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 2024-12-06.

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% 8% 3 6% 4 4 4 3 5% 7% 36%	101682
	Night	20% 9% 3 6% 4 4 4 4 5% 6% 35%	98064
Transparency	Day	25% 48% 8% 12% 7% 1	105214
	Night	26% 47% 7% 12% 7% 1	102622
Seeing	Day	23% 12% 28% 25% 10% 2	16310
	Night	24% 12% 29% 25% 8% 1	24360

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	17% 10% 2 6% 5% 4 5% 3 5% 6% 38%	2304
	11th to 20th	Day	17% 9% 3 8% 3 3 3 2 3 4 45%	2520
	21st to end	Day	15% 10% 3 8% 5% 5% 5% 4 6% 6% 34%	2997
February	1st to 10th	Day	18% 11% 4 8% 4 4 5% 4 4 6% 31%	1710
	11th to 20th	Day	14% 11% 3 9% 4 5% 5% 4 3 7% 35%	1672
	21st to end	Day	13% 11% 2 7% 4 5% 5% 3 4 7% 38%	1570
March	1st to 10th	Day	25% 12% 3 6% 4 4 3 3 4 5% 31%	3258
	11th to 20th	Day	18% 12% 3 8% 6% 5% 5% 4 6% 6% 27%	3480
	21st to end	Day	27% 11% 4 7% 4 4 4 3 4 4 27%	3816
April	1st to 10th	Day	21% 11% 3 7% 5% 4 4 3 4 6% 32%	2214
	11th to 20th	Day	21% 8% 2 6% 3 4 3 3 5% 7% 38%	2442
	21st to end	Day	22% 9% 2 6% 5% 3 4 3 4 6% 37%	2470
May	1st to 10th	Day	20% 10% 3 8% 4 4 4 4 5% 7% 32%	4106
	11th to 20th	Day	18% 8% 2 7% 5% 4 4 4 5% 7% 36%	4162
	21st to end	Day	17% 7% 2 7% 5% 4 5% 4 6% 9% 34%	4964
June	1st to 10th	Day	23% 8% 3 7% 5% 5% 4 3 5% 7% 30%	2850
	11th to 20th	Day	20% 8% 3 5% 4 4 4 3 5% 7% 38%	2850
	21st to end	Day	25% 8% 3 7% 5% 5% 4 3 5% 6% 29%	2850
July	1st to 10th	Day	16% 7% 3 6% 5% 4 5% 4 6% 9% 34%	4760
	11th to 20th	Day	17% 7% 2 6% 4 4 4 4 5% 9% 38%	4570
	21st to end	Day	19% 7% 2 6% 4 4 4 4 6% 8% 35%	4578
August	1st to 10th	Day	21% 8% 3 7% 5% 4 4 4 5% 7% 32%	4105
	11th to 20th	Day	18% 7% 3 7% 4 5% 4 3 7% 7% 35%	4045
	21st to end	Day	10% 4 2 5% 4 4 5% 4 6% 9% 47%	4532
September	1st to 10th	Day	13% 4 2 5% 4 3 4 3 4 7% 51%	2227
	11th to 20th	Day	16% 5% 2 5% 3 4 3 3 5% 6% 50%	2090
	21st to end	Day	18% 6% 3 5% 3 3 3 2 4 7% 45%	2090
October	1st to 10th	Day	16% 7% 3 7% 4 4 4 3 6% 6% 41%	1967
	11th to 20th	Day	19% 6% 1 5% 4 3 5% 4 4 7% 41%	1900
	21st to end	Day	21% 7% 2 6% 5% 4 4 3 4 6% 37%	1952
November	1st to 10th	Day	21% 8% 2 5% 3 3 5% 4 5% 5% 39%	1761
	11th to 20th	Day	21% 6% 2 5% 3 3 4 4 5% 8% 39%	1615
	21st to end	Day	20% 10% 3 6% 4 3 4 3 5% 6% 35%	1600
December	1st to 10th	Day	24% 9% 2 6% 4 4 5% 3 6% 6% 30%	1556
	11th to 20th	Day	25% 12% 3 7% 4 4 4 3 4 6% 29%	1668
	21st to end	Day	24% 10% 2 6% 3 5% 4 3 4 5% 34%	2431
January	1st to 10th	Night	14% 11% 3 7% 5% 4 4 3 5% 6% 38%	4643
	11th to 20th	Night	14% 10% 4 8% 5% 5% 4 3 5% 6% 36%	4440
	21st to end	Night	15% 12% 4 9% 5% 4 4 4 5% 6% 32%	4656
February	1st to 10th	Night	18% 11% 2 6% 4 5% 5% 4 5% 7% 32%	2470
	11th to 20th	Night	16% 10% 3 9% 5% 4 5% 4 5% 5% 33%	2393
	21st to end	Night	16% 11% 2 7% 5% 4 4 3 4 5% 39%	1884
