Joseph H. Rogers Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Joseph H. Rogers Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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% 11% 4 10% 6% 5% 4 3 5% 6% 32%	143216
	Night	13% 13% 5% 12% 6% 5% 4 3 5% 6% 28%	139296
Transparency	Day	52% 3 10% 22% 12% 1	146715
	Night	57% 3 11% 19% 9% 1	143874
Seeing	Day	18% 13% 29% 24% 12% 4	18258
	Night	20% 10% 29% 29% 11% 1	28272

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% 22% 11% 28% 8% 4 3 2 3 3 6%	4691
	11th to 20th	Day	9% 18% 9% 25% 13% 11% 5% 2 2 2 4	4710
	21st to end	Day	10% 14% 8% 17% 10% 10% 7% 3 4 5% 12%	5156
February	1st to 10th	Day	9% 18% 6% 16% 16% 7% 5% 3 3 4 13%	1801
	11th to 20th	Day	11% 15% 7% 17% 11% 8% 5% 3 4 3 15%	2048
	21st to end	Day	10% 19% 8% 13% 10% 5% 3 4 5% 4 18%	1730
March	1st to 10th	Day	11% 11% 4 12% 8% 4 4 4 5% 5% 31%	4580
	11th to 20th	Day	11% 12% 6% 13% 5% 4 5% 4 6% 8% 27%	4941
	21st to end	Day	21% 12% 8% 16% 5% 4 4 3 4 4 21%	5813
April	1st to 10th	Day	17% 16% 4 8% 5% 5% 5% 3 4 6% 28%	2520
	11th to 20th	Day	19% 13% 3 7% 4 3 3 3 4 6% 33%	2591
	21st to end	Day	15% 12% 4 8% 5% 5% 4 4 6% 6% 32%	2730
May	1st to 10th	Day	13% 9% 3 6% 4 4 4 3 5% 8% 41%	7616
	11th to 20th	Day	13% 9% 3 7% 4 4 4 3 5% 5% 42%	7918
	21st to end	Day	14% 7% 2 6% 4 4 3 3 4 6% 47%	9047
June	1st to 10th	Day	14% 6% 2 5% 4 5% 4 4 5% 7% 44%	2940
	11th to 20th	Day	12% 6% 2 7% 5% 4 5% 4 6% 8% 41%	2940
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 6% 9% 40%	2940
July	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 6% 9% 42%	8280
	11th to 20th	Day	10% 4 2 4 3 4 4 4 6% 9% 50%	8280
	21st to end	Day	11% 6% 2 5% 4 4 4 3 5% 9% 47%	8990
August	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 41%	4516
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 5% 6% 47%	4304
	21st to end	Day	12% 7% 3 7% 4 5% 5% 3 5% 6% 44%	4565
September	1st to 10th	Day	15% 4 2 6% 4 5% 5% 4 6% 8% 41%	2380
	11th to 20th	Day	13% 5% 2 6% 4 4 3 4 6% 9% 45%	2200
	21st to end	Day	16% 6% 3 6% 4 5% 5% 4 6% 8% 38%	2200
October	1st to 10th	Day	18% 8% 3 7% 4 4 4 3 5% 7% 37%	2200
	11th to 20th	Day	21% 14% 4 10% 5% 5% 5% 4 5% 5% 23%	1895
	21st to end	Day	22% 16% 3 10% 6% 6% 4 4 5% 5% 19%	1980
November	1st to 10th	Day	15% 19% 5% 7% 5% 4 4 3 5% 6% 27%	1852
	11th to 20th	Day	12% 22% 7% 11% 6% 6% 4 3 6% 6% 18%	1890
	21st to end	Day	15% 24% 9% 13% 5% 4 3 2 4 5% 15%	1812
December	1st to 10th	Day	11% 29% 12% 18% 7% 5% 4 3 3 2 6%	2350
	11th to 20th	Day	10% 22% 10% 19% 10% 5% 5% 3 4 4 8%	2740
	21st to end	Day	17% 22% 10% 13% 8% 6% 5% 3 3 3 8%	4070
January	1st to 10th	Night	12% 19% 9% 28% 8% 4 3 2 3 3 9%	8232
	11th to 20th	Night	8% 19% 9% 20% 16% 9% 5% 2 3 3 6%	8280
	21st to end	Night	8% 15% 8% 17% 12% 9% 6% 3 5% 4 11%	9058
February	1st to 10th	Night	9% 18% 6% 17% 14% 8% 4 3 4 3 15%	2812
