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 2025-04-26.

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% 12% 5% 10% 6% 5% 4 3 5% 6% 31%	112785
	Night	13% 13% 5% 12% 6% 5% 4 3 5% 5% 27%	116007
Transparency	Day	54% 3 10% 21% 11% 1	116331
	Night	59% 3 11% 18% 9% 1	120604
Seeing	Day	18% 12% 29% 25% 12% 4	16301
	Night	20% 10% 29% 29% 11% 1	26535

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	9% 21% 12% 30% 6% 4 3 2 3 3 7%	3617
	11th to 20th	Day	10% 16% 8% 24% 14% 10% 6% 3 3 3 5%	3620
	21st to end	Day	13% 17% 9% 18% 8% 5% 7% 3 4 5% 11%	3982
February	1st to 10th	Day	9% 18% 6% 16% 15% 7% 5% 3 3 4 13%	1715
	11th to 20th	Day	11% 16% 7% 17% 11% 8% 6% 3 3 3 14%	1948
	21st to end	Day	11% 18% 8% 13% 9% 5% 3 4 5% 4 18%	1650
March	1st to 10th	Day	11% 11% 4 12% 8% 4 4 4 5% 5% 31%	4480
	11th to 20th	Day	11% 12% 6% 13% 5% 4 5% 4 6% 8% 27%	4834
	21st to end	Day	21% 12% 8% 16% 5% 4 3 3 4 4 21%	5687
April	1st to 10th	Day	16% 16% 4 8% 5% 5% 5% 3 4 6% 28%	2400
	11th to 20th	Day	18% 13% 3 7% 4 3 3 3 5% 6% 33%	2466
	21st to end	Day	16% 11% 4 8% 5% 4 4 4 6% 7% 33%	2535
May	1st to 10th	Day	15% 9% 3 6% 4 4 3 3 5% 7% 42%	4082
	11th to 20th	Day	12% 7% 2 6% 4 4 4 3 5% 6% 48%	4233
	21st to end	Day	13% 7% 2 6% 4 4 4 4 6% 8% 43%	4824
June	1st to 10th	Day	14% 6% 2 5% 4 5% 4 4 6% 7% 43%	2660
	11th to 20th	Day	12% 6% 2 7% 5% 4 4 4 6% 9% 43%	2660
	21st to end	Day	14% 6% 2 7% 4 4 5% 4 6% 8% 40%	2660
July	1st to 10th	Day	17% 7% 2 6% 4 4 3 3 6% 8% 40%	4440
	11th to 20th	Day	10% 4 1 4 4 4 4 4 6% 8% 50%	4440
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 9% 48%	4813
August	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	4246
	11th to 20th	Day	14% 6% 2 5% 4 3 4 3 5% 6% 47%	4044
	21st to end	Day	13% 7% 3 7% 4 4 5% 3 4 6% 44%	4317
September	1st to 10th	Day	15% 4 2 6% 4 4 5% 4 5% 8% 42%	2261
	11th to 20th	Day	13% 5% 2 5% 3 4 3 4 6% 9% 45%	2090
	21st to end	Day	16% 6% 3 6% 4 5% 4 4 6% 8% 37%	2090
October	1st to 10th	Day	18% 8% 3 7% 4 4 4 3 5% 7% 37%	2090
	11th to 20th	Day	21% 15% 4 10% 6% 4 5% 4 5% 5% 22%	1801
	21st to end	Day	23% 16% 3 11% 6% 6% 5% 4 5% 5% 17%	1881
November	1st to 10th	Day	16% 19% 4 7% 5% 4 4 3 5% 6% 27%	1762
	11th to 20th	Day	12% 22% 7% 11% 6% 6% 4 3 6% 6% 17%	1800
	21st to end	Day	15% 24% 10% 13% 5% 3 4 2 4 5% 14%	1726
December	1st to 10th	Day	11% 29% 12% 18% 6% 5% 4 3 3 2 6%	2280
	11th to 20th	Day	10% 23% 11% 18% 10% 5% 5% 2 4 4 8%	2669
	21st to end	Day	17% 22% 10% 13% 9% 6% 5% 3 3 3 8%	3982
January	1st to 10th	Night	10% 19% 9% 30% 7% 4 3 2 3 3 9%	6348
	11th to 20th	Night	9% 16% 8% 21% 15% 10% 5% 2 3 4 7%	6360
	21st to end	Night	11% 20% 10% 16% 8% 7% 6% 3 4 4 11%	6996
February	1st to 10th	Night	9% 18% 6% 17% 13% 8% 5% 3 4 3 14%	2679
	11th to 20th	Night	11% 15% 6% 17% 10% 6% 5% 4 5% 4 17%	2338
