Wrights Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wrights Lake Observatory clear sky chart from 2005-11-06 to 2025-04-11.

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	21% 12% 4 9% 5% 4 4 3 5% 6% 28%	114064
	Night	21% 12% 4 9% 5% 4 4 3 5% 6% 28%	114311
Transparency	Day	56% 4 10% 19% 10% 1	117693
	Night	56% 4 10% 19% 10% 1	118846
Seeing	Day	30% 17% 30% 16% 5% 1	17871
	Night	28% 15% 29% 21% 6%	25511

By Month

Percentage Cloud Cover NE (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	14% 19% 5% 17% 9% 4 4 3 3 3 19%	3616
	11th to 20th	Day	15% 17% 7% 17% 8% 6% 4 3 4 5% 14%	3735
	21st to end	Day	15% 12% 5% 9% 7% 6% 5% 3 5% 6% 28%	4466
February	1st to 10th	Day	20% 14% 5% 10% 8% 5% 5% 4 4 5% 20%	1800
	11th to 20th	Day	17% 16% 5% 12% 6% 4 4 4 5% 5% 22%	1753
	21st to end	Day	13% 15% 4 8% 8% 5% 4 3 4 6% 30%	1518
March	1st to 10th	Day	24% 15% 4 12% 6% 5% 3 2 3 4 22%	4918
	11th to 20th	Day	17% 17% 5% 9% 4 3 4 3 4 5% 29%	4980
	21st to end	Day	28% 16% 4 9% 5% 4 4 3 4 4 20%	5466
April	1st to 10th	Day	26% 16% 3 8% 4 3 3 2 4 5% 24%	2394
	11th to 20th	Day	22% 12% 3 6% 4 4 4 4 5% 6% 31%	2313
	21st to end	Day	22% 13% 3 6% 5% 4 4 3 5% 6% 29%	2470
May	1st to 10th	Day	24% 15% 4 8% 6% 4 4 3 5% 6% 20%	4202
	11th to 20th	Day	22% 12% 4 8% 5% 4 4 3 4 5% 29%	4398
	21st to end	Day	19% 9% 3 7% 4 4 4 4 7% 8% 29%	4836
June	1st to 10th	Day	25% 11% 3 8% 5% 5% 4 4 5% 7% 24%	2660
	11th to 20th	Day	22% 9% 3 7% 5% 4 4 3 5% 7% 32%	2706
	21st to end	Day	29% 10% 2 6% 5% 4 4 3 5% 6% 25%	2850
July	1st to 10th	Day	22% 10% 3 7% 4 4 4 4 5% 7% 29%	4550
	11th to 20th	Day	20% 13% 3 6% 3 4 3 3 6% 8% 30%	4440
	21st to end	Day	21% 9% 3 7% 4 4 4 4 6% 8% 30%	4669
August	1st to 10th	Day	27% 10% 3 7% 5% 4 4 4 5% 7% 25%	4105
	11th to 20th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 8% 31%	4045
	21st to end	Day	11% 6% 3 6% 3 4 5% 4 7% 8% 44%	4344
September	1st to 10th	Day	19% 7% 2 6% 4 3 4 3 5% 8% 39%	2280
	11th to 20th	Day	20% 7% 2 6% 3 3 3 3 4 6% 41%	2214
	21st to end	Day	20% 7% 3 5% 4 4 3 3 4 7% 40%	2025
October	1st to 10th	Day	19% 7% 3 6% 4 4 3 4 6% 6% 37%	1900
	11th to 20th	Day	20% 8% 3 6% 5% 3 4 3 6% 7% 34%	1900
	21st to end	Day	21% 10% 3 7% 5% 4 3 4 4 6% 31%	2090
November	1st to 10th	Day	22% 11% 3 8% 5% 5% 3 3 5% 6% 29%	1633
	11th to 20th	Day	23% 13% 4 10% 5% 4 4 3 5% 6% 24%	1600
