Hillsman's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hillsman's Observatory clear sky chart from 2005-11-06 to 2024-12-09.

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 6% 4 4 4 3 5% 7% 45%	101391
	Night	12% 7% 3 6% 4 4 4 3 5% 6% 47%	98454
Transparency	Day	38% 5% 12% 26% 17% 2	105042
	Night	36% 4 11% 26% 19% 3	102944
Seeing	Day	16% 8% 27% 28% 17% 5%	15768
	Night	15% 7% 30% 33% 14% 1	25642

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	14% 9% 4 8% 5% 5% 4 4 5% 7% 36%	2847
	11th to 20th	Day	12% 12% 4 8% 5% 7% 5% 3 3 4 38%	2850
	21st to end	Day	16% 16% 5% 7% 3 4 4 3 3 5% 35%	3135
February	1st to 10th	Day	15% 12% 4 7% 5% 3 3 3 5% 6% 36%	1710
	11th to 20th	Day	13% 10% 3 7% 5% 5% 3 3 4 6% 41%	1664
	21st to end	Day	13% 9% 2 6% 4 4 4 3 5% 7% 42%	1614
March	1st to 10th	Day	14% 12% 3 6% 4 3 3 2 4 5% 47%	3436
	11th to 20th	Day	12% 7% 2 5% 3 3 3 3 5% 6% 51%	3480
	21st to end	Day	17% 13% 3 8% 5% 4 4 3 5% 6% 34%	3814
April	1st to 10th	Day	14% 8% 3 7% 4 4 3 3 5% 6% 44%	2230
	11th to 20th	Day	16% 9% 2 5% 5% 4 4 3 4 6% 41%	2254
	21st to end	Day	20% 7% 3 8% 5% 4 5% 3 5% 6% 35%	2351
May	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 3 6% 7% 38%	4082
	11th to 20th	Day	16% 8% 3 6% 5% 4 4 3 5% 7% 39%	4082
	21st to end	Day	13% 6% 2 6% 4 4 5% 4 5% 8% 44%	4725
June	1st to 10th	Day	7% 5% 2 4 3 3 4 5% 8% 10% 49%	2660
	11th to 20th	Day	7% 5% 2 5% 5% 5% 4 4 6% 9% 48%	2660
	21st to end	Day	6% 4 1 5% 4 4 5% 4 7% 9% 51%	2660
July	1st to 10th	Day	12% 7% 2 7% 3 4 4 3 5% 8% 44%	4262
	11th to 20th	Day	5% 4 2 5% 4 4 4 4 6% 10% 53%	4120
	21st to end	Day	6% 4 2 5% 5% 5% 7% 5% 8% 10% 43%	4465
August	1st to 10th	Day	5% 4 2 5% 4 4 4 3 6% 10% 54%	4104
	11th to 20th	Day	11% 4 2 6% 4 4 5% 4 5% 8% 47%	4041
	21st to end	Day	5% 3 1 4 4 4 4 3 6% 9% 55%	4194
September	1st to 10th	Day	11% 5% 1 5% 3 3 3 3 5% 7% 54%	2280
	11th to 20th	Day	10% 4 2 4 3 3 4 4 5% 9% 52%	2171
	21st to end	Day	9% 4 2 5% 3 3 4 4 5% 6% 55%	2043
October	1st to 10th	Day	11% 5% 2 5% 3 3 3 2 5% 6% 55%	1900
	11th to 20th	Day	13% 6% 2 5% 2 2 3 3 3 6% 55%	1900
	21st to end	Day	17% 8% 2 7% 3 3 3 2 4 5% 46%	2090
November	1st to 10th	Day	10% 7% 2 5% 3 3 4 3 4 7% 52%	1958
	11th to 20th	Day	16% 7% 2 4 3 3 3 3 4 6% 50%	1710
	21st to end	Day	15% 9% 3 6% 4 3 3 3 4 5% 46%	1600
December	1st to 10th	Day	11% 8% 4 6% 5% 3 4 3 4 7% 46%	1584
	11th to 20th	Day	12% 10% 4 8% 4 4 3 3 5% 5% 41%	1918
	21st to end	Day	16% 16% 7% 11% 5% 4 4 3 4 5% 27%	2797
January	1st to 10th	Night	10% 10% 4 13% 5% 4 4 2 4 4 39%	4108
	11th to 20th	Night	12% 9% 4 9% 6% 7% 5% 4 4 5% 35%	4120
	21st to end	Night	18% 13% 5% 9% 4 4 3 2 4 5% 34%	4532
February	1st to 10th	Night	13% 10% 4 8% 5% 4 4 3 6% 6% 36%	2470
	11th to 20th	Night	12% 9% 3 8% 4 4 4 2 5% 7% 43%	2403
	21st to end	Night	13% 9% 3 5% 4 3 4 3 4 5% 45%	1855
