Orson Pratt Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Orson Pratt Observatory clear sky chart from 2005-11-06 to 2024-12-24.

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	7% 8% 2 5% 3 3 4 3 5% 6% 53%	105193
	Night	9% 9% 2 5% 3 3 3 3 5% 6% 51%	103621
Transparency	Day	29% 7% 11% 18% 20% 14%	104958
	Night	31% 8% 12% 20% 18% 11%	103445
Seeing	Day	11% 7% 20% 32% 23% 7%	14948
	Night	12% 4 16% 39% 26% 3	25573

By Month

Percentage Cloud Cover SW (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% 18% 5% 12% 5% 4 3 4 5% 5% 30%	2858
	11th to 20th	Day	13% 22% 5% 11% 4 3 3 2 3 4 30%	2860
	21st to end	Day	11% 16% 5% 9% 3 2 3 2 4 6% 37%	3146
February	1st to 10th	Day	13% 15% 3 7% 5% 4 4 3 5% 6% 35%	1710
	11th to 20th	Day	12% 15% 3 7% 3 3 3 2 5% 7% 39%	1731
	21st to end	Day	10% 15% 5% 8% 4 4 4 3 5% 6% 36%	1570
March	1st to 10th	Day	7% 16% 5% 9% 3 3 3 4 5% 7% 38%	3376
	11th to 20th	Day	7% 10% 4 5% 4 4 4 3 6% 7% 45%	3490
	21st to end	Day	13% 19% 4 7% 5% 5% 5% 4 6% 7% 26%	3825
April	1st to 10th	Day	11% 10% 3 6% 4 4 4 3 5% 6% 44%	2113
	11th to 20th	Day	9% 11% 2 6% 4 5% 4 3 5% 6% 45%	2376
	21st to end	Day	10% 9% 2 4 4 3 4 3 4 6% 49%	2470
May	1st to 10th	Day	9% 12% 3 5% 4 4 5% 4 6% 7% 41%	4110
	11th to 20th	Day	7% 4 1 4 3 3 4 4 6% 8% 56%	4071
	21st to end	Day	9% 6% 2 4 3 4 4 4 6% 7% 51%	4477
June	1st to 10th	Day	5% 3 1 4 4 3 4 3 6% 8% 58%	2470
	11th to 20th	Day	6% 4 1 3 3 2 3 2 4 6% 65%	2544
	21st to end	Day	3 3 1 3 2 3 3 3 4 6% 71%	2470
July	1st to 10th	Day	2 1 2 2 2 3 2 3 4 79%	4110
	11th to 20th	Day	2 3 1 4 3 4 5% 5% 7% 8% 58%	4110
	21st to end	Day	3 2 1 3 3 3 3 3 5% 7% 67%	4454
August	1st to 10th	Day	3 2 1 4 3 3 4 3 6% 6% 65%	4092
	11th to 20th	Day	2 2 1 4 3 4 5% 3 4 5% 67%	4028
	21st to end	Day	3 2 1 3 2 3 3 2 5% 5% 70%	4464
September	1st to 10th	Day	4 3 1 3 2 2 4 3 5% 5% 70%	2152
	11th to 20th	Day	6% 2 1 3 2 2 3 3 5% 6% 67%	2090
	21st to end	Day	6% 3 1 3 2 2 3 2 3 5% 70%	2090
October	1st to 10th	Day	6% 3 1 3 2 2 3 2 3 4 70%	3313
	11th to 20th	Day	7% 7% 2 4 3 3 3 3 4 6% 57%	3150
	21st to end	Day	8% 7% 2 6% 3 2 2 2 4 4 59%	3438
November	1st to 10th	Day	12% 9% 3 4 3 3 3 3 4 6% 51%	1781
	11th to 20th	Day	14% 7% 3 5% 4 3 4 3 4 6% 47%	1800