	11th to 20th	Night	11% 14% 6% 17% 10% 5% 5% 4 5% 4 19%	2458
	21st to end	Night	14% 16% 6% 14% 7% 5% 4 3 5% 6% 21%	2076
March	1st to 10th	Night	14% 11% 4 10% 6% 4 3 3 5% 6% 35%	5493
	11th to 20th	Night	11% 13% 4 10% 5% 4 4 4 6% 7% 33%	5238
	21st to end	Night	18% 13% 5% 10% 5% 4 4 3 4 5% 28%	5401
April	1st to 10th	Night	17% 13% 4 9% 5% 4 4 3 5% 6% 29%	2100
	11th to 20th	Night	21% 12% 3 7% 4 3 3 3 4 5% 35%	1977
	21st to end	Night	16% 11% 3 6% 4 3 4 4 5% 6% 38%	1890
May	1st to 10th	Night	12% 7% 2 7% 4 4 5% 4 6% 8% 40%	5237
	11th to 20th	Night	12% 9% 2 6% 4 4 5% 4 6% 7% 41%	4955
	21st to end	Night	15% 7% 2 6% 4 3 4 3 4 6% 45%	5139
June	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 6% 7% 44%	1680
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 4 7% 10% 41%	1680
	21st to end	Night	11% 6% 3 6% 5% 4 5% 3 6% 8% 44%	1680
July	1st to 10th	Night	12% 8% 2 6% 4 5% 4 4 6% 9% 41%	4708
	11th to 20th	Night	8% 5% 2 4 3 3 4 4 6% 9% 53%	4707
	21st to end	Night	10% 6% 2 6% 4 4 4 4 5% 7% 49%	5117
August	1st to 10th	Night	12% 6% 2 7% 5% 5% 4 4 5% 7% 43%	2855
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 5% 7% 49%	2965
	21st to end	Night	11% 5% 2 6% 4 4 5% 4 5% 7% 47%	3528
September	1st to 10th	Night	11% 7% 3 6% 4 4 4 3 4 7% 49%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 7% 49%	2200
	21st to end	Night	17% 7% 2 6% 5% 4 3 3 5% 7% 40%	2200
October	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 5% 6% 41%	2200
	11th to 20th	Night	21% 12% 4 8% 5% 4 4 3 5% 6% 27%	2502
	21st to end	Night	22% 15% 4 9% 5% 4 4 3 4 5% 25%	2860
November	1st to 10th	Night	14% 16% 4 9% 6% 4 5% 4 5% 6% 29%	2666
	11th to 20th	Night	13% 21% 7% 11% 6% 5% 5% 3 5% 5% 19%	2730
	21st to end	Night	16% 22% 8% 11% 6% 5% 5% 3 5% 5% 15%	2795
December	1st to 10th	Night	11% 20% 11% 18% 7% 5% 5% 3 4 4 11%	5060
	11th to 20th	Night	10% 23% 9% 18% 9% 5% 5% 3 4 4 9%	5667
	21st to end	Night	20% 21% 9% 14% 6% 4 4 3 4 4 12%	7150

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	87% 2 6% 4 1	4692
	11th to 20th	Day	89% 3 3 4 1	4710
	21st to end	Day	77% 3 9% 10% 3	5156
February	1st to 10th	Day	77% 2 8% 11% 2	1801
	11th to 20th	Day	74% 2 8% 12% 4	2058
	21st to end	Day	70% 2 8% 15% 4 1	1730
March	1st to 10th	Day	55% 2 14% 17% 10% 1	4580
	11th to 20th	Day	56% 3 13% 17% 9% 1	4941
	21st to end	Day	69% 2 7% 15% 6%	5815
April	1st to 10th	Day	59% 3 8% 19% 11%	2520
	11th to 20th	Day	54% 2 7% 22% 14% 1	2591
	21st to end	Day	52% 3 10% 16% 16% 2	2730
May	1st to 10th	Day	42% 3 8% 27% 18% 2	7651
	11th to 20th	Day	45% 3 7% 26% 18% 2	7922
	21st to end	Day	40% 2 7% 24% 24% 3	9061
June	1st to 10th	Day	39% 3 10% 29% 17% 2	2940
	11th to 20th	Day	40% 3 11% 25% 19% 1	2940
	21st to end	Day	40% 4 13% 27% 15% 1	2940
July	1st to 10th	Day	39% 4 14% 26% 16% 1	8211
	11th to 20th	Day	30% 3 11% 38% 16% 1	8205
	21st to end	Day	34% 4 12% 32% 16% 1	8794
