Salewsky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Salewsky Observatory clear sky chart from 2005-11-06 to 2025-01-07.

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	13% 9% 3 8% 5% 4 5% 4 5% 7% 37%	105199
	Night	13% 10% 4 8% 5% 4 4 4 5% 6% 37%	103955
Transparency	Day	47% 2 11% 26% 13% 1	108717
	Night	48% 2 11% 24% 13% 1	108577
Seeing	Day	20% 10% 30% 24% 13% 4	16159
	Night	19% 9% 30% 30% 11% 1	26117

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	9% 17% 8% 15% 7% 5% 4 2 3 5% 24%	2575
	11th to 20th	Day	9% 13% 5% 14% 6% 6% 8% 7% 7% 6% 18%	2530
	21st to end	Day	15% 19% 8% 13% 5% 3 4 3 4 5% 21%	2893
February	1st to 10th	Day	9% 14% 5% 10% 5% 5% 5% 4 5% 7% 31%	1710
	11th to 20th	Day	11% 11% 4 9% 6% 4 5% 4 5% 6% 36%	1739
	21st to end	Day	11% 13% 4 10% 6% 5% 5% 4 4 7% 30%	1570
March	1st to 10th	Day	14% 9% 3 5% 4 3 3 2 3 6% 48%	3120
	11th to 20th	Day	9% 9% 3 8% 5% 4 4 4 4 7% 43%	3406
	21st to end	Day	21% 13% 3 9% 5% 4 3 3 4 6% 29%	4087
April	1st to 10th	Day	17% 13% 4 8% 6% 4 5% 3 5% 5% 32%	2280
	11th to 20th	Day	20% 12% 3 7% 5% 4 4 3 4 6% 33%	2254
	21st to end	Day	19% 10% 3 7% 5% 4 4 3 5% 6% 32%	2342
May	1st to 10th	Day	15% 7% 3 7% 5% 4 5% 4 5% 7% 38%	4349
	11th to 20th	Day	15% 6% 2 7% 4 4 3 3 6% 8% 43%	4399
	21st to end	Day	16% 7% 3 7% 5% 4 5% 4 6% 7% 37%	4824
June	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 4 6% 8% 43%	2660
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 4 7% 8% 40%	2660
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 7% 9% 40%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 9% 37%	4440
	11th to 20th	Day	4 3 1 4 3 4 5% 5% 9% 12% 49%	4440
	21st to end	Day	8% 6% 2 8% 5% 5% 5% 4 6% 8% 43%	4813
August	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 4 6% 8% 40%	4368
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 5% 8% 45%	3896
	21st to end	Day	11% 6% 2 6% 4 5% 5% 4 6% 9% 42%	4180
September	1st to 10th	Day	14% 6% 2 6% 5% 4 3 3 5% 8% 45%	2280
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 6% 47%	2242
	21st to end	Day	14% 7% 2 6% 4 4 5% 3 6% 7% 43%	2090
October	1st to 10th	Day	14% 7% 2 6% 4 3 4 3 5% 7% 44%	1900
	11th to 20th	Day	19% 10% 3 7% 3 3 4 3 6% 7% 34%	1900
	21st to end	Day	17% 11% 2 8% 5% 3 3 3 4 5% 39%	1919
November	1st to 10th	Day	14% 9% 2 6% 4 4 5% 3 5% 6% 41%	1762
	11th to 20th	Day	15% 13% 4 12% 6% 4 4 3 4 6% 28%	1711
	21st to end	Day	15% 11% 4 8% 5% 5% 4 3 5% 5% 36%	1600
December	1st to 10th	Day	7% 10% 5% 13% 7% 8% 6% 4 6% 6% 27%	2610
	11th to 20th	Day	13% 17% 6% 11% 6% 4 4 2 4 5% 27%	3008
	21st to end	Day	18% 21% 8% 13% 5% 5% 4 3 3 4 15%	3982
January	1st to 10th	Night	11% 14% 8% 12% 7% 4 4 2 3 4 31%	4513
	11th to 20th	Night	10% 13% 6% 16% 6% 6% 7% 6% 7% 4 19%	4440
	21st to end	Night	14% 19% 8% 11% 6% 5% 3 3 4 4 22%	4756
February	1st to 10th	Night	9% 13% 5% 10% 5% 6% 4 4 5% 5% 33%	2470