	21st to end	Day	9% 10% 4 6% 3 3 4 3 5% 7% 44%	1675
December	1st to 10th	Day	12% 11% 4 6% 3 4 3 2 4 7% 43%	1600
	11th to 20th	Day	14% 14% 3 8% 4 4 4 3 6% 7% 33%	2006
	21st to end	Day	11% 14% 4 8% 4 4 4 4 5% 7% 36%	3173
January	1st to 10th	Night	9% 15% 6% 9% 3 3 4 3 5% 8% 33%	4097
	11th to 20th	Night	15% 20% 5% 11% 4 4 3 2 4 5% 28%	4110
	21st to end	Night	11% 16% 4 8% 3 3 4 3 4 6% 37%	4521
February	1st to 10th	Night	12% 14% 4 7% 4 3 4 4 4 6% 39%	2470
	11th to 20th	Night	11% 14% 3 6% 4 3 3 3 4 7% 43%	2334
	21st to end	Night	8% 17% 4 7% 4 3 3 4 5% 6% 39%	1884
March	1st to 10th	Night	10% 17% 6% 9% 4 4 4 3 5% 6% 34%	3496
	11th to 20th	Night	7% 9% 2 5% 4 3 4 3 5% 7% 50%	3490
	21st to end	Night	13% 18% 4 7% 4 3 4 3 4 6% 34%	3828
April	1st to 10th	Night	13% 12% 1 4 3 3 3 3 4 5% 50%	2066
	11th to 20th	Night	9% 8% 2 5% 4 4 4 4 4 6% 50%	1748
	21st to end	Night	12% 9% 1 5% 3 3 4 3 5% 5% 51%	1710
May	1st to 10th	Night	12% 11% 2 4 3 3 4 4 5% 7% 44%	2860
	11th to 20th	Night	8% 3 1 3 3 3 3 3 5% 6% 62%	2834
	21st to end	Night	9% 6% 2 4 3 2 3 4 5% 7% 55%	3110
June	1st to 10th	Night	6% 4 1 3 4 3 4 4 6% 7% 58%	1704
	11th to 20th	Night	5% 4 1 3 2 3 3 2 5% 4 67%	1578
	21st to end	Night	3 2 1 3 2 2 3 3 5% 6% 69%	1710
July	1st to 10th	Night	3 3 1 3 2 3 3 2 4 4 72%	2860
	11th to 20th	Night	3 3 1 5% 4 5% 6% 6% 8% 9% 48%	2860
	21st to end	Night	3 4 1 5% 4 5% 4 4 5% 6% 58%	3109
August	1st to 10th	Night	3 3 1 4 3 4 5% 4 5% 7% 60%	2847
	11th to 20th	Night	4 4 2 5% 3 4 4 3 6% 6% 60%	2811
	21st to end	Night	4 3 2 2 2 2 2 4 5% 72%	3203
September	1st to 10th	Night	3 2 1 3 3 3 3 3 4 5% 69%	2023
	11th to 20th	Night	6% 3 1 3 2 2 3 3 4 8% 65%	2090
	21st to end	Night	7% 3 1 3 2 2 2 2 3 5% 70%	2090
October	1st to 10th	Night	5% 3 1 2 2 2 2 2 3 4 73%	3660
	11th to 20th	Night	10% 6% 1 3 2 3 3 2 4 5% 60%	3778
	21st to end	Night	6% 7% 2 4 3 2 3 3 4 7% 59%	4120
November	1st to 10th	Night	10% 8% 2 4 3 3 2 3 5% 7% 54%	2516
	11th to 20th	Night	12% 7% 2 4 3 3 3 3 5% 7% 52%	2600
	21st to end	Night	10% 11% 3 5% 3 3 3 3 5% 6% 48%	2699
December	1st to 10th	Night	12% 12% 2 6% 3 3 3 3 4 6% 46%	2800
	11th to 20th	Night	15% 14% 3 6% 3 3 4 3 6% 7% 36%	3411
	21st to end	Night	10% 13% 4 10% 5% 4 4 4 5% 7% 33%	4594

