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 2026-04-17.

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%	140199
	Night	8% 9% 2 5% 3 3 3 3 5% 6% 52%	139450
Transparency	Day	28% 7% 11% 19% 20% 14%	134947
	Night	30% 7% 12% 21% 19% 11%	135707
Seeing	Day	12% 8% 22% 31% 22% 6%	16678
	Night	12% 5% 18% 38% 24% 3	27359

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	12% 19% 4 10% 4 3 3 3 5% 6% 31%	5318
	11th to 20th	Day	7% 12% 3 8% 6% 4 3 2 3 6% 46%	5340
	21st to end	Day	6% 12% 4 8% 3 3 3 3 4 7% 48%	5844
February	1st to 10th	Day	13% 14% 3 7% 4 4 4 3 5% 6% 36%	1890
	11th to 20th	Day	12% 15% 4 8% 3 4 4 2 5% 7% 37%	1919
	21st to end	Day	10% 15% 4 8% 4 4 4 3 5% 6% 38%	1730
March	1st to 10th	Day	7% 16% 5% 9% 3 3 3 3 5% 7% 40%	4968
	11th to 20th	Day	7% 11% 4 7% 5% 4 5% 3 6% 6% 41%	5110
	21st to end	Day	10% 17% 4 9% 4 5% 4 3 5% 6% 31%	5607
April	1st to 10th	Day	10% 9% 3 5% 4 4 4 3 5% 7% 45%	2335
	11th to 20th	Day	9% 10% 2 6% 4 5% 4 4 5% 6% 44%	2579
	21st to end	Day	10% 9% 2 5% 4 3 4 3 5% 6% 49%	2600
May	1st to 10th	Day	8% 10% 2 5% 3 4 4 4 6% 6% 48%	5880
	11th to 20th	Day	9% 6% 2 6% 3 4 4 4 6% 8% 50%	5841
	21st to end	Day	6% 5% 2 3 2 3 4 4 6% 7% 57%	6423
June	1st to 10th	Day	5% 3 1 4 4 3 4 4 6% 8% 58%	2600
	11th to 20th	Day	6% 4 1 3 3 2 3 2 4 6% 66%	2676
	21st to end	Day	3 3 1 3 2 2 3 2 4 6% 71%	2600
July	1st to 10th	Day	2 2 1 3 2 3 3 3 3 4 75%	5879
	11th to 20th	Day	2 3 1 4 3 4 4 4 6% 7% 61%	5880
	21st to end	Day	2 2 1 3 2 3 3 3 4 6% 71%	6401
August	1st to 10th	Day	2 2 1 3 3 3 3 3 5% 6% 70%	5779
	11th to 20th	Day	2 2 1 3 3 3 4 3 3 5% 72%	5798
	21st to end	Day	4 3 1 4 4 4 4 4 6% 6% 60%	6385
September	1st to 10th	Day	4 3 1 3 2 3 4 3 5% 5% 69%	2265
	11th to 20th	Day	5% 3 1 3 3 3 3 3 5% 6% 66%	2200
	21st to end	Day	6% 3 1 3 2 2 3 2 4 5% 69%	2200
October	1st to 10th	Day	6% 3 1 3 2 2 3 2 4 4 70%	3418
	11th to 20th	Day	7% 7% 2 4 3 3 3 3 4 6% 58%	3250
	21st to end	Day	8% 7% 2 6% 3 2 2 2 4 4 59%	3548
November	1st to 10th	Day	11% 8% 3 4 3 3 3 3 4 6% 52%	1872
	11th to 20th	Day	14% 7% 3 6% 4 3 4 3 4 6% 46%	1890
	21st to end	Day	9% 10% 4 6% 3 3 4 3 6% 8% 44%	1759
December	1st to 10th	Day	8% 7% 2 6% 4 3 3 1 4 9% 53%	2700
	11th to 20th	Day	10% 13% 3 7% 4 4 4 3 6% 9% 38%	3106
	21st to end	Day	15% 15% 4 8% 4 4 4 4 5% 6% 33%	4609
January	1st to 10th	Night	13% 15% 5% 8% 3 3 3 3 4 8% 34%	7605
	11th to 20th	Night	9% 11% 3 8% 4 3 3 2 3 7% 46%	7650
	21st to end	Night	6% 11% 3 7% 2 3 3 3 4 7% 50%	8370
February	1st to 10th	Night	12% 14% 4 7% 3 3 4 4 4 6% 39%	2730
	11th to 20th	Night	11% 15% 3 6% 4 3 3 3 4 7% 41%	2586
	21st to end	Night	8% 16% 4 7% 4 3 2 3 4 6% 42%	2076
March	1st to 10th	Night	11% 17% 5% 7% 3 3 3 3 4 6% 37%	5144
	11th to 20th	Night	8% 12% 3 7% 4 4 4 3 5% 7% 43%	5110
	21st to end	Night	12% 15% 3 7% 4 3 4 3 5% 6% 40%	5610