	11th to 20th	Night	10% 10% 3 8% 5% 5% 4 4 5% 6% 40%	2327
	21st to end	Night	15% 11% 4 8% 6% 4 4 3 4 6% 33%	1884
March	1st to 10th	Night	13% 9% 3 5% 4 4 4 3 5% 7% 44%	3733
	11th to 20th	Night	10% 9% 2 7% 5% 5% 5% 4 6% 7% 41%	3582
	21st to end	Night	22% 12% 3 10% 5% 4 3 3 3 5% 30%	3543
April	1st to 10th	Night	19% 13% 3 7% 4 4 4 4 5% 5% 34%	1900
	11th to 20th	Night	20% 9% 3 6% 4 4 3 3 5% 6% 36%	1875
	21st to end	Night	17% 8% 2 6% 4 4 4 4 6% 7% 37%	1836
May	1st to 10th	Night	16% 8% 3 7% 5% 5% 5% 4 5% 8% 34%	2551
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 6% 6% 47%	2506
	21st to end	Night	13% 7% 3 7% 5% 4 4 3 6% 7% 42%	2743
June	1st to 10th	Night	9% 5% 2 7% 5% 6% 4 4 6% 8% 45%	1520
	11th to 20th	Night	11% 6% 2 7% 5% 3 4 3 5% 9% 44%	1520
	21st to end	Night	8% 5% 2 6% 5% 3 4 4 7% 8% 47%	1520
July	1st to 10th	Night	13% 6% 2 5% 4 4 5% 4 6% 8% 44%	2530
	11th to 20th	Night	5% 3 1 4 4 4 4 4 6% 8% 56%	2530
	21st to end	Night	7% 5% 2 6% 4 4 5% 5% 6% 9% 47%	2750
August	1st to 10th	Night	10% 5% 2 6% 4 4 5% 3 6% 8% 46%	2570
	11th to 20th	Night	10% 7% 2 6% 4 3 4 4 5% 7% 47%	2938
	21st to end	Night	10% 6% 2 6% 3 4 4 4 5% 8% 48%	3476
September	1st to 10th	Night	10% 6% 2 4 4 3 4 3 5% 6% 53%	1900
	11th to 20th	Night	13% 4 2 5% 4 4 3 3 4 7% 51%	1919
	21st to end	Night	12% 6% 2 5% 4 4 4 3 5% 7% 49%	2090
October	1st to 10th	Night	11% 7% 2 5% 4 4 4 2 5% 7% 49%	2278
	11th to 20th	Night	16% 9% 3 6% 4 3 4 3 6% 7% 40%	2280
	21st to end	Night	17% 10% 3 7% 4 4 5% 4 5% 6% 34%	2679
November	1st to 10th	Night	15% 10% 3 6% 4 3 3 4 4 6% 42%	2536
	11th to 20th	Night	14% 13% 4 10% 6% 4 4 3 4 5% 32%	2685
	21st to end	Night	16% 11% 5% 9% 5% 3 4 3 4 5% 35%	2800
December	1st to 10th	Night	7% 9% 4 10% 6% 8% 4 4 7% 8% 33%	4580
	11th to 20th	Night	14% 18% 7% 13% 6% 4 4 4 4 4 23%	5199
	21st to end	Night	20% 18% 6% 12% 5% 4 3 3 4 5% 20%	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	65% 1 12% 15% 6%	2576
	11th to 20th	Day	66% 3 9% 17% 5%	2530
	21st to end	Day	68% 2 11% 15% 4	2893
February	1st to 10th	Day	52% 3 12% 20% 11% 1	1710
	11th to 20th	Day	50% 3 12% 25% 10% 1	1748
	21st to end	Day	55% 3 9% 21% 9% 2	1570
March	1st to 10th	Day	40% 1 11% 28% 18% 2	3120
	11th to 20th	Day	43% 2 10% 29% 15% 2	3406
	21st to end	Day	58% 2 9% 21% 9% 1	4087
April	1st to 10th	Day	55% 2 9% 21% 11% 1	2280
	11th to 20th	Day	55% 2 6% 22% 13% 2	2254
	21st to end	Day	53% 2 10% 21% 13% 1	2342
May	1st to 10th	Day	46% 2 9% 26% 16% 1	4390
	11th to 20th	Day	41% 2 8% 35% 13% 1	4403
	21st to end	Day	47% 2 8% 24% 16% 2	4837
June	1st to 10th	Day	39% 3 12% 29% 16% 2	2660
	11th to 20th	Day	41% 3 12% 26% 16% 1	2660
	21st to end	Day	41% 3 15% 27% 13% 2	2660
July	1st to 10th	Day	42% 3 16% 29% 8% 1	4371
	11th to 20th	Day	26% 3 16% 37% 17% 2	4365
	21st to end	Day	38% 3 13% 26% 18% 2	4662
August	1st to 10th	Day	42% 3 13% 27% 13% 1	4353