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	49% 15% 14% 17% 5%	2859
	11th to 20th	Day	57% 9% 16% 12% 5% 1	2860
	21st to end	Day	47% 10% 20% 15% 6% 1	3146
February	1st to 10th	Day	49% 9% 16% 16% 9% 1	1710
	11th to 20th	Day	45% 9% 14% 19% 12% 1	1740
	21st to end	Day	46% 6% 15% 21% 11% 1	1570
March	1st to 10th	Day	37% 12% 15% 21% 14% 1	3376
	11th to 20th	Day	30% 12% 16% 28% 11% 2	3490
	21st to end	Day	50% 13% 16% 14% 6% 1	3825
April	1st to 10th	Day	39% 7% 14% 19% 16% 5%	2113
	11th to 20th	Day	38% 8% 12% 20% 19% 3	2379
	21st to end	Day	34% 6% 13% 25% 20% 3	2470
May	1st to 10th	Day	33% 11% 14% 19% 19% 5%	4110
	11th to 20th	Day	19% 8% 12% 28% 29% 4	4074
	21st to end	Day	25% 9% 12% 25% 24% 5%	4477
June	1st to 10th	Day	20% 7% 10% 20% 30% 13%	2470
	11th to 20th	Day	19% 5% 8% 15% 30% 24%	2544
	21st to end	Day	14% 4 5% 11% 26% 40%	2470
July	1st to 10th	Day	10% 3 5% 8% 13% 61%	4046
	11th to 20th	Day	19% 5% 8% 11% 25% 32%	4041
	21st to end	Day	15% 5% 8% 11% 22% 39%	4314
August	1st to 10th	Day	17% 5% 8% 19% 27% 25%	4078
	11th to 20th	Day	17% 5% 6% 15% 29% 26%	4067
	21st to end	Day	14% 5% 7% 14% 28% 32%	4463
September	1st to 10th	Day	15% 5% 9% 14% 29% 30%	2152
	11th to 20th	Day	17% 6% 9% 18% 26% 24%	2090
	21st to end	Day	18% 3 7% 20% 32% 20%	2090
October	1st to 10th	Day	18% 4 6% 12% 39% 20%	3313
	11th to 20th	Day	28% 5% 7% 25% 28% 8%	3150
	21st to end	Day	26% 6% 11% 26% 26% 5%	3438
November	1st to 10th	Day	33% 7% 11% 25% 20% 4	1781
	11th to 20th	Day	36% 7% 14% 27% 16% 1	1800
	21st to end	Day	36% 8% 16% 25% 13% 3	1675
December	1st to 10th	Day	41% 7% 18% 24% 9% 2	1600
	11th to 20th	Day	45% 10% 17% 20% 9%	2004
	21st to end	Day	42% 11% 17% 18% 12% 1	3173
January	1st to 10th	Night	44% 16% 14% 18% 7%	4097
	11th to 20th	Night	57% 10% 15% 13% 5% 1	4110
	21st to end	Night	43% 13% 23% 13% 7% 1	4521
February	1st to 10th	Night	45% 7% 20% 19% 8% 1	2470
	11th to 20th	Night	41% 7% 14% 23% 13% 2	2335
	21st to end	Night	42% 8% 11% 24% 12% 2	1884
March	1st to 10th	Night	40% 13% 14% 22% 11%	3496
	11th to 20th	Night	28% 10% 15% 31% 15% 2	3490
	21st to end	Night	45% 11% 11% 18% 13% 2	3828
April	1st to 10th	Night	36% 5% 13% 22% 17% 7%	2066
	11th to 20th	Night	32% 6% 14% 23% 22% 3	1748
	21st to end	Night	32% 6% 15% 24% 20% 2	1710
May	1st to 10th	Night	33% 8% 13% 20% 21% 4	2860
	11th to 20th	Night	19% 7% 10% 28% 30% 6%	2835
	21st to end	Night	23% 7% 11% 28% 28% 3	3110
June	1st to 10th	Night	20% 8% 11% 21% 29% 12%	1704
	11th to 20th	Night	18% 4 8% 16% 29% 24%	1578
	21st to end	Night	14% 4 6% 10% 26% 40%	1710
July	1st to 10th	Night	14% 4 5% 7% 10% 59%	2815
	11th to 20th	Night	24% 7% 9% 10% 23% 27%	2806
	21st to end	Night	24% 6% 8% 11% 20% 31%	3011
August	1st to 10th	Night	19% 6% 10% 20% 21% 24%	2838
	11th to 20th	Night	22% 6% 8% 15% 24% 26%	2838
	21st to end	Night	14% 4 8% 13% 27% 34%	3203
September	1st to 10th	Night	16% 4 7% 15% 26% 32%	2023
	11th to 20th	Night	17% 5% 9% 19% 26% 24%	2090
	21st to end	Night	18% 3 6% 16% 32% 25%	2090
October	1st to 10th	Night	15% 4 6% 14% 35% 26%	3660
	11th to 20th	Night	25% 5% 7% 25% 29% 10%	3778
	21st to end	Night	24% 5% 9% 31% 28% 4	4120
November	1st to 10th	Night	28% 7% 11% 25% 23% 6%	2516
	11th to 20th	Night	30% 6% 15% 30% 17% 3	2600
	21st to end	Night	37% 5% 11% 27% 16% 3	2699
December	1st to 10th	Night	39% 7% 17% 26% 11% 2	2800
	11th to 20th	Night	43% 10% 18% 20% 9%	3412
	21st to end	Night	46% 11% 15% 17% 10%	4594

