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 2026-06-16.

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% 4 8% 5% 5% 5% 4 6% 7% 35%	138353
	Night	14% 10% 4 8% 5% 4 4 3 5% 7% 36%	133697
Transparency	Day	48% 4 11% 23% 14% 1	141994
	Night	48% 3 10% 23% 14% 2	138115
Seeing	Day	17% 10% 26% 23% 16% 7%	17407
	Night	18% 8% 27% 30% 15% 2	26903

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% 15% 8% 13% 5% 4 4 3 4 6% 22%	5258
	11th to 20th	Day	12% 14% 5% 17% 7% 6% 4 3 3 4 26%	5280
	21st to end	Day	19% 13% 7% 10% 7% 7% 6% 3 5% 5% 18%	5780
February	1st to 10th	Day	18% 14% 5% 11% 5% 4 5% 3 4 5% 27%	1964
	11th to 20th	Day	17% 12% 5% 11% 6% 5% 4 3 4 6% 28%	2048
	21st to end	Day	17% 13% 4 11% 5% 4 3 3 4 5% 30%	1730
March	1st to 10th	Day	17% 14% 7% 9% 4 4 4 3 3 4 30%	4669
	11th to 20th	Day	13% 8% 3 8% 4 4 4 4 5% 8% 39%	5090
	21st to end	Day	17% 10% 3 7% 5% 4 4 3 6% 7% 33%	5586
April	1st to 10th	Day	15% 10% 3 7% 5% 4 5% 4 5% 7% 34%	2441
	11th to 20th	Day	15% 8% 3 8% 5% 4 4 3 5% 6% 38%	2580
	21st to end	Day	17% 8% 2 7% 5% 5% 5% 3 5% 7% 36%	2730
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 5% 7% 31%	7616
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 3 6% 8% 35%	7626
	21st to end	Day	18% 8% 3 8% 5% 5% 5% 5% 7% 8% 29%	8391
June	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 8% 39%	2730
	11th to 20th	Day	10% 5% 2 7% 5% 5% 4 4 7% 10% 41%	2809
	21st to end	Day	9% 5% 2 7% 6% 5% 5% 4 7% 9% 40%	2800
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 4 7% 9% 42%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 5% 8% 11% 37%	6002
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 7% 9% 32%	6424
August	1st to 10th	Day	8% 6% 2 6% 4 4 5% 4 7% 10% 43%	4234
	11th to 20th	Day	8% 5% 2 7% 4 5% 5% 5% 7% 11% 40%	4174
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 8% 54%	4415
September	1st to 10th	Day	11% 5% 2 4 4 4 5% 3 5% 7% 49%	2255
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 7% 8% 50%	2200
	21st to end	Day	16% 7% 3 6% 3 4 3 2 5% 6% 46%	2200
October	1st to 10th	Day	10% 4 2 6% 4 5% 5% 3 5% 7% 49%	2200
	11th to 20th	Day	14% 7% 2 5% 3 4 4 4 4 6% 47%	2006
	21st to end	Day	20% 9% 3 6% 3 4 3 3 5% 7% 37%	2075
November	1st to 10th	Day	10% 7% 2 5% 4 4 4 3 5% 6% 50%	1852
	11th to 20th	Day	15% 11% 4 7% 5% 4 4 4 5% 6% 35%	1890
	21st to end	Day	18% 11% 5% 8% 5% 4 3 3 4 6% 32%	1890
December	1st to 10th	Day	18% 10% 4 9% 5% 4 4 3 4 5% 35%	3020
	11th to 20th	Day	19% 15% 5% 10% 5% 4 5% 3 4 5% 24%	3463
	21st to end	Day	20% 20% 6% 11% 5% 4 4 3 4 5% 18%	4565
January	1st to 10th	Night	13% 16% 6% 13% 8% 6% 5% 4 5% 5% 20%	7635
	11th to 20th	Night	12% 15% 7% 15% 7% 5% 4 3 4 5% 22%	7680
	21st to end	Night	18% 13% 4 9% 5% 5% 4 3 4 5% 30%	8401
February	1st to 10th	Night	19% 13% 4 9% 4 4 5% 3 4 6% 29%	2652
	11th to 20th	Night	18% 11% 4 9% 5% 4 5% 3 5% 6% 30%	2458