	11th to 20th	Day	37% 3 11% 35% 14%	3935
	21st to end	Day	40% 2 14% 31% 13% 1	4179
September	1st to 10th	Day	38% 3 12% 28% 18% 1	2280
	11th to 20th	Day	38% 2 15% 34% 10% 1	2242
	21st to end	Day	41% 2 13% 30% 13% 1	2090
October	1st to 10th	Day	26% 1 7% 43% 23% 1	3160
	11th to 20th	Day	41% 2 9% 28% 17% 2	3140
	21st to end	Day	44% 1 13% 30% 11% 1	3144
November	1st to 10th	Day	44% 3 13% 23% 16% 1	1762
	11th to 20th	Day	58% 2 10% 19% 9% 1	1711
	21st to end	Day	52% 2 15% 22% 8% 1	1600
December	1st to 10th	Day	58% 5% 12% 19% 6% 1	2610
	11th to 20th	Day	61% 2 9% 18% 8% 2	3005
	21st to end	Day	75% 2 6% 11% 6%	3982
January	1st to 10th	Night	60% 1 9% 21% 8% 1	4513
	11th to 20th	Night	66% 2 9% 15% 8%	4440
	21st to end	Night	68% 2 8% 17% 6%	4756
February	1st to 10th	Night	54% 2 10% 23% 10% 2	2470
	11th to 20th	Night	46% 3 11% 27% 11% 2	2328
	21st to end	Night	53% 3 11% 21% 11% 2	1884
March	1st to 10th	Night	42% 2 11% 30% 11% 4	3733
	11th to 20th	Night	44% 1 8% 28% 18% 2	3582
	21st to end	Night	59% 2 6% 22% 9% 1	3543
April	1st to 10th	Night	52% 3 8% 19% 16% 2	1900
	11th to 20th	Night	51% 2 7% 21% 15% 4	1875
	21st to end	Night	47% 2 11% 23% 15% 2	1836
May	1st to 10th	Night	50% 2 12% 17% 18%	2575
	11th to 20th	Night	38% 2 10% 29% 20% 2	2507
	21st to end	Night	43% 2 10% 24% 19% 1	2751
June	1st to 10th	Night	37% 3 13% 30% 17% 1	1520
	11th to 20th	Night	39% 3 11% 27% 19% 2	1520
	21st to end	Night	34% 4 14% 30% 14% 4	1520
July	1st to 10th	Night	39% 3 17% 24% 13% 3	2490
	11th to 20th	Night	26% 3 16% 35% 18% 1	2482
	21st to end	Night	33% 4 14% 25% 20% 3	2662
August	1st to 10th	Night	36% 3 13% 28% 18% 2	2561
	11th to 20th	Night	36% 3 14% 31% 15% 1	2965
	21st to end	Night	36% 2 14% 27% 19% 2	3476
September	1st to 10th	Night	32% 3 11% 30% 23% 1	1900
	11th to 20th	Night	35% 2 11% 36% 15% 1	1919
	21st to end	Night	37% 2 9% 32% 18% 1	2090
October	1st to 10th	Night	27% 1 6% 37% 26% 2	3798
	11th to 20th	Night	32% 2 11% 33% 21% 1	3779
	21st to end	Night	48% 2 10% 25% 15%	4404
November	1st to 10th	Night	45% 2 11% 26% 14% 2	2536
	11th to 20th	Night	55% 2 11% 19% 11% 1	2685
	21st to end	Night	53% 2 14% 24% 6% 1	2800
December	1st to 10th	Night	51% 4 16% 23% 5% 2	4580
	11th to 20th	Night	66% 2 9% 15% 8% 1	5201
	21st to end	Night	69% 2 7% 15% 7%	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	23% 13% 33% 21% 10% 1	288
	11th to 20th	Day	25% 12% 38% 18% 5% 1	270
	21st to end	Day	24% 14% 33% 24% 5%	297
February	1st to 10th	Day	17% 20% 39% 20% 4 1	270
	11th to 20th	Day	21% 24% 33% 15% 7% 1	270
	21st to end	Day	24% 20% 40% 14% 1	225
March	1st to 10th	Day	24% 15% 39% 16% 5%	270
	11th to 20th	Day	26% 14% 32% 18% 7% 3	270
	21st to end	Day	31% 10% 29% 20% 8% 2	454
April	1st to 10th	Day	28% 11% 37% 18% 5%	460
	11th to 20th	Day	32% 15% 25% 17% 9% 2	452
	21st to end	Day	27% 11% 28% 22% 10% 3	460