March	1st to 10th	Night	26% 12% 3 6% 4 5% 4 3 3 4 29%	3581
	11th to 20th	Night	17% 11% 3 8% 6% 5% 5% 4 7% 7% 25%	3479
	21st to end	Night	30% 9% 2 6% 5% 4 4 4 4 4 27%	3816
April	1st to 10th	Night	24% 11% 3 6% 4 4 4 3 5% 4 33%	1963
	11th to 20th	Night	22% 9% 3 6% 3 3 4 4 6% 5% 35%	1688
	21st to end	Night	27% 7% 3 6% 4 4 4 3 5% 7% 31%	1710
May	1st to 10th	Night	23% 7% 3 5% 3 4 3 3 5% 8% 37%	2834
	11th to 20th	Night	18% 8% 3 7% 5% 5% 6% 5% 7% 6% 30%	2732
	21st to end	Night	16% 9% 3 8% 5% 4 4 4 6% 8% 33%	2635
June	1st to 10th	Night	25% 7% 3 7% 4 4 4 4 5% 8% 29%	1330
	11th to 20th	Night	21% 7% 3 6% 4 3 4 4 7% 6% 35%	1330
	21st to end	Night	28% 8% 3 6% 5% 4 4 3 3 6% 29%	1330
July	1st to 10th	Night	16% 9% 2 8% 5% 5% 7% 5% 7% 9% 27%	2210
	11th to 20th	Night	20% 7% 2 5% 4 4 5% 5% 6% 9% 33%	2390
	21st to end	Night	19% 8% 2 6% 4 4 5% 4 7% 7% 35%	2929
August	1st to 10th	Night	23% 10% 3 7% 4 4 5% 4 6% 7% 28%	2825
	11th to 20th	Night	16% 9% 2 6% 4 5% 5% 4 7% 9% 34%	2782
	21st to end	Night	11% 5% 2 5% 4 3 4 3 5% 8% 49%	3124
September	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 6% 6% 46%	1949
	11th to 20th	Night	17% 5% 2 5% 3 2 3 2 4 7% 49%	2090
	21st to end	Night	19% 6% 2 4 3 4 3 3 4 6% 45%	2090
October	1st to 10th	Night	20% 6% 2 6% 3 3 4 3 6% 7% 41%	2213
	11th to 20th	Night	18% 7% 2 5% 4 3 4 3 5% 6% 42%	2280
	21st to end	Night	23% 9% 2 6% 4 4 3 3 5% 5% 37%	2643
November	1st to 10th	Night	19% 6% 2 5% 5% 4 4 4 4 7% 39%	2537
	11th to 20th	Night	19% 7% 2 6% 5% 4 4 3 5% 6% 39%	2784
	21st to end	Night	21% 7% 2 6% 5% 3 5% 4 6% 7% 35%	2800
December	1st to 10th	Night	25% 10% 4 7% 4 2 3 3 5% 7% 30%	2737
	11th to 20th	Night	25% 9% 3 6% 3 4 4 4 5% 6% 32%	3531
	21st to end	Night	24% 8% 3 6% 3 3 4 3 5% 6% 34%	5236

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	30% 39% 12% 15% 4	2305
	11th to 20th	Day	24% 37% 5% 20% 10% 2	2520
	21st to end	Day	33% 42% 8% 9% 8%	2997
February	1st to 10th	Day	20% 65% 4 6% 4 1	1710
	11th to 20th	Day	17% 65% 2 9% 6%	1680
	21st to end	Day	14% 65% 5% 8% 5% 3	1570
March	1st to 10th	Day	36% 41% 5% 9% 6% 3	3262
	11th to 20th	Day	34% 45% 8% 7% 6% 1	3480
	21st to end	Day	40% 41% 2 6% 8% 1	3816
April	1st to 10th	Day	19% 65% 4 7% 5% 2	2214
	11th to 20th	Day	16% 65% 5% 9% 4 1	2442
	21st to end	Day	18% 65% 3 7% 6% 1	2470
May	1st to 10th	Day	28% 44% 11% 14% 3	4120
	11th to 20th	Day	28% 43% 11% 14% 3 1	4166
	21st to end	Day	26% 44% 12% 8% 9%	4964
June	1st to 10th	Day	16% 66% 6% 8% 4	2850
	11th to 20th	Day	15% 65% 5% 9% 6%	2850
	21st to end	Day	18% 64% 4 8% 6%	2850
July	1st to 10th	Day	30% 40% 8% 13% 8% 1	4685
	11th to 20th	Day	29% 38% 9% 17% 6%	4490
	21st to end	Day	29% 38% 10% 16% 7% 1	4432
August	1st to 10th	Day	34% 39% 8% 15% 4	4092
	11th to 20th	Day	30% 41% 12% 12% 5%	4084
	21st to end	Day	19% 40% 10% 16% 14% 1	4532
