Three Guys Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Three Guys Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	14% 8% 3 7% 5% 4 5% 4 6% 7% 38%	137104
	Night	15% 8% 3 7% 4 4 4 3 5% 7% 40%	135104
Transparency	Day	43% 6% 11% 23% 15% 2	140708
	Night	44% 5% 10% 22% 16% 2	139577
Seeing	Day	17% 8% 24% 25% 18% 8%	17672
	Night	18% 7% 26% 31% 16% 2	27504

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	14% 11% 6% 11% 5% 5% 5% 3 5% 6% 29%	4691
	11th to 20th	Day	11% 11% 5% 13% 7% 5% 5% 4 4 4 32%	4710
	21st to end	Day	19% 14% 5% 9% 6% 4 4 3 4 4 26%	5636
February	1st to 10th	Day	17% 11% 5% 8% 5% 3 5% 3 4 6% 32%	2100
	11th to 20th	Day	17% 10% 4 9% 5% 4 4 3 4 6% 34%	2048
	21st to end	Day	15% 10% 5% 10% 4 5% 4 3 4 6% 35%	1730
March	1st to 10th	Day	20% 11% 4 7% 4 3 3 2 3 4 40%	4580
	11th to 20th	Day	13% 10% 3 6% 4 3 3 3 5% 8% 43%	4803
	21st to end	Day	16% 9% 3 7% 5% 4 5% 4 6% 7% 35%	6004
April	1st to 10th	Day	15% 8% 3 6% 5% 4 4 4 5% 8% 38%	2520
	11th to 20th	Day	17% 8% 3 6% 4 4 3 4 5% 6% 40%	2494
	21st to end	Day	17% 7% 3 7% 4 4 4 3 5% 7% 39%	2607
May	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 4 7% 10% 30%	7616
	11th to 20th	Day	16% 7% 3 6% 5% 4 5% 3 5% 7% 39%	7626
	21st to end	Day	17% 8% 3 8% 6% 6% 6% 5% 7% 9% 28%	8628
June	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 7% 8% 45%	2940
	11th to 20th	Day	7% 6% 2 6% 4 5% 5% 4 8% 8% 46%	2871
	21st to end	Day	8% 6% 2 6% 4 5% 5% 5% 7% 9% 44%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 5% 4 4 6% 9% 44%	6360
	11th to 20th	Day	8% 5% 2 7% 5% 5% 5% 4 7% 12% 39%	6360
	21st to end	Day	10% 6% 3 8% 6% 6% 6% 6% 8% 10% 33%	6789
August	1st to 10th	Day	7% 4 2 4 3 3 5% 3 6% 10% 52%	4154
	11th to 20th	Day	8% 4 2 6% 4 5% 5% 4 8% 9% 43%	3849
	21st to end	Day	5% 3 2 5% 4 4 5% 3 6% 9% 55%	4312
September	1st to 10th	Day	10% 5% 1 6% 5% 4 4 3 5% 6% 50%	2400
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 7% 51%	2209
	21st to end	Day	12% 5% 2 4 3 4 4 3 5% 6% 52%	2200
October	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 4 7% 55%	2200
	11th to 20th	Day	13% 5% 2 6% 4 3 4 3 5% 6% 49%	2061
	21st to end	Day	20% 7% 2 6% 4 4 3 3 4 6% 42%	1980
November	1st to 10th	Day	9% 6% 2 5% 3 3 4 4 5% 6% 53%	1852
	11th to 20th	Day	17% 8% 3 6% 4 3 3 3 4 6% 42%	1890
	21st to end	Day	20% 10% 4 7% 4 3 3 3 4 5% 38%	1890
December	1st to 10th	Day	14% 10% 3 7% 4 3 4 2 4 5% 44%	3020
	11th to 20th	Day	17% 12% 6% 9% 5% 4 4 4 5% 5% 29%	3104
	21st to end	Day	27% 16% 6% 9% 6% 4 4 2 3 5% 19%	4070
January	1st to 10th	Night	16% 12% 5% 9% 4 4 4 3 4 6% 32%	8232
	11th to 20th	Night	13% 13% 5% 10% 5% 5% 6% 3 4 5% 32%	8280
	21st to end	Night	18% 11% 4 7% 5% 4 4 3 4 6% 33%	8562
February	1st to 10th	Night	20% 10% 4 8% 6% 4 4 4 5% 5% 29%	2520
	11th to 20th	Night	17% 9% 3 8% 5% 5% 4 3 5% 7% 35%	2458