	21st to end	Day	20% 14% 5% 10% 6% 5% 4 3 4 5% 24%	1600
December	1st to 10th	Day	22% 12% 6% 13% 5% 5% 4 3 4 6% 20%	2610
	11th to 20th	Day	30% 11% 3 7% 5% 5% 5% 3 4 4 24%	2996
	21st to end	Day	21% 13% 7% 12% 5% 3 3 3 4 6% 23%	3982
January	1st to 10th	Night	14% 16% 5% 15% 8% 5% 4 3 4 6% 19%	6349
	11th to 20th	Night	17% 14% 6% 19% 6% 5% 5% 3 4 5% 16%	6159
	21st to end	Night	13% 11% 5% 12% 8% 6% 5% 4 6% 7% 24%	6490
February	1st to 10th	Night	19% 14% 4 10% 7% 6% 4 4 5% 5% 21%	2600
	11th to 20th	Night	17% 13% 4 10% 7% 6% 5% 4 6% 6% 22%	2533
	21st to end	Night	14% 13% 3 10% 7% 5% 5% 4 5% 6% 27%	2111
March	1st to 10th	Night	25% 13% 4 12% 7% 5% 3 3 3 4 21%	4920
	11th to 20th	Night	20% 15% 7% 8% 4 4 3 3 4 4 27%	4979
	21st to end	Night	29% 14% 5% 10% 4 3 3 2 3 4 23%	5466
April	1st to 10th	Night	25% 13% 3 7% 5% 4 4 3 4 5% 27%	2005
	11th to 20th	Night	23% 11% 3 7% 5% 4 4 2 4 5% 32%	1822
	21st to end	Night	25% 12% 3 6% 5% 4 3 3 5% 6% 30%	1710
May	1st to 10th	Night	21% 17% 5% 8% 4 4 3 4 5% 7% 23%	2741
	11th to 20th	Night	23% 11% 3 7% 4 4 4 4 5% 6% 30%	2506
	21st to end	Night	20% 12% 4 9% 5% 5% 4 3 4 6% 28%	2749
June	1st to 10th	Night	28% 11% 2 6% 4 4 3 2 5% 7% 27%	1520
	11th to 20th	Night	21% 11% 3 7% 4 3 5% 3 5% 5% 34%	1483
	21st to end	Night	28% 12% 3 7% 5% 5% 4 3 5% 4 25%	1330
July	1st to 10th	Night	20% 12% 5% 10% 6% 6% 5% 3 5% 7% 23%	2365
	11th to 20th	Night	21% 12% 3 8% 5% 4 4 4 4 7% 27%	2530
	21st to end	Night	20% 9% 2 6% 4 4 4 3 4 7% 36%	2888
August	1st to 10th	Night	24% 11% 3 6% 4 3 4 3 4 6% 33%	2825
	11th to 20th	Night	18% 9% 3 7% 5% 4 4 3 6% 8% 32%	2782
	21st to end	Night	11% 7% 2 6% 3 3 3 3 5% 7% 50%	3311
September	1st to 10th	Night	18% 6% 2 5% 4 3 4 3 4 6% 43%	1900
	11th to 20th	Night	19% 7% 2 6% 4 3 3 2 4 6% 44%	1963
	21st to end	Night	21% 8% 3 7% 4 3 3 3 5% 6% 38%	2154
October	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 5% 6% 40%	2280
	11th to 20th	Night	20% 6% 2 7% 3 4 4 3 6% 7% 38%	2280
	21st to end	Night	25% 9% 2 5% 4 4 4 3 5% 6% 33%	2508
November	1st to 10th	Night	21% 10% 3 7% 4 3 4 3 5% 7% 33%	2663
	11th to 20th	Night	22% 11% 3 9% 6% 4 4 4 4 6% 26%	2800
	21st to end	Night	21% 11% 5% 9% 6% 4 5% 4 4 6% 26%	2800
December	1st to 10th	Night	17% 11% 4 13% 6% 5% 5% 4 6% 7% 24%	4580
	11th to 20th	Night	28% 12% 3 7% 4 4 4 3 4 5% 26%	5213
	21st to end	Night	25% 11% 4 10% 4 3 3 3 4 5% 28%	6996