	21st to end	Night	20% 13% 6% 8% 3 4 4 3 4 5% 31%	2076
March	1st to 10th	Night	18% 13% 3 6% 4 4 3 2 4 5% 36%	5404
	11th to 20th	Night	11% 7% 3 7% 4 4 5% 4 5% 8% 41%	5090
	21st to end	Night	18% 9% 4 8% 5% 5% 4 3 5% 5% 34%	5588
April	1st to 10th	Night	16% 8% 2 6% 5% 4 4 4 6% 6% 38%	2195
	11th to 20th	Night	14% 8% 2 6% 4 3 5% 4 6% 7% 41%	1988
	21st to end	Night	15% 8% 3 6% 4 4 4 4 6% 8% 38%	1890
May	1st to 10th	Night	20% 9% 3 8% 5% 4 5% 4 6% 7% 30%	5237
	11th to 20th	Night	14% 7% 2 7% 4 5% 4 4 6% 8% 37%	5234
	21st to end	Night	16% 8% 3 7% 4 4 5% 4 7% 8% 34%	5761
June	1st to 10th	Night	9% 5% 2 6% 5% 4 4 3 5% 9% 49%	1890
	11th to 20th	Night	8% 4 2 5% 4 5% 5% 4 7% 10% 46%	1694
	21st to end	Night	6% 4 1 4 5% 5% 6% 5% 8% 9% 46%	1600
July	1st to 10th	Night	9% 6% 2 6% 4 4 4 3 6% 8% 48%	3618
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 5% 7% 9% 43%	3942
	21st to end	Night	10% 5% 3 7% 5% 5% 5% 4 8% 9% 40%	4427
August	1st to 10th	Night	7% 6% 3 7% 5% 4 5% 5% 6% 9% 43%	2915
	11th to 20th	Night	8% 8% 3 7% 5% 3 4 4 6% 8% 45%	2875
	21st to end	Night	4 3 1 5% 3 4 4 4 6% 10% 57%	3479
September	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 7% 52%	2124
	11th to 20th	Night	8% 4 1 6% 4 3 4 3 5% 8% 55%	2200
	21st to end	Night	15% 5% 2 5% 4 3 3 3 5% 6% 49%	2200
October	1st to 10th	Night	8% 5% 2 5% 4 4 4 3 5% 6% 54%	2200
	11th to 20th	Night	13% 7% 3 6% 4 4 4 3 5% 6% 45%	2393
	21st to end	Night	19% 7% 3 6% 4 3 4 4 4 7% 40%	2760
November	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 46%	2666
	11th to 20th	Night	15% 10% 3 7% 4 4 4 3 5% 7% 39%	2730
	21st to end	Night	19% 13% 4 8% 5% 4 4 3 4 7% 30%	2730
December	1st to 10th	Night	16% 11% 4 9% 4 4 4 3 4 5% 36%	4380
	11th to 20th	Night	19% 15% 6% 10% 5% 4 4 3 4 6% 25%	4952
	21st to end	Night	21% 19% 7% 11% 6% 5% 4 3 4 5% 17%	6633

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	65% 2 12% 16% 5%	5259
	11th to 20th	Day	64% 2 8% 14% 12% 1	5280
	21st to end	Day	68% 2 9% 14% 6% 1	5780
February	1st to 10th	Day	60% 2 9% 18% 9% 1	1964
	11th to 20th	Day	59% 3 9% 19% 10% 1	2058
	21st to end	Day	57% 2 9% 22% 8% 1	1730
March	1st to 10th	Day	59% 3 6% 20% 11% 2	4669
	11th to 20th	Day	44% 3 9% 28% 15% 1	5090
	21st to end	Day	51% 3 11% 22% 11% 2	5586
April	1st to 10th	Day	49% 4 11% 23% 12% 1	2441
	11th to 20th	Day	47% 3 11% 23% 14% 3	2580
	21st to end	Day	48% 3 11% 24% 12% 2	2730
May	1st to 10th	Day	52% 4 11% 21% 11% 1	7651
	11th to 20th	Day	47% 4 12% 25% 12%	7630
	21st to end	Day	51% 4 10% 20% 12% 3	8405
June	1st to 10th	Day	40% 5% 11% 25% 17% 2	2730
	11th to 20th	Day	37% 5% 15% 25% 17% 1	2809
	21st to end	Day	39% 6% 14% 23% 15% 3	2800
July	1st to 10th	Day	38% 5% 11% 26% 18% 1	6291
	11th to 20th	Day	39% 6% 13% 27% 13% 1	5930
	21st to end	Day	45% 6% 13% 20% 13% 2	6240
August	1st to 10th	Day	34% 6% 16% 32% 11% 1	4221
