Milky Whey Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Milky Whey Observatory clear sky chart from 2005-11-06 to 2025-01-07.

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	13% 5% 2 5% 3 3 4 3 5% 7% 50%	104991
	Night	12% 5% 2 4 3 3 3 3 5% 7% 55%	104205
Transparency	Day	33% 5% 10% 21% 20% 12%	108916
	Night	29% 5% 10% 20% 20% 15%	108407
Seeing	Day	13% 6% 22% 33% 21% 4	14906
	Night	13% 6% 26% 40% 15% 1	24976

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	11% 7% 2 3 2 2 2 2 3 4 63%	2901
	11th to 20th	Day	10% 6% 2 3 2 2 2 2 5% 6% 59%	3052
	21st to end	Day	13% 8% 2 5% 3 3 3 3 4 9% 48%	3487
February	1st to 10th	Day	13% 8% 2 5% 3 3 3 3 4 5% 51%	1900
	11th to 20th	Day	16% 6% 2 5% 3 3 3 3 4 6% 49%	1849
	21st to end	Day	13% 5% 2 3 2 3 3 3 4 6% 55%	1570
March	1st to 10th	Day	18% 8% 2 5% 3 3 3 3 3 5% 47%	3187
	11th to 20th	Day	24% 9% 3 6% 3 4 3 3 3 5% 38%	3480
	21st to end	Day	15% 5% 2 4 3 3 3 3 6% 6% 50%	3814
April	1st to 10th	Day	15% 6% 2 5% 3 3 3 4 5% 6% 48%	2128
	11th to 20th	Day	16% 5% 2 5% 3 3 3 3 4 7% 50%	2254
	21st to end	Day	16% 6% 2 5% 3 3 3 3 4 6% 49%	2280
May	1st to 10th	Day	22% 6% 2 6% 4 3 3 3 4 7% 41%	4036
	11th to 20th	Day	16% 5% 2 5% 3 4 4 3 4 7% 47%	4082
	21st to end	Day	13% 6% 2 5% 4 4 5% 4 6% 9% 42%	4477
June	1st to 10th	Day	7% 4 1 4 3 3 4 4 7% 9% 53%	2470
	11th to 20th	Day	9% 4 1 4 3 4 4 4 7% 9% 50%	2470
	21st to end	Day	9% 6% 2 6% 6% 5% 6% 4 6% 8% 42%	2470
July	1st to 10th	Day	8% 4 2 5% 4 4 4 4 6% 8% 51%	4120
	11th to 20th	Day	6% 4 1 4 4 4 5% 4 7% 10% 51%	4120
	21st to end	Day	9% 4 2 5% 4 4 5% 4 7% 10% 46%	4288
August	1st to 10th	Day	2 2 1 3 3 3 3 2 5% 8% 70%	3785
	11th to 20th	Day	4 2 1 3 3 3 3 3 6% 9% 64%	3726
	21st to end	Day	5% 3 1 5% 4 5% 5% 4 6% 9% 51%	4180
September	1st to 10th	Day	12% 5% 2 7% 5% 5% 5% 3 6% 8% 41%	2111
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 41%	2090
	21st to end	Day	9% 5% 2 5% 4 5% 6% 4 5% 7% 49%	2090
October	1st to 10th	Day	10% 4 2 5% 4 3 4 3 4 7% 55%	2090
	11th to 20th	Day	15% 5% 2 4 3 3 3 3 4 6% 50%	2090
	21st to end	Day	14% 5% 2 4 3 2 2 2 3 5% 58%	2118
November	1st to 10th	Day	16% 7% 2 4 4 3 3 3 3 7% 47%	1857
	11th to 20th	Day	14% 7% 1 4 3 2 4 3 5% 5% 53%	1800
	21st to end	Day	15% 8% 2 5% 3 3 3 2 4 4 50%	1800
December	1st to 10th	Day	24% 9% 3 6% 4 3 3 2 3 4 39%	2940
	11th to 20th	Day	18% 8% 2 5% 3 3 4 3 4 4 45%	3391
	21st to end	Day	14% 5% 2 3 2 2 2 2 3 4 61%	4488
January	1st to 10th	Night	12% 4 2 3 3 2 2 2 3 7% 60%	4187
	11th to 20th	Night	10% 5% 1 3 2 2 2 1 3 4 69%	3946
	21st to end	Night	18% 7% 2 4 3 2 2 2 4 5% 49%	4180
February	1st to 10th	Night	13% 6% 1 3 2 3 3 3 5% 6% 56%	2280
	11th to 20th	Night	14% 6% 2 4 3 3 2 2 4 5% 55%	2218
	21st to end	Night	13% 4 1 4 3 3 3 3 5% 7% 52%	1884
March	1st to 10th	Night	11% 6% 2 5% 3 4 3 3 5% 7% 50%	3673
