Rachel A. Hutzel Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rachel A. Hutzel Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	14% 9% 3 8% 5% 4 4 4 5% 7% 35%	109776
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 36%	108838
Transparency	Day	47% 3 11% 23% 14% 2	113461
	Night	47% 3 10% 23% 15% 2	113279
Seeing	Day	17% 10% 26% 24% 16% 7%	15605
	Night	18% 8% 26% 31% 15% 2	25208

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	15% 17% 9% 13% 5% 3 3 3 4 5% 22%	4056
	11th to 20th	Day	14% 16% 5% 15% 6% 6% 4 3 3 4 24%	4060
	21st to end	Day	21% 13% 7% 10% 5% 5% 5% 3 4 5% 21%	4466
February	1st to 10th	Day	18% 15% 5% 11% 5% 4 5% 3 4 5% 26%	1870
	11th to 20th	Day	17% 11% 5% 11% 6% 5% 4 3 4 6% 27%	1948
	21st to end	Day	19% 14% 4 10% 5% 4 3 3 4 5% 29%	1570
March	1st to 10th	Day	22% 16% 5% 9% 4 4 4 3 3 4 25%	3179
	11th to 20th	Day	15% 8% 3 8% 4 4 4 4 5% 7% 39%	3480
	21st to end	Day	18% 9% 3 6% 5% 4 4 3 6% 7% 34%	3815
April	1st to 10th	Day	15% 10% 3 8% 5% 4 5% 4 6% 7% 34%	2209
	11th to 20th	Day	16% 8% 3 7% 5% 4 4 3 5% 6% 39%	2332
	21st to end	Day	18% 8% 3 7% 5% 5% 4 3 5% 7% 35%	2470
May	1st to 10th	Day	22% 9% 3 7% 5% 4 4 4 4 6% 31%	4082
	11th to 20th	Day	20% 8% 2 6% 5% 4 4 3 6% 7% 34%	4081
	21st to end	Day	11% 6% 3 7% 6% 6% 6% 5% 8% 9% 35%	4476
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 5% 8% 9% 38%	2470
	11th to 20th	Day	10% 5% 2 7% 5% 4 4 4 7% 10% 42%	2604
	21st to end	Day	10% 5% 2 7% 6% 5% 5% 4 7% 9% 40%	2660
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 4 7% 9% 38%	4440
	11th to 20th	Day	7% 5% 2 6% 4 4 4 5% 7% 11% 45%	4194
	21st to end	Day	11% 7% 3 7% 6% 6% 6% 5% 7% 9% 32%	4466
August	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 7% 10% 43%	4104
	11th to 20th	Day	8% 5% 2 7% 5% 5% 5% 5% 7% 11% 40%	4044
	21st to end	Day	6% 4 2 5% 3 4 4 4 6% 8% 54%	4280
September	1st to 10th	Day	12% 5% 2 4 4 4 5% 3 6% 7% 48%	2142
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 7% 8% 49%	2090
	21st to end	Day	16% 7% 3 6% 3 4 3 2 4 6% 46%	2090
October	1st to 10th	Day	10% 4 2 6% 4 5% 4 3 5% 7% 48%	2090
	11th to 20th	Day	14% 7% 2 6% 3 4 4 4 4 6% 47%	1906
	21st to end	Day	20% 9% 3 6% 3 4 3 3 5% 7% 37%	1971
November	1st to 10th	Day	10% 6% 2 5% 4 4 4 3 5% 6% 51%	1762
	11th to 20th	Day	15% 11% 4 7% 5% 4 4 4 5% 7% 35%	1800
	21st to end	Day	18% 11% 4 8% 5% 4 3 3 4 7% 33%	1800
December	1st to 10th	Day	18% 10% 4 8% 5% 4 3 3 4 5% 36%	2930
	11th to 20th	Day	19% 15% 5% 10% 5% 4 4 3 4 5% 24%	3373
	21st to end	Day	20% 20% 6% 11% 5% 4 4 3 4 5% 18%	4466
January	1st to 10th	Night	15% 17% 7% 14% 8% 5% 4 3 4 4 18%	5889
	11th to 20th	Night	14% 16% 7% 13% 8% 5% 5% 3 4 5% 20%	5900
	21st to end	Night	21% 13% 5% 8% 5% 4 4 3 4 4 29%	6490
February	1st to 10th	Night	20% 13% 4 9% 5% 4 5% 3 4 5% 28%	2526
	11th to 20th	Night	18% 11% 4 9% 6% 4 5% 3 5% 6% 29%	2338