April	1st to 10th	Night	13% 11% 1 4 3 2 3 3 4 5% 51%	2284
	11th to 20th	Night	9% 8% 2 5% 4 4 4 4 4 6% 50%	1898
	21st to end	Night	11% 9% 1 5% 4 3 4 3 5% 6% 50%	1800
May	1st to 10th	Night	10% 9% 2 4 3 3 4 4 5% 6% 51%	4100
	11th to 20th	Night	7% 4 1 4 3 3 3 3 6% 7% 58%	4074
	21st to end	Night	7% 5% 1 4 3 3 3 3 5% 7% 59%	4474
June	1st to 10th	Night	6% 4 1 3 4 3 4 4 6% 6% 59%	1794
	11th to 20th	Night	5% 4 1 3 2 3 3 2 4 4 68%	1662
	21st to end	Night	3 2 2 3 2 2 3 3 5% 6% 70%	1800
July	1st to 10th	Night	3 2 1 5% 3 4 3 2 3 4 70%	4100
	11th to 20th	Night	3 4 1 5% 4 5% 5% 5% 7% 8% 52%	4100
	21st to end	Night	2 3 1 4 3 4 4 3 5% 6% 64%	4472
August	1st to 10th	Night	2 2 1 3 2 3 4 3 5% 7% 67%	4033
	11th to 20th	Night	3 3 1 4 3 4 4 3 5% 5% 63%	4051
	21st to end	Night	6% 5% 2 5% 4 4 4 3 5% 5% 59%	4592
September	1st to 10th	Night	3 2 1 3 3 3 3 3 5% 6% 68%	2130
	11th to 20th	Night	6% 3 1 3 2 2 4 3 4 7% 64%	2200
	21st to end	Night	6% 3 1 3 2 2 2 3 3 5% 69%	2200
October	1st to 10th	Night	5% 3 1 3 2 2 2 2 3 4 73%	3775
	11th to 20th	Night	11% 6% 1 3 2 3 3 2 4 5% 60%	3898
	21st to end	Night	6% 7% 2 4 3 2 3 2 4 7% 59%	4252
November	1st to 10th	Night	10% 8% 2 4 2 3 2 3 5% 7% 55%	2645
	11th to 20th	Night	12% 7% 2 4 3 3 3 3 5% 7% 52%	2730
	21st to end	Night	10% 11% 3 6% 3 3 3 3 5% 6% 48%	2834
December	1st to 10th	Night	7% 8% 2 5% 3 3 3 2 5% 9% 54%	4710
	11th to 20th	Night	12% 13% 3 5% 2 3 3 3 5% 6% 45%	5321
	21st to end	Night	13% 14% 4 9% 4 4 4 4 5% 7% 33%	6640

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	48% 12% 17% 15% 7%	5321
	11th to 20th	Day	37% 9% 14% 27% 13% 1	5340
	21st to end	Day	35% 7% 18% 21% 17% 1	5844
February	1st to 10th	Day	48% 8% 16% 17% 10% 2	1890
	11th to 20th	Day	46% 9% 14% 18% 12% 1	1928
	21st to end	Day	45% 6% 14% 22% 12% 1	1730
March	1st to 10th	Day	37% 11% 12% 21% 15% 4	4968
	11th to 20th	Day	35% 11% 14% 26% 13% 2	5110
	21st to end	Day	47% 11% 17% 17% 7% 1	5607
April	1st to 10th	Day	37% 7% 14% 20% 17% 5%	2335
	11th to 20th	Day	37% 8% 12% 20% 19% 3	2582
	21st to end	Day	34% 7% 12% 25% 20% 3	2600
May	1st to 10th	Day	29% 10% 12% 22% 22% 5%	5880
	11th to 20th	Day	25% 8% 11% 25% 27% 4	5844
	21st to end	Day	21% 7% 10% 25% 32% 5%	6424
June	1st to 10th	Day	19% 6% 10% 21% 32% 13%	2600
	11th to 20th	Day	18% 5% 7% 14% 30% 26%	2676
	21st to end	Day	13% 4 5% 11% 28% 39%	2600
July	1st to 10th	Day	13% 3 4 7% 14% 59%	5816
	11th to 20th	Day	18% 5% 7% 10% 24% 36%	5811
	21st to end	Day	12% 5% 6% 10% 23% 44%	6217
August	1st to 10th	Day	17% 5% 8% 19% 26% 26%	4208
	11th to 20th	Day	17% 5% 6% 15% 30% 27%	4197
	21st to end	Day	15% 5% 7% 14% 28% 31%	4604
September	1st to 10th	Day	15% 5% 9% 14% 28% 29%	2265
	11th to 20th	Day	18% 6% 8% 18% 27% 23%	2200
	21st to end	Day	18% 3 7% 20% 32% 20%	2200