	11th to 20th	Night	19% 8% 3 6% 4 3 3 3 4 5% 41%	3480
	21st to end	Night	13% 7% 2 4 3 3 3 2 4 6% 52%	3817
April	1st to 10th	Night	10% 4 2 4 4 3 4 4 5% 7% 53%	2047
	11th to 20th	Night	16% 6% 1 4 4 3 3 3 5% 6% 50%	1877
	21st to end	Night	16% 7% 2 4 4 2 3 2 4 5% 52%	1900
May	1st to 10th	Night	19% 7% 2 4 3 3 3 3 5% 7% 45%	2869
	11th to 20th	Night	12% 4 2 5% 2 3 3 3 5% 6% 55%	2823
	21st to end	Night	11% 4 2 5% 3 3 4 3 5% 8% 52%	3090
June	1st to 10th	Night	3 2 1 3 2 4 2 3 7% 11% 62%	1710
	11th to 20th	Night	5% 2 1 4 4 4 4 4 7% 9% 58%	1710
	21st to end	Night	7% 4 1 4 3 4 4 4 7% 11% 52%	1710
July	1st to 10th	Night	5% 4 1 4 3 3 4 3 5% 9% 59%	2850
	11th to 20th	Night	5% 3 1 5% 3 3 4 5% 6% 9% 56%	2850
	21st to end	Night	11% 5% 2 4 3 3 4 4 6% 8% 51%	3270
August	1st to 10th	Night	1 1 1 2 2 2 3 2 5% 9% 74%	3155
	11th to 20th	Night	3 2 1 3 2 3 4 3 5% 9% 65%	3113
	21st to end	Night	6% 3 1 5% 3 4 5% 4 6% 10% 53%	3487
September	1st to 10th	Night	9% 5% 2 5% 4 5% 4 4 6% 8% 47%	2067
	11th to 20th	Night	10% 5% 2 6% 3 4 4 4 6% 8% 48%	2090
	21st to end	Night	8% 5% 2 5% 3 3 4 3 5% 8% 54%	2090
October	1st to 10th	Night	8% 4 1 4 2 3 3 2 4 6% 62%	2090
	11th to 20th	Night	13% 4 2 4 2 3 3 3 4 5% 57%	2090
	21st to end	Night	12% 3 1 4 2 2 2 2 2 4 68%	2460
November	1st to 10th	Night	16% 5% 2 4 2 2 2 2 4 5% 56%	2436
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 4 6% 58%	2600
	21st to end	Night	15% 5% 2 4 3 3 4 3 4 6% 52%	2600
December	1st to 10th	Night	26% 6% 2 4 3 3 4 3 5% 5% 39%	4250
	11th to 20th	Night	20% 6% 2 4 3 3 3 2 3 5% 49%	4816
	21st to end	Night	14% 5% 1 3 2 2 2 2 4 6% 59%	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	27% 2 4 15% 26% 26%	2902
	11th to 20th	Day	27% 3 5% 17% 26% 22%	3052
	21st to end	Day	36% 4 7% 15% 27% 11%	3487
February	1st to 10th	Day	38% 3 6% 13% 21% 19%	1900
	11th to 20th	Day	37% 4 6% 16% 22% 15%	1859
	21st to end	Day	32% 4 7% 16% 21% 19%	1570
March	1st to 10th	Day	42% 4 5% 13% 20% 16%	3187
	11th to 20th	Day	51% 4 5% 11% 16% 13%	3480
	21st to end	Day	33% 5% 10% 15% 16% 22%	3814
April	1st to 10th	Day	36% 5% 8% 16% 18% 17%	2128
	11th to 20th	Day	35% 5% 8% 20% 16% 17%	2254
	21st to end	Day	38% 4 7% 18% 17% 15%	2280
May	1st to 10th	Day	42% 6% 10% 16% 15% 11%	4074
	11th to 20th	Day	36% 6% 8% 21% 18% 10%	4086
	21st to end	Day	36% 7% 11% 26% 15% 4	4489
June	1st to 10th	Day	25% 6% 14% 30% 19% 5%	2470
	11th to 20th	Day	28% 8% 12% 25% 20% 7%	2470
	21st to end	Day	36% 8% 12% 22% 16% 5%	2470
July	1st to 10th	Day	30% 6% 16% 34% 13% 1	4056
	11th to 20th	Day	25% 7% 20% 31% 16% 2	4051
	21st to end	Day	29% 8% 18% 27% 17% 2	4153
August	1st to 10th	Day	13% 5% 11% 40% 29% 2	3772
	11th to 20th	Day	17% 5% 21% 42% 13% 2	3762
	21st to end	Day	26% 8% 16% 26% 22% 2	4179
September	1st to 10th	Day	41% 8% 11% 18% 16% 7%	2111