	21st to end	Night	21% 13% 6% 8% 3 4 4 3 4 5% 30%	1885
March	1st to 10th	Night	21% 12% 3 6% 4 4 3 3 5% 6% 33%	3674
	11th to 20th	Night	12% 7% 3 6% 4 4 4 3 6% 7% 42%	3480
	21st to end	Night	18% 8% 3 6% 5% 4 4 3 4 5% 39%	3817
April	1st to 10th	Night	15% 8% 3 7% 5% 4 4 4 6% 6% 39%	1986
	11th to 20th	Night	14% 8% 2 6% 4 3 4 4 5% 7% 41%	1796
	21st to end	Night	16% 8% 3 6% 4 4 4 4 6% 7% 37%	1710
May	1st to 10th	Night	23% 11% 3 9% 5% 4 4 4 5% 5% 27%	2813
	11th to 20th	Night	18% 7% 2 6% 4 4 4 3 6% 8% 38%	2812
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 7% 9% 46%	3079
June	1st to 10th	Night	8% 5% 2 6% 5% 4 5% 3 5% 9% 49%	1710
	11th to 20th	Night	8% 4 2 5% 4 5% 5% 5% 7% 10% 47%	1569
	21st to end	Night	6% 5% 1 4 5% 5% 6% 5% 8% 9% 46%	1520
July	1st to 10th	Night	10% 6% 2 5% 3 4 4 3 5% 7% 50%	2530
	11th to 20th	Night	7% 4 2 5% 3 4 4 4 7% 9% 51%	2764
	21st to end	Night	11% 5% 3 7% 6% 5% 5% 4 7% 9% 38%	3087
August	1st to 10th	Night	7% 7% 3 7% 5% 4 5% 5% 6% 9% 43%	2825
	11th to 20th	Night	8% 8% 3 7% 5% 3 4 4 5% 8% 45%	2785
	21st to end	Night	4 3 1 5% 3 4 4 4 6% 10% 57%	3372
September	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 7% 51%	2018
	11th to 20th	Night	8% 4 1 6% 4 3 4 3 5% 8% 55%	2090
	21st to end	Night	14% 5% 2 6% 4 3 3 2 5% 6% 49%	2090
October	1st to 10th	Night	8% 5% 2 6% 4 4 4 3 5% 6% 53%	2090
	11th to 20th	Night	13% 7% 3 6% 4 4 4 3 5% 6% 44%	2273
	21st to end	Night	18% 7% 3 6% 4 3 4 4 5% 6% 40%	2622
November	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 46%	2536
	11th to 20th	Night	16% 10% 3 7% 5% 4 4 3 5% 6% 39%	2600
	21st to end	Night	18% 13% 4 8% 5% 4 4 3 4 7% 30%	2600
December	1st to 10th	Night	16% 11% 4 9% 4 4 4 3 4 5% 36%	4250
	11th to 20th	Night	19% 15% 6% 10% 5% 4 4 3 4 6% 25%	4822
	21st to end	Night	21% 19% 7% 11% 6% 5% 4 3 4 5% 16%	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	66% 2 8% 17% 7%	4057
	11th to 20th	Day	66% 2 8% 13% 10% 1	4060
	21st to end	Day	67% 2 9% 15% 7% 2	4466
February	1st to 10th	Day	61% 2 9% 16% 10% 1	1870
	11th to 20th	Day	59% 3 9% 18% 10% 1	1958
	21st to end	Day	59% 2 8% 21% 8% 2	1570
March	1st to 10th	Day	63% 3 7% 19% 8% 1	3179
	11th to 20th	Day	45% 3 9% 27% 16% 1	3480
	21st to end	Day	50% 3 11% 25% 11% 1	3815
April	1st to 10th	Day	49% 4 11% 23% 12% 1	2209
	11th to 20th	Day	47% 3 10% 23% 14% 3	2332
	21st to end	Day	49% 3 11% 23% 12% 2	2470
May	1st to 10th	Day	54% 3 11% 23% 8% 1	4120
	11th to 20th	Day	49% 3 14% 23% 11% 1	4085
	21st to end	Day	43% 5% 15% 21% 15% 2	4489
June	1st to 10th	Day	40% 5% 11% 26% 16% 2	2470
	11th to 20th	Day	36% 5% 15% 25% 17% 1	2604
	21st to end	Day	39% 6% 15% 23% 15% 3	2660
July	1st to 10th	Day	42% 5% 12% 22% 18% 2	4371
	11th to 20th	Day	32% 5% 11% 33% 18% 1	4122
	21st to end	Day	45% 6% 13% 19% 13% 3	4324
August	1st to 10th	Day	34% 6% 16% 32% 11% 1	4091
