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 2024-02-25.

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% 5% 8% 7% 4 5% 4 5% 6% 30%	77911
	Night	14% 13% 6% 10% 7% 4 5% 4 4 6% 27%	81425
Transparency	Day	55% 3 10% 20% 10% 1	77611
	Night	60% 2 10% 18% 8% 1	81246
Seeing	Day	18% 11% 28% 25% 13% 4	14706
	Night	20% 9% 29% 30% 11% 1	24959

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% 24% 14% 19% 9% 4 4 3 3 3 7%	2527
	11th to 20th	Day	12% 18% 9% 19% 15% 9% 6% 3 3 2 5%	2530
	21st to end	Day	19% 20% 11% 11% 9% 4 6% 4 3 5% 8%	2783
February	1st to 10th	Day	9% 16% 8% 13% 17% 6% 6% 4 4 4 13%	1628
	11th to 20th	Day	11% 15% 7% 15% 13% 6% 7% 4 4 3 15%	1848
	21st to end	Day	12% 16% 10% 13% 10% 3 5% 4 5% 5% 18%	1520
March	1st to 10th	Day	12% 12% 5% 11% 11% 3 4 5% 5% 6% 25%	1800
	11th to 20th	Day	11% 12% 7% 8% 6% 4 5% 5% 5% 6% 31%	1918
	21st to end	Day	16% 13% 5% 7% 7% 3 4 5% 5% 6% 29%	2264
April	1st to 10th	Day	18% 15% 4 8% 5% 3 5% 4 4 6% 28%	2160
	11th to 20th	Day	19% 13% 4 6% 5% 3 4 4 5% 6% 33%	2216
	21st to end	Day	16% 11% 4 7% 5% 3 4 5% 5% 7% 32%	2340
May	1st to 10th	Day	16% 9% 3 5% 6% 3 4 4 5% 7% 38%	2340
	11th to 20th	Day	16% 7% 3 6% 5% 3 4 4 5% 6% 41%	2383
	21st to end	Day	11% 6% 3 5% 5% 3 5% 5% 6% 8% 43%	2725
June	1st to 10th	Day	14% 5% 2 4 4 4 4 5% 5% 7% 44%	2520
	11th to 20th	Day	12% 6% 3 5% 5% 3 4 4 6% 8% 43%	2520
	21st to end	Day	13% 5% 2 6% 5% 3 5% 5% 6% 9% 41%	2520
July	1st to 10th	Day	8% 4 1 3 4 3 4 5% 6% 10% 51%	2520
	11th to 20th	Day	13% 4 2 4 5% 3 5% 5% 5% 9% 44%	2520
	21st to end	Day	9% 5% 2 4 5% 3 5% 5% 6% 9% 48%	2743
August	1st to 10th	Day	9% 5% 2 5% 6% 3 6% 5% 7% 10% 42%	2414
	11th to 20th	Day	11% 6% 2 5% 6% 3 4 5% 6% 8% 45%	2303
	21st to end	Day	13% 7% 2 5% 5% 3 5% 4 5% 7% 44%	2457
September	1st to 10th	Day	15% 4 2 5% 5% 4 5% 5% 5% 7% 42%	2142
	11th to 20th	Day	14% 5% 2 5% 4 3 4 4 6% 8% 44%	1980
	21st to end	Day	17% 6% 3 5% 5% 3 5% 6% 6% 8% 36%	1980
October	1st to 10th	Day	19% 8% 3 6% 5% 3 5% 4 5% 7% 35%	1980
	11th to 20th	Day	23% 14% 4 9% 7% 3 5% 5% 5% 5% 21%	1707
	21st to end	Day	24% 16% 4 9% 8% 4 5% 5% 5% 5% 17%	1782
November	1st to 10th	Day	15% 17% 5% 6% 6% 3 4 4 5% 7% 27%	1672
	11th to 20th	Day	13% 21% 7% 10% 7% 5% 5% 4 5% 6% 17%	1710
	21st to end	Day	16% 22% 10% 12% 6% 3 4 3 4 5% 15%	1641
December	1st to 10th	Day	14% 20% 13% 16% 9% 4 4 4 3 3 9%	1330
