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 2024-12-02.

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	16% 9% 3 7% 5% 4 5% 4 6% 7% 35%	101431
	Night	17% 9% 3 7% 4 4 4 3 5% 7% 37%	98167
Transparency	Day	47% 6% 11% 21% 13% 3	105160
	Night	46% 5% 11% 21% 14% 3	102577
Seeing	Day	22% 14% 23% 22% 14% 6%	15155
	Night	21% 10% 20% 29% 17% 2	25234

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	18% 15% 8% 10% 5% 3 3 2 4 6% 27%	2836
	11th to 20th	Day	18% 18% 8% 10% 8% 3 3 3 4 4 22%	2840
	21st to end	Day	19% 18% 4 8% 4 3 4 2 4 4 28%	3124
February	1st to 10th	Day	20% 13% 3 7% 4 4 5% 4 4 6% 31%	1730
	11th to 20th	Day	23% 12% 5% 9% 3 4 3 3 4 5% 29%	1848
	21st to end	Day	24% 12% 4 7% 3 3 3 2 4 6% 32%	1570
March	1st to 10th	Day	20% 13% 3 7% 4 3 3 3 4 6% 34%	3391
	11th to 20th	Day	15% 8% 3 6% 3 4 3 3 4 6% 45%	3480
	21st to end	Day	19% 9% 3 6% 4 4 4 4 5% 6% 37%	3815
April	1st to 10th	Day	15% 8% 3 6% 4 4 4 4 7% 7% 38%	2090
	11th to 20th	Day	16% 8% 2 6% 4 4 4 4 5% 6% 42%	2296
	21st to end	Day	18% 7% 2 6% 5% 4 5% 3 5% 6% 39%	2470
May	1st to 10th	Day	22% 8% 3 8% 6% 5% 5% 4 5% 6% 28%	4082
	11th to 20th	Day	21% 8% 3 7% 5% 5% 5% 4 5% 6% 30%	4081
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 10% 37%	4476
June	1st to 10th	Day	13% 6% 2 7% 4 5% 5% 5% 7% 9% 36%	2470
	11th to 20th	Day	9% 6% 2 9% 6% 6% 6% 6% 8% 9% 33%	2470
	21st to end	Day	11% 6% 2 6% 6% 5% 6% 5% 7% 9% 35%	2470
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 7% 6% 9% 11% 36%	4120
	11th to 20th	Day	12% 6% 2 7% 5% 5% 5% 5% 7% 8% 37%	4120
	21st to end	Day	21% 10% 4 9% 7% 7% 6% 4 5% 8% 20%	4466
August	1st to 10th	Day	12% 7% 3 8% 6% 6% 7% 5% 8% 11% 28%	4104
	11th to 20th	Day	12% 6% 2 8% 5% 6% 6% 4 7% 11% 34%	4044
	21st to end	Day	6% 3 2 4 4 4 4 4 6% 8% 55%	4291
September	1st to 10th	Day	12% 5% 2 6% 4 5% 6% 4 6% 8% 42%	2090
	11th to 20th	Day	14% 7% 2 6% 4 5% 5% 4 6% 7% 40%	2090
	21st to end	Day	15% 6% 3 6% 4 5% 5% 4 6% 7% 39%	2090
October	1st to 10th	Day	14% 5% 2 5% 4 3 3 3 5% 6% 49%	2024
	11th to 20th	Day	16% 6% 2 5% 3 4 3 3 5% 5% 49%	1900
	21st to end	Day	18% 7% 2 6% 3 3 3 3 3 6% 46%	2090
November	1st to 10th	Day	14% 6% 2 5% 4 3 3 3 5% 6% 50%	1867
	11th to 20th	Day	12% 10% 2 7% 4 4 5% 3 5% 7% 42%	1800
	21st to end	Day	14% 10% 4 6% 4 4 3 3 5% 6% 41%	1800
December	1st to 10th	Day	21% 14% 4 8% 4 3 4 3 4 5% 30%	1719
	11th to 20th	Day	17% 16% 3 6% 3 3 3 3 5% 7% 33%	2153
	21st to end	Day	17% 16% 5% 7% 5% 5% 6% 4 5% 6% 25%	3124
January	1st to 10th	Night	21% 17% 6% 8% 4 3 3 3 4 4 26%	4109
	11th to 20th	Night	22% 12% 4 10% 5% 3 3 3 4 4 33%	4120
	21st to end	Night	23% 11% 4 8% 5% 4 3 3 4 5% 30%	4532
February	1st to 10th	Night	18% 12% 3 7% 3 4 4 2 5% 6% 35%	2447
	11th to 20th	Night	22% 13% 4 7% 5% 4 4 3 3 6% 30%	2218
	21st to end	Night	23% 13% 3 8% 4 3 3 3 4 6% 30%	1884