	11th to 20th	Day	36% 6% 10% 32% 16% 1	4083
	21st to end	Day	27% 4 15% 30% 20% 3	4279
September	1st to 10th	Day	35% 3 10% 25% 20% 5%	2142
	11th to 20th	Day	32% 4 12% 27% 22% 3	2090
	21st to end	Day	41% 3 9% 24% 22% 2	2090
October	1st to 10th	Day	24% 3 9% 31% 31% 2	3480
	11th to 20th	Day	31% 2 8% 32% 25% 2	3146
	21st to end	Day	37% 2 11% 24% 24% 2	3221
November	1st to 10th	Day	35% 3 9% 30% 20% 3	1762
	11th to 20th	Day	50% 3 8% 21% 15% 3	1800
	21st to end	Day	53% 3 8% 23% 12% 1	1800
December	1st to 10th	Day	52% 3 10% 26% 9%	2930
	11th to 20th	Day	64% 3 7% 19% 7%	3370
	21st to end	Day	70% 2 10% 12% 5%	4466
January	1st to 10th	Night	70% 2 7% 14% 8%	5889
	11th to 20th	Night	68% 2 8% 12% 8% 2	5900
	21st to end	Night	60% 2 7% 20% 9% 1	6490
February	1st to 10th	Night	59% 3 9% 19% 8% 1	2526
	11th to 20th	Night	55% 3 8% 20% 12% 1	2339
	21st to end	Night	58% 3 10% 17% 10% 2	1885
March	1st to 10th	Night	54% 3 6% 25% 11% 1	3674
	11th to 20th	Night	41% 3 11% 29% 14% 2	3480
	21st to end	Night	48% 3 8% 29% 11% 1	3817
April	1st to 10th	Night	44% 4 11% 25% 13% 2	1986
	11th to 20th	Night	41% 3 11% 20% 19% 5%	1796
	21st to end	Night	45% 3 12% 23% 14% 3	1710
May	1st to 10th	Night	59% 3 8% 20% 8% 2	2840
	11th to 20th	Night	44% 5% 13% 28% 9% 1	2813
	21st to end	Night	33% 5% 14% 30% 13% 5%	3087
June	1st to 10th	Night	32% 5% 12% 28% 21% 3	1710
	11th to 20th	Night	30% 6% 17% 23% 20% 4	1569
	21st to end	Night	30% 8% 16% 24% 17% 5%	1520
July	1st to 10th	Night	34% 5% 11% 25% 19% 6%	2490
	11th to 20th	Night	29% 5% 12% 33% 19% 2	2713
	21st to end	Night	41% 4 12% 23% 15% 4	2988
August	1st to 10th	Night	36% 7% 13% 31% 12% 1	2814
	11th to 20th	Night	37% 5% 10% 28% 19% 1	2812
	21st to end	Night	23% 5% 15% 32% 22% 3	3372
September	1st to 10th	Night	33% 3 13% 24% 20% 7%	2018
	11th to 20th	Night	29% 3 9% 28% 28% 3	2090
	21st to end	Night	38% 3 8% 23% 26% 3	2090
October	1st to 10th	Night	24% 3 6% 28% 36% 3	3480
	11th to 20th	Night	29% 2 9% 27% 30% 3	3772
	21st to end	Night	33% 3 8% 24% 27% 5%	4309
November	1st to 10th	Night	40% 2 10% 23% 21% 4	2536
	11th to 20th	Night	46% 3 8% 23% 17% 3	2600
	21st to end	Night	55% 2 11% 20% 11% 1	2600
December	1st to 10th	Night	51% 2 11% 25% 9%	4250
	11th to 20th	Night	62% 2 10% 18% 7%	4824
	21st to end	Night	71% 3 8% 15% 4	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% 19% 36% 17% 4	300
	11th to 20th	Day	26% 20% 37% 14% 3	300
	21st to end	Day	30% 17% 35% 15% 3	330
February	1st to 10th	Day	27% 17% 36% 16% 3	300
	11th to 20th	Day	29% 22% 33% 13% 3	300
	21st to end	Day	27% 18% 45% 9% 1	225
March	1st to 10th	Day	21% 23% 36% 17% 3	303
	11th to 20th	Day	25% 13% 35% 19% 7% 1	460
	21st to end	Day	29% 15% 32% 19% 5%	506
April	1st to 10th	Day	20% 16% 31% 23% 10% 1	460
	11th to 20th	Day	23% 15% 26% 22% 11% 3	452
	21st to end	Day	22% 11% 36% 18% 9% 3	460