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	69% 3 11% 10% 6%	3617
	11th to 20th	Day	70% 5% 7% 14% 3	3765
	21st to end	Day	56% 3 8% 14% 16% 3	4466
February	1st to 10th	Day	65% 4 8% 12% 9% 2	1800
	11th to 20th	Day	64% 5% 8% 15% 8% 1	1761
	21st to end	Day	56% 3 9% 18% 11% 3	1518
March	1st to 10th	Day	66% 3 7% 12% 8% 3	4922
	11th to 20th	Day	58% 3 11% 20% 7% 1	4980
	21st to end	Day	69% 3 4 14% 9% 1	5466
April	1st to 10th	Day	62% 3 7% 13% 12% 3	2394
	11th to 20th	Day	53% 4 8% 21% 13% 1	2313
	21st to end	Day	55% 3 9% 19% 12% 2	2470
May	1st to 10th	Day	65% 2 10% 16% 6% 1	4216
	11th to 20th	Day	57% 4 11% 20% 8%	4402
	21st to end	Day	52% 4 16% 19% 9% 1	4836
June	1st to 10th	Day	60% 4 10% 20% 7% 1	2660
	11th to 20th	Day	52% 4 12% 21% 11% 1	2706
	21st to end	Day	60% 4 11% 15% 9% 1	2850
July	1st to 10th	Day	55% 4 12% 17% 11% 1	4480
	11th to 20th	Day	52% 4 14% 22% 8%	4364
	21st to end	Day	52% 4 14% 20% 9% 1	4520
August	1st to 10th	Day	58% 4 12% 18% 7%	4092
	11th to 20th	Day	49% 5% 14% 26% 7%	4084
	21st to end	Day	36% 4 13% 30% 17% 1	4344
September	1st to 10th	Day	43% 4 11% 26% 14% 2	2280
	11th to 20th	Day	45% 3 10% 25% 16% 1	2214
	21st to end	Day	45% 3 8% 26% 17% 1	2025
October	1st to 10th	Day	45% 6% 10% 26% 13% 1	3160
	11th to 20th	Day	42% 4 9% 26% 18% 1	3140
	21st to end	Day	46% 3 13% 23% 13% 2	3428
November	1st to 10th	Day	57% 3 9% 21% 8% 1	1633
	11th to 20th	Day	60% 4 11% 18% 7% 1	1600
	21st to end	Day	62% 5% 11% 15% 7%	1600
December	1st to 10th	Day	64% 4 10% 19% 3	2610
	11th to 20th	Day	62% 4 8% 15% 11%	2995
	21st to end	Day	61% 4 12% 15% 8%	3982
January	1st to 10th	Night	65% 4 9% 18% 4	6349
	11th to 20th	Night	69% 5% 9% 13% 4	6204
	21st to end	Night	57% 5% 10% 16% 11% 1	6490
February	1st to 10th	Night	63% 4 9% 13% 9% 1	2600
	11th to 20th	Night	60% 5% 10% 16% 8% 1	2536
	21st to end	Night	57% 3 9% 20% 10% 2	2111
March	1st to 10th	Night	67% 3 5% 12% 11% 1	4929
	11th to 20th	Night	61% 3 8% 19% 8% 1	4980
	21st to end	Night	67% 2 5% 16% 8% 1	5466
April	1st to 10th	Night	59% 4 7% 14% 13% 3	2005
	11th to 20th	Night	54% 3 7% 20% 14% 2	1822
	21st to end	Night	55% 3 9% 19% 12% 3	1710
May	1st to 10th	Night	61% 2 14% 15% 7% 1	2747
	11th to 20th	Night	57% 2 11% 22% 8% 1	2507
	21st to end	Night	58% 3 13% 19% 6% 1	2750
June	1st to 10th	Night	59% 3 9% 21% 7%	1520
	11th to 20th	Night	53% 3 9% 23% 12% 1	1483
	21st to end	Night	64% 4 9% 15% 8% 1	1330
July	1st to 10th	Night	62% 4 11% 13% 9% 1	2326
	11th to 20th	Night	59% 3 12% 16% 9% 1	2482
	21st to end	Night	50% 4 12% 21% 12% 1	2797
August	1st to 10th	Night	55% 4 12% 24% 6%	2813
	11th to 20th	Night	49% 5% 16% 23% 6%	2809
	21st to end	Night	34% 3 15% 29% 18%	3310
September	1st to 10th	Night	42% 3 11% 26% 16% 2	1900
	11th to 20th	Night	43% 2 9% 29% 16% 1	1963
	21st to end	Night	48% 3 8% 26% 14% 1	2154
October	1st to 10th	Night	46% 4 11% 25% 12% 1	3800
	11th to 20th	Night	37% 4 11% 29% 19%	3779
	21st to end	Night	43% 4 12% 23% 17% 1	4120
November	1st to 10th	Night	51% 4 13% 20% 12% 1	2663
	11th to 20th	Night	57% 4 10% 18% 10% 1	2800
	21st to end	Night	58% 5% 13% 16% 7%	2800
December	1st to 10th	Night	57% 4 12% 23% 5%	4580
	11th to 20th	Night	59% 4 8% 17% 11% 1	5215
	21st to end	Night	59% 4 10% 18% 9% 1	6996