	11th to 20th	Day	35% 6% 11% 32% 15% 1	4213
	21st to end	Day	27% 4 14% 31% 20% 3	4414
September	1st to 10th	Day	34% 3 10% 27% 20% 5%	2255
	11th to 20th	Day	31% 4 11% 27% 23% 3	2200
	21st to end	Day	41% 3 9% 23% 22% 2	2200
October	1st to 10th	Day	24% 3 9% 31% 32% 2	3590
	11th to 20th	Day	31% 2 8% 32% 25% 2	3246
	21st to end	Day	37% 2 10% 24% 24% 2	3325
November	1st to 10th	Day	36% 3 9% 30% 20% 3	1852
	11th to 20th	Day	50% 3 8% 21% 15% 3	1890
	21st to end	Day	54% 2 8% 23% 12% 1	1890
December	1st to 10th	Day	53% 3 10% 26% 9%	3020
	11th to 20th	Day	64% 3 7% 19% 6%	3460
	21st to end	Day	70% 2 9% 12% 5%	4565
January	1st to 10th	Night	67% 2 9% 14% 7%	7636
	11th to 20th	Night	66% 2 7% 13% 10% 2	7680
	21st to end	Night	58% 2 9% 21% 9% 1	8401
February	1st to 10th	Night	58% 3 9% 20% 9% 1	2652
	11th to 20th	Night	55% 3 9% 21% 11% 1	2459
	21st to end	Night	57% 3 10% 18% 10% 2	2076
March	1st to 10th	Night	52% 2 5% 28% 11% 2	5404
	11th to 20th	Night	41% 3 10% 30% 14% 1	5090
	21st to end	Night	53% 3 7% 25% 11% 2	5588
April	1st to 10th	Night	45% 4 11% 25% 12% 2	2195
	11th to 20th	Night	41% 3 13% 21% 18% 5%	1988
	21st to end	Night	44% 3 12% 24% 14% 3	1890
May	1st to 10th	Night	53% 4 9% 22% 8% 3	5264
	11th to 20th	Night	44% 5% 12% 25% 14% 1	5235
	21st to end	Night	46% 4 13% 22% 10% 5%	5771
June	1st to 10th	Night	33% 5% 12% 27% 22% 2	1890
	11th to 20th	Night	32% 6% 16% 23% 19% 4	1694
	21st to end	Night	30% 8% 17% 24% 17% 5%	1600
July	1st to 10th	Night	35% 6% 11% 24% 20% 4	3580
	11th to 20th	Night	35% 7% 14% 29% 14% 1	3894
	21st to end	Night	39% 5% 13% 25% 15% 3	4297
August	1st to 10th	Night	36% 7% 13% 31% 12% 1	2904
	11th to 20th	Night	36% 6% 10% 28% 19% 1	2902
	21st to end	Night	23% 5% 15% 33% 22% 3	3479
September	1st to 10th	Night	32% 3 13% 24% 21% 6%	2124
	11th to 20th	Night	29% 3 8% 27% 29% 3	2200
	21st to end	Night	38% 3 8% 22% 26% 3	2200
October	1st to 10th	Night	24% 3 6% 28% 37% 3	3590
	11th to 20th	Night	29% 2 8% 27% 30% 3	3892
	21st to end	Night	34% 3 8% 24% 27% 5%	4447
November	1st to 10th	Night	39% 2 10% 23% 21% 3	2666
	11th to 20th	Night	45% 3 8% 24% 17% 3	2730
	21st to end	Night	56% 2 11% 20% 10% 1	2730
December	1st to 10th	Night	52% 2 11% 25% 9%	4380
	11th to 20th	Night	62% 2 10% 18% 7%	4954
	21st to end	Night	71% 3 8% 15% 4	6633

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% 16% 5%	330
	11th to 20th	Day	26% 22% 36% 13% 3	330
	21st to end	Day	29% 18% 34% 15% 3	363
February	1st to 10th	Day	26% 16% 37% 17% 3 1	330
	11th to 20th	Day	28% 21% 34% 14% 3	330
	21st to end	Day	23% 19% 46% 11% 1	273
March	1st to 10th	Day	22% 23% 37% 16% 3	367
	11th to 20th	Day	26% 14% 36% 18% 6% 1	540
	21st to end	Day	27% 16% 33% 19% 5%	594
April	1st to 10th	Day	21% 16% 32% 21% 9% 1	540
	11th to 20th	Day	21% 17% 29% 21% 10% 3	532
	21st to end	Day	21% 12% 34% 21% 10% 2	540