August	1st to 10th	Day	37% 5% 13% 31% 14% 1	4501
	11th to 20th	Day	37% 4 11% 33% 14% 1	4343
	21st to end	Day	41% 4 11% 31% 12% 1	4564
September	1st to 10th	Day	40% 4 13% 26% 14% 2	2380
	11th to 20th	Day	37% 4 14% 31% 14% 1	2200
	21st to end	Day	44% 3 14% 27% 11%	2200
October	1st to 10th	Day	34% 3 8% 35% 20% 1	3590
	11th to 20th	Day	56% 2 10% 21% 11%	3059
	21st to end	Day	63% 4 12% 17% 4	3180
November	1st to 10th	Day	59% 3 9% 20% 9%	1852
	11th to 20th	Day	68% 3 12% 13% 4	1890
	21st to end	Day	75% 2 11% 10% 2	1812
December	1st to 10th	Day	85% 3 6% 5% 1	2350
	11th to 20th	Day	82% 3 8% 6% 1	2736
	21st to end	Day	82% 2 7% 7% 2	4070
January	1st to 10th	Night	83% 2 7% 6% 2	8233
	11th to 20th	Night	87% 2 6% 4 1	8280
	21st to end	Night	77% 3 7% 10% 3	9058
February	1st to 10th	Night	75% 2 7% 13% 4	2812
	11th to 20th	Night	69% 3 8% 15% 5%	2459
	21st to end	Night	66% 2 11% 17% 4	2076
March	1st to 10th	Night	52% 2 17% 21% 9%	5493
	11th to 20th	Night	51% 2 17% 21% 8% 1	5238
	21st to end	Night	61% 1 10% 19% 8% 1	5401
April	1st to 10th	Night	56% 3 8% 22% 11% 1	2100
	11th to 20th	Night	54% 2 6% 21% 15% 2	1977
	21st to end	Night	49% 2 11% 22% 14% 2	1890
May	1st to 10th	Night	42% 3 10% 25% 16% 3	5264
	11th to 20th	Night	44% 3 7% 28% 17% 1	4956
	21st to end	Night	41% 2 11% 22% 20% 5%	5149
June	1st to 10th	Night	39% 3 10% 29% 17% 2	1680
	11th to 20th	Night	39% 4 13% 26% 16% 2	1680
	21st to end	Night	38% 4 14% 27% 16% 2	1680
July	1st to 10th	Night	41% 4 12% 24% 15% 3	4670
	11th to 20th	Night	28% 3 13% 38% 17% 1	4662
	21st to end	Night	34% 4 13% 30% 18% 1	5002
August	1st to 10th	Night	41% 4 14% 27% 13% 1	2846
	11th to 20th	Night	36% 3 16% 31% 15% 1	2992
	21st to end	Night	36% 4 14% 25% 20% 1	3528
September	1st to 10th	Night	36% 3 14% 26% 20% 1	2000
	11th to 20th	Night	35% 3 14% 33% 14%	2200
	21st to end	Night	45% 3 14% 26% 11% 1	2200
October	1st to 10th	Night	40% 3 9% 35% 13% 1	3590
	11th to 20th	Night	51% 3 9% 24% 13%	4077
	21st to end	Night	54% 3 12% 24% 7%	4610
November	1st to 10th	Night	57% 3 11% 21% 8% 1	2666
	11th to 20th	Night	68% 2 12% 12% 5% 1	2730
	21st to end	Night	73% 3 11% 11% 1	2795
December	1st to 10th	Night	76% 5% 9% 8% 1	5060
	11th to 20th	Night	79% 3 8% 8% 2	5670
	21st to end	Night	77% 3 9% 8% 3	7150

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% 17% 37% 20% 6% 1	330
	11th to 20th	Day	21% 16% 40% 18% 5%	330
	21st to end	Day	22% 17% 32% 23% 5% 1	363
February	1st to 10th	Day	17% 26% 34% 18% 3 2	330
	11th to 20th	Day	20% 21% 38% 17% 4	330
	21st to end	Day	24% 25% 37% 12% 1	273
March	1st to 10th	Day	23% 21% 38% 16% 3	330
	11th to 20th	Day	26% 16% 29% 20% 8% 1	469
	21st to end	Day	29% 14% 28% 20% 8% 1	594
April	1st to 10th	Day	27% 15% 31% 21% 5% 1	540
	11th to 20th	Day	27% 19% 27% 16% 8% 3	532
	21st to end	Day	21% 15% 27% 24% 10% 3	540
