Windy Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windy Hill Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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	12% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	108620
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	110486
Transparency	Day	24% 44% 6% 11% 11% 5%	112270
	Night	23% 43% 6% 11% 12% 5%	114976
Seeing	Day	15% 9% 22% 24% 21% 9%	15315
	Night	15% 8% 24% 29% 20% 4	25182

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	17% 9% 3 7% 3 3 3 3 4 6% 43%	4077
	11th to 20th	Day	12% 7% 3 6% 4 4 3 2 3 5% 52%	4080
	21st to end	Day	21% 10% 3 7% 4 4 4 3 4 4 36%	4507
February	1st to 10th	Day	17% 8% 3 7% 5% 4 5% 3 5% 5% 39%	2000
	11th to 20th	Day	18% 8% 4 7% 4 4 4 3 3 5% 40%	1948
	21st to end	Day	18% 8% 3 8% 4 4 4 3 5% 7% 36%	1650
March	1st to 10th	Day	21% 9% 4 8% 5% 5% 5% 4 5% 6% 28%	3536
	11th to 20th	Day	17% 8% 3 7% 5% 5% 4 4 6% 7% 34%	3491
	21st to end	Day	13% 7% 3 7% 6% 5% 4 4 4 5% 40%	3906
April	1st to 10th	Day	10% 7% 3 6% 5% 4 6% 4 5% 7% 42%	2280
	11th to 20th	Day	16% 7% 2 7% 5% 5% 3 4 5% 8% 39%	2254
	21st to end	Day	9% 5% 2 6% 5% 6% 5% 4 6% 8% 44%	2280
May	1st to 10th	Day	13% 8% 2 7% 5% 6% 6% 4 6% 8% 34%	3765
	11th to 20th	Day	14% 7% 2 6% 4 4 5% 4 5% 7% 42%	3764
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 6% 7% 10% 39%	4129
June	1st to 10th	Day	9% 5% 3 8% 6% 7% 6% 6% 8% 10% 32%	2458
	11th to 20th	Day	6% 6% 2 8% 7% 6% 7% 6% 8% 9% 35%	2470
	21st to end	Day	7% 6% 2 8% 8% 7% 8% 6% 8% 10% 29%	2470
July	1st to 10th	Day	6% 6% 3 9% 7% 8% 9% 8% 9% 10% 25%	4120
	11th to 20th	Day	7% 6% 2 9% 8% 8% 9% 6% 9% 11% 25%	4120
	21st to end	Day	12% 10% 4 10% 6% 7% 6% 5% 6% 8% 26%	4354
August	1st to 10th	Day	3 3 2 4 4 5% 5% 4 5% 7% 58%	3785
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 5% 8% 10% 44%	3729
	21st to end	Day	10% 6% 3 8% 6% 6% 6% 4 6% 7% 37%	4180
September	1st to 10th	Day	11% 6% 3 7% 6% 6% 6% 4 8% 8% 35%	2280
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 5% 5% 8% 39%	2280
	21st to end	Day	10% 5% 2 6% 4 4 3 4 4 7% 51%	2073
October	1st to 10th	Day	7% 4 1 5% 4 5% 3 4 5% 6% 55%	1900
	11th to 20th	Day	8% 5% 2 5% 3 3 3 3 5% 6% 58%	1900
	21st to end	Day	11% 4 2 5% 4 3 3 3 3 5% 57%	2090
November	1st to 10th	Day	11% 6% 2 4 4 3 4 4 5% 6% 52%	1958
	11th to 20th	Day	10% 6% 2 5% 4 4 3 3 5% 5% 52%	2000
	21st to end	Day	14% 7% 3 6% 3 4 4 3 5% 6% 46%	1839
December	1st to 10th	Day	16% 9% 2 6% 4 4 4 4 4 5% 42%	2930
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 3 5% 6% 43%	3529
	21st to end	Day	15% 7% 2 6% 3 4 3 3 5% 5% 47%	4488
January	1st to 10th	Night	19% 7% 3 4 3 3 3 3 4 5% 47%	5888
	11th to 20th	Night	17% 8% 3 5% 3 3 2 2 3 4 48%	5900
	21st to end	Night	25% 9% 2 5% 3 4 3 3 4 5% 37%	6466
February	1st to 10th	Night	15% 8% 3 6% 4 4 5% 3 5% 5% 42%	2400
