Sawyer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sawyer Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	19% 9% 3 7% 4 4 4 3 5% 7% 35%	130804
	Night	19% 9% 3 7% 4 4 4 3 5% 6% 35%	130746
Transparency	Day	30% 39% 8% 14% 8% 1	134321
	Night	32% 37% 8% 14% 8% 1	135317
Seeing	Day	22% 12% 29% 25% 10% 1	17930
	Night	24% 13% 29% 25% 8% 1	26105

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	15% 11% 3 8% 6% 4 5% 4 5% 6% 33%	4281
	11th to 20th	Day	18% 12% 4 9% 4 3 3 2 3 4 38%	4650
	21st to end	Day	12% 9% 3 7% 4 4 5% 4 6% 6% 40%	5556
February	1st to 10th	Day	16% 11% 4 8% 4 5% 5% 4 4 7% 32%	1890
	11th to 20th	Day	14% 10% 3 9% 4 5% 5% 3 4 7% 36%	1854
	21st to end	Day	13% 11% 2 6% 5% 5% 5% 3 5% 7% 38%	1730
March	1st to 10th	Day	25% 11% 2 8% 5% 4 3 3 4 5% 29%	4778
	11th to 20th	Day	20% 13% 2 7% 5% 4 4 3 5% 7% 30%	5057
	21st to end	Day	27% 10% 4 7% 4 4 4 3 5% 5% 26%	5466
April	1st to 10th	Day	22% 10% 3 7% 5% 4 4 3 4 6% 32%	2331
	11th to 20th	Day	21% 9% 2 6% 3 4 3 3 4 7% 37%	2572
	21st to end	Day	22% 9% 2 6% 4 3 4 3 4 6% 37%	2600
May	1st to 10th	Day	28% 11% 4 9% 4 3 3 3 3 5% 25%	5886
	11th to 20th	Day	23% 8% 2 6% 4 4 4 4 5% 6% 34%	5970
	21st to end	Day	20% 8% 2 7% 5% 4 4 4 6% 8% 32%	7132
June	1st to 10th	Day	23% 8% 3 7% 5% 5% 4 3 5% 7% 31%	3000
	11th to 20th	Day	20% 8% 3 5% 4 4 4 3 5% 7% 38%	3000
	21st to end	Day	25% 8% 3 6% 5% 5% 4 3 5% 6% 30%	3000
July	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 7% 10% 32%	6820
	11th to 20th	Day	16% 8% 3 6% 4 4 4 4 5% 8% 39%	6546
	21st to end	Day	15% 7% 2 6% 4 4 4 4 6% 9% 39%	6592
August	1st to 10th	Day	20% 8% 3 6% 5% 4 4 4 5% 7% 33%	4235
	11th to 20th	Day	17% 7% 3 7% 4 5% 4 3 6% 7% 36%	4175
	21st to end	Day	10% 4 2 5% 4 4 5% 4 6% 8% 48%	4675
September	1st to 10th	Day	12% 4 2 5% 4 3 4 3 5% 7% 50%	2343
	11th to 20th	Day	16% 5% 2 5% 3 4 3 3 5% 6% 50%	2200
	21st to end	Day	18% 6% 3 5% 3 3 3 2 4 8% 45%	2200
October	1st to 10th	Day	16% 7% 2 6% 3 4 3 3 6% 6% 43%	2070
	11th to 20th	Day	19% 6% 2 5% 4 3 5% 4 4 8% 40%	2000
	21st to end	Day	21% 7% 2 7% 5% 4 4 3 4 6% 37%	2055
November	1st to 10th	Day	20% 8% 2 6% 4 4 5% 4 5% 5% 39%	1851
	11th to 20th	Day	20% 7% 2 5% 3 3 4 4 5% 9% 39%	1696
	21st to end	Day	20% 10% 3 6% 4 3 4 3 5% 6% 35%	1680
December	1st to 10th	Day	21% 9% 2 4 3 4 4 4 7% 7% 35%	2672
	11th to 20th	Day	23% 10% 3 6% 4 3 3 3 3 5% 37%	2688
	21st to end	Day	21% 12% 2 5% 3 4 3 3 5% 6% 37%	3553
January	1st to 10th	Night	13% 12% 3 8% 5% 4 4 3 4 6% 36%	8614
	11th to 20th	Night	19% 12% 3 8% 5% 4 4 3 5% 6% 31%	8235
	21st to end	Night	10% 11% 3 7% 4 4 4 4 5% 6% 42%	8622
February	1st to 10th	Night	17% 11% 2 6% 4 5% 5% 4 5% 7% 34%	2730
	11th to 20th	Night	15% 10% 3 9% 5% 4 4 4 5% 5% 35%	2651
	21st to end	Night	15% 12% 2 6% 5% 4 4 4 5% 5% 38%	2076
