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 2024-01-16.

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	8% 7% 3 5% 4 2 4 4 5% 7% 51%	74488
	Night	9% 8% 2 5% 4 2 4 4 5% 7% 50%	75299
Transparency	Day	31% 7% 12% 19% 19% 12%	74213
	Night	32% 7% 13% 21% 18% 9%	75095
Seeing	Day	11% 6% 19% 33% 24% 7%	13688
	Night	12% 3 15% 40% 27% 3	24326

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	16% 11% 3 5% 6% 2 3 5% 5% 6% 37%	1708
	11th to 20th	Day	11% 14% 5% 8% 5% 2 3 3 4 5% 38%	1669
	21st to end	Day	11% 15% 6% 9% 5% 2 4 4 4 5% 35%	1782
February	1st to 10th	Day	13% 13% 4 6% 5% 2 5% 4 5% 6% 37%	1620
	11th to 20th	Day	12% 14% 4 7% 4 2 4 3 5% 7% 39%	1638
	21st to end	Day	11% 14% 6% 7% 5% 3 5% 4 5% 6% 35%	1480
March	1st to 10th	Day	9% 12% 5% 8% 3 3 4 4 5% 8% 39%	1935
	11th to 20th	Day	9% 10% 4 4 5% 3 4 5% 7% 7% 43%	1980
	21st to end	Day	12% 14% 3 7% 5% 3 5% 4 5% 7% 34%	2178
April	1st to 10th	Day	12% 9% 3 5% 5% 2 5% 4 5% 7% 43%	2001
	11th to 20th	Day	10% 10% 3 5% 4 3 5% 5% 5% 6% 44%	2249
	21st to end	Day	11% 9% 2 4 5% 2 3 4 4 6% 50%	2340
May	1st to 10th	Day	8% 7% 2 5% 4 3 5% 6% 7% 8% 45%	2340
	11th to 20th	Day	8% 5% 2 4 4 3 5% 5% 6% 9% 49%	2301
	21st to end	Day	8% 6% 3 4 4 3 5% 6% 8% 9% 44%	2530
June	1st to 10th	Day	5% 3 1 3 4 2 4 4 6% 8% 58%	2340
	11th to 20th	Day	6% 4 1 3 3 2 3 3 5% 6% 65%	2411
	21st to end	Day	3 3 1 2 3 2 3 3 4 6% 70%	2340
July	1st to 10th	Day	3 2 1 3 3 2 4 4 5% 7% 66%	2340
	11th to 20th	Day	3 4 1 3 4 2 5% 6% 7% 7% 58%	2340
	21st to end	Day	4 3 1 4 4 3 5% 5% 7% 9% 56%	2545
August	1st to 10th	Day	4 3 1 4 4 3 4 5% 7% 7% 58%	2328
	11th to 20th	Day	4 3 1 3 3 2 4 5% 4 6% 66%	2299
	21st to end	Day	4 3 1 3 4 2 4 3 6% 6% 64%	2543
September	1st to 10th	Day	4 2 1 2 2 2 4 3 4 5% 70%	2039
	11th to 20th	Day	6% 2 1 3 3 2 3 4 5% 5% 67%	1980
	21st to end	Day	7% 3 1 2 3 2 3 3 3 5% 69%	1980
October	1st to 10th	Day	10% 6% 2 3 4 1 3 4 5% 6% 56%	1887
	11th to 20th	Day	7% 6% 2 4 3 2 3 3 4 7% 61%	1800
	21st to end	Day	8% 6% 2 3 4 2 2 4 4 6% 58%	1980
November	1st to 10th	Day	12% 8% 3 4 4 2 3 4 4 6% 51%	1691
	11th to 20th	Day	15% 6% 3 5% 4 2 4 3 4 7% 47%	1710
	21st to end	Day	9% 10% 5% 6% 3 2 4 5% 5% 7% 45%	1592
December	1st to 10th	Day	13% 11% 5% 6% 3 4 3 3 4 6% 42%	1520
	11th to 20th	Day	15% 13% 4 7% 5% 3 4 5% 6% 7% 33%	1926
	21st to end	Day	11% 13% 4 7% 5% 3 4 5% 5% 7% 36%	3146
January	1st to 10th	Night	15% 12% 3 6% 4 2 4 4 4 8% 37%	2457
	11th to 20th	Night	12% 14% 4 7% 5% 3 4 4 4 5% 39%	2418
	21st to end	Night	11% 15% 4 7% 4 2 4 5% 4 7% 37%	2574
February	1st to 10th	Night	12% 12% 4 7% 4 2 3 5% 4 6% 41%	2340
	11th to 20th	Night	11% 13% 4 5% 4 2 3 3 4 7% 43%	2207
	21st to end	Night	8% 15% 6% 7% 5% 2 3 4 5% 6% 39%	1776
March	1st to 10th	Night	13% 14% 5% 6% 4 3 3 4 5% 6% 37%	2023