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	35% 22% 28% 13% 2	400
	11th to 20th	Day	36% 19% 28% 13% 4	400
	21st to end	Day	25% 24% 35% 13% 3	440
February	1st to 10th	Day	34% 23% 27% 14% 2 1	400
	11th to 20th	Day	35% 27% 26% 11% 2	396
	21st to end	Day	29% 22% 31% 12% 5%	331
March	1st to 10th	Day	31% 25% 32% 10% 2	496
	11th to 20th	Day	31% 29% 26% 9% 4 1	500
	21st to end	Day	36% 25% 26% 11% 2 1	550
April	1st to 10th	Day	43% 24% 25% 6% 2	500
	11th to 20th	Day	34% 25% 29% 9% 3	452
	21st to end	Day	33% 21% 29% 13% 4	460
May	1st to 10th	Day	35% 22% 28% 11% 3 1	520
	11th to 20th	Day	25% 19% 30% 20% 6% 1	674
	21st to end	Day	31% 14% 34% 17% 3 1	735
June	1st to 10th	Day	34% 13% 30% 18% 5%	680
	11th to 20th	Day	27% 14% 32% 19% 6% 1	680
	21st to end	Day	37% 9% 29% 18% 8% 1	680
July	1st to 10th	Day	27% 8% 32% 25% 8% 1	670
	11th to 20th	Day	27% 7% 32% 23% 10% 1	668
	21st to end	Day	31% 7% 33% 21% 7% 1	656
August	1st to 10th	Day	23% 8% 35% 25% 8% 1	486
	11th to 20th	Day	25% 9% 32% 22% 9% 3	479
	21st to end	Day	21% 11% 31% 25% 7% 4	539
September	1st to 10th	Day	20% 12% 38% 21% 7% 1	490
	11th to 20th	Day	27% 15% 31% 15% 10% 2	490
	21st to end	Day	24% 14% 31% 22% 7% 2	455
October	1st to 10th	Day	23% 15% 35% 16% 10% 1	390
	11th to 20th	Day	26% 21% 28% 16% 6% 2	390
	21st to end	Day	28% 22% 33% 11% 4 2	429
November	1st to 10th	Day	29% 18% 31% 14% 7% 1	395
	11th to 20th	Day	34% 20% 28% 13% 5% 1	400
	21st to end	Day	34% 24% 28% 12% 2 1	400
December	1st to 10th	Day	35% 19% 29% 14% 2	400
	11th to 20th	Day	42% 22% 26% 9% 2	400
	21st to end	Day	32% 23% 31% 10% 3 1	440
January	1st to 10th	Night	30% 21% 32% 14% 3	900
	11th to 20th	Night	32% 19% 30% 16% 3	867
	21st to end	Night	25% 24% 33% 15% 3	880
February	1st to 10th	Night	31% 19% 29% 18% 2	800
	11th to 20th	Night	31% 25% 29% 13% 2	783
	21st to end	Night	23% 22% 32% 19% 4	660
March	1st to 10th	Night	28% 23% 32% 15% 2	800
	11th to 20th	Night	25% 20% 30% 19% 5%	800
	21st to end	Night	29% 21% 29% 16% 5%	880
April	1st to 10th	Night	34% 18% 30% 15% 3	605
	11th to 20th	Night	29% 19% 34% 15% 4	567
	21st to end	Night	32% 14% 31% 18% 4 1	570
May	1st to 10th	Night	32% 13% 31% 18% 5%	570
	11th to 20th	Night	29% 11% 28% 24% 7% 1	562
	21st to end	Night	25% 12% 35% 23% 5%	616
June	1st to 10th	Night	35% 8% 29% 24% 5%	570
	11th to 20th	Night	26% 8% 36% 24% 6%	533
	21st to end	Night	35% 4 21% 30% 9%	380
July	1st to 10th	Night	25% 5% 29% 32% 9%	456
	11th to 20th	Night	26% 5% 29% 31% 10%	552
	21st to end	Night	24% 3 27% 34% 11%	623
August	1st to 10th	Night	21% 4 25% 35% 13% 1	568
	11th to 20th	Night	23% 5% 25% 33% 13% 2	562
	21st to end	Night	22% 5% 27% 31% 14% 1	627
September	1st to 10th	Night	21% 8% 31% 28% 12% 1	570
	11th to 20th	Night	24% 9% 25% 28% 14% 1	633
	21st to end	Night	27% 11% 27% 25% 9% 1	760
October	1st to 10th	Night	24% 9% 29% 27% 10% 1	760
	11th to 20th	Night	24% 15% 30% 21% 9% 1	760
	21st to end	Night	32% 16% 28% 20% 5%	836
November	1st to 10th	Night	28% 14% 30% 21% 5% 1	871
	11th to 20th	Night	30% 20% 26% 19% 5%	900
	21st to end	Night	31% 22% 26% 16% 5%	900
December	1st to 10th	Night	32% 18% 31% 16% 3	900
	11th to 20th	Night	36% 20% 29% 12% 3	900
	21st to end	Night	32% 18% 29% 16% 3	990

Page generated at 2025-04-08 10:17:25. Version ac07

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