March	1st to 10th	Night	26% 12% 4 8% 5% 4 4 3 3 4 28%	5281
	11th to 20th	Night	19% 12% 3 7% 5% 4 5% 4 6% 7% 29%	5057
	21st to end	Night	30% 9% 2 5% 4 4 4 3 4 5% 29%	5466
April	1st to 10th	Night	24% 11% 3 6% 4 4 4 3 5% 4 32%	2066
	11th to 20th	Night	23% 9% 3 6% 3 3 4 4 6% 5% 35%	1778
	21st to end	Night	27% 7% 3 6% 4 4 4 3 5% 7% 31%	1800
May	1st to 10th	Night	29% 10% 3 6% 4 4 3 2 4 6% 29%	4054
	11th to 20th	Night	21% 9% 3 6% 4 4 4 4 6% 6% 31%	3913
	21st to end	Night	18% 10% 3 8% 5% 4 4 4 6% 7% 31%	3792
June	1st to 10th	Night	25% 7% 2 7% 4 4 4 4 5% 9% 29%	1400
	11th to 20th	Night	20% 7% 3 6% 4 3 4 4 7% 6% 36%	1400
	21st to end	Night	27% 8% 3 6% 5% 4 4 3 3 7% 30%	1400
July	1st to 10th	Night	15% 9% 3 9% 5% 6% 7% 5% 7% 8% 26%	3160
	11th to 20th	Night	19% 9% 3 6% 4 4 5% 4 6% 8% 32%	3418
	21st to end	Night	15% 7% 2 6% 4 4 4 4 7% 8% 39%	4200
August	1st to 10th	Night	22% 10% 3 7% 4 4 4 4 5% 7% 29%	2915
	11th to 20th	Night	16% 9% 2 6% 4 4 5% 4 7% 9% 34%	2872
	21st to end	Night	11% 5% 2 5% 4 3 4 3 5% 8% 49%	3223
September	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 6% 6% 46%	2053
	11th to 20th	Night	17% 5% 2 5% 3 2 3 2 4 7% 49%	2200
	21st to end	Night	19% 6% 2 4 3 4 3 3 5% 7% 45%	2200
October	1st to 10th	Night	19% 5% 2 6% 3 3 4 3 5% 7% 43%	2330
	11th to 20th	Night	18% 7% 2 5% 4 3 4 3 5% 6% 42%	2400
	21st to end	Night	22% 9% 2 6% 4 4 3 3 5% 5% 36%	2782
November	1st to 10th	Night	20% 6% 2 5% 4 4 4 4 4 7% 39%	2667
	11th to 20th	Night	19% 7% 2 6% 5% 4 4 3 5% 6% 39%	2923
	21st to end	Night	21% 7% 2 6% 5% 4 5% 4 6% 7% 35%	2940
December	1st to 10th	Night	18% 9% 3 6% 4 3 4 3 5% 7% 37%	4720
	11th to 20th	Night	23% 8% 2 5% 3 3 4 3 4 5% 39%	5741
	21st to end	Night	22% 10% 3 7% 3 3 4 3 5% 6% 35%	7667

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	40% 23% 11% 19% 6% 1	4282
	11th to 20th	Day	40% 22% 7% 19% 10% 1	4680
	21st to end	Day	34% 24% 7% 16% 16% 3	5556
February	1st to 10th	Day	22% 59% 4 9% 5% 1	1890
	11th to 20th	Day	19% 59% 3 11% 7% 1	1862
	21st to end	Day	16% 60% 6% 9% 7% 3	1730
March	1st to 10th	Day	43% 30% 6% 7% 11% 4	4782
	11th to 20th	Day	39% 33% 11% 11% 5% 1	5057
	21st to end	Day	44% 30% 4 10% 10% 1	5466
April	1st to 10th	Day	20% 62% 4 7% 6% 2	2331
	11th to 20th	Day	18% 62% 5% 9% 4 1	2572
	21st to end	Day	19% 62% 3 9% 6% 2	2600
May	1st to 10th	Day	45% 31% 8% 12% 3	5897
	11th to 20th	Day	37% 32% 9% 16% 2 2	5974
	21st to end	Day	35% 33% 11% 10% 9% 1	7132
June	1st to 10th	Day	17% 63% 6% 9% 5%	3000
	11th to 20th	Day	16% 62% 6% 10% 7%	3000
	21st to end	Day	19% 61% 4 9% 7% 1	3000
July	1st to 10th	Day	36% 32% 11% 14% 7%	6745
	11th to 20th	Day	32% 29% 11% 18% 9% 1	6466
	21st to end	Day	28% 28% 13% 20% 10%	6404
August	1st to 10th	Day	33% 38% 8% 16% 5%	4222
	11th to 20th	Day	30% 40% 12% 13% 5%	4214
	21st to end	Day	20% 39% 10% 17% 14% 1	4675