May	1st to 10th	Day	22% 11% 34% 24% 7% 2	616
	11th to 20th	Day	21% 10% 31% 24% 10% 3	675
	21st to end	Day	21% 8% 26% 26% 15% 4	734
June	1st to 10th	Day	15% 8% 29% 29% 16% 4	680
	11th to 20th	Day	16% 8% 31% 28% 14% 4	680
	21st to end	Day	16% 8% 26% 30% 15% 5%	680
July	1st to 10th	Day	10% 3 21% 31% 25% 10%	671
	11th to 20th	Day	9% 5% 28% 32% 17% 9%	668
	21st to end	Day	9% 4 29% 29% 22% 7%	739
August	1st to 10th	Day	10% 4 24% 32% 23% 7%	676
	11th to 20th	Day	15% 4 20% 27% 25% 9%	585
	21st to end	Day	14% 7% 22% 29% 22% 6%	539
September	1st to 10th	Day	19% 6% 24% 29% 16% 7%	490
	11th to 20th	Day	14% 4 24% 33% 17% 6%	490
	21st to end	Day	20% 9% 24% 28% 17% 2	490
October	1st to 10th	Day	25% 9% 27% 25% 11% 2	300
	11th to 20th	Day	28% 16% 33% 15% 7%	300
	21st to end	Day	28% 16% 34% 16% 5% 1	330
November	1st to 10th	Day	21% 16% 35% 19% 7% 1	300
	11th to 20th	Day	24% 19% 40% 14% 3	300
	21st to end	Day	27% 14% 39% 15% 5%	300
December	1st to 10th	Day	20% 18% 39% 15% 6% 1	300
	11th to 20th	Day	24% 17% 37% 18% 3 1	300
	21st to end	Day	29% 15% 35% 18% 3	330
January	1st to 10th	Night	17% 11% 36% 27% 7% 1	981
	11th to 20th	Night	22% 11% 31% 29% 5% 1	950
	21st to end	Night	20% 9% 34% 29% 7%	1045
February	1st to 10th	Night	17% 12% 35% 28% 7%	950
	11th to 20th	Night	17% 15% 34% 26% 8%	853
	21st to end	Night	23% 15% 32% 25% 4	628
March	1st to 10th	Night	20% 12% 33% 26% 8% 1	760
	11th to 20th	Night	20% 10% 34% 27% 8%	633
	21st to end	Night	24% 8% 30% 28% 8% 2	627
April	1st to 10th	Night	27% 9% 38% 21% 5%	570
	11th to 20th	Night	26% 9% 28% 30% 5% 1	563
	21st to end	Night	22% 9% 32% 29% 7% 1	570
May	1st to 10th	Night	26% 7% 30% 31% 6% 1	570
	11th to 20th	Night	18% 7% 31% 35% 8% 1	561
	21st to end	Night	14% 4 28% 38% 16%	617
June	1st to 10th	Night	11% 4 31% 41% 12%	570
	11th to 20th	Night	16% 3 29% 39% 12% 1	570
	21st to end	Night	11% 4 25% 41% 17% 1	570
July	1st to 10th	Night	10% 1 18% 41% 27% 3	555
	11th to 20th	Night	10% 3 19% 41% 24% 3	552
	21st to end	Night	8% 2 18% 45% 25% 2	624
August	1st to 10th	Night	8% 2 16% 47% 25% 2	567
	11th to 20th	Night	13% 2 15% 38% 28% 4	564
	21st to end	Night	14% 4 18% 44% 18% 3	627
September	1st to 10th	Night	12% 4 22% 39% 18% 6%	570
	11th to 20th	Night	15% 3 25% 32% 21% 4	589
	21st to end	Night	16% 7% 26% 34% 16% 1	760
October	1st to 10th	Night	16% 8% 28% 31% 17% 1	760
	11th to 20th	Night	23% 10% 33% 26% 9%	760
	21st to end	Night	24% 11% 29% 27% 8% 1	836
November	1st to 10th	Night	21% 11% 34% 24% 10% 1	780
	11th to 20th	Night	22% 16% 35% 22% 5%	885
	21st to end	Night	24% 14% 33% 22% 7%	1000
December	1st to 10th	Night	21% 13% 37% 22% 6% 1	1000
	11th to 20th	Night	25% 12% 31% 24% 6% 1	1000
	21st to end	Night	30% 11% 29% 25% 5%	1100

Page generated at 2025-01-04 02:24:02. Version ac07

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