March	1st to 10th	Night	14% 10% 3 6% 4 3 3 3 4 6% 45%	3419
	11th to 20th	Night	11% 7% 2 4 3 3 3 3 4 7% 54%	3480
	21st to end	Night	16% 10% 4 7% 4 5% 5% 3 5% 6% 35%	3817
April	1st to 10th	Night	11% 8% 2 4 3 3 3 3 5% 6% 51%	1948
	11th to 20th	Night	13% 8% 3 6% 3 3 3 4 4 6% 47%	1875
	21st to end	Night	17% 6% 3 6% 4 3 4 3 6% 7% 42%	1825
May	1st to 10th	Night	15% 7% 3 6% 5% 4 4 4 5% 7% 40%	2823
	11th to 20th	Night	14% 7% 3 6% 4 4 3 4 5% 9% 41%	2823
	21st to end	Night	11% 4 2 6% 4 3 4 3 5% 7% 52%	2837
June	1st to 10th	Night	7% 3 1 3 3 4 4 4 8% 9% 54%	1520
	11th to 20th	Night	4 4 2 5% 4 4 4 5% 8% 12% 50%	1520
	21st to end	Night	4 2 1 3 3 4 4 3 7% 9% 59%	1520
July	1st to 10th	Night	12% 6% 2 5% 5% 3 4 3 6% 8% 47%	2689
	11th to 20th	Night	3 3 1 4 3 3 4 4 6% 10% 60%	2850
	21st to end	Night	6% 4 2 5% 4 4 5% 4 7% 10% 49%	3098
August	1st to 10th	Night	2 1 1 3 3 3 4 4 6% 10% 63%	2836
	11th to 20th	Night	12% 6% 2 4 4 3 4 3 4 6% 52%	2798
	21st to end	Night	4 3 1 5% 4 3 4 3 5% 8% 59%	3462
September	1st to 10th	Night	9% 5% 1 5% 3 3 4 2 4 6% 57%	1900
	11th to 20th	Night	10% 4 2 4 3 3 3 3 6% 7% 55%	2005
	21st to end	Night	7% 4 2 5% 4 3 3 3 5% 6% 57%	2118
October	1st to 10th	Night	10% 4 1 4 3 3 3 3 5% 6% 58%	2280
	11th to 20th	Night	13% 5% 2 5% 3 2 3 2 4 5% 57%	2280
	21st to end	Night	16% 6% 2 4 4 3 3 3 3 4 52%	2508
November	1st to 10th	Night	11% 4 2 4 3 3 3 2 3 6% 59%	2340
	11th to 20th	Night	11% 6% 3 5% 4 3 4 3 5% 6% 52%	2685
	21st to end	Night	16% 8% 2 5% 4 3 3 2 4 5% 49%	2800
December	1st to 10th	Night	12% 8% 3 5% 4 4 4 3 5% 6% 46%	2779
	11th to 20th	Night	13% 8% 3 7% 4 3 4 3 5% 7% 44%	3279
	21st to end	Night	19% 17% 5% 10% 4 4 3 3 4 4 28%	4852

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	48% 5% 10% 25% 12% 1	2848
	11th to 20th	Day	52% 3 11% 23% 11%	2850
	21st to end	Day	54% 4 10% 18% 14% 1	3135
February	1st to 10th	Day	50% 3 12% 22% 12% 2	1710
	11th to 20th	Day	45% 3 13% 22% 15% 2	1673
	21st to end	Day	43% 3 11% 23% 16% 4	1614
March	1st to 10th	Day	43% 2 8% 24% 20% 2	3436
	11th to 20th	Day	34% 5% 12% 27% 18% 4	3480
	21st to end	Day	50% 6% 16% 21% 6% 1	3814
April	1st to 10th	Day	41% 4 12% 25% 16% 2	2230
	11th to 20th	Day	44% 4 13% 24% 15% 1	2254
	21st to end	Day	50% 5% 11% 24% 11% 1	2351
May	1st to 10th	Day	45% 7% 11% 23% 13% 1	4120
	11th to 20th	Day	44% 7% 13% 24% 13%	4086
	21st to end	Day	38% 6% 11% 30% 14% 1	4738
June	1st to 10th	Day	27% 8% 17% 32% 15% 2	2660
	11th to 20th	Day	32% 6% 13% 29% 18% 1	2660
	21st to end	Day	27% 6% 16% 31% 15% 4	2660
July	1st to 10th	Day	39% 5% 13% 24% 16% 3	4198
	11th to 20th	Day	27% 5% 12% 36% 18% 2	4051
	21st to end	Day	33% 6% 17% 31% 11% 1	4325
August	1st to 10th	Day	27% 4 16% 30% 22% 1	4090
	11th to 20th	Day	35% 5% 16% 27% 15% 1	4080
	21st to end	Day	26% 4 15% 36% 15% 4	4193