	11th to 20th	Day	13% 19% 9% 15% 10% 5% 5% 3 4 5% 11%	1705
	21st to end	Day	18% 21% 13% 12% 9% 5% 4 3 3 3 9%	2783
January	1st to 10th	Night	13% 22% 11% 18% 8% 4 4 3 3 3 12%	4428
	11th to 20th	Night	10% 17% 8% 17% 16% 8% 5% 3 3 3 8%	4440
	21st to end	Night	15% 22% 11% 13% 9% 5% 5% 4 4 4 8%	4884
February	1st to 10th	Night	9% 16% 8% 13% 15% 7% 6% 4 4 3 15%	2546
	11th to 20th	Night	11% 13% 7% 16% 11% 5% 5% 5% 5% 4 17%	2218
	21st to end	Night	15% 15% 6% 14% 9% 3 5% 3 4 5% 21%	1831
March	1st to 10th	Night	11% 11% 5% 9% 7% 4 5% 5% 5% 5% 33%	2160
	11th to 20th	Night	12% 12% 5% 9% 7% 3 4 4 5% 5% 35%	2041
	21st to end	Night	17% 9% 5% 8% 7% 3 5% 4 5% 6% 33%	2106
April	1st to 10th	Night	17% 12% 5% 8% 5% 2 4 4 5% 7% 30%	1800
	11th to 20th	Night	21% 10% 4 6% 4 2 3 4 4 6% 35%	1692
	21st to end	Night	17% 9% 3 5% 5% 2 3 5% 5% 6% 38%	1620
May	1st to 10th	Night	15% 8% 3 7% 6% 3 5% 6% 5% 7% 34%	1620
	11th to 20th	Night	16% 8% 4 7% 6% 3 5% 5% 5% 6% 35%	1508
	21st to end	Night	12% 5% 2 6% 3 2 4 5% 6% 9% 45%	1551
June	1st to 10th	Night	14% 5% 3 5% 5% 3 5% 5% 5% 6% 45%	1440
	11th to 20th	Night	11% 6% 2 5% 4 3 4 5% 6% 11% 43%	1440
	21st to end	Night	9% 5% 3 6% 5% 3 4 4 5% 8% 47%	1440
July	1st to 10th	Night	8% 5% 1 3 3 2 5% 4 6% 10% 53%	1440
	11th to 20th	Night	7% 4 2 4 5% 4 5% 5% 5% 9% 50%	1440
	21st to end	Night	9% 4 2 5% 6% 3 4 5% 5% 9% 49%	1575
August	1st to 10th	Night	8% 5% 2 5% 6% 3 5% 5% 6% 9% 47%	1522
	11th to 20th	Night	9% 4 2 5% 5% 2 4 5% 6% 8% 49%	1598
	21st to end	Night	12% 6% 2 6% 5% 3 5% 4 5% 7% 45%	1897
September	1st to 10th	Night	11% 7% 3 5% 5% 2 4 4 4 6% 48%	1800
	11th to 20th	Night	15% 6% 3 4 4 2 4 3 5% 6% 48%	1980
	21st to end	Night	18% 7% 2 5% 6% 4 3 4 5% 7% 39%	1980
October	1st to 10th	Night	17% 9% 3 7% 5% 3 3 3 4 5% 39%	1980
	11th to 20th	Night	23% 11% 4 7% 6% 3 5% 3 5% 6% 26%	2250
	21st to end	Night	24% 15% 4 9% 6% 3 4 3 4 5% 23%	2574
November	1st to 10th	Night	13% 15% 5% 8% 6% 4 5% 4 5% 6% 29%	2406
	11th to 20th	Night	14% 19% 8% 9% 7% 4 6% 4 5% 5% 19%	2470
	21st to end	Night	17% 18% 9% 10% 7% 4 5% 5% 4 5% 16%	2528
December	1st to 10th	Night	13% 19% 12% 15% 9% 4 5% 4 3 4 11%	2850
	11th to 20th	Night	12% 20% 8% 18% 9% 4 5% 5% 4 4 10%	3486
	21st to end	Night	21% 23% 9% 12% 7% 4 5% 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	70% 2 8% 15% 5% 1	1520
