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 2024-10-25.

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% 11% 4 9% 6% 5% 4 3 5% 6% 33%	100858
	Night	14% 13% 5% 10% 6% 5% 4 3 5% 6% 29%	98007
Transparency	Day	53% 3 10% 22% 12% 1	100648
	Night	58% 2 11% 19% 9% 1	97889
Seeing	Day	18% 12% 29% 25% 13% 4	15732
	Night	20% 9% 29% 30% 11% 1	25765

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% 26% 13% 21% 7% 5% 3 2 3 3 7%	2527
	11th to 20th	Day	12% 19% 8% 21% 13% 11% 5% 2 3 2 5%	2530
	21st to end	Day	18% 22% 10% 13% 7% 5% 6% 3 3 5% 8%	2783
February	1st to 10th	Day	9% 18% 6% 15% 16% 7% 5% 3 4 4 13%	1628
	11th to 20th	Day	11% 16% 7% 17% 11% 7% 6% 4 4 3 15%	1848
	21st to end	Day	11% 18% 8% 14% 9% 5% 3 4 5% 4 18%	1570
March	1st to 10th	Day	12% 14% 5% 10% 7% 3 4 4 5% 5% 31%	3120
	11th to 20th	Day	9% 12% 7% 16% 5% 4 4 4 5% 6% 27%	3376
	21st to end	Day	16% 14% 7% 16% 6% 4 4 3 4 4 22%	3969
April	1st to 10th	Day	17% 16% 3 8% 5% 5% 5% 3 4 6% 29%	2280
	11th to 20th	Day	19% 13% 3 7% 4 3 3 3 5% 5% 33%	2341
	21st to end	Day	15% 11% 4 8% 5% 4 4 4 6% 6% 33%	2470
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% 5% 4 4 5% 5% 17%	1824
November	1st to 10th	Day	15% 18% 4 7% 5% 4 4 3 5% 7% 27%	1672
	11th to 20th	Day	12% 23% 7% 11% 6% 6% 4 3 6% 6% 17%	1710
	21st to end	Day	15% 24% 9% 13% 5% 4 3 2 4 5% 15%	1641
December	1st to 10th	Day	14% 23% 11% 17% 8% 5% 4 3 3 2 9%	1330
	11th to 20th	Day	13% 22% 7% 16% 9% 6% 5% 2 5% 5% 11%	1705
	21st to end	Day	17% 22% 12% 13% 8% 6% 4 3 3 3 9%	2783
January	1st to 10th	Night	12% 23% 10% 19% 7% 5% 4 2 3 3 12%	4428
	11th to 20th	Night	10% 18% 8% 20% 14% 10% 4 2 3 3 8%	4440
	21st to end	Night	15% 23% 10% 14% 8% 6% 5% 3 4 4 8%	4884
February	1st to 10th	Night	9% 18% 6% 16% 13% 8% 5% 3 4 3 15%	2546
	11th to 20th	Night	11% 14% 6% 17% 10% 6% 5% 4 5% 4 17%	2218
	21st to end	Night	14% 16% 5% 14% 7% 5% 4 3 4 5% 21%	1884
March	1st to 10th	Night	14% 10% 4 7% 4 4 4 3 5% 6% 38%	3733
	11th to 20th	Night	8% 15% 5% 10% 5% 4 4 3 5% 6% 36%	3583
	21st to end	Night	15% 12% 6% 11% 6% 5% 5% 3 5% 5% 26%	3690
April	1st to 10th	Night	17% 14% 4 8% 4 4 4 3 5% 6% 30%	1900
	11th to 20th	Night	21% 11% 3 7% 4 4 3 3 4 6% 35%	1787
	21st to end	Night	17% 10% 3 6% 4 3 4 4 5% 5% 38%	1710
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%	2638
November	1st to 10th	Night	13% 16% 4 9% 6% 4 5% 4 5% 6% 29%	2406
	11th to 20th	Night	13% 21% 7% 11% 6% 5% 5% 3 5% 5% 19%	2470
	21st to end	Night	16% 20% 8% 12% 6% 5% 5% 3 5% 5% 16%	2528
December	1st to 10th	Night	13% 21% 10% 17% 8% 5% 5% 3 4 4 11%	2850
	11th to 20th	Night	12% 21% 8% 19% 7% 6% 5% 4 4 4 10%	3486
	21st to end	Night	21% 24% 9% 13% 6% 4 4 3 4 4 10%	4884