	11th to 20th	Night	10% 8% 2 4 4 2 4 4 5% 8% 48%	1980
	21st to end	Night	12% 12% 3 5% 4 2 4 4 5% 8% 40%	2178
April	1st to 10th	Night	14% 11% 2 3 3 1 4 4 4 5% 49%	1958
	11th to 20th	Night	9% 8% 2 4 5% 3 4 5% 4 7% 50%	1655
	21st to end	Night	11% 8% 2 4 4 2 4 4 4 6% 52%	1620
May	1st to 10th	Night	10% 7% 2 5% 4 2 5% 5% 5% 8% 47%	1620
	11th to 20th	Night	10% 3 2 3 4 3 3 4 6% 8% 54%	1594
	21st to end	Night	10% 6% 2 4 4 2 4 5% 6% 9% 48%	1746
June	1st to 10th	Night	6% 4 1 3 5% 2 4 5% 5% 7% 59%	1615
	11th to 20th	Night	5% 4 1 2 2 3 3 3 5% 4 68%	1494
	21st to end	Night	3 2 1 2 2 1 3 4 5% 6% 70%	1620
July	1st to 10th	Night	4 2 1 4 4 3 5% 5% 6% 8% 58%	1620
	11th to 20th	Night	4 3 1 4 5% 3 6% 6% 6% 9% 52%	1620
	21st to end	Night	4 3 1 4 6% 3 5% 6% 7% 8% 53%	1773
August	1st to 10th	Night	4 3 1 3 4 3 4 5% 6% 7% 59%	1611
	11th to 20th	Night	3 3 2 4 4 3 3 4 6% 7% 61%	1602
	21st to end	Night	3 4 1 3 4 2 4 4 5% 6% 64%	1813
September	1st to 10th	Night	3 2 1 3 3 2 3 4 4 5% 70%	1916
	11th to 20th	Night	6% 3 1 2 3 2 3 4 4 8% 65%	1980
	21st to end	Night	7% 3 1 3 2 2 2 3 2 5% 70%	1980
October	1st to 10th	Night	9% 5% 2 3 3 2 3 3 4 6% 61%	2072
	11th to 20th	Night	9% 6% 2 3 2 2 3 4 3 5% 63%	2160
	21st to end	Night	7% 7% 1 3 3 2 3 3 4 7% 61%	2376
November	1st to 10th	Night	10% 7% 2 4 3 2 3 4 5% 6% 54%	2387
	11th to 20th	Night	12% 7% 2 3 3 2 3 4 5% 7% 52%	2470
	21st to end	Night	10% 10% 4 5% 3 2 3 4 5% 6% 48%	2564
December	1st to 10th	Night	13% 12% 3 5% 3 2 3 4 4 6% 45%	2660
	11th to 20th	Night	16% 13% 3 6% 4 2 4 4 6% 7% 35%	3271
	21st to end	Night	11% 13% 4 9% 6% 3 4 5% 5% 7% 33%	4549

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	45% 10% 20% 20% 5% 1	1709
	11th to 20th	Day	48% 8% 17% 18% 8% 2	1669
	21st to end	Day	49% 10% 17% 17% 5% 2	1782
February	1st to 10th	Day	48% 9% 17% 17% 9% 1	1620
	11th to 20th	Day	45% 8% 14% 20% 12% 1	1647
	21st to end	Day	48% 6% 15% 20% 11% 1	1480
March	1st to 10th	Day	42% 7% 15% 23% 12% 1	1935
	11th to 20th	Day	38% 7% 15% 24% 13% 3	1980
	21st to end	Day	46% 8% 16% 18% 9% 3	2178
April	1st to 10th	Day	40% 7% 14% 18% 16% 6%	2001
	11th to 20th	Day	39% 8% 13% 19% 19% 3	2252
	21st to end	Day	34% 6% 12% 24% 20% 3	2340
May	1st to 10th	Day	30% 8% 14% 22% 21% 5%	2340
	11th to 20th	Day	27% 8% 12% 23% 23% 6%	2304
	21st to end	Day	30% 9% 15% 21% 19% 5%	2530
June	1st to 10th	Day	20% 6% 10% 20% 30% 13%	2340
	11th to 20th	Day	19% 5% 8% 15% 30% 24%	2411
	21st to end	Day	14% 4 5% 11% 26% 39%	2340
July	1st to 10th	Day	17% 4 8% 14% 22% 35%	2276
	11th to 20th	Day	21% 7% 9% 14% 21% 28%	2271
	21st to end	Day	20% 8% 12% 15% 20% 24%	2404
August	1st to 10th	Day	21% 7% 10% 17% 19% 25%	2313
	11th to 20th	Day	18% 5% 7% 13% 26% 30%	2299
	21st to end	Day	19% 4 9% 18% 28% 23%	2543
September	1st to 10th	Day	15% 5% 9% 13% 28% 31%	2039