March	1st to 10th	Night	24% 13% 3 7% 3 3 3 2 3 5% 32%	3462
	11th to 20th	Night	16% 7% 4 5% 4 2 3 3 4 6% 45%	3480
	21st to end	Night	16% 7% 2 5% 4 4 4 4 5% 6% 42%	3817
April	1st to 10th	Night	16% 9% 2 6% 4 4 4 3 6% 7% 40%	2090
	11th to 20th	Night	16% 8% 2 6% 4 4 4 4 4 7% 41%	1848
	21st to end	Night	15% 7% 3 6% 5% 5% 4 4 5% 7% 41%	1710
May	1st to 10th	Night	21% 9% 3 8% 5% 4 4 3 5% 7% 30%	2813
	11th to 20th	Night	22% 8% 2 7% 4 4 4 4 5% 8% 31%	2812
	21st to end	Night	12% 6% 2 8% 5% 4 5% 4 8% 9% 36%	3079
June	1st to 10th	Night	13% 5% 2 6% 5% 4 6% 3 8% 10% 38%	1710
	11th to 20th	Night	6% 5% 3 6% 6% 6% 5% 5% 7% 10% 41%	1710
	21st to end	Night	12% 6% 2 6% 5% 6% 4 5% 6% 12% 37%	1710
July	1st to 10th	Night	11% 5% 2 7% 5% 5% 6% 5% 10% 12% 33%	2840
	11th to 20th	Night	11% 6% 3 5% 5% 4 5% 5% 6% 11% 39%	2840
	21st to end	Night	17% 11% 3 10% 7% 5% 5% 4 7% 8% 23%	3087
August	1st to 10th	Night	13% 7% 4 7% 5% 5% 6% 5% 7% 11% 32%	2825
	11th to 20th	Night	14% 7% 2 6% 5% 6% 5% 4 6% 8% 36%	2785
	21st to end	Night	7% 3 1 4 3 4 4 4 7% 9% 54%	3328
September	1st to 10th	Night	13% 7% 2 7% 5% 4 4 3 6% 7% 44%	2090
	11th to 20th	Night	14% 6% 3 6% 4 4 3 4 5% 8% 44%	2090
	21st to end	Night	15% 6% 2 5% 4 4 4 3 5% 6% 45%	2090
October	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 4 6% 50%	2155
	11th to 20th	Night	16% 8% 2 4 3 3 3 4 4 4 49%	2280
	21st to end	Night	16% 7% 2 5% 4 3 3 3 4 6% 47%	2508
November	1st to 10th	Night	14% 7% 3 4 3 3 3 3 5% 5% 50%	2429
	11th to 20th	Night	16% 9% 3 6% 3 3 3 3 4 6% 44%	2600
	21st to end	Night	15% 10% 2 5% 4 3 3 3 4 5% 45%	2600
December	1st to 10th	Night	21% 13% 5% 7% 5% 4 4 2 4 6% 29%	2495
	11th to 20th	Night	19% 12% 5% 9% 4 3 3 3 4 4 34%	3042
	21st to end	Night	17% 17% 5% 8% 5% 4 4 4 6% 7% 23%	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	60% 4 5% 16% 12% 3	2837
	11th to 20th	Day	67% 3 5% 13% 10% 1	2840
	21st to end	Day	61% 3 5% 16% 15% 1	3124
February	1st to 10th	Day	56% 4 6% 18% 14% 1	1730
	11th to 20th	Day	59% 4 6% 15% 14% 2	1858
	21st to end	Day	55% 3 7% 14% 16% 4	1570
March	1st to 10th	Day	52% 4 6% 18% 15% 3	3391
	11th to 20th	Day	42% 4 7% 25% 19% 4	3480
	21st to end	Day	48% 4 8% 25% 13% 2	3815
April	1st to 10th	Day	44% 5% 10% 23% 14% 3	2090
	11th to 20th	Day	43% 4 8% 20% 19% 4	2296
	21st to end	Day	47% 5% 12% 23% 11% 2	2470
May	1st to 10th	Day	56% 6% 10% 18% 8% 1	4120
	11th to 20th	Day	55% 6% 10% 22% 6% 1	4085
	21st to end	Day	39% 8% 16% 23% 11% 2	4489
June	1st to 10th	Day	43% 10% 14% 20% 12% 1	2470
	11th to 20th	Day	43% 11% 14% 19% 12% 2	2470
	21st to end	Day	43% 9% 13% 19% 15% 1	2470
July	1st to 10th	Day	40% 10% 21% 18% 10% 1	4055
	11th to 20th	Day	43% 7% 17% 25% 6% 1	4051
	21st to end	Day	63% 7% 10% 14% 7%	4324
August	1st to 10th	Day	49% 8% 19% 18% 6%	4091
	11th to 20th	Day	44% 7% 18% 21% 9% 1	4083
	21st to end	Day	27% 6% 19% 35% 11% 1	4291