May	1st to 10th	Day	20% 16% 32% 19% 10% 3	543
	11th to 20th	Day	16% 13% 29% 28% 10% 3	664
	21st to end	Day	15% 10% 30% 27% 15% 4	844
June	1st to 10th	Day	14% 10% 31% 28% 13% 4	780
	11th to 20th	Day	15% 11% 30% 29% 11% 3	780
	21st to end	Day	16% 8% 30% 29% 14% 3	780
July	1st to 10th	Day	9% 6% 28% 32% 18% 6%	771
	11th to 20th	Day	11% 8% 27% 31% 16% 7%	768
	21st to end	Day	9% 6% 29% 34% 18% 5%	849
August	1st to 10th	Day	10% 6% 22% 31% 23% 8%	667
	11th to 20th	Day	12% 6% 20% 30% 21% 10%	559
	21st to end	Day	16% 9% 22% 28% 19% 6%	619
September	1st to 10th	Day	17% 9% 22% 27% 17% 8%	530
	11th to 20th	Day	12% 5% 18% 29% 25% 11%	530
	21st to end	Day	18% 9% 18% 27% 21% 6%	530
October	1st to 10th	Day	21% 11% 26% 24% 14% 4	530
	11th to 20th	Day	26% 16% 31% 23% 5%	400
	21st to end	Day	29% 17% 32% 16% 5% 1	363
November	1st to 10th	Day	28% 18% 32% 14% 8%	330
	11th to 20th	Day	23% 19% 37% 15% 5%	330
	21st to end	Day	27% 22% 34% 15% 2	316
December	1st to 10th	Day	29% 19% 30% 15% 6% 1	220
	11th to 20th	Day	23% 24% 36% 14% 3	231
	21st to end	Day	27% 19% 37% 13% 3 1	363
January	1st to 10th	Night	20% 15% 35% 24% 7% 1	1050
	11th to 20th	Night	23% 14% 34% 22% 6% 1	1050
	21st to end	Night	18% 12% 35% 28% 7% 1	1155
February	1st to 10th	Night	18% 11% 36% 27% 7% 1	938
	11th to 20th	Night	19% 16% 34% 25% 6%	823
	21st to end	Night	23% 16% 32% 24% 5%	692
March	1st to 10th	Night	17% 12% 34% 27% 9% 1	840
	11th to 20th	Night	19% 13% 34% 28% 5% 1	840
	21st to end	Night	20% 11% 30% 31% 7% 1	840
April	1st to 10th	Night	25% 9% 31% 28% 6% 1	630
	11th to 20th	Night	24% 14% 26% 26% 9% 1	623
	21st to end	Night	19% 9% 30% 32% 9% 1	630
May	1st to 10th	Night	16% 6% 31% 35% 11% 1	630
	11th to 20th	Night	19% 8% 26% 34% 13%	621
	21st to end	Night	15% 5% 26% 36% 17% 1	683
June	1st to 10th	Night	17% 5% 28% 37% 13% 1	630
	11th to 20th	Night	14% 6% 23% 42% 13% 1	630
	21st to end	Night	14% 4 25% 37% 19% 1	630
July	1st to 10th	Night	11% 3 17% 42% 25% 2	615
	11th to 20th	Night	10% 4 20% 39% 25% 2	612
	21st to end	Night	10% 4 20% 41% 23% 2	690
August	1st to 10th	Night	10% 3 17% 42% 27% 1	627
	11th to 20th	Night	9% 4 18% 39% 26% 4	624
	21st to end	Night	13% 5% 19% 40% 22% 2	687
September	1st to 10th	Night	15% 5% 22% 32% 22% 4	600
	11th to 20th	Night	16% 3 20% 34% 24% 4	800
	21st to end	Night	20% 6% 20% 32% 19% 4	800
October	1st to 10th	Night	20% 8% 26% 29% 16% 1	800
	11th to 20th	Night	29% 10% 28% 27% 6% 1	800
	21st to end	Night	29% 12% 28% 24% 6%	880
November	1st to 10th	Night	25% 13% 30% 26% 6%	820
	11th to 20th	Night	25% 15% 34% 21% 5%	840
	21st to end	Night	28% 12% 36% 21% 3	887
December	1st to 10th	Night	23% 16% 36% 21% 4	1050
	11th to 20th	Night	23% 16% 36% 20% 4 1	1050
	21st to end	Night	27% 13% 36% 21% 3	1155

Page generated at 2026-08-27 04:07:12. Version ac07

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