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 5% 5% 1	2528
	11th to 20th	Day	90% 2 3 4 1	2530
	21st to end	Day	83% 2 9% 5% 1	2783
February	1st to 10th	Day	77% 2 7% 11% 3	1628
	11th to 20th	Day	74% 2 8% 11% 4	1858
	21st to end	Day	69% 2 8% 16% 4 1	1570
March	1st to 10th	Day	56% 2 11% 21% 9% 1	3120
	11th to 20th	Day	58% 2 12% 19% 8% 1	3376
	21st to end	Day	68% 2 7% 17% 6% 1	3969
April	1st to 10th	Day	58% 3 8% 19% 11%	2280
	11th to 20th	Day	55% 1 7% 21% 15% 1	2341
	21st to end	Day	51% 3 10% 16% 17% 3	2470
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	48% 3 10% 26% 12% 1	2090
	11th to 20th	Day	65% 2 11% 17% 5%	1801
	21st to end	Day	70% 3 10% 12% 4	1824
November	1st to 10th	Day	59% 3 10% 20% 9%	1672
	11th to 20th	Day	69% 2 12% 13% 4	1710
	21st to end	Day	74% 2 12% 10% 2	1641
December	1st to 10th	Day	83% 2 6% 7% 1	1330
	11th to 20th	Day	78% 2 10% 8% 1	1701
	21st to end	Day	83% 1 7% 6% 2	2783
January	1st to 10th	Night	81% 2 7% 10% 1	4428
	11th to 20th	Night	86% 1 6% 6% 1	4440
	21st to end	Night	83% 2 8% 7% 1	4884
February	1st to 10th	Night	75% 1 6% 13% 4	2546
	11th to 20th	Night	70% 3 8% 14% 4	2219
	21st to end	Night	67% 2 10% 16% 5%	1884
March	1st to 10th	Night	48% 2 16% 24% 9%	3733
	11th to 20th	Night	52% 1 12% 25% 9% 1	3583
	21st to end	Night	61% 1 9% 22% 7% 1	3690
April	1st to 10th	Night	55% 3 8% 22% 11% 1	1900
	11th to 20th	Night	53% 2 7% 21% 14% 2	1787
	21st to end	Night	49% 2 10% 22% 15% 3	1710
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	47% 2 9% 27% 13% 1	2090
	11th to 20th	Night	59% 2 10% 21% 7% 1	2376
	21st to end	Night	64% 2 11% 16% 6%	2638
November	1st to 10th	Night	57% 3 11% 20% 8% 1	2406
	11th to 20th	Night	69% 2 12% 12% 5% 1	2470
	21st to end	Night	72% 2 12% 12% 2	2528
December	1st to 10th	Night	78% 2 8% 10% 2	2850
	11th to 20th	Night	79% 3 9% 7% 2	3489
	21st to end	Night	80% 2 10% 6% 1	4884

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% 15% 38% 21% 7% 1	270
	11th to 20th	Day	21% 13% 40% 20% 6%	270
	21st to end	Day	24% 14% 31% 25% 6% 1	297
February	1st to 10th	Day	17% 23% 37% 17% 4 3	270
	11th to 20th	Day	19% 22% 37% 19% 4	270
	21st to end	Day	21% 25% 40% 12% 2	225
March	1st to 10th	Day	21% 20% 39% 17% 3	270
	11th to 20th	Day	24% 13% 30% 24% 9% 2	395
	21st to end	Day	27% 13% 28% 22% 9% 1	506
April	1st to 10th	Day	26% 14% 31% 23% 5% 2	460
	11th to 20th	Day	25% 20% 27% 16% 9% 3	452
	21st to end	Day	20% 15% 27% 24% 10% 3	460
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	32% 15% 33% 15% 5% 1	311
November	1st to 10th	Day	25% 17% 32% 16% 10%	270
	11th to 20th	Day	19% 20% 41% 16% 4	270
	21st to end	Day	24% 23% 36% 15% 3	259
December	1st to 10th	Day	29% 16% 33% 16% 6% 1	180
	11th to 20th	Day	22% 22% 37% 16% 3 1	189
	21st to end	Day	23% 20% 38% 15% 3 1	297
January	1st to 10th	Night	20% 13% 34% 24% 7% 1	950
	11th to 20th	Night	23% 13% 34% 23% 7% 1	950
	21st to end	Night	18% 10% 34% 29% 8% 1	1045
February	1st to 10th	Night	17% 11% 36% 28% 7% 1	850
	11th to 20th	Night	20% 14% 36% 24% 6%	743
	21st to end	Night	23% 15% 33% 24% 5%	628
March	1st to 10th	Night	16% 10% 34% 28% 10% 1	760
	11th to 20th	Night	17% 12% 34% 31% 5% 1	760
	21st to end	Night	19% 10% 29% 33% 7% 1	760
April	1st to 10th	Night	24% 8% 32% 29% 6% 1	570
	11th to 20th	Night	24% 13% 28% 26% 9% 1	563
	21st to end	Night	19% 9% 30% 32% 9% 1	570
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% 23% 6%	812
November	1st to 10th	Night	25% 11% 30% 27% 6%	740
	11th to 20th	Night	24% 15% 35% 22% 4	760
	21st to end	Night	27% 11% 36% 22% 3	802
December	1st to 10th	Night	24% 15% 36% 21% 4	950
	11th to 20th	Night	22% 15% 36% 21% 4 1	950
	21st to end	Night	27% 12% 37% 21% 3	1045

Page generated at 2024-10-22 20:21:02. Version ac07

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