	11th to 20th	Night	21% 9% 3 6% 3 3 3 3 4 6% 41%	2338
	21st to end	Night	20% 8% 3 5% 4 4 4 3 5% 5% 39%	1980
March	1st to 10th	Night	22% 12% 3 7% 4 4 4 3 5% 6% 31%	3566
	11th to 20th	Night	16% 8% 3 7% 5% 5% 4 4 6% 7% 36%	3497
	21st to end	Night	16% 8% 2 6% 5% 4 4 3 5% 6% 42%	3722
April	1st to 10th	Night	12% 7% 2 5% 4 5% 3 3 6% 8% 44%	1900
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 5% 7% 45%	1875
	21st to end	Night	9% 5% 2 5% 4 3 4 4 6% 7% 53%	1900
May	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 6% 9% 43%	3130
	11th to 20th	Night	15% 6% 3 6% 4 3 4 4 6% 6% 43%	3129
	21st to end	Night	9% 5% 2 7% 5% 5% 6% 5% 7% 10% 41%	3426
June	1st to 10th	Night	8% 5% 2 6% 6% 5% 5% 4 7% 11% 42%	1716
	11th to 20th	Night	3 4 2 5% 4 4 5% 5% 9% 10% 49%	1710
	21st to end	Night	6% 4 1 6% 6% 5% 5% 6% 9% 12% 39%	1710
July	1st to 10th	Night	5% 6% 3 9% 6% 6% 6% 6% 9% 12% 33%	2850
	11th to 20th	Night	6% 5% 2 5% 5% 6% 7% 7% 10% 13% 35%	2850
	21st to end	Night	8% 8% 4 7% 7% 6% 7% 5% 7% 11% 31%	3202
August	1st to 10th	Night	2 2 1 3 3 3 3 3 7% 10% 63%	3144
	11th to 20th	Night	3 3 1 6% 5% 5% 5% 5% 8% 13% 47%	3100
	21st to end	Night	7% 5% 3 7% 5% 6% 6% 5% 6% 8% 43%	3476
September	1st to 10th	Night	8% 5% 2 5% 5% 5% 5% 4 7% 9% 43%	1900
	11th to 20th	Night	8% 6% 2 5% 5% 4 5% 5% 6% 10% 44%	1900
	21st to end	Night	9% 4 2 5% 4 3 4 3 6% 7% 53%	2082
October	1st to 10th	Night	6% 4 1 4 3 3 3 4 5% 7% 60%	2280
	11th to 20th	Night	9% 4 2 4 3 3 3 4 4 6% 59%	2280
	21st to end	Night	11% 6% 1 4 3 2 2 3 4 5% 57%	2508
November	1st to 10th	Night	12% 4 2 4 4 4 3 2 5% 6% 55%	2340
	11th to 20th	Night	14% 6% 1 4 4 3 3 3 4 5% 55%	2400
	21st to end	Night	14% 6% 2 6% 4 3 4 3 4 6% 48%	2551
December	1st to 10th	Night	19% 7% 2 6% 4 4 3 2 3 5% 44%	4250
	11th to 20th	Night	12% 11% 2 7% 4 4 3 3 5% 5% 44%	4630
	21st to end	Night	19% 10% 2 5% 3 3 2 2 3 4 46%	6490

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	33% 27% 4 15% 19% 2	4078
	11th to 20th	Day	26% 26% 3 13% 24% 7%	4080
	21st to end	Day	40% 28% 3 7% 11% 11%	4507
February	1st to 10th	Day	19% 62% 3 5% 6% 5%	2000
	11th to 20th	Day	20% 61% 2 7% 8% 2	1958
	21st to end	Day	21% 62% 3 3 6% 5%	1650
March	1st to 10th	Day	40% 41% 5% 8% 6% 2	3536
	11th to 20th	Day	32% 41% 4 13% 8% 2	3491
	21st to end	Day	29% 41% 5% 7% 13% 5%	3906
April	1st to 10th	Day	16% 65% 2 6% 8% 4	2280
	11th to 20th	Day	19% 66% 3 5% 4 3	2254
	21st to end	Day	13% 66% 4 7% 7% 3	2280
May	1st to 10th	Day	29% 45% 6% 12% 6% 2	3800
	11th to 20th	Day	26% 41% 6% 9% 16% 2	3768
	21st to end	Day	21% 43% 10% 20% 6% 1	4141
June	1st to 10th	Day	16% 66% 4 6% 6% 1	2458
	11th to 20th	Day	19% 67% 4 5% 4 1	2470
	21st to end	Day	17% 67% 7% 6% 3	2470
July	1st to 10th	Day	29% 43% 12% 13% 3	4056
	11th to 20th	Day	32% 43% 17% 8% 1	4051
	21st to end	Day	37% 38% 7% 13% 5% 1	4216
August	1st to 10th	Day	9% 38% 6% 24% 22% 1	3773
