12% 7% 28% 29% 21% 4	730
July	1st to 10th	Day	6% 6% 24% 36% 23% 5%	721
	11th to 20th	Day	5% 7% 27% 37% 19% 5%	718
	21st to end	Day	6% 6% 24% 34% 24% 6%	794
August	1st to 10th	Day	9% 7% 21% 37% 21% 5%	667
	11th to 20th	Day	7% 6% 24% 36% 21% 6%	517
	21st to end	Day	7% 7% 27% 35% 18% 6%	583
September	1st to 10th	Day	11% 10% 29% 29% 16% 5%	530
	11th to 20th	Day	12% 8% 28% 28% 20% 3	530
	21st to end	Day	15% 10% 34% 25% 12% 5%	465
October	1st to 10th	Day	16% 9% 27% 24% 19% 4	330
	11th to 20th	Day	9% 12% 35% 28% 14% 2	330
	21st to end	Day	14% 13% 39% 25% 8% 2	363
November	1st to 10th	Day	15% 15% 35% 27% 8%	330
	11th to 20th	Day	10% 21% 40% 20% 8% 1	321
	21st to end	Day	9% 12% 39% 26% 10% 4	300
December	1st to 10th	Day	9% 13% 43% 28% 6% 1	247
	11th to 20th	Day	11% 14% 43% 27% 6%	200
	21st to end	Day	8% 6% 35% 36% 13% 2	220
January	1st to 10th	Night	7% 13% 37% 31% 11% 2	1000
	11th to 20th	Night	7% 15% 39% 29% 9% 1	1000
	21st to end	Night	7% 15% 37% 29% 10% 2	1044
February	1st to 10th	Night	9% 10% 30% 34% 15% 2	800
	11th to 20th	Night	7% 9% 37% 36% 9% 2	784
	21st to end	Night	6% 8% 30% 37% 16% 3	660
March	1st to 10th	Night	7% 9% 36% 33% 13% 2	800
	11th to 20th	Night	8% 8% 34% 34% 13% 3	800
	21st to end	Night	12% 7% 31% 34% 13% 3	850
April	1st to 10th	Night	10% 8% 34% 36% 12% 2	600
	11th to 20th	Night	15% 7% 31% 37% 9% 1	593
	21st to end	Night	15% 8% 27% 37% 13% 1	600
May	1st to 10th	Night	13% 6% 21% 40% 18% 2	600
	11th to 20th	Night	13% 7% 24% 37% 18% 2	591
	21st to end	Night	18% 6% 25% 31% 17% 3	514
June	1st to 10th	Night	15% 9% 23% 33% 18% 2	400
	11th to 20th	Night	11% 9% 33% 35% 13%	400
	21st to end	Night	7% 6% 26% 42% 18% 2	400
July	1st to 10th	Night	4 5% 24% 44% 22% 2	390
	11th to 20th	Night	5% 7% 25% 45% 17% 2	388
	21st to end	Night	6% 4 20% 47% 23% 1	528
August	1st to 10th	Night	8% 5% 20% 46% 19% 2	597
	11th to 20th	Night	6% 3 21% 43% 26% 2	594
	21st to end	Night	11% 5% 21% 40% 22% 1	660
September	1st to 10th	Night	10% 4 23% 40% 20% 2	600
	11th to 20th	Night	9% 6% 25% 39% 19% 2	645
	21st to end	Night	8% 7% 28% 39% 16% 2	800
October	1st to 10th	Night	13% 7% 25% 35% 18% 2	800
	11th to 20th	Night	9% 10% 32% 32% 15% 2	800
	21st to end	Night	10% 10% 36% 34% 8% 1	973
November	1st to 10th	Night	14% 13% 35% 30% 8%	1025
	11th to 20th	Night	8% 14% 40% 28% 9% 1	1036
	21st to end	Night	8% 13% 35% 35% 9%	1000
December	1st to 10th	Night	7% 10% 38% 30% 13% 1	1000
	11th to 20th	Night	8% 11% 39% 31% 10% 1	1000
	21st to end	Night	10% 10% 34% 35% 10% 2	1100

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