	21st to end	Night	14% 16% 5% 14% 7% 5% 4 3 5% 6% 21%	1980
March	1st to 10th	Night	15% 11% 4 10% 6% 4 3 3 5% 6% 35%	5373
	11th to 20th	Night	10% 12% 4 9% 5% 4 4 4 6% 8% 33%	5125
	21st to end	Night	18% 13% 5% 10% 5% 4 4 3 4 5% 28%	5284
April	1st to 10th	Night	17% 13% 4 8% 4 4 4 3 5% 6% 31%	2000
	11th to 20th	Night	20% 12% 3 7% 4 3 3 3 4 5% 35%	1882
	21st to end	Night	17% 11% 4 6% 4 3 4 4 5% 5% 37%	1761
May	1st to 10th	Night	15% 8% 2 7% 5% 4 4 4 5% 6% 39%	2813
	11th to 20th	Night	13% 7% 2 6% 5% 4 5% 4 5% 7% 42%	2663
	21st to end	Night	18% 6% 2 6% 3 3 4 3 5% 8% 43%	2743
June	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 6% 7% 43%	1520
	11th to 20th	Night	11% 6% 2 6% 3 4 4 4 6% 10% 43%	1520
	21st to end	Night	10% 6% 2 6% 5% 4 4 3 6% 8% 45%	1520
July	1st to 10th	Night	12% 8% 2 6% 3 4 5% 4 6% 8% 42%	2530
	11th to 20th	Night	8% 4 2 5% 4 4 4 4 5% 7% 54%	2530
	21st to end	Night	9% 5% 2 6% 5% 4 4 4 5% 7% 48%	2750
August	1st to 10th	Night	12% 6% 2 7% 5% 5% 4 4 5% 7% 43%	2685
	11th to 20th	Night	14% 5% 1 5% 4 3 4 3 5% 7% 49%	2785
	21st to end	Night	12% 5% 2 6% 4 4 4 4 5% 7% 48%	3337
September	1st to 10th	Night	11% 7% 3 6% 4 4 4 3 4 6% 49%	1900
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 5% 6% 49%	2090
	21st to end	Night	18% 7% 2 6% 5% 4 3 4 5% 7% 40%	2090
October	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 4 5% 40%	2090
	11th to 20th	Night	21% 12% 3 8% 5% 4 4 3 5% 6% 27%	2376
	21st to end	Night	23% 15% 4 9% 5% 4 4 3 4 5% 24%	2717
November	1st to 10th	Night	13% 16% 4 9% 6% 4 5% 4 5% 6% 29%	2536
	11th to 20th	Night	14% 21% 7% 11% 6% 5% 5% 3 5% 5% 19%	2600
	21st to end	Night	16% 21% 8% 12% 6% 5% 5% 3 5% 5% 15%	2662
December	1st to 10th	Night	11% 20% 11% 18% 7% 5% 5% 3 4 4 11%	4910
	11th to 20th	Night	10% 23% 9% 18% 9% 5% 5% 3 4 4 10%	5518
	21st to end	Night	20% 21% 8% 14% 6% 4 4 3 4 4 12%	6996

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	86% 2 5% 6% 1	3618
	11th to 20th	Day	87% 3 4 5% 1	3620
	21st to end	Day	78% 2 10% 8% 2	3982
February	1st to 10th	Day	77% 2 8% 11% 2	1715
	11th to 20th	Day	75% 2 8% 11% 4	1958
	21st to end	Day	69% 2 8% 16% 5% 1	1650
March	1st to 10th	Day	55% 2 14% 17% 10% 1	4480
	11th to 20th	Day	56% 3 13% 18% 9% 1	4834
	21st to end	Day	69% 2 7% 15% 6%	5689
April	1st to 10th	Day	59% 3 8% 19% 11%	2400
	11th to 20th	Day	54% 2 7% 22% 14% 1	2466
	21st to end	Day	51% 3 10% 16% 16% 3	2535
May	1st to 10th	Day	45% 1 7% 26% 19% 2	4120
	11th to 20th	Day	39% 1 8% 31% 18% 2	4237
	21st to end	Day	41% 2 10% 23% 21% 2	4837
June	1st to 10th	Day	39% 3 10% 29% 17% 2	2660
	11th to 20th	Day	39% 3 12% 25% 20% 2	2660
	21st to end	Day	40% 4 12% 27% 15% 1	2660
July	1st to 10th	Day	43% 3 13% 25% 14% 2	4371
	11th to 20th	Day	30% 4 13% 36% 16% 2	4365
	21st to end	Day	32% 5% 12% 29% 21% 1	4662
August	1st to 10th	Day	37% 5% 13% 30% 14% 1	4231