October	1st to 10th	Day	18% 4 6% 13% 39% 20%	3418
	11th to 20th	Day	28% 5% 7% 25% 28% 7%	3250
	21st to end	Day	26% 6% 11% 26% 26% 5%	3548
November	1st to 10th	Day	32% 6% 11% 26% 20% 5%	1872
	11th to 20th	Day	36% 7% 14% 27% 15% 1	1890
	21st to end	Day	35% 8% 16% 26% 13% 2	1759
December	1st to 10th	Day	28% 7% 13% 42% 8% 1	2700
	11th to 20th	Day	38% 9% 18% 24% 11% 1	3104
	21st to end	Day	47% 9% 13% 20% 10%	4609
January	1st to 10th	Night	46% 12% 17% 16% 9%	7605
	11th to 20th	Night	37% 7% 15% 29% 11% 1	7650
	21st to end	Night	31% 9% 18% 23% 19% 1	8370
February	1st to 10th	Night	44% 7% 19% 20% 9% 1	2730
	11th to 20th	Night	42% 7% 14% 22% 13% 1	2587
	21st to end	Night	41% 7% 11% 26% 13% 3	2076
March	1st to 10th	Night	41% 10% 11% 22% 12% 4	5144
	11th to 20th	Night	35% 9% 14% 25% 16% 2	5110
	21st to end	Night	39% 10% 16% 20% 14% 2	5610
April	1st to 10th	Night	35% 5% 12% 23% 19% 6%	2284
	11th to 20th	Night	32% 6% 13% 23% 22% 4	1898
	21st to end	Night	32% 7% 14% 24% 20% 2	1800
May	1st to 10th	Night	29% 8% 12% 21% 27% 4	4097
	11th to 20th	Night	21% 7% 10% 27% 31% 5%	4075
	21st to end	Night	20% 6% 9% 26% 36% 3	4474
June	1st to 10th	Night	20% 8% 10% 20% 30% 13%	1794
	11th to 20th	Night	18% 4 8% 16% 29% 26%	1662
	21st to end	Night	14% 4 6% 10% 27% 39%	1800
July	1st to 10th	Night	17% 4 4 6% 11% 57%	4055
	11th to 20th	Night	23% 6% 7% 9% 21% 34%	4046
	21st to end	Night	18% 5% 6% 9% 20% 41%	4344
August	1st to 10th	Night	18% 6% 10% 19% 21% 25%	2928
	11th to 20th	Night	22% 6% 8% 14% 24% 26%	2928
	21st to end	Night	15% 4 8% 13% 26% 33%	3304
September	1st to 10th	Night	16% 4 7% 16% 26% 31%	2130
	11th to 20th	Night	18% 5% 9% 18% 26% 24%	2200
	21st to end	Night	19% 3 6% 16% 31% 24%	2200
October	1st to 10th	Night	15% 4 6% 14% 35% 26%	3775
	11th to 20th	Night	25% 5% 7% 25% 28% 9%	3898
	21st to end	Night	24% 5% 9% 30% 28% 3	4252
November	1st to 10th	Night	27% 7% 11% 26% 23% 6%	2645
	11th to 20th	Night	30% 6% 15% 30% 16% 2	2730
	21st to end	Night	37% 6% 12% 27% 16% 3	2834
December	1st to 10th	Night	28% 6% 15% 36% 14% 1	4710
	11th to 20th	Night	37% 8% 22% 24% 9%	5322
	21st to end	Night	47% 11% 15% 19% 8%	6640

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	24% 10% 22% 27% 13% 5%	330
	11th to 20th	Day	16% 7% 23% 33% 16% 5%	330
	21st to end	Day	15% 7% 25% 31% 18% 5%	363
February	1st to 10th	Day	25% 6% 25% 31% 10% 2	330
	11th to 20th	Day	20% 10% 26% 29% 12% 3	404
	21st to end	Day	14% 9% 31% 33% 12% 2	446
March	1st to 10th	Day	16% 14% 23% 31% 13% 3	540
	11th to 20th	Day	15% 10% 26% 30% 16% 2	540
	21st to end	Day	20% 12% 30% 26% 11% 1	594
April	1st to 10th	Day	16% 10% 24% 29% 17% 3	540
	11th to 20th	Day	15% 16% 27% 26% 14% 3	517
	21st to end	Day	15% 14% 24% 27% 17% 4	500
May	1st to 10th	Day	10% 12% 26% 34% 15% 3	503