March	1st to 10th	Day	60% 3 11% 17% 7% 1	1800
	11th to 20th	Day	54% 4 11% 20% 9% 2	1918
	21st to end	Day	57% 3 10% 21% 8% 1	2264
April	1st to 10th	Day	59% 3 9% 19% 11%	2160
	11th to 20th	Day	55% 2 7% 20% 15% 1	2216
	21st to end	Day	52% 3 11% 16% 17% 3	2340
May	1st to 10th	Day	48% 2 8% 27% 13% 2	2340
	11th to 20th	Day	45% 3 10% 23% 17% 3	2387
	21st to end	Day	39% 4 13% 27% 16% 2	2725
June	1st to 10th	Day	39% 3 11% 29% 16% 2	2520
	11th to 20th	Day	39% 3 12% 25% 20% 2	2520
	21st to end	Day	39% 4 13% 27% 16% 1	2520
July	1st to 10th	Day	29% 3 13% 33% 20% 2	2451
	11th to 20th	Day	36% 4 11% 30% 17% 2	2445
	21st to end	Day	33% 4 12% 29% 20% 2	2591
August	1st to 10th	Day	36% 4 14% 33% 13% 1	2399
	11th to 20th	Day	37% 4 11% 31% 14% 2	2303
	21st to end	Day	40% 4 13% 26% 16% 2	2457
September	1st to 10th	Day	41% 4 11% 26% 15% 2	2142
	11th to 20th	Day	38% 3 14% 32% 11% 1	1980
	21st to end	Day	46% 3 15% 27% 9%	1980
October	1st to 10th	Day	50% 4 10% 25% 11% 1	1980
	11th to 20th	Day	66% 2 11% 17% 4	1707
	21st to end	Day	70% 3 11% 11% 4	1782
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	68% 2 10% 16% 5%	1831
March	1st to 10th	Night	53% 3 12% 23% 9%	2160
	11th to 20th	Night	53% 2 11% 20% 12% 2	2041
	21st to end	Night	53% 2 10% 24% 10% 1	2106
April	1st to 10th	Night	55% 3 8% 22% 12% 1	1800
	11th to 20th	Night	53% 2 7% 21% 14% 3	1692
	21st to end	Night	48% 2 11% 22% 15% 3	1620
May	1st to 10th	Night	49% 4 10% 22% 12% 3	1620
	11th to 20th	Night	50% 3 7% 22% 15% 3	1509
	21st to end	Night	37% 4 15% 27% 16% 2	1552
June	1st to 10th	Night	39% 3 11% 30% 15% 1	1440
	11th to 20th	Night	37% 4 14% 26% 17% 2	1440
	21st to end	Night	36% 4 14% 28% 17% 2	1440
July	1st to 10th	Night	28% 3 14% 32% 20% 3	1400
	11th to 20th	Night	32% 3 16% 27% 21% 2	1392
	21st to end	Night	34% 3 13% 32% 17% 1	1487
August	1st to 10th	Night	35% 4 16% 32% 12% 2	1513
	11th to 20th	Night	33% 3 16% 31% 16% 1	1598
	21st to end	Night	38% 3 13% 27% 17% 1	1897
September	1st to 10th	Night	37% 3 13% 25% 21% 2	1800
	11th to 20th	Night	38% 3 15% 33% 12%	1980
	21st to end	Night	46% 3 16% 27% 8% 1	1980
October	1st to 10th	Night	48% 2 9% 26% 13% 1	1980
	11th to 20th	Night	61% 2 11% 20% 6% 1	2250
	21st to end	Night	65% 2 11% 15% 6%	2574
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	22% 24% 38% 13% 3	210
March	1st to 10th	Day	22% 19% 38% 17% 4	240