	21st to end	Night	19% 11% 4 7% 3 4 4 3 5% 5% 35%	2076
March	1st to 10th	Night	21% 11% 3 6% 4 3 3 3 4 5% 36%	5493
	11th to 20th	Night	13% 6% 3 6% 4 3 4 3 4 6% 49%	5377
	21st to end	Night	18% 8% 3 8% 5% 5% 5% 3 5% 6% 34%	5198
April	1st to 10th	Night	13% 7% 2 7% 4 4 4 4 5% 8% 41%	2100
	11th to 20th	Night	15% 7% 2 5% 4 4 4 3 5% 7% 44%	2075
	21st to end	Night	17% 5% 2 6% 5% 4 5% 4 6% 7% 38%	2011
May	1st to 10th	Night	17% 8% 3 8% 4 5% 4 3 5% 9% 33%	5237
	11th to 20th	Night	14% 6% 3 7% 5% 4 5% 4 6% 8% 39%	5234
	21st to end	Night	15% 8% 3 8% 5% 5% 5% 4 6% 9% 32%	5519
June	1st to 10th	Night	9% 6% 3 5% 3 5% 4 3 6% 9% 47%	1680
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 48%	1640
	21st to end	Night	5% 4 1 4 4 4 5% 3 8% 10% 51%	1600
July	1st to 10th	Night	9% 5% 2 7% 4 4 4 4 5% 7% 47%	3618
	11th to 20th	Night	7% 4 2 5% 4 5% 5% 5% 6% 9% 47%	3617
	21st to end	Night	8% 5% 2 8% 6% 5% 6% 5% 7% 11% 37%	4076
August	1st to 10th	Night	7% 4 2 5% 3 3 4 3 5% 9% 56%	2978
	11th to 20th	Night	6% 3 2 6% 4 5% 5% 4 6% 8% 51%	3200
	21st to end	Night	4 3 1 4 3 3 4 3 6% 9% 60%	3586
September	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 7% 52%	2000
	11th to 20th	Night	7% 4 1 6% 4 4 3 4 5% 6% 55%	2170
	21st to end	Night	12% 6% 2 5% 3 4 3 3 5% 7% 50%	2200
October	1st to 10th	Night	9% 4 2 4 3 3 3 3 4 6% 60%	2200
	11th to 20th	Night	14% 6% 2 4 4 3 3 2 4 5% 52%	2332
	21st to end	Night	19% 8% 2 6% 3 4 3 2 4 6% 43%	2860
November	1st to 10th	Night	12% 6% 2 6% 4 3 3 3 4 6% 51%	2666
	11th to 20th	Night	14% 7% 3 5% 4 3 3 3 4 7% 46%	2730
	21st to end	Night	20% 9% 3 6% 4 3 3 2 4 6% 40%	2730
December	1st to 10th	Night	18% 10% 3 7% 4 3 3 3 4 5% 41%	4380
	11th to 20th	Night	19% 9% 3 7% 4 4 4 3 5% 6% 37%	5319
	21st to end	Night	29% 15% 4 8% 4 4 3 3 3 4 22%	7150

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	56% 4 12% 21% 6%	4692
	11th to 20th	Day	56% 4 9% 20% 11% 1	4710
	21st to end	Day	62% 3 8% 16% 7% 4	5636
February	1st to 10th	Day	54% 4 10% 21% 10% 2	2100
	11th to 20th	Day	52% 3 11% 21% 12% 2	2058
	21st to end	Day	53% 4 9% 21% 11% 3	1730
March	1st to 10th	Day	50% 3 4 19% 19% 4	4580
	11th to 20th	Day	40% 4 10% 30% 15% 2	4803
	21st to end	Day	47% 6% 12% 23% 11% 1	6006
April	1st to 10th	Day	44% 6% 13% 21% 15% 1	2520
	11th to 20th	Day	45% 5% 11% 23% 14% 3	2494
	21st to end	Day	45% 5% 11% 26% 11% 2	2607
May	1st to 10th	Day	48% 7% 14% 20% 11% 1	7651
	11th to 20th	Day	45% 6% 11% 23% 15%	7630
	21st to end	Day	50% 7% 12% 17% 13% 1	8642
June	1st to 10th	Day	35% 7% 11% 25% 18% 3	2940
	11th to 20th	Day	33% 9% 12% 24% 19% 4	2871
	21st to end	Day	34% 9% 14% 22% 17% 4	2800
July	1st to 10th	Day	36% 6% 9% 27% 20% 2	6291
	11th to 20th	Day	37% 9% 14% 26% 13% 1	6285
	21st to end	Day	43% 10% 16% 22% 8% 1	6595
August	1st to 10th	Day	27% 7% 15% 35% 15% 1	4141
	11th to 20th	Day	35% 7% 15% 26% 17% 1	3885