	11th to 20th	Day	17% 6% 9% 18% 25% 24%	1980
	21st to end	Day	18% 3 7% 20% 32% 19%	1980
October	1st to 10th	Day	27% 7% 9% 20% 28% 9%	1887
	11th to 20th	Day	24% 5% 8% 28% 27% 8%	1800
	21st to end	Day	24% 6% 15% 28% 22% 3	1980
November	1st to 10th	Day	34% 7% 11% 24% 20% 4	1691
	11th to 20th	Day	36% 7% 14% 26% 16% 1	1710
	21st to end	Day	36% 8% 16% 25% 12% 3	1592
December	1st to 10th	Day	42% 7% 18% 21% 9% 3	1520
	11th to 20th	Day	46% 10% 17% 20% 8%	1924
	21st to end	Day	42% 11% 17% 18% 12% 1	3146
January	1st to 10th	Night	45% 10% 18% 21% 5% 1	2457
	11th to 20th	Night	47% 8% 17% 18% 8% 2	2418
	21st to end	Night	46% 10% 18% 17% 6% 2	2574
February	1st to 10th	Night	44% 7% 20% 19% 9% 1	2340
	11th to 20th	Night	41% 6% 14% 23% 14% 2	2208
	21st to end	Night	43% 8% 12% 23% 12% 2	1776
March	1st to 10th	Night	44% 7% 13% 24% 11% 1	2023
	11th to 20th	Night	32% 6% 16% 26% 17% 3	1980
	21st to end	Night	40% 8% 14% 24% 10% 3	2178
April	1st to 10th	Night	37% 5% 13% 21% 17% 7%	1958
	11th to 20th	Night	32% 6% 14% 23% 22% 3	1655
	21st to end	Night	31% 6% 15% 25% 21% 2	1620
May	1st to 10th	Night	32% 8% 13% 22% 20% 5%	1620
	11th to 20th	Night	27% 6% 11% 24% 25% 7%	1595
	21st to end	Night	29% 7% 15% 26% 19% 4	1746
June	1st to 10th	Night	20% 7% 10% 21% 29% 12%	1615
	11th to 20th	Night	18% 4 8% 17% 28% 25%	1494
	21st to end	Night	14% 4 6% 11% 26% 40%	1620
July	1st to 10th	Night	21% 6% 8% 13% 18% 34%	1575
	11th to 20th	Night	24% 7% 11% 13% 19% 27%	1566
	21st to end	Night	25% 8% 12% 15% 17% 23%	1674
August	1st to 10th	Night	20% 7% 13% 20% 17% 23%	1602
	11th to 20th	Night	20% 5% 9% 16% 21% 29%	1602
	21st to end	Night	18% 5% 9% 18% 25% 24%	1813
September	1st to 10th	Night	16% 4 7% 15% 26% 32%	1916
	11th to 20th	Night	17% 5% 9% 18% 25% 25%	1980
	21st to end	Night	19% 3 6% 16% 32% 24%	1980
October	1st to 10th	Night	24% 6% 9% 22% 28% 11%	2072
	11th to 20th	Night	24% 5% 9% 24% 31% 9%	2160
	21st to end	Night	23% 6% 12% 30% 24% 5%	2376
November	1st to 10th	Night	28% 7% 11% 26% 22% 6%	2387
	11th to 20th	Night	30% 6% 16% 29% 17% 3	2470
	21st to end	Night	37% 6% 12% 27% 16% 4	2564
December	1st to 10th	Night	39% 7% 17% 24% 10% 2	2660
	11th to 20th	Night	43% 10% 18% 20% 9%	3272
	21st to end	Night	47% 11% 15% 17% 10%	4549

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% 4 19% 37% 18% 5%	257
	21st to end	Day	17% 3 19% 36% 19% 6%	264
February	1st to 10th	Day	22% 3 23% 38% 11% 3	240
	11th to 20th	Day	15% 6% 27% 35% 14% 4	303
	21st to end	Day	13% 7% 30% 34% 13% 2	346
March	1st to 10th	Day	15% 10% 24% 34% 14% 4	420
	11th to 20th	Day	13% 7% 25% 33% 20% 3	420
	21st to end	Day	20% 10% 31% 26% 13% 1	462
April	1st to 10th	Day	16% 9% 23% 30% 19% 4	420
	11th to 20th	Day	14% 14% 28% 26% 14% 4	412
	21st to end	Day	16% 12% 25% 25% 18% 4	420