	11th to 20th	Day	38% 8% 13% 20% 18% 5%	2090
	21st to end	Day	32% 7% 10% 22% 18% 12%	2090
October	1st to 10th	Day	19% 4 6% 14% 22% 35%	3480
	11th to 20th	Day	32% 4 7% 13% 19% 25%	3458
	21st to end	Day	29% 4 4 16% 35% 13%	3468
November	1st to 10th	Day	38% 4 10% 18% 19% 12%	1857
	11th to 20th	Day	34% 3 7% 18% 19% 18%	1800
	21st to end	Day	39% 4 5% 19% 18% 15%	1800
December	1st to 10th	Day	52% 3 5% 13% 17% 10%	2940
	11th to 20th	Day	42% 4 6% 14% 19% 15%	3389
	21st to end	Day	30% 3 4 15% 29% 20%	4488
January	1st to 10th	Night	26% 4 5% 13% 27% 26%	4187
	11th to 20th	Night	23% 2 4 17% 26% 28%	3946
	21st to end	Night	38% 3 5% 13% 26% 15%	4180
February	1st to 10th	Night	30% 4 8% 15% 20% 23%	2280
	11th to 20th	Night	33% 4 7% 18% 20% 18%	2218
	21st to end	Night	32% 4 7% 16% 21% 20%	1884
March	1st to 10th	Night	33% 6% 10% 16% 20% 15%	3673
	11th to 20th	Night	45% 5% 8% 12% 14% 15%	3480
	21st to end	Night	32% 6% 10% 17% 16% 19%	3817
April	1st to 10th	Night	29% 6% 11% 18% 17% 18%	2047
	11th to 20th	Night	36% 5% 9% 16% 15% 19%	1877
	21st to end	Night	36% 4 10% 19% 16% 15%	1900
May	1st to 10th	Night	38% 5% 11% 19% 15% 12%	2896
	11th to 20th	Night	30% 6% 9% 22% 22% 10%	2824
	21st to end	Night	30% 6% 13% 27% 16% 8%	3099
June	1st to 10th	Night	16% 6% 19% 34% 22% 4	1710
	11th to 20th	Night	21% 6% 13% 28% 24% 7%	1710
	21st to end	Night	24% 8% 18% 26% 18% 6%	1710
July	1st to 10th	Night	21% 7% 22% 37% 12% 1	2805
	11th to 20th	Night	23% 8% 20% 28% 18% 3	2796
	21st to end	Night	27% 8% 21% 30% 14% 1	3166
August	1st to 10th	Night	9% 5% 16% 42% 27% 2	3144
	11th to 20th	Night	17% 6% 25% 40% 11% 2	3143
	21st to end	Night	25% 10% 18% 27% 18% 2	3487
September	1st to 10th	Night	33% 9% 12% 21% 17% 8%	2067
	11th to 20th	Night	32% 9% 17% 21% 16% 6%	2090
	21st to end	Night	29% 7% 11% 22% 18% 12%	2090
October	1st to 10th	Night	16% 4 7% 15% 21% 36%	3480
	11th to 20th	Night	27% 3 6% 13% 24% 26%	3460
	21st to end	Night	22% 2 6% 15% 39% 16%	4048
November	1st to 10th	Night	32% 3 7% 19% 23% 15%	2436
	11th to 20th	Night	28% 5% 8% 19% 20% 19%	2600
	21st to end	Night	35% 4 7% 17% 19% 18%	2600
December	1st to 10th	Night	48% 3 9% 13% 13% 14%	4250
	11th to 20th	Night	39% 4 6% 14% 20% 18%	4817
	21st to end	Night	28% 3 5% 17% 26% 20%	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	19% 9% 28% 32% 11% 1	288
	11th to 20th	Day	20% 9% 30% 30% 10% 1	270
	21st to end	Day	13% 12% 37% 26% 11%	297
February	1st to 10th	Day	24% 13% 25% 26% 11% 1	270
	11th to 20th	Day	24% 10% 36% 23% 7% 1	270
	21st to end	Day	18% 15% 41% 21% 5%	225
March	1st to 10th	Day	20% 13% 42% 21% 3	270
	11th to 20th	Day	20% 13% 42% 21% 5%	270
	21st to end	Day	20% 15% 39% 22% 3	297
April	1st to 10th	Day	16% 16% 41% 25% 3	308
	11th to 20th	Day	12% 18% 39% 29% 3	452