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% 6% 19% 31% 16% 6%	270
	11th to 20th	Day	18% 5% 18% 36% 18% 5%	270
	21st to end	Day	16% 3 18% 35% 22% 6%	297
February	1st to 10th	Day	23% 5% 23% 36% 11% 3	270
	11th to 20th	Day	15% 7% 26% 34% 14% 4	336
	21st to end	Day	13% 7% 29% 35% 13% 2	382
March	1st to 10th	Day	15% 11% 23% 33% 15% 4	460
	11th to 20th	Day	12% 7% 26% 33% 19% 3	460
	21st to end	Day	20% 10% 29% 28% 12% 1	506
April	1st to 10th	Day	15% 10% 23% 30% 18% 4	460
	11th to 20th	Day	13% 13% 27% 27% 15% 4	452
	21st to end	Day	15% 12% 24% 28% 17% 4	460
May	1st to 10th	Day	11% 12% 25% 33% 16% 3	463
	11th to 20th	Day	9% 6% 25% 30% 23% 7%	488
	21st to end	Day	10% 6% 22% 27% 26% 8%	529
June	1st to 10th	Day	3 8% 18% 37% 27% 8%	490
	11th to 20th	Day	9% 7% 23% 32% 24% 5%	564
	21st to end	Day	4 6% 25% 35% 24% 6%	490
July	1st to 10th	Day	3 5% 15% 36% 33% 8%	486
	11th to 20th	Day	4 3 14% 41% 30% 7%	484
	21st to end	Day	4 2 12% 36% 35% 12%	531
August	1st to 10th	Day	5% 2 15% 39% 31% 8%	487
	11th to 20th	Day	4 5% 16% 30% 35% 10%	477
	21st to end	Day	5% 4 13% 31% 37% 9%	539
September	1st to 10th	Day	4 3 14% 34% 33% 12%	490
	11th to 20th	Day	8% 7% 19% 29% 28% 9%	490
	21st to end	Day	7% 6% 18% 27% 33% 9%	490
October	1st to 10th	Day	8% 7% 20% 28% 27% 9%	391
	11th to 20th	Day	8% 7% 20% 26% 26% 12%	300
	21st to end	Day	11% 7% 21% 35% 22% 4	330
November	1st to 10th	Day	18% 9% 18% 27% 19% 9%	300
	11th to 20th	Day	13% 7% 19% 34% 18% 9%	300
	21st to end	Day	16% 5% 18% 30% 24% 8%	300
December	1st to 10th	Day	16% 3 17% 37% 18% 9%	300
	11th to 20th	Day	15% 7% 17% 38% 19% 4	300
	21st to end	Day	23% 5% 16% 36% 16% 3	306
January	1st to 10th	Night	19% 3 13% 41% 19% 5%	760
	11th to 20th	Night	16% 4 17% 37% 23% 3	760
	21st to end	Night	15% 4 14% 41% 22% 4	836
February	1st to 10th	Night	17% 3 18% 44% 15% 3	760
	11th to 20th	Night	15% 4 18% 44% 17% 2	744
	21st to end	Night	13% 5% 20% 45% 16% 1	628
March	1st to 10th	Night	19% 5% 18% 41% 16% 2	760
	11th to 20th	Night	10% 3 18% 44% 23% 2	760
	21st to end	Night	15% 7% 19% 41% 16% 2	836
April	1st to 10th	Night	16% 8% 18% 39% 17% 1	736
	11th to 20th	Night	11% 5% 25% 43% 15%	563
	21st to end	Night	13% 5% 21% 37% 22% 2	570
May	1st to 10th	Night	11% 5% 22% 39% 21% 1	570
	11th to 20th	Night	11% 3 13% 40% 32% 2	561
	21st to end	Night	10% 7% 16% 36% 29% 3	617
June	1st to 10th	Night	7% 6% 14% 38% 33% 2	570
	11th to 20th	Night	8% 4 21% 41% 25% 2	570
	21st to end	Night	4 4 14% 42% 34% 2	570
July	1st to 10th	Night	5% 2 14% 31% 44% 5%	555
	11th to 20th	Night	4 1 9% 43% 38% 5%	552
	21st to end	Night	5% 1 8% 29% 53% 4	624
August	1st to 10th	Night	5% 1 11% 41% 39% 2	567
	11th to 20th	Night	4 3 10% 39% 39% 5%	564
	21st to end	Night	5% 3 9% 38% 43% 3	627
September	1st to 10th	Night	4 3 11% 39% 36% 7%	693
	11th to 20th	Night	7% 4 12% 35% 36% 6%	760
	21st to end	Night	9% 3 14% 30% 37% 7%	760
October	1st to 10th	Night	10% 4 18% 38% 25% 5%	760
	11th to 20th	Night	11% 3 15% 37% 29% 5%	760
	21st to end	Night	10% 4 16% 39% 27% 3	836
November	1st to 10th	Night	12% 4 17% 38% 25% 3	780
	11th to 20th	Night	15% 5% 15% 41% 21% 4	800
	21st to end	Night	13% 5% 17% 37% 21% 6%	899
December	1st to 10th	Night	16% 3 17% 43% 19% 3	1000
	11th to 20th	Night	20% 5% 17% 40% 17% 1	1000
	21st to end	Night	21% 4 18% 42% 14% 1	865

Page generated at 2024-12-21 02:13:51. Version ac07

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