September	1st to 10th	Day	40% 7% 13% 21% 14% 5%	2090
	11th to 20th	Day	44% 6% 9% 22% 17% 2	2090
	21st to end	Day	43% 6% 11% 20% 16% 4	2090
October	1st to 10th	Day	31% 3 7% 21% 31% 7%	3362
	11th to 20th	Day	29% 2 8% 25% 17% 18%	3140
	21st to end	Day	31% 2 5% 32% 17% 13%	3428
November	1st to 10th	Day	37% 3 8% 20% 27% 5%	1867
	11th to 20th	Day	41% 4 6% 20% 23% 5%	1800
	21st to end	Day	45% 4 7% 22% 19% 4	1800
December	1st to 10th	Day	57% 4 9% 18% 10% 2	1719
	11th to 20th	Day	50% 4 9% 18% 16% 3	2150
	21st to end	Day	59% 4 8% 20% 8% 1	3124
January	1st to 10th	Night	63% 3 6% 14% 11% 3	4109
	11th to 20th	Night	57% 4 6% 13% 18% 2	4120
	21st to end	Night	58% 4 7% 16% 14% 1	4532
February	1st to 10th	Night	51% 4 9% 20% 15% 2	2447
	11th to 20th	Night	58% 4 6% 17% 11% 4	2219
	21st to end	Night	56% 4 7% 18% 11% 4	1884
March	1st to 10th	Night	56% 3 8% 17% 14% 2	3462
	11th to 20th	Night	41% 4 10% 23% 15% 7%	3480
	21st to end	Night	41% 5% 10% 26% 15% 2	3817
April	1st to 10th	Night	44% 5% 11% 22% 14% 3	2090
	11th to 20th	Night	43% 5% 8% 21% 18% 5%	1848
	21st to end	Night	43% 5% 12% 26% 12% 1	1710
May	1st to 10th	Night	53% 7% 13% 19% 7% 1	2840
	11th to 20th	Night	50% 8% 13% 24% 5% 1	2813
	21st to end	Night	40% 9% 16% 22% 10% 1	3087
June	1st to 10th	Night	39% 9% 17% 22% 13% 1	1710
	11th to 20th	Night	36% 9% 19% 24% 11% 1	1710
	21st to end	Night	39% 10% 15% 20% 12% 3	1710
July	1st to 10th	Night	42% 8% 20% 20% 9% 1	2795
	11th to 20th	Night	38% 7% 23% 25% 6% 2	2786
	21st to end	Night	58% 8% 13% 16% 5%	2988
August	1st to 10th	Night	46% 6% 22% 20% 5%	2814
	11th to 20th	Night	45% 6% 17% 23% 9%	2812
	21st to end	Night	25% 5% 18% 38% 13% 1	3328
September	1st to 10th	Night	39% 6% 14% 20% 16% 4	2090
	11th to 20th	Night	40% 7% 12% 22% 18% 1	2090
	21st to end	Night	39% 6% 11% 24% 18% 2	2090
October	1st to 10th	Night	36% 3 7% 20% 26% 8%	3597
	11th to 20th	Night	31% 3 7% 22% 20% 17%	3779
	21st to end	Night	27% 2 5% 31% 21% 13%	4120
November	1st to 10th	Night	38% 4 9% 20% 24% 6%	2429
	11th to 20th	Night	43% 3 7% 19% 23% 6%	2600
	21st to end	Night	42% 3 6% 25% 20% 3	2600
December	1st to 10th	Night	59% 4 7% 17% 10% 2	2495
	11th to 20th	Night	55% 4 6% 16% 16% 3	3044
	21st to end	Night	60% 5% 9% 15% 11% 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	31% 30% 27% 9% 3	270
	11th to 20th	Day	41% 25% 24% 9%	270
	21st to end	Day	26% 36% 25% 12% 1	297
February	1st to 10th	Day	29% 26% 30% 11% 4	270
	11th to 20th	Day	48% 25% 19% 6% 1	270
	21st to end	Day	44% 25% 23% 8%	225
March	1st to 10th	Day	26% 23% 27% 18% 6% 1	427
	11th to 20th	Day	29% 26% 25% 15% 3 1	460
	21st to end	Day	31% 21% 25% 16% 6%	506
April	1st to 10th	Day	27% 23% 22% 18% 8% 2	460
	11th to 20th	Day	23% 20% 26% 20% 9% 2	452
	21st to end	Day	26% 15% 28% 19% 9% 4	460
May	1st to 10th	Day	25% 21% 23% 17% 10% 5%	463