September	1st to 10th	Day	31% 4 12% 27% 20% 6%	2280
	11th to 20th	Day	29% 6% 12% 27% 25% 2	2171
	21st to end	Day	30% 4 10% 30% 23% 3	2043
October	1st to 10th	Day	18% 2 7% 22% 39% 12%	3160
	11th to 20th	Day	21% 2 4 24% 41% 8%	3140
	21st to end	Day	35% 3 7% 26% 26% 3	3428
November	1st to 10th	Day	33% 4 12% 27% 20% 4	1958
	11th to 20th	Day	37% 3 9% 25% 22% 3	1710
	21st to end	Day	43% 2 9% 28% 18% 1	1600
December	1st to 10th	Day	40% 3 12% 31% 13% 1	1584
	11th to 20th	Day	45% 3 13% 27% 9% 2	1915
	21st to end	Day	61% 4 10% 16% 8% 1	2797
January	1st to 10th	Night	48% 4 13% 23% 12% 1	4108
	11th to 20th	Night	52% 3 11% 21% 11% 1	4120
	21st to end	Night	53% 4 11% 20% 11% 1	4532
February	1st to 10th	Night	48% 4 11% 22% 14% 3	2470
	11th to 20th	Night	42% 4 11% 21% 18% 3	2404
	21st to end	Night	41% 3 10% 22% 19% 5%	1855
March	1st to 10th	Night	41% 4 9% 23% 19% 4	3419
	11th to 20th	Night	31% 5% 8% 27% 25% 4	3480
	21st to end	Night	48% 7% 14% 23% 7% 1	3817
April	1st to 10th	Night	34% 3 11% 26% 21% 4	1948
	11th to 20th	Night	38% 4 9% 28% 17% 4	1875
	21st to end	Night	41% 4 11% 27% 16% 1	1825
May	1st to 10th	Night	42% 7% 14% 21% 15% 1	2850
	11th to 20th	Night	40% 7% 12% 28% 12% 1	2824
	21st to end	Night	32% 6% 11% 27% 24% 1	2845
June	1st to 10th	Night	24% 7% 17% 32% 17% 4	1520
	11th to 20th	Night	25% 7% 14% 29% 22% 3	1520
	21st to end	Night	21% 5% 15% 34% 21% 5%	1520
July	1st to 10th	Night	36% 5% 11% 26% 18% 3	2645
	11th to 20th	Night	20% 4 13% 41% 20% 2	2796
	21st to end	Night	28% 4 22% 33% 12% 1	2999
August	1st to 10th	Night	16% 5% 22% 30% 24% 3	2826
	11th to 20th	Night	34% 4 10% 29% 21% 1	2825
	21st to end	Night	25% 3 12% 39% 17% 4	3462
September	1st to 10th	Night	29% 3 11% 26% 22% 8%	1900
	11th to 20th	Night	28% 4 10% 29% 25% 4	2005
	21st to end	Night	28% 4 8% 28% 29% 4	2118
October	1st to 10th	Night	17% 2 6% 22% 44% 9%	3800
	11th to 20th	Night	22% 2 5% 25% 31% 15%	3779
	21st to end	Night	34% 2 7% 29% 25% 3	4120
November	1st to 10th	Night	29% 3 12% 28% 22% 6%	2340
	11th to 20th	Night	34% 3 8% 26% 24% 5%	2685
	21st to end	Night	40% 3 8% 29% 18% 3	2800
December	1st to 10th	Night	39% 4 12% 28% 15% 1	2779
	11th to 20th	Night	41% 3 10% 31% 14% 1	3281
	21st to end	Night	59% 5% 8% 18% 9% 2	4852

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	22% 12% 35% 24% 7%	270
	11th to 20th	Day	30% 14% 40% 14% 2	270
	21st to end	Day	21% 12% 42% 21% 3	297
February	1st to 10th	Day	26% 13% 40% 16% 4	270
	11th to 20th	Day	21% 17% 37% 17% 6% 1	270
	21st to end	Day	20% 23% 40% 13% 3 1	225
March	1st to 10th	Day	20% 13% 46% 17% 4	270
	11th to 20th	Day	25% 12% 38% 20% 4 1	270
	21st to end	Day	36% 14% 29% 15% 5% 1	297
April	1st to 10th	Day	21% 15% 35% 21% 8%	410
	11th to 20th	Day	21% 14% 33% 23% 8% 1	452
	21st to end	Day	24% 13% 31% 23% 9% 1	531