	11th to 20th	Day	23% 11% 30% 25% 9% 1	358
	21st to end	Day	27% 11% 27% 23% 10% 1	462
April	1st to 10th	Day	27% 12% 30% 24% 5% 2	420
	11th to 20th	Day	25% 19% 26% 17% 10% 3	412
	21st to end	Day	21% 14% 26% 25% 10% 3	420
May	1st to 10th	Day	21% 15% 30% 19% 11% 4	423
	11th to 20th	Day	16% 9% 31% 29% 12% 4	529
	21st to end	Day	13% 9% 28% 29% 17% 5%	679
June	1st to 10th	Day	15% 8% 29% 30% 15% 4	630
	11th to 20th	Day	14% 9% 28% 32% 13% 4	630
	21st to end	Day	14% 6% 27% 32% 16% 4	630
July	1st to 10th	Day	8% 5% 27% 32% 21% 7%	621
	11th to 20th	Day	12% 7% 24% 32% 17% 7%	618
	21st to end	Day	8% 5% 28% 34% 19% 6%	684
August	1st to 10th	Day	11% 5% 21% 31% 23% 9%	533
	11th to 20th	Day	12% 5% 18% 29% 24% 12%	439
	21st to end	Day	17% 8% 20% 28% 21% 6%	495
September	1st to 10th	Day	17% 7% 22% 28% 17% 9%	450
	11th to 20th	Day	13% 6% 20% 30% 22% 8%	450
	21st to end	Day	19% 10% 18% 26% 20% 6%	450
October	1st to 10th	Day	22% 11% 24% 25% 14% 4	450
	11th to 20th	Day	28% 16% 30% 21% 5%	334
	21st to end	Day	32% 15% 32% 15% 5% 1	297
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	24% 15% 32% 25% 5%	611
March	1st to 10th	Night	16% 10% 34% 28% 11% 1	720
	11th to 20th	Night	17% 12% 34% 31% 6% 1	720
	21st to end	Night	20% 10% 29% 33% 8% 1	720
April	1st to 10th	Night	24% 8% 32% 29% 6% 1	540
	11th to 20th	Night	23% 11% 28% 27% 9% 1	533
	21st to end	Night	19% 8% 30% 33% 9% 1	540
May	1st to 10th	Night	18% 6% 31% 34% 11% 1	540
	11th to 20th	Night	20% 6% 26% 35% 13%	531
	21st to end	Night	14% 4 25% 36% 19% 1	584
June	1st to 10th	Night	17% 4 26% 36% 15% 1	540
	11th to 20th	Night	13% 5% 22% 43% 15% 2	540
	21st to end	Night	12% 3 24% 38% 21% 1	540
July	1st to 10th	Night	10% 3 16% 42% 27% 2	525
	11th to 20th	Night	11% 3 21% 38% 25% 2	522
	21st to end	Night	11% 3 19% 41% 24% 2	591
August	1st to 10th	Night	9% 1 17% 44% 27% 1	537
	11th to 20th	Night	9% 3 18% 38% 28% 4	534
	21st to end	Night	13% 4 19% 40% 22% 2	594
September	1st to 10th	Night	15% 4 22% 33% 22% 4	540
	11th to 20th	Night	17% 3 21% 33% 22% 4	720
	21st to end	Night	21% 7% 20% 31% 18% 4	720
October	1st to 10th	Night	21% 7% 25% 29% 17% 1	720
	11th to 20th	Night	30% 9% 28% 27% 6% 1	720
	21st to end	Night	31% 12% 27% 23% 6% 1	792
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 2024-02-22 06:05:41. Version ac07

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