September	1st to 10th	Day	11% 61% 6% 13% 9%	2227
	11th to 20th	Day	17% 59% 4 11% 7% 1	2090
	21st to end	Day	13% 60% 4 14% 9% 1	2090
October	1st to 10th	Day	29% 41% 10% 15% 6%	3266
	11th to 20th	Day	18% 39% 6% 26% 11%	3140
	21st to end	Day	19% 37% 14% 16% 12% 2	3189
November	1st to 10th	Day	22% 56% 5% 11% 5% 1	1761
	11th to 20th	Day	21% 57% 5% 9% 7% 1	1615
	21st to end	Day	22% 58% 6% 10% 4	1600
December	1st to 10th	Day	22% 60% 4 10% 4	1556
	11th to 20th	Day	34% 49% 3 9% 6%	1668
	21st to end	Day	33% 39% 8% 9% 7% 2	2431
January	1st to 10th	Night	28% 38% 9% 18% 5% 1	4643
	11th to 20th	Night	29% 39% 8% 14% 8% 1	4440
	21st to end	Night	35% 41% 6% 12% 6%	4656
February	1st to 10th	Night	19% 66% 3 6% 5% 1	2470
	11th to 20th	Night	18% 64% 2 8% 7% 1	2396
	21st to end	Night	17% 64% 2 7% 8% 2	1884
March	1st to 10th	Night	38% 41% 4 8% 7% 2	3590
	11th to 20th	Night	33% 46% 7% 7% 6% 1	3480
	21st to end	Night	41% 40% 3 8% 6% 1	3816
April	1st to 10th	Night	19% 64% 3 6% 6% 2	1963
	11th to 20th	Night	18% 66% 4 7% 5% 1	1688
	21st to end	Night	19% 65% 4 6% 6% 1	1710
May	1st to 10th	Night	26% 43% 15% 12% 3	2840
	11th to 20th	Night	28% 46% 13% 11% 3	2733
	21st to end	Night	28% 44% 13% 11% 4	2636
June	1st to 10th	Night	16% 65% 5% 8% 5% 1	1330
	11th to 20th	Night	16% 65% 5% 9% 5% 1	1330
	21st to end	Night	19% 63% 4 9% 5% 1	1330
July	1st to 10th	Night	34% 40% 8% 11% 7%	2175
	11th to 20th	Night	31% 40% 9% 15% 4	2347
	21st to end	Night	29% 37% 7% 15% 11%	2835
August	1st to 10th	Night	40% 39% 6% 11% 5%	2813
	11th to 20th	Night	29% 41% 11% 14% 5%	2809
	21st to end	Night	19% 40% 9% 17% 13% 2	3124
September	1st to 10th	Night	13% 60% 4 14% 8% 1	1949
	11th to 20th	Night	17% 60% 5% 11% 6% 1	2090
	21st to end	Night	14% 59% 4 14% 7% 1	2090
October	1st to 10th	Night	28% 39% 9% 17% 7% 1	3694
	11th to 20th	Night	19% 38% 7% 21% 15%	3779
	21st to end	Night	25% 37% 8% 19% 10% 2	4356
November	1st to 10th	Night	21% 57% 6% 9% 6% 1	2537
	11th to 20th	Night	20% 58% 4 10% 8% 1	2784
	21st to end	Night	22% 58% 5% 7% 6% 1	2800
December	1st to 10th	Night	24% 59% 3 9% 5%	2737
	11th to 20th	Night	29% 50% 4 9% 7% 1	3532
	21st to end	Night	31% 39% 10% 11% 5% 3	5236

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	28% 17% 32% 18% 5%	180
	11th to 20th	Day	27% 25% 34% 11% 3 1	180
	21st to end	Day	21% 18% 33% 23% 5%	332
February	1st to 10th	Day	25% 20% 31% 19% 4 1	370
	11th to 20th	Day	21% 23% 37% 15% 3	370
	21st to end	Day	20% 20% 33% 23% 4	382
March	1st to 10th	Day	22% 20% 36% 19% 3	460
	11th to 20th	Day	22% 20% 30% 20% 7%	460
	21st to end	Day	24% 20% 30% 19% 6% 2	506
April	1st to 10th	Day	28% 19% 29% 18% 5% 1	460
	11th to 20th	Day	23% 14% 33% 23% 5% 1	452
	21st to end	Day	24% 14% 31% 23% 8% 1	460
May	1st to 10th	Day	25% 13% 32% 21% 9%	463