September	1st to 10th	Day	11% 58% 6% 14% 9%	2343
	11th to 20th	Day	17% 56% 5% 13% 8% 1	2200
	21st to end	Day	13% 57% 5% 15% 10% 1	2200
October	1st to 10th	Day	28% 40% 10% 16% 6%	3369
	11th to 20th	Day	19% 38% 6% 26% 11%	3240
	21st to end	Day	20% 36% 14% 16% 12% 2	3292
November	1st to 10th	Day	24% 53% 6% 12% 5% 1	1851
	11th to 20th	Day	22% 54% 6% 10% 7% 1	1696
	21st to end	Day	24% 55% 7% 10% 4	1680
December	1st to 10th	Day	28% 37% 12% 17% 7%	2672
	11th to 20th	Day	35% 31% 9% 10% 10% 4	2688
	21st to end	Day	36% 28% 10% 13% 11% 2	3553
January	1st to 10th	Night	39% 23% 9% 20% 8% 1	8614
	11th to 20th	Night	44% 23% 9% 16% 8% 1	8280
	21st to end	Night	33% 23% 7% 19% 14% 4	8622
February	1st to 10th	Night	21% 60% 4 8% 7% 1	2730
	11th to 20th	Night	20% 58% 3 11% 8% 1	2654
	21st to end	Night	19% 59% 3 9% 8% 2	2076
March	1st to 10th	Night	45% 29% 4 8% 10% 4	5290
	11th to 20th	Night	38% 33% 13% 10% 5% 1	5058
	21st to end	Night	44% 29% 4 13% 9% 1	5466
April	1st to 10th	Night	21% 61% 3 6% 7% 2	2066
	11th to 20th	Night	20% 63% 4 7% 5% 1	1778
	21st to end	Night	21% 62% 4 7% 6% 1	1800
May	1st to 10th	Night	43% 31% 11% 10% 5% 1	4060
	11th to 20th	Night	36% 33% 11% 14% 4 1	3914
	21st to end	Night	36% 32% 12% 14% 5% 1	3793
June	1st to 10th	Night	18% 63% 5% 9% 5% 1	1400
	11th to 20th	Night	17% 62% 5% 10% 5% 1	1400
	21st to end	Night	19% 60% 4 10% 6% 1	1400
July	1st to 10th	Night	42% 31% 9% 13% 5%	3125
	11th to 20th	Night	38% 30% 9% 17% 6%	3375
	21st to end	Night	30% 29% 11% 19% 11% 1	4074
August	1st to 10th	Night	39% 38% 6% 12% 6%	2903
	11th to 20th	Night	29% 40% 12% 15% 5%	2899
	21st to end	Night	19% 39% 10% 17% 13% 2	3223
September	1st to 10th	Night	14% 58% 4 14% 9% 1	2053
	11th to 20th	Night	17% 57% 5% 12% 7% 1	2200
	21st to end	Night	15% 57% 5% 15% 8% 1	2200
October	1st to 10th	Night	27% 38% 9% 17% 8% 1	3811
	11th to 20th	Night	20% 37% 7% 21% 15%	3899
	21st to end	Night	26% 36% 8% 19% 10% 2	4495
November	1st to 10th	Night	22% 54% 6% 10% 7% 1	2667
	11th to 20th	Night	21% 55% 4 11% 7% 1	2923
	21st to end	Night	24% 55% 6% 8% 7% 1	2940
December	1st to 10th	Night	28% 37% 7% 21% 7%	4720
	11th to 20th	Night	33% 33% 7% 14% 12% 2	5742
	21st to end	Night	37% 28% 9% 13% 10% 2	7667

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% 17% 37% 16% 4	220
	11th to 20th	Day	25% 27% 32% 12% 3 1	220
	21st to end	Day	20% 18% 35% 22% 5%	391
February	1st to 10th	Day	23% 21% 31% 20% 5%	430
	11th to 20th	Day	20% 23% 37% 16% 4	432
	21st to end	Day	19% 20% 35% 22% 4	446
March	1st to 10th	Day	22% 21% 36% 18% 3	540
	11th to 20th	Day	24% 19% 31% 19% 7%	528
	21st to end	Day	25% 19% 30% 18% 5% 1	550
April	1st to 10th	Day	28% 19% 30% 17% 4 1	500
	11th to 20th	Day	25% 14% 33% 23% 5% 1	492
	21st to end	Day	25% 14% 31% 22% 7% 1	500
May	1st to 10th	Day	27% 13% 31% 20% 9%	503