May	1st to 10th	Day	18% 11% 36% 25% 9% 1	653
	11th to 20th	Day	20% 6% 27% 30% 14% 3	675
	21st to end	Day	18% 6% 25% 27% 18% 5%	734
June	1st to 10th	Day	8% 2 20% 36% 26% 9%	680
	11th to 20th	Day	9% 2 24% 35% 24% 5%	680
	21st to end	Day	8% 3 30% 35% 20% 4	680
July	1st to 10th	Day	14% 1 12% 27% 31% 15%	671
	11th to 20th	Day	6% 1 18% 36% 29% 11%	668
	21st to end	Day	9% 1 14% 34% 32% 10%	739
August	1st to 10th	Day	7% 1 15% 40% 29% 7%	676
	11th to 20th	Day	10% 1 16% 36% 29% 7%	665
	21st to end	Day	9% 1 12% 29% 37% 12%	553
September	1st to 10th	Day	13% 2 11% 36% 28% 10%	490
	11th to 20th	Day	7% 3 17% 36% 30% 7%	381
	21st to end	Day	10% 7% 25% 31% 21% 6%	300
October	1st to 10th	Day	13% 6% 27% 26% 19% 9%	300
	11th to 20th	Day	12% 13% 40% 26% 8% 1	300
	21st to end	Day	25% 13% 34% 23% 5%	330
November	1st to 10th	Day	18% 10% 35% 24% 8% 5%	300
	11th to 20th	Day	20% 15% 32% 22% 9% 2	300
	21st to end	Day	24% 12% 34% 23% 7%	300
December	1st to 10th	Day	11% 16% 38% 25% 7% 3	294
	11th to 20th	Day	23% 14% 33% 23% 7% 1	270
	21st to end	Day	23% 19% 40% 18% 1	297
January	1st to 10th	Night	15% 11% 38% 29% 7%	950
	11th to 20th	Night	17% 13% 39% 25% 5% 1	950
	21st to end	Night	16% 11% 40% 29% 4	1045
February	1st to 10th	Night	17% 11% 41% 26% 4	950
	11th to 20th	Night	18% 13% 38% 29% 3	929
	21st to end	Night	20% 13% 43% 21% 3	599
March	1st to 10th	Night	15% 11% 42% 27% 5%	570
	11th to 20th	Night	19% 9% 35% 30% 6% 1	570
	21st to end	Night	27% 9% 34% 26% 4	627
April	1st to 10th	Night	16% 10% 37% 30% 6%	570
	11th to 20th	Night	19% 9% 32% 30% 9% 1	563
	21st to end	Night	19% 6% 35% 31% 8%	570
May	1st to 10th	Night	17% 5% 31% 38% 8% 1	570
	11th to 20th	Night	16% 5% 25% 36% 18% 1	561
	21st to end	Night	13% 4 25% 34% 20% 3	617
June	1st to 10th	Night	9% 2 16% 42% 28% 2	570
	11th to 20th	Night	6% 1 22% 46% 23% 2	570
	21st to end	Night	5% 2 25% 49% 18% 2	570
July	1st to 10th	Night	10% 2 9% 41% 34% 4	555
	11th to 20th	Night	6% 1 13% 44% 34% 3	552
	21st to end	Night	9% 1 10% 40% 37% 2	624
August	1st to 10th	Night	5% 1 9% 47% 37% 2	567
	11th to 20th	Night	7% 2 13% 43% 32% 4	564
	21st to end	Night	7% 1 11% 39% 39% 4	627
September	1st to 10th	Night	12% 1 13% 40% 29% 4	570
	11th to 20th	Night	11% 2 15% 40% 25% 6%	570
	21st to end	Night	8% 4 23% 39% 22% 4	598
October	1st to 10th	Night	12% 3 23% 39% 21% 3	760
	11th to 20th	Night	15% 6% 37% 33% 9% 1	760
	21st to end	Night	20% 9% 33% 28% 10%	836
November	1st to 10th	Night	14% 7% 33% 35% 11%	780
	11th to 20th	Night	14% 11% 36% 29% 10%	940
	21st to end	Night	20% 9% 37% 29% 6%	1000
December	1st to 10th	Night	16% 10% 38% 30% 6% 1	993
	11th to 20th	Night	20% 7% 36% 32% 4	950
	21st to end	Night	20% 14% 36% 26% 4	1045

Page generated at 2024-12-06 02:54:51. Version ac07

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