	11th to 20th	Day	12% 7% 24% 29% 22% 6%	528
	21st to end	Day	9% 7% 23% 27% 25% 8%	573
June	1st to 10th	Day	3 9% 18% 36% 26% 7%	530
	11th to 20th	Day	10% 8% 24% 31% 22% 5%	606
	21st to end	Day	5% 7% 25% 34% 23% 6%	530
July	1st to 10th	Day	3 6% 18% 35% 31% 7%	526
	11th to 20th	Day	4 4 17% 40% 28% 7%	524
	21st to end	Day	4 3 15% 35% 32% 11%	575
August	1st to 10th	Day	6% 4 17% 36% 29% 8%	527
	11th to 20th	Day	4 5% 18% 30% 33% 9%	517
	21st to end	Day	6% 5% 14% 31% 35% 9%	583
September	1st to 10th	Day	5% 3 16% 33% 32% 11%	530
	11th to 20th	Day	8% 8% 20% 28% 27% 9%	530
	21st to end	Day	7% 7% 19% 28% 31% 8%	530
October	1st to 10th	Day	9% 8% 22% 27% 25% 8%	426
	11th to 20th	Day	10% 10% 21% 25% 24% 11%	330
	21st to end	Day	11% 8% 21% 35% 20% 4	363
November	1st to 10th	Day	17% 10% 20% 26% 18% 8%	330
	11th to 20th	Day	16% 8% 20% 33% 16% 8%	330
	21st to end	Day	16% 6% 20% 28% 22% 8%	330
December	1st to 10th	Day	16% 4 21% 35% 16% 8%	330
	11th to 20th	Day	16% 7% 18% 38% 18% 3	330
	21st to end	Day	26% 8% 17% 32% 13% 3	363
January	1st to 10th	Night	20% 5% 17% 37% 17% 4	840
	11th to 20th	Night	15% 4 20% 36% 21% 3	840
	21st to end	Night	14% 6% 18% 38% 20% 3	924
February	1st to 10th	Night	18% 4 21% 41% 13% 2	840
	11th to 20th	Night	17% 6% 19% 41% 16% 1	824
	21st to end	Night	13% 7% 22% 42% 15% 1	692
March	1st to 10th	Night	18% 6% 20% 38% 14% 2	840
	11th to 20th	Night	11% 4 22% 41% 21% 2	840
	21st to end	Night	15% 7% 23% 38% 14% 1	924
April	1st to 10th	Night	16% 8% 21% 38% 15% 1	814
	11th to 20th	Night	12% 6% 27% 40% 14%	611
	21st to end	Night	13% 6% 23% 36% 21% 2	600
May	1st to 10th	Night	11% 5% 24% 40% 20% 1	600
	11th to 20th	Night	11% 4 14% 38% 30% 2	591
	21st to end	Night	10% 7% 17% 35% 28% 3	650
June	1st to 10th	Night	7% 6% 15% 38% 32% 2	600
	11th to 20th	Night	8% 5% 23% 39% 24% 2	600
	21st to end	Night	4 4 15% 42% 33% 2	600
July	1st to 10th	Night	5% 2 15% 31% 42% 5%	585
	11th to 20th	Night	4 1 12% 42% 36% 5%	582
	21st to end	Night	5% 2 11% 29% 51% 4	657
August	1st to 10th	Night	5% 2 15% 40% 37% 2	597
	11th to 20th	Night	5% 3 12% 39% 37% 4	594
	21st to end	Night	5% 3 11% 38% 41% 2	660
September	1st to 10th	Night	4 3 13% 39% 34% 7%	730
	11th to 20th	Night	7% 5% 14% 34% 34% 6%	800
	21st to end	Night	9% 4 15% 30% 36% 7%	800
October	1st to 10th	Night	10% 5% 19% 37% 24% 5%	800
	11th to 20th	Night	12% 4 16% 36% 28% 5%	800
	21st to end	Night	10% 4 18% 39% 26% 3	880
November	1st to 10th	Night	12% 4 19% 37% 24% 3	820
	11th to 20th	Night	15% 5% 16% 40% 20% 4	840
	21st to end	Night	13% 6% 18% 37% 21% 5%	944
December	1st to 10th	Night	16% 3 19% 42% 18% 2	1050
	11th to 20th	Night	20% 5% 18% 39% 16% 1	1050
	21st to end	Night	22% 5% 19% 39% 13% 1	940

Page generated at 2026-04-14 00:44:23. Version ac07

Copyright © 2026 Attilla Danko.