	11th to 20th	Day	37% 4 11% 33% 14% 1	4083
	21st to end	Day	42% 4 11% 30% 12% 1	4316
September	1st to 10th	Day	40% 4 12% 27% 15% 2	2261
	11th to 20th	Day	37% 3 14% 31% 14% 1	2090
	21st to end	Day	44% 3 15% 27% 11%	2090
October	1st to 10th	Day	33% 3 8% 35% 20% 1	3480
	11th to 20th	Day	56% 2 10% 21% 11%	2965
	21st to end	Day	64% 4 12% 16% 4	3081
November	1st to 10th	Day	59% 3 9% 20% 9%	1762
	11th to 20th	Day	69% 3 11% 13% 5%	1800
	21st to end	Day	75% 2 12% 9% 2	1726
December	1st to 10th	Day	85% 3 6% 5% 1	2280
	11th to 20th	Day	82% 3 8% 6% 1	2665
	21st to end	Day	82% 2 7% 7% 2	3982
January	1st to 10th	Night	83% 2 6% 8% 2	6348
	11th to 20th	Night	85% 2 6% 5% 1	6360
	21st to end	Night	79% 2 8% 10% 1	6996
February	1st to 10th	Night	76% 1 6% 13% 4	2679
	11th to 20th	Night	71% 3 8% 14% 4	2339
	21st to end	Night	66% 2 11% 17% 5%	1980
March	1st to 10th	Night	52% 2 17% 21% 8%	5373
	11th to 20th	Night	50% 2 17% 21% 8% 1	5125
	21st to end	Night	61% 1 10% 20% 8% 1	5284
April	1st to 10th	Night	55% 3 8% 23% 11% 1	2000
	11th to 20th	Night	53% 2 7% 21% 15% 2	1882
	21st to end	Night	50% 2 10% 22% 14% 2	1761
May	1st to 10th	Night	46% 2 11% 20% 18% 3	2840
	11th to 20th	Night	44% 2 7% 29% 17% 2	2664
	21st to end	Night	42% 2 14% 21% 17% 4	2751
June	1st to 10th	Night	40% 3 10% 30% 16% 1	1520
	11th to 20th	Night	37% 3 13% 26% 18% 2	1520
	21st to end	Night	37% 3 14% 28% 16% 2	1520
July	1st to 10th	Night	41% 3 12% 26% 14% 4	2490
	11th to 20th	Night	29% 3 16% 32% 19% 1	2482
	21st to end	Night	35% 4 13% 27% 21% 1	2662
August	1st to 10th	Night	41% 4 14% 27% 13% 1	2676
	11th to 20th	Night	35% 3 15% 31% 15% 1	2812
	21st to end	Night	36% 4 14% 25% 20% 1	3337
September	1st to 10th	Night	36% 3 13% 25% 21% 1	1900
	11th to 20th	Night	36% 3 14% 32% 14%	2090
	21st to end	Night	44% 3 15% 27% 10% 1	2090
October	1st to 10th	Night	41% 3 9% 34% 13% 1	3480
	11th to 20th	Night	51% 3 9% 25% 13%	3951
	21st to end	Night	55% 3 12% 24% 6%	4467
November	1st to 10th	Night	56% 3 11% 21% 8% 1	2536
	11th to 20th	Night	69% 2 11% 11% 6% 1	2600
	21st to end	Night	73% 2 12% 11% 2	2662
December	1st to 10th	Night	76% 5% 10% 8% 1	4910
	11th to 20th	Night	79% 3 8% 8% 2	5521
	21st to end	Night	77% 3 9% 7% 3	6996

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	18% 16% 38% 21% 6% 1	300
	11th to 20th	Day	21% 14% 41% 19% 5%	300
	21st to end	Day	23% 17% 31% 23% 5% 1	330
February	1st to 10th	Day	18% 26% 34% 16% 3 2	300
	11th to 20th	Day	20% 22% 38% 17% 4	300
	21st to end	Day	23% 23% 39% 13% 2	249
March	1st to 10th	Day	22% 22% 37% 16% 3	300
	11th to 20th	Day	24% 15% 29% 22% 8% 1	432
	21st to end	Day	29% 13% 28% 20% 9% 1	550
April	1st to 10th	Day	26% 14% 31% 22% 5% 1	500
	11th to 20th	Day	26% 20% 28% 16% 9% 3	492
	21st to end	Day	21% 14% 28% 24% 10% 3	480
May	1st to 10th	Day	20% 15% 31% 20% 11% 3	463