May	1st to 10th	Day	22% 13% 30% 22% 8% 5%	463
	11th to 20th	Day	21% 6% 27% 24% 16% 7%	487
	21st to end	Day	12% 5% 24% 28% 20% 12%	529
June	1st to 10th	Day	12% 4 21% 28% 25% 9%	490
	11th to 20th	Day	10% 4 20% 32% 26% 8%	624
	21st to end	Day	10% 4 24% 31% 21% 10%	680
July	1st to 10th	Day	13% 4 16% 25% 23% 20%	671
	11th to 20th	Day	9% 3 19% 26% 26% 16%	529
	21st to end	Day	11% 3 14% 28% 28% 16%	532
August	1st to 10th	Day	10% 4 10% 27% 28% 20%	486
	11th to 20th	Day	11% 4 17% 29% 25% 13%	479
	21st to end	Day	9% 3 11% 28% 32% 17%	539
September	1st to 10th	Day	12% 2 14% 27% 32% 14%	490
	11th to 20th	Day	10% 2 14% 23% 35% 15%	490
	21st to end	Day	19% 8% 18% 24% 22% 9%	490
October	1st to 10th	Day	9% 5% 23% 34% 21% 8%	490
	11th to 20th	Day	14% 12% 31% 24% 14% 5%	490
	21st to end	Day	22% 11% 34% 24% 8%	420
November	1st to 10th	Day	13% 10% 35% 27% 10% 4	300
	11th to 20th	Day	22% 17% 29% 20% 10% 2	300
	21st to end	Day	23% 13% 39% 21% 5%	300
December	1st to 10th	Day	22% 19% 35% 16% 6% 1	300
	11th to 20th	Day	24% 16% 37% 19% 5%	300
	21st to end	Day	29% 16% 33% 18% 3	330
January	1st to 10th	Night	22% 17% 34% 22% 5% 1	800
	11th to 20th	Night	26% 15% 35% 21% 3	800
	21st to end	Night	23% 11% 39% 25% 3	880
February	1st to 10th	Night	28% 10% 34% 25% 3	800
	11th to 20th	Night	25% 12% 38% 23% 2	783
	21st to end	Night	27% 11% 37% 23% 2	629
March	1st to 10th	Night	21% 12% 36% 26% 6%	760
	11th to 20th	Night	20% 12% 34% 27% 6% 1	760
	21st to end	Night	24% 8% 33% 26% 8%	836
April	1st to 10th	Night	18% 12% 34% 30% 6% 1	656
	11th to 20th	Night	17% 9% 33% 28% 11% 2	563
	21st to end	Night	18% 8% 36% 30% 8% 1	570
May	1st to 10th	Night	21% 7% 30% 29% 11% 1	570
	11th to 20th	Night	18% 3 22% 36% 17% 3	561
	21st to end	Night	9% 2 20% 38% 26% 5%	617
June	1st to 10th	Night	9% 2 18% 44% 25% 2	570
	11th to 20th	Night	9% 4 14% 43% 28% 3	570
	21st to end	Night	6% 2 19% 43% 27% 2	570
July	1st to 10th	Night	11% 2 12% 33% 33% 9%	555
	11th to 20th	Night	7% 3 11% 39% 35% 6%	552
	21st to end	Night	11% 2 9% 42% 33% 4	624
August	1st to 10th	Night	8% 2 9% 40% 35% 6%	567
	11th to 20th	Night	10% 1 10% 44% 30% 5%	564
	21st to end	Night	7% 1 11% 35% 39% 7%	627
September	1st to 10th	Night	14% 2 13% 32% 35% 5%	688
	11th to 20th	Night	9% 3 12% 36% 35% 6%	760
	21st to end	Night	16% 4 20% 34% 23% 2	760
October	1st to 10th	Night	12% 4 22% 36% 24% 3	760
	11th to 20th	Night	18% 8% 26% 33% 14% 1	760
	21st to end	Night	22% 6% 33% 28% 12%	836
November	1st to 10th	Night	19% 6% 29% 29% 15% 1	780
	11th to 20th	Night	19% 12% 32% 26% 10% 1	800
	21st to end	Night	24% 11% 28% 29% 8%	800
December	1st to 10th	Night	26% 10% 34% 25% 5%	800
	11th to 20th	Night	25% 9% 35% 25% 6% 1	800
	21st to end	Night	29% 13% 30% 25% 3	880

Page generated at 2025-02-18 03:26:29. Version ac07

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