	11th to 20th	Day	15% 40% 13% 23% 8% 1	3765
	21st to end	Day	28% 39% 9% 10% 13% 1	4179
September	1st to 10th	Day	17% 62% 6% 7% 6% 2	2280
	11th to 20th	Day	14% 61% 5% 10% 8% 2	2280
	21st to end	Day	13% 61% 5% 12% 6% 2	2073
October	1st to 10th	Day	13% 37% 4 7% 12% 27%	3160
	11th to 20th	Day	6% 36% 4 11% 17% 26%	3140
	21st to end	Day	13% 36% 2 7% 28% 15%	3428
November	1st to 10th	Day	19% 57% 4 8% 8% 4	1958
	11th to 20th	Day	18% 57% 3 6% 10% 6%	2000
	21st to end	Day	16% 57% 3 8% 11% 4	1839
December	1st to 10th	Day	31% 36% 3 8% 17% 4	2930
	11th to 20th	Day	25% 33% 6% 10% 17% 9%	3527
	21st to end	Day	30% 27% 4 15% 21% 2	4488
January	1st to 10th	Night	31% 27% 4 16% 18% 5%	5888
	11th to 20th	Night	31% 27% 3 9% 23% 8%	5900
	21st to end	Night	38% 28% 3 10% 13% 8%	6466
February	1st to 10th	Night	17% 62% 3 7% 9% 4	2400
	11th to 20th	Night	18% 61% 2 8% 8% 3	2339
	21st to end	Night	19% 62% 3 5% 5% 6%	1980
March	1st to 10th	Night	40% 41% 6% 6% 5% 2	3566
	11th to 20th	Night	30% 42% 5% 11% 9% 3	3497
	21st to end	Night	28% 42% 4 11% 13% 3	3722
April	1st to 10th	Night	15% 66% 3 5% 8% 4	1900
	11th to 20th	Night	16% 66% 4 7% 5% 3	1875
	21st to end	Night	12% 65% 4 7% 8% 3	1900
May	1st to 10th	Night	25% 44% 8% 11% 9% 3	3160
	11th to 20th	Night	27% 42% 6% 14% 11% 1	3130
	21st to end	Night	22% 44% 9% 17% 7% 1	3435
June	1st to 10th	Night	12% 67% 5% 9% 7% 1	1716
	11th to 20th	Night	11% 68% 8% 8% 4 1	1710
	21st to end	Night	15% 66% 7% 9% 3	1710
July	1st to 10th	Night	27% 43% 15% 12% 2	2805
	11th to 20th	Night	25% 43% 22% 8% 2	2796
	21st to end	Night	33% 41% 8% 13% 5%	3100
August	1st to 10th	Night	8% 37% 9% 23% 22% 1	3132
	11th to 20th	Night	15% 41% 16% 24% 5% 1	3130
	21st to end	Night	25% 40% 8% 12% 12% 3	3476
September	1st to 10th	Night	14% 63% 7% 8% 6% 3	1900
	11th to 20th	Night	12% 61% 7% 10% 6% 3	1900
	21st to end	Night	14% 61% 5% 12% 6% 3	2082
October	1st to 10th	Night	12% 38% 5% 7% 14% 25%	3800
	11th to 20th	Night	7% 36% 2 13% 17% 25%	3779
	21st to end	Night	12% 36% 3 6% 28% 15%	4120
November	1st to 10th	Night	20% 57% 5% 7% 8% 4	2340
	11th to 20th	Night	18% 57% 2 6% 11% 6%	2400
	21st to end	Night	16% 57% 3 8% 11% 5%	2551
December	1st to 10th	Night	32% 35% 2 10% 18% 2	4250
	11th to 20th	Night	28% 33% 5% 11% 15% 8%	4631
	21st to end	Night	33% 27% 4 14% 18% 5%	6490

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% 25% 33% 14% 3 1	300
	11th to 20th	Day	24% 18% 39% 14% 4	300
	21st to end	Day	28% 23% 36% 10% 3	330
February	1st to 10th	Day	25% 21% 35% 15% 3 1	300
	11th to 20th	Day	38% 18% 29% 11% 4	300
	21st to end	Day	25% 28% 38% 7% 2	249
March	1st to 10th	Day	26% 20% 37% 14% 2	300
	11th to 20th	Day	26% 21% 38% 13% 2	273
	21st to end	Day	18% 24% 32% 21% 5% 1	351
April	1st to 10th	Day	20% 15% 33% 23% 8% 1	460
	11th to 20th	Day	25% 16% 28% 17% 10% 2	452
	21st to end	Day	12% 11% 32% 31% 11% 3	460