	11th to 20th	Day	14% 10% 30% 31% 11% 4	574
	21st to end	Day	13% 9% 28% 29% 16% 4	734
June	1st to 10th	Day	15% 9% 29% 29% 14% 4	680
	11th to 20th	Day	14% 10% 29% 31% 12% 4	680
	21st to end	Day	15% 8% 28% 30% 15% 4	680
July	1st to 10th	Day	9% 6% 27% 31% 19% 7%	671
	11th to 20th	Day	12% 7% 25% 32% 17% 7%	668
	21st to end	Day	9% 5% 27% 34% 19% 5%	739
August	1st to 10th	Day	10% 6% 22% 31% 23% 8%	577
	11th to 20th	Day	12% 5% 20% 29% 23% 11%	479
	21st to end	Day	17% 8% 21% 28% 21% 6%	539
September	1st to 10th	Day	17% 8% 22% 28% 18% 8%	490
	11th to 20th	Day	12% 6% 19% 30% 24% 9%	490
	21st to end	Day	19% 9% 19% 27% 20% 6%	490
October	1st to 10th	Day	22% 12% 26% 24% 14% 4	490
	11th to 20th	Day	27% 16% 29% 23% 5%	367
	21st to end	Day	31% 17% 32% 15% 5% 1	330
November	1st to 10th	Day	27% 18% 31% 15% 9%	300
	11th to 20th	Day	22% 19% 40% 16% 4	300
	21st to end	Day	26% 22% 34% 16% 2	287
December	1st to 10th	Day	30% 18% 31% 16% 6% 1	200
	11th to 20th	Day	21% 23% 37% 15% 3	210
	21st to end	Day	25% 19% 37% 14% 3 2	330
January	1st to 10th	Night	20% 14% 35% 24% 7% 1	1000
	11th to 20th	Night	23% 14% 34% 22% 7% 1	1000
	21st to end	Night	18% 11% 34% 28% 8% 1	1100
February	1st to 10th	Night	18% 12% 36% 27% 6% 1	894
	11th to 20th	Night	20% 15% 35% 24% 6%	783
	21st to end	Night	23% 16% 33% 24% 5%	660
March	1st to 10th	Night	17% 12% 34% 27% 10% 1	800
	11th to 20th	Night	17% 13% 34% 30% 5% 1	800
	21st to end	Night	21% 10% 29% 32% 7% 1	800
April	1st to 10th	Night	25% 9% 32% 29% 6% 1	600
	11th to 20th	Night	24% 14% 26% 27% 9% 1	593
	21st to end	Night	19% 9% 30% 32% 9% 1	587
May	1st to 10th	Night	17% 6% 31% 35% 11% 1	570
	11th to 20th	Night	19% 6% 26% 35% 13%	561
	21st to end	Night	15% 5% 25% 36% 18% 1	617
June	1st to 10th	Night	17% 5% 28% 35% 14% 1	570
	11th to 20th	Night	14% 5% 22% 43% 14% 2	570
	21st to end	Night	14% 4 25% 37% 20% 1	570
July	1st to 10th	Night	11% 3 17% 41% 26% 2	555
	11th to 20th	Night	11% 3 20% 38% 26% 2	552
	21st to end	Night	10% 3 18% 41% 25% 2	624
August	1st to 10th	Night	10% 2 16% 43% 28% 1	567
	11th to 20th	Night	9% 3 18% 38% 27% 4	564
	21st to end	Night	13% 4 18% 41% 22% 2	627
September	1st to 10th	Night	15% 5% 22% 33% 22% 4	570
	11th to 20th	Night	16% 3 20% 34% 23% 4	760
	21st to end	Night	21% 6% 20% 31% 18% 4	760
October	1st to 10th	Night	20% 8% 25% 29% 16% 1	760
	11th to 20th	Night	29% 10% 28% 27% 6% 1	760
	21st to end	Night	30% 12% 28% 24% 6%	836
November	1st to 10th	Night	25% 12% 30% 26% 6%	780
	11th to 20th	Night	25% 15% 35% 22% 4	800
	21st to end	Night	27% 11% 36% 22% 3	845
December	1st to 10th	Night	23% 15% 36% 22% 4	1000
	11th to 20th	Night	23% 15% 36% 20% 4 1	1000
	21st to end	Night	27% 13% 36% 21% 3	1100

Page generated at 2025-04-23 09:25:07. Version ac07

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