	11th to 20th	Day	21% 11% 28% 28% 11% 2	571
	21st to end	Day	24% 9% 30% 26% 11% 1	790
June	1st to 10th	Day	25% 7% 29% 28% 10% 1	730
	11th to 20th	Day	22% 8% 26% 28% 14% 2	730
	21st to end	Day	29% 6% 25% 27% 12% 2	730
July	1st to 10th	Day	21% 5% 25% 33% 15% 1	720
	11th to 20th	Day	21% 4 23% 34% 15% 2	601
	21st to end	Day	18% 4 24% 38% 15% 1	576
August	1st to 10th	Day	16% 4 23% 37% 17% 3	526
	11th to 20th	Day	18% 4 19% 37% 19% 3	519
	21st to end	Day	17% 5% 23% 32% 19% 3	583
September	1st to 10th	Day	13% 4 28% 38% 16% 2	530
	11th to 20th	Day	20% 8% 22% 27% 18% 5%	530
	21st to end	Day	18% 6% 30% 28% 15% 3	530
October	1st to 10th	Day	17% 13% 22% 25% 20% 2	458
	11th to 20th	Day	20% 10% 29% 26% 11% 3	420
	21st to end	Day	24% 14% 31% 23% 8% 1	462
November	1st to 10th	Day	24% 16% 31% 19% 8% 1	425
	11th to 20th	Day	25% 19% 31% 18% 7%	430
	21st to end	Day	25% 20% 31% 19% 4	430
December	1st to 10th	Day	27% 18% 33% 17% 4	425
	11th to 20th	Day	29% 26% 30% 11% 4	220
	21st to end	Day	22% 27% 28% 17% 5% 1	242
January	1st to 10th	Night	24% 20% 31% 21% 3	911
	11th to 20th	Night	26% 19% 31% 19% 4	840
	21st to end	Night	17% 23% 38% 18% 4	924
February	1st to 10th	Night	24% 19% 36% 18% 4	840
	11th to 20th	Night	23% 24% 32% 18% 3 1	823
	21st to end	Night	23% 17% 36% 20% 4	692
March	1st to 10th	Night	23% 16% 38% 21% 2	840
	11th to 20th	Night	23% 16% 31% 24% 5%	829
	21st to end	Night	25% 14% 34% 21% 6% 1	880
April	1st to 10th	Night	30% 14% 33% 19% 4	746
	11th to 20th	Night	26% 8% 36% 25% 6%	594
	21st to end	Night	30% 7% 32% 22% 8% 1	600
May	1st to 10th	Night	26% 6% 30% 28% 9% 1	600
	11th to 20th	Night	27% 5% 28% 30% 9% 1	592
	21st to end	Night	21% 7% 32% 31% 8% 2	584
June	1st to 10th	Night	28% 5% 23% 34% 9% 1	400
	11th to 20th	Night	23% 5% 26% 33% 13% 1	400
	21st to end	Night	32% 4 18% 35% 11% 1	400
July	1st to 10th	Night	22% 4 19% 42% 13%	390
	11th to 20th	Night	23% 5% 16% 39% 16% 1	388
	21st to end	Night	22% 3 18% 42% 14% 1	558
August	1st to 10th	Night	18% 4 17% 46% 14% 1	598
	11th to 20th	Night	22% 4 16% 40% 18% 1	592
	21st to end	Night	17% 4 21% 36% 20% 2	660
September	1st to 10th	Night	19% 3 22% 36% 18% 2	687
	11th to 20th	Night	19% 6% 23% 30% 20% 2	800
	21st to end	Night	21% 5% 26% 32% 15% 2	800
October	1st to 10th	Night	22% 7% 25% 29% 15% 2	800
	11th to 20th	Night	20% 11% 31% 24% 12% 2	800
	21st to end	Night	26% 15% 29% 22% 7%	880
November	1st to 10th	Night	23% 14% 32% 23% 8% 1	820
	11th to 20th	Night	24% 16% 32% 19% 8% 1	942
	21st to end	Night	27% 19% 31% 16% 6% 1	950
December	1st to 10th	Night	31% 14% 32% 17% 4	950
	11th to 20th	Night	27% 24% 32% 15% 2 1	950
	21st to end	Night	28% 20% 31% 16% 4	1045

Page generated at 2026-03-15 02:23:55. Version ac07

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