	11th to 20th	Day	21% 10% 29% 28% 11% 2	529
	21st to end	Day	23% 9% 30% 26% 10% 1	735
June	1st to 10th	Day	26% 7% 29% 27% 9% 1	680
	11th to 20th	Day	23% 7% 24% 29% 14% 2	680
	21st to end	Day	29% 6% 24% 26% 11% 3	680
July	1st to 10th	Day	22% 4 24% 33% 15% 1	670
	11th to 20th	Day	21% 4 23% 34% 16% 3	557
	21st to end	Day	18% 4 23% 38% 15% 1	532
August	1st to 10th	Day	17% 4 24% 36% 17% 2	486
	11th to 20th	Day	20% 4 20% 37% 18% 2	479
	21st to end	Day	18% 5% 22% 32% 20% 3	539
September	1st to 10th	Day	14% 4 27% 39% 15% 2	490
	11th to 20th	Day	21% 8% 22% 26% 17% 5%	490
	21st to end	Day	18% 6% 29% 28% 16% 3	490
October	1st to 10th	Day	18% 14% 23% 25% 18% 3	425
	11th to 20th	Day	21% 9% 29% 27% 10% 3	390
	21st to end	Day	24% 13% 31% 23% 7% 1	429
November	1st to 10th	Day	24% 16% 30% 20% 8% 2	395
	11th to 20th	Day	25% 20% 31% 18% 7% 1	400
	21st to end	Day	26% 21% 30% 20% 4 1	400
December	1st to 10th	Day	28% 16% 33% 18% 5%	381
	11th to 20th	Day	31% 26% 28% 12% 3	180
	21st to end	Day	24% 27% 30% 13% 5% 2	198
January	1st to 10th	Night	24% 19% 31% 21% 3	819
	11th to 20th	Night	26% 19% 32% 19% 4	760
	21st to end	Night	18% 22% 37% 19% 4	836
February	1st to 10th	Night	25% 19% 35% 18% 4	760
	11th to 20th	Night	24% 23% 32% 17% 3 1	743
	21st to end	Night	23% 17% 36% 20% 4	628
March	1st to 10th	Night	24% 16% 37% 22% 2	760
	11th to 20th	Night	23% 16% 31% 24% 5%	760
	21st to end	Night	25% 14% 33% 21% 6% 1	836
April	1st to 10th	Night	30% 13% 33% 20% 4	709
	11th to 20th	Night	25% 8% 36% 25% 6%	564
	21st to end	Night	30% 6% 32% 22% 9% 1	570
May	1st to 10th	Night	25% 6% 31% 28% 9% 1	570
	11th to 20th	Night	26% 5% 28% 31% 9% 1	562
	21st to end	Night	21% 6% 32% 31% 8% 2	554
June	1st to 10th	Night	28% 5% 23% 34% 8% 1	380
	11th to 20th	Night	22% 5% 26% 32% 14% 1	380
	21st to end	Night	32% 3 18% 35% 10% 1	380
July	1st to 10th	Night	22% 4 19% 42% 13%	370
	11th to 20th	Night	23% 4 16% 39% 16% 1	368
	21st to end	Night	23% 3 17% 42% 14% 1	530
August	1st to 10th	Night	18% 4 17% 46% 14% 1	568
	11th to 20th	Night	22% 4 16% 39% 18% 1	562
	21st to end	Night	17% 4 20% 36% 21% 2	627
September	1st to 10th	Night	19% 3 23% 37% 17% 1	652
	11th to 20th	Night	19% 6% 23% 30% 20% 2	760
	21st to end	Night	22% 4 26% 31% 15% 2	760
October	1st to 10th	Night	23% 6% 25% 29% 15% 2	760
	11th to 20th	Night	20% 11% 31% 24% 12% 2	760
	21st to end	Night	27% 15% 29% 22% 7%	836
November	1st to 10th	Night	22% 13% 32% 24% 8% 1	780
	11th to 20th	Night	25% 16% 32% 19% 8% 1	893
	21st to end	Night	27% 19% 30% 17% 6%	900
December	1st to 10th	Night	32% 13% 33% 17% 5%	878
	11th to 20th	Night	27% 22% 32% 15% 2 1	850
	21st to end	Night	29% 20% 31% 16% 4	935

Page generated at 2024-12-03 06:45:10. Version ac07

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