May	1st to 10th	Day	21% 15% 31% 21% 8% 4	543
	11th to 20th	Day	21% 6% 28% 22% 16% 6%	567
	21st to end	Day	14% 6% 25% 27% 18% 10%	617
June	1st to 10th	Day	12% 5% 22% 28% 25% 8%	570
	11th to 20th	Day	10% 5% 21% 31% 25% 8%	694
	21st to end	Day	10% 4 23% 31% 23% 10%	730
July	1st to 10th	Day	12% 4 15% 25% 24% 20%	721
	11th to 20th	Day	10% 4 19% 25% 27% 15%	571
	21st to end	Day	10% 3 14% 28% 28% 17%	576
August	1st to 10th	Day	10% 4 11% 28% 28% 20%	526
	11th to 20th	Day	11% 3 17% 28% 27% 14%	519
	21st to end	Day	9% 3 12% 28% 32% 17%	583
September	1st to 10th	Day	11% 3 14% 26% 32% 14%	530
	11th to 20th	Day	10% 2 14% 22% 37% 16%	530
	21st to end	Day	18% 7% 18% 25% 22% 10%	530
October	1st to 10th	Day	9% 5% 22% 33% 22% 10%	530
	11th to 20th	Day	14% 11% 30% 26% 14% 5%	530
	21st to end	Day	23% 12% 34% 23% 7%	458
November	1st to 10th	Day	14% 12% 35% 26% 10% 4	330
	11th to 20th	Day	22% 18% 30% 19% 9% 2	330
	21st to end	Day	24% 14% 38% 20% 4	330
December	1st to 10th	Day	22% 20% 37% 15% 5% 1	330
	11th to 20th	Day	23% 16% 38% 18% 4	330
	21st to end	Day	30% 17% 33% 17% 2	363
January	1st to 10th	Night	21% 17% 35% 22% 5%	840
	11th to 20th	Night	25% 16% 35% 21% 3	840
	21st to end	Night	22% 12% 40% 24% 3	924
February	1st to 10th	Night	27% 10% 34% 26% 3	840
	11th to 20th	Night	24% 12% 38% 23% 2	823
	21st to end	Night	26% 12% 37% 23% 2	692
March	1st to 10th	Night	22% 12% 36% 25% 6%	840
	11th to 20th	Night	20% 13% 34% 26% 6%	840
	21st to end	Night	24% 10% 34% 26% 7%	924
April	1st to 10th	Night	19% 12% 34% 29% 5% 1	725
	11th to 20th	Night	16% 10% 35% 27% 10% 1	623
	21st to end	Night	17% 8% 36% 29% 9% 1	630
May	1st to 10th	Night	21% 7% 31% 28% 11% 1	630
	11th to 20th	Night	18% 4 24% 35% 16% 3	621
	21st to end	Night	11% 3 21% 37% 24% 5%	683
June	1st to 10th	Night	10% 2 19% 43% 25% 1	630
	11th to 20th	Night	10% 4 14% 42% 27% 2	615
	21st to end	Night	6% 3 19% 42% 28% 3	600
July	1st to 10th	Night	10% 2 12% 34% 33% 9%	585
	11th to 20th	Night	8% 3 11% 38% 35% 6%	582
	21st to end	Night	11% 2 9% 41% 34% 4	657
August	1st to 10th	Night	8% 3 10% 39% 35% 6%	597
	11th to 20th	Night	10% 1 10% 43% 30% 5%	594
	21st to end	Night	7% 1 11% 36% 38% 6%	660
September	1st to 10th	Night	13% 2 13% 32% 35% 5%	724
	11th to 20th	Night	9% 3 12% 36% 35% 6%	800
	21st to end	Night	17% 5% 20% 34% 24% 2	800
October	1st to 10th	Night	12% 4 21% 36% 24% 4	800
	11th to 20th	Night	17% 8% 25% 35% 14% 1	800
	21st to end	Night	22% 7% 33% 27% 11%	880
November	1st to 10th	Night	18% 7% 30% 30% 15% 1	820
	11th to 20th	Night	19% 13% 32% 26% 9% 1	840
	21st to end	Night	25% 11% 27% 29% 8%	840
December	1st to 10th	Night	25% 11% 34% 25% 5%	840
	11th to 20th	Night	25% 9% 36% 24% 5% 1	840
	21st to end	Night	29% 13% 31% 25% 2	924

Page generated at 2026-06-13 02:31:40. Version ac07

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