May	1st to 10th	Day	15% 12% 30% 29% 11% 3	463
	11th to 20th	Day	14% 5% 25% 28% 19% 8%	487
	21st to end	Day	8% 4 24% 32% 22% 10%	529
June	1st to 10th	Day	10% 4 16% 32% 28% 10%	668
	11th to 20th	Day	8% 3 17% 35% 29% 9%	680
	21st to end	Day	10% 2 14% 28% 30% 16%	680
July	1st to 10th	Day	6% 2 8% 25% 41% 18%	671
	11th to 20th	Day	5% 2 7% 24% 41% 21%	668
	21st to end	Day	9% 3 7% 24% 42% 15%	677
August	1st to 10th	Day	5% 2 6% 21% 39% 27%	486
	11th to 20th	Day	9% 1 4 16% 44% 25%	479
	21st to end	Day	12% 4 9% 24% 28% 24%	539
September	1st to 10th	Day	11% 3 11% 25% 32% 18%	490
	11th to 20th	Day	11% 3 13% 25% 34% 14%	490
	21st to end	Day	12% 3 11% 30% 31% 12%	473
October	1st to 10th	Day	6% 7% 25% 41% 17% 5%	300
	11th to 20th	Day	9% 5% 28% 34% 19% 5%	300
	21st to end	Day	18% 11% 23% 28% 15% 4	330
November	1st to 10th	Day	15% 6% 33% 32% 11% 2	300
	11th to 20th	Day	16% 13% 35% 25% 11%	300
	21st to end	Day	17% 16% 37% 20% 7% 3	300
December	1st to 10th	Day	21% 14% 35% 22% 6% 1	300
	11th to 20th	Day	20% 16% 37% 21% 5% 1	300
	21st to end	Day	26% 17% 32% 19% 4 1	330
January	1st to 10th	Night	24% 15% 33% 23% 5%	1000
	11th to 20th	Night	23% 15% 31% 24% 7% 1	1000
	21st to end	Night	25% 14% 33% 24% 4	1089
February	1st to 10th	Night	20% 17% 37% 23% 4	800
	11th to 20th	Night	28% 16% 29% 23% 5%	783
	21st to end	Night	23% 22% 33% 19% 3	660
March	1st to 10th	Night	19% 17% 34% 25% 5%	626
	11th to 20th	Night	17% 14% 35% 28% 7%	575
	21st to end	Night	16% 12% 39% 26% 7%	627
April	1st to 10th	Night	17% 13% 36% 25% 9% 1	570
	11th to 20th	Night	17% 10% 35% 26% 12%	563
	21st to end	Night	11% 9% 36% 31% 14% 1	570
May	1st to 10th	Night	11% 5% 31% 38% 14% 1	570
	11th to 20th	Night	11% 4 25% 37% 22% 2	561
	21st to end	Night	7% 4 18% 39% 26% 6%	617
June	1st to 10th	Night	7% 2 13% 38% 35% 5%	570
	11th to 20th	Night	3 1 12% 38% 38% 7%	570
	21st to end	Night	5% 2 9% 32% 37% 15%	570
July	1st to 10th	Night	5% 2 5% 26% 47% 15%	555
	11th to 20th	Night	5% 1 4 27% 45% 18%	552
	21st to end	Night	7% 1 6% 26% 48% 12%	624
August	1st to 10th	Night	3 1 4 22% 53% 17%	567
	11th to 20th	Night	5% 1 6% 22% 50% 16%	564
	21st to end	Night	11% 1 5% 27% 40% 16%	627
September	1st to 10th	Night	9% 2 6% 28% 43% 12%	570
	11th to 20th	Night	11% 2 9% 29% 42% 7%	570
	21st to end	Night	10% 3 14% 31% 37% 6%	741
October	1st to 10th	Night	8% 2 18% 37% 31% 3	760
	11th to 20th	Night	10% 5% 25% 35% 21% 3	760
	21st to end	Night	16% 7% 21% 33% 21% 2	836
November	1st to 10th	Night	14% 7% 22% 35% 21% 2	780
	11th to 20th	Night	16% 9% 28% 33% 13% 1	800
	21st to end	Night	18% 10% 31% 28% 12% 1	800
December	1st to 10th	Night	21% 12% 29% 29% 10%	800
	11th to 20th	Night	20% 11% 33% 29% 7%	855
	21st to end	Night	27% 13% 28% 26% 6%	1100

Page generated at 2025-03-09 07:08:43. Version ac07

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