	21st to end	Day	26% 6% 14% 34% 16% 4	4311
September	1st to 10th	Day	35% 5% 10% 25% 21% 4	2400
	11th to 20th	Day	31% 5% 12% 28% 22% 3	2209
	21st to end	Day	34% 5% 8% 24% 26% 4	2200
October	1st to 10th	Day	21% 3 5% 28% 41% 3	3590
	11th to 20th	Day	25% 4 8% 26% 33% 3	3328
	21st to end	Day	36% 3 8% 24% 27% 3	3180
November	1st to 10th	Day	34% 4 10% 29% 17% 5%	1852
	11th to 20th	Day	44% 4 8% 22% 18% 4	1890
	21st to end	Day	50% 4 8% 22% 15% 1	1890
December	1st to 10th	Day	44% 3 13% 26% 13%	3020
	11th to 20th	Day	58% 4 9% 21% 8% 1	3101
	21st to end	Day	70% 4 10% 11% 5%	4070
January	1st to 10th	Night	55% 4 12% 22% 7%	8233
	11th to 20th	Night	56% 4 7% 16% 15% 3	8280
	21st to end	Night	53% 3 11% 21% 12% 1	8562
February	1st to 10th	Night	57% 4 11% 19% 8% 1	2520
	11th to 20th	Night	50% 4 10% 21% 14% 1	2459
	21st to end	Night	51% 5% 9% 21% 11% 3	2076
March	1st to 10th	Night	52% 4 6% 20% 15% 3	5493
	11th to 20th	Night	37% 5% 10% 25% 22% 2	5377
	21st to end	Night	51% 5% 10% 24% 9% 1	5198
April	1st to 10th	Night	41% 5% 10% 26% 15% 2	2100
	11th to 20th	Night	40% 5% 11% 19% 19% 5%	2075
	21st to end	Night	44% 6% 11% 23% 14% 2	2011
May	1st to 10th	Night	47% 8% 12% 23% 9% 1	5264
	11th to 20th	Night	43% 7% 11% 22% 16%	5235
	21st to end	Night	47% 8% 14% 19% 11% 2	5529
June	1st to 10th	Night	33% 7% 10% 24% 22% 4	1680
	11th to 20th	Night	32% 7% 10% 22% 21% 7%	1640
	21st to end	Night	27% 7% 16% 23% 20% 7%	1600
July	1st to 10th	Night	36% 6% 9% 25% 21% 3	3580
	11th to 20th	Night	32% 9% 14% 28% 15% 2	3572
	21st to end	Night	39% 9% 17% 23% 11% 1	3959
August	1st to 10th	Night	26% 7% 14% 38% 13% 2	2967
	11th to 20th	Night	30% 7% 13% 30% 19% 1	3230
	21st to end	Night	22% 6% 14% 29% 25% 4	3586
September	1st to 10th	Night	31% 6% 8% 26% 22% 6%	2000
	11th to 20th	Night	29% 6% 8% 24% 28% 5%	2170
	21st to end	Night	34% 5% 10% 21% 24% 6%	2200
October	1st to 10th	Night	18% 3 6% 18% 50% 4	3590
	11th to 20th	Night	25% 3 6% 26% 37% 4	3804
	21st to end	Night	33% 3 7% 22% 28% 6%	4610
November	1st to 10th	Night	36% 4 9% 25% 22% 5%	2666
	11th to 20th	Night	39% 4 9% 22% 22% 4	2730
	21st to end	Night	48% 3 10% 23% 15% 2	2730
December	1st to 10th	Night	48% 3 10% 24% 14%	4380
	11th to 20th	Night	49% 4 13% 21% 12% 1	5321
	21st to end	Night	67% 4 9% 13% 5% 1	7150

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	25% 14% 32% 22% 7% 1	330
	11th to 20th	Day	25% 19% 36% 16% 3	330
	21st to end	Day	25% 15% 37% 15% 6% 1	363
February	1st to 10th	Day	25% 15% 38% 18% 5%	330
	11th to 20th	Day	26% 18% 35% 17% 3 1	330
	21st to end	Day	26% 11% 42% 17% 4	273
March	1st to 10th	Day	23% 15% 38% 19% 4 1	330
	11th to 20th	Day	29% 14% 35% 14% 6% 1	409
	21st to end	Day	29% 12% 35% 17% 6% 1	594
April	1st to 10th	Day	19% 15% 32% 24% 9% 1	540
	11th to 20th	Day	23% 15% 27% 23% 10% 3	532
	21st to end	Day	20% 9% 36% 22% 10% 2	540