May	1st to 10th	Day	10% 11% 25% 34% 17% 4	423
	11th to 20th	Day	9% 6% 25% 31% 22% 7%	448
	21st to end	Day	10% 6% 22% 29% 25% 8%	485
June	1st to 10th	Day	3 8% 18% 37% 26% 8%	450
	11th to 20th	Day	7% 6% 23% 33% 25% 6%	521
	21st to end	Day	4 4 22% 37% 26% 7%	450
July	1st to 10th	Day	2 3 12% 39% 35% 9%	446
	11th to 20th	Day	4 3 13% 41% 32% 7%	444
	21st to end	Day	4 1 9% 36% 38% 13%	487
August	1st to 10th	Day	5% 1 13% 39% 33% 9%	447
	11th to 20th	Day	3 3 14% 32% 38% 11%	437
	21st to end	Day	4 3 11% 33% 39% 10%	495
September	1st to 10th	Day	5% 2 12% 33% 35% 12%	450
	11th to 20th	Day	6% 4 19% 29% 31% 10%	450
	21st to end	Day	7% 6% 17% 26% 35% 9%	450
October	1st to 10th	Day	9% 8% 20% 26% 27% 10%	357
	11th to 20th	Day	7% 7% 16% 28% 29% 13%	270
	21st to end	Day	9% 5% 20% 36% 24% 5%	297
November	1st to 10th	Day	18% 7% 17% 28% 21% 10%	270
	11th to 20th	Day	13% 6% 15% 37% 20% 10%	270
	21st to end	Day	15% 3 15% 32% 26% 9%	270
December	1st to 10th	Day	17% 3 16% 36% 18% 10%	270
	11th to 20th	Day	14% 6% 15% 40% 21% 4	270
	21st to end	Day	23% 5% 16% 36% 16% 3	297
January	1st to 10th	Night	19% 3 13% 41% 19% 5%	760
	11th to 20th	Night	16% 4 17% 37% 23% 3	744
	21st to end	Night	16% 4 14% 42% 21% 4	792
February	1st to 10th	Night	17% 2 18% 45% 15% 3	720
	11th to 20th	Night	15% 3 18% 44% 18% 2	704
	21st to end	Night	13% 4 20% 46% 16% 1	592
March	1st to 10th	Night	19% 4 18% 40% 16% 2	720
	11th to 20th	Night	11% 3 18% 44% 23% 2	720
	21st to end	Night	15% 7% 19% 42% 16% 2	792
April	1st to 10th	Night	17% 8% 18% 38% 17% 1	698
	11th to 20th	Night	11% 5% 25% 43% 16%	533
	21st to end	Night	13% 5% 21% 36% 22% 2	540
May	1st to 10th	Night	11% 5% 22% 40% 22% 1	540
	11th to 20th	Night	11% 2 12% 39% 32% 3	531
	21st to end	Night	10% 7% 15% 36% 28% 3	584
June	1st to 10th	Night	7% 6% 15% 37% 33% 2	540
	11th to 20th	Night	7% 4 19% 42% 26% 2	540
	21st to end	Night	4 3 12% 43% 36% 2	540
July	1st to 10th	Night	4 1 10% 32% 46% 6%	525
	11th to 20th	Night	4 1 8% 43% 39% 5%	522
	21st to end	Night	5% 1 6% 28% 55% 4	591
August	1st to 10th	Night	6% 1 9% 41% 40% 2	537
	11th to 20th	Night	3 1 9% 41% 41% 5%	534
	21st to end	Night	5% 2 8% 38% 45% 3	594
September	1st to 10th	Night	4 2 10% 39% 38% 7%	656
	11th to 20th	Night	7% 2 10% 36% 38% 7%	720
	21st to end	Night	9% 3 12% 30% 39% 8%	720
October	1st to 10th	Night	10% 4 17% 37% 27% 6%	720
	11th to 20th	Night	10% 3 13% 38% 31% 5%	720
	21st to end	Night	10% 3 14% 41% 29% 4	792
November	1st to 10th	Night	13% 3 15% 39% 26% 4	740
	11th to 20th	Night	14% 3 13% 43% 22% 4	760
	21st to end	Night	13% 4 16% 39% 22% 6%	854
December	1st to 10th	Night	17% 3 17% 43% 19% 3	950
	11th to 20th	Night	20% 5% 14% 42% 17% 1	950
	21st to end	Night	21% 4 17% 42% 15% 1	851

Page generated at 2024-01-13 17:28:29. Version ac07

Copyright 2024 Attilla Danko