	21st to end	Day	14% 14% 44% 24% 5%	460
May	1st to 10th	Day	16% 8% 37% 31% 7% 1	620
	11th to 20th	Day	19% 7% 34% 31% 7% 1	675
	21st to end	Day	17% 7% 30% 37% 9% 1	734
June	1st to 10th	Day	6% 3 20% 51% 18% 2	680
	11th to 20th	Day	10% 1 20% 49% 18% 2	680
	21st to end	Day	9% 2 15% 39% 29% 6%	680
July	1st to 10th	Day	10% 3 11% 34% 36% 6%	671
	11th to 20th	Day	6% 1 5% 39% 44% 6%	668
	21st to end	Day	4 1 6% 37% 46% 6%	637
August	1st to 10th	Day	2 5% 40% 44% 9%	487
	11th to 20th	Day	6% 1 7% 34% 42% 10%	477
	21st to end	Day	4 2 7% 32% 42% 13%	539
September	1st to 10th	Day	10% 2 5% 31% 40% 12%	321
	11th to 20th	Day	7% 1 7% 25% 43% 17%	300
	21st to end	Day	9% 1 5% 35% 39% 11%	300
October	1st to 10th	Day	8% 4 11% 36% 33% 8%	300
	11th to 20th	Day	13% 5% 14% 42% 24% 2	300
	21st to end	Day	18% 10% 22% 31% 16% 3	330
November	1st to 10th	Day	22% 5% 25% 30% 15% 2	300
	11th to 20th	Day	21% 7% 20% 29% 18% 4	300
	21st to end	Day	20% 12% 29% 27% 10% 3	300
December	1st to 10th	Day	22% 8% 22% 31% 14% 3	300
	11th to 20th	Day	22% 14% 24% 26% 14% 2	300
	21st to end	Day	20% 8% 28% 30% 12% 2	330
January	1st to 10th	Night	15% 6% 28% 40% 10% 1	981
	11th to 20th	Night	18% 7% 28% 39% 9%	841
	21st to end	Night	19% 9% 28% 37% 8%	836
February	1st to 10th	Night	16% 9% 31% 35% 8%	760
	11th to 20th	Night	18% 8% 36% 29% 7%	744
	21st to end	Night	15% 10% 41% 32% 2	628
March	1st to 10th	Night	20% 8% 39% 27% 5%	725
	11th to 20th	Night	17% 10% 35% 34% 4	570
	21st to end	Night	14% 12% 45% 25% 4	627
April	1st to 10th	Night	9% 10% 46% 33% 2	570
	11th to 20th	Night	15% 11% 51% 22% 2	563
	21st to end	Night	14% 10% 48% 23% 3 1	570
May	1st to 10th	Night	13% 9% 44% 32% 2	570
	11th to 20th	Night	13% 8% 41% 34% 5%	561
	21st to end	Night	13% 5% 42% 33% 7%	617
June	1st to 10th	Night	4 2 30% 54% 11%	570
	11th to 20th	Night	5% 1 36% 51% 7%	570
	21st to end	Night	5% 2 23% 53% 17%	570
July	1st to 10th	Night	8% 2 11% 59% 19% 1	555
	11th to 20th	Night	3 1 14% 56% 26% 1	552
	21st to end	Night	5% 1 9% 56% 28% 1	624
August	1st to 10th	Night	1 5% 57% 35% 1	567
	11th to 20th	Night	4 1 8% 54% 32% 1	564
	21st to end	Night	6% 1 7% 48% 34% 3	627
September	1st to 10th	Night	8% 1 10% 42% 36% 2	570
	11th to 20th	Night	12% 1 9% 43% 33% 2	570
	21st to end	Night	8% 2 12% 42% 32% 4	570
October	1st to 10th	Night	8% 2 13% 49% 26% 1	570
	11th to 20th	Night	13% 3 20% 40% 22% 1	570
	21st to end	Night	12% 6% 23% 36% 22% 2	788
November	1st to 10th	Night	17% 4 18% 38% 22% 2	876
	11th to 20th	Night	12% 5% 22% 42% 16% 2	1000
	21st to end	Night	17% 7% 23% 39% 13% 1	1000
December	1st to 10th	Night	18% 6% 21% 42% 13% 1	1000
	11th to 20th	Night	20% 9% 22% 38% 12%	1000
	21st to end	Night	16% 7% 25% 42% 9% 1	1100

Page generated at 2025-01-04 02:23:57. Version ac07

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