May	1st to 10th	Day	21% 9% 29% 28% 10% 4	543
	11th to 20th	Day	21% 6% 26% 26% 14% 7%	567
	21st to end	Day	14% 4 26% 27% 19% 11%	698
June	1st to 10th	Day	15% 4 18% 28% 26% 9%	780
	11th to 20th	Day	10% 4 21% 30% 24% 12%	756
	21st to end	Day	11% 3 23% 30% 20% 13%	730
July	1st to 10th	Day	13% 1 11% 25% 28% 21%	721
	11th to 20th	Day	8% 3 13% 33% 27% 16%	718
	21st to end	Day	10% 2 10% 29% 34% 15%	734
August	1st to 10th	Day	11% 2 10% 25% 33% 19%	526
	11th to 20th	Day	13% 3 12% 26% 33% 13%	519
	21st to end	Day	7% 2 8% 27% 36% 20%	583
September	1st to 10th	Day	10% 2 13% 27% 33% 15%	530
	11th to 20th	Day	8% 3 8% 29% 35% 18%	530
	21st to end	Day	16% 5% 19% 24% 23% 13%	530
October	1st to 10th	Day	11% 3 19% 34% 24% 9%	530
	11th to 20th	Day	13% 9% 31% 29% 15% 4	400
	21st to end	Day	28% 10% 32% 22% 7%	363
November	1st to 10th	Day	13% 9% 35% 25% 12% 6%	330
	11th to 20th	Day	20% 18% 29% 22% 8% 3	330
	21st to end	Day	26% 11% 26% 27% 8% 1	330
December	1st to 10th	Day	18% 18% 37% 18% 7% 2	330
	11th to 20th	Day	26% 12% 35% 18% 8% 1	330
	21st to end	Day	32% 13% 31% 20% 4	363
January	1st to 10th	Night	23% 12% 33% 28% 4	1050
	11th to 20th	Night	25% 12% 36% 23% 4	1050
	21st to end	Night	22% 11% 38% 26% 3	1103
February	1st to 10th	Night	25% 10% 35% 26% 3	840
	11th to 20th	Night	24% 13% 35% 25% 3	823
	21st to end	Night	25% 10% 37% 25% 2	692
March	1st to 10th	Night	23% 13% 33% 28% 4	840
	11th to 20th	Night	23% 10% 34% 26% 7%	840
	21st to end	Night	22% 10% 31% 31% 5% 1	754
April	1st to 10th	Night	17% 10% 33% 31% 7%	630
	11th to 20th	Night	19% 9% 30% 30% 11%	623
	21st to end	Night	19% 5% 33% 34% 8% 1	630
May	1st to 10th	Night	18% 5% 31% 32% 13% 1	630
	11th to 20th	Night	16% 5% 21% 38% 18% 2	621
	21st to end	Night	12% 3 19% 39% 25% 3	683
June	1st to 10th	Night	13% 1 17% 40% 27% 2	630
	11th to 20th	Night	10% 3 17% 44% 25% 2	615
	21st to end	Night	7% 1 16% 41% 33% 2	600
July	1st to 10th	Night	10% 2 9% 32% 38% 9%	585
	11th to 20th	Night	6% 2 10% 40% 37% 5%	582
	21st to end	Night	9% 2 8% 35% 40% 5%	657
August	1st to 10th	Night	7% 2 8% 40% 38% 6%	597
	11th to 20th	Night	7% 2 9% 35% 41% 5%	594
	21st to end	Night	6% 1 5% 37% 44% 7%	660
September	1st to 10th	Night	12% 3 10% 34% 38% 5%	600
	11th to 20th	Night	8% 1 12% 35% 38% 6%	770
	21st to end	Night	14% 4 17% 31% 28% 5%	800
October	1st to 10th	Night	11% 3 17% 38% 25% 5%	800
	11th to 20th	Night	17% 7% 25% 32% 18% 2	800
	21st to end	Night	23% 7% 32% 29% 10%	880
November	1st to 10th	Night	16% 7% 29% 35% 13% 1	820
	11th to 20th	Night	18% 10% 31% 31% 9%	840
	21st to end	Night	26% 10% 26% 30% 9%	840
December	1st to 10th	Night	25% 7% 35% 27% 5% 1	840
	11th to 20th	Night	25% 8% 32% 28% 6% 1	1030
	21st to end	Night	31% 10% 30% 24% 4	1155

Page generated at 2026-06-13 02:31:44. Version ac07

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