	11th to 20th	Day	20% 11% 26% 23% 15% 5%	487
	21st to end	Day	17% 9% 20% 24% 18% 12%	529
June	1st to 10th	Day	15% 8% 23% 28% 18% 8%	490
	11th to 20th	Day	12% 7% 22% 28% 22% 9%	490
	21st to end	Day	17% 7% 24% 26% 18% 8%	490
July	1st to 10th	Day	16% 4 18% 26% 23% 13%	485
	11th to 20th	Day	13% 5% 15% 34% 23% 12%	484
	21st to end	Day	17% 4 18% 27% 23% 11%	532
August	1st to 10th	Day	16% 4 22% 23% 21% 14%	486
	11th to 20th	Day	15% 4 20% 26% 25% 10%	479
	21st to end	Day	12% 3 12% 29% 28% 16%	539
September	1st to 10th	Day	12% 4 18% 28% 27% 12%	490
	11th to 20th	Day	17% 4 16% 25% 27% 11%	490
	21st to end	Day	21% 7% 19% 29% 18% 6%	490
October	1st to 10th	Day	19% 6% 23% 28% 18% 6%	490
	11th to 20th	Day	21% 16% 19% 25% 16% 3	490
	21st to end	Day	22% 12% 25% 26% 12% 3	539
November	1st to 10th	Day	18% 17% 27% 23% 11% 3	405
	11th to 20th	Day	23% 24% 24% 21% 8%	300
	21st to end	Day	27% 19% 30% 17% 6%	300
December	1st to 10th	Day	30% 25% 30% 12% 3	273
	11th to 20th	Day	28% 21% 28% 18% 4	270
	21st to end	Day	27% 26% 29% 14% 4	297
January	1st to 10th	Night	25% 22% 25% 24% 3	760
	11th to 20th	Night	31% 18% 25% 22% 4	760
	21st to end	Night	25% 20% 29% 23% 3	836
February	1st to 10th	Night	26% 16% 28% 28% 3	760
	11th to 20th	Night	31% 23% 29% 17% 1	743
	21st to end	Night	31% 22% 25% 20% 2	628
March	1st to 10th	Night	26% 16% 28% 26% 4	760
	11th to 20th	Night	30% 16% 25% 22% 7% 1	760
	21st to end	Night	21% 15% 28% 28% 8%	836
April	1st to 10th	Night	23% 16% 24% 28% 8%	760
	11th to 20th	Night	20% 14% 26% 28% 10% 1	672
	21st to end	Night	20% 9% 25% 36% 9% 1	570
May	1st to 10th	Night	21% 11% 22% 31% 15% 1	570
	11th to 20th	Night	19% 5% 18% 35% 21% 2	561
	21st to end	Night	13% 3 14% 33% 32% 5%	617
June	1st to 10th	Night	13% 4 13% 38% 28% 4	570
	11th to 20th	Night	9% 3 14% 37% 31% 5%	570
	21st to end	Night	13% 2 14% 36% 32% 3	570
July	1st to 10th	Night	15% 1 9% 32% 38% 5%	555
	11th to 20th	Night	15% 1 7% 30% 40% 6%	552
	21st to end	Night	13% 2 10% 33% 32% 10%	624
August	1st to 10th	Night	15% 2 8% 31% 41% 4	567
	11th to 20th	Night	16% 4 7% 34% 34% 5%	564
	21st to end	Night	13% 1 9% 32% 37% 7%	689
September	1st to 10th	Night	15% 3 12% 30% 36% 5%	760
	11th to 20th	Night	16% 3 14% 29% 33% 5%	760
	21st to end	Night	18% 5% 17% 33% 24% 3	760
October	1st to 10th	Night	20% 5% 16% 28% 26% 5%	760
	11th to 20th	Night	20% 8% 22% 33% 15% 2	760
	21st to end	Night	21% 10% 20% 35% 12% 2	836
November	1st to 10th	Night	20% 7% 22% 34% 15% 2	780
	11th to 20th	Night	23% 12% 22% 33% 10% 1	800
	21st to end	Night	22% 13% 24% 30% 10% 1	800
December	1st to 10th	Night	29% 14% 24% 26% 7%	768
	11th to 20th	Night	28% 13% 24% 29% 6% 1	760
	21st to end	Night	28% 15% 24% 29% 5%	836

Page generated at 2024-11-29 10:43:04. Version ac07

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