Kelsar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Kelsar Observatory clear sky chart from 2005-11-06 to 2024-12-02.

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	18% 9% 3 8% 5% 5% 4 4 5% 6% 32%	105933
	Night	19% 10% 3 8% 5% 4 4 3 5% 6% 32%	102308
Transparency	Day	49% 10% 14% 17% 8% 2	105689
	Night	50% 10% 14% 16% 7% 1	102198
Seeing	Day	21% 11% 21% 25% 17% 6%	15225
	Night	23% 9% 19% 28% 19% 2	23611

By Month

Percentage Cloud Cover NW (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	34% 13% 4 10% 6% 5% 4 3 5% 4 11%	2219
	11th to 20th	Day	26% 10% 5% 10% 5% 4 4 3 4 5% 24%	2512
	21st to end	Day	39% 13% 4 11% 5% 4 4 3 4 4 9%	2919
February	1st to 10th	Day	27% 13% 4 9% 7% 6% 5% 3 6% 6% 15%	1710
	11th to 20th	Day	25% 14% 5% 9% 5% 5% 4 3 4 5% 20%	1665
	21st to end	Day	22% 13% 4 8% 6% 5% 6% 4 5% 5% 22%	1464
March	1st to 10th	Day	17% 12% 5% 12% 8% 7% 7% 5% 6% 6% 16%	3250
	11th to 20th	Day	18% 8% 3 8% 5% 4 4 3 4 6% 36%	3490
	21st to end	Day	20% 12% 4 11% 7% 6% 5% 5% 6% 6% 19%	3934
April	1st to 10th	Day	22% 12% 4 9% 6% 5% 5% 5% 6% 7% 19%	2280
	11th to 20th	Day	19% 11% 4 7% 7% 5% 4 4 5% 6% 26%	2288
	21st to end	Day	17% 10% 4 11% 6% 5% 5% 5% 6% 7% 24%	2491
May	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 4 6% 7% 37%	4420
	11th to 20th	Day	15% 7% 3 6% 5% 5% 4 4 6% 7% 37%	4377
	21st to end	Day	19% 12% 4 10% 6% 6% 5% 5% 6% 6% 21%	4813
June	1st to 10th	Day	17% 8% 4 8% 6% 6% 4 5% 7% 8% 27%	2720
	11th to 20th	Day	13% 12% 4 9% 9% 6% 6% 5% 6% 7% 21%	2850
	21st to end	Day	12% 8% 3 7% 5% 5% 5% 5% 6% 7% 38%	2850
July	1st to 10th	Day	7% 5% 2 5% 3 3 3 3 3 5% 60%	4740
	11th to 20th	Day	5% 4 1 3 3 3 3 3 4 5% 66%	4558
	21st to end	Day	10% 6% 2 5% 4 3 3 3 5% 6% 53%	4790
August	1st to 10th	Day	4 5% 2 5% 3 4 3 3 5% 6% 59%	4282
	11th to 20th	Day	12% 6% 3 7% 4 4 5% 3 6% 8% 40%	4026
	21st to end	Day	14% 9% 3 7% 4 3 4 3 4 6% 42%	4348
September	1st to 10th	Day	10% 6% 2 6% 4 4 3 3 5% 7% 49%	2280
	11th to 20th	Day	13% 8% 2 8% 5% 4 5% 4 6% 7% 38%	2280
	21st to end	Day	16% 8% 3 6% 4 4 6% 4 5% 8% 37%	2003
October	1st to 10th	Day	13% 7% 2 6% 4 5% 6% 4 7% 10% 35%	3180
	11th to 20th	Day	26% 8% 3 7% 5% 4 4 4 6% 7% 26%	3161
	21st to end	Day	27% 11% 3 9% 5% 4 4 3 5% 6% 21%	3291
November	1st to 10th	Day	29% 13% 4 8% 5% 5% 4 4 6% 6% 18%	1597
	11th to 20th	Day	29% 12% 3 8% 6% 5% 4 3 4 6% 20%	1600
	21st to end	Day	27% 12% 4 10% 6% 5% 5% 3 5% 6% 18%	1600
December	1st to 10th	Day	27% 10% 4 8% 6% 5% 5% 4 4 5% 22%	1528
	11th to 20th	Day	31% 12% 5% 14% 7% 5% 5% 3 4 4 10%	1925
	21st to end	Day	26% 14% 5% 14% 8% 6% 5% 3 5% 4 11%	2492
January	1st to 10th	Night	33% 13% 5% 10% 6% 5% 4 3 4 4 12%	4726
	11th to 20th	Night	26% 11% 4 8% 4 3 3 3 5% 7% 26%	4382
	21st to end	Night	35% 14% 4 10% 5% 4 4 3 5% 5% 12%	4738
February	1st to 10th	Night	26% 13% 4 9% 5% 4 3 3 5% 7% 21%	2470
	11th to 20th	Night	22% 12% 4 8% 6% 5% 4 3 5% 6% 25%	2402
	21st to end	Night	22% 12% 3 9% 6% 4 5% 5% 5% 6% 24%	1989
March	1st to 10th	Night	19% 16% 5% 11% 6% 6% 5% 5% 5% 6% 17%	3622
	11th to 20th	Night	17% 9% 3 5% 3 3 3 3 5% 7% 41%	3489
	21st to end	Night	23% 14% 5% 10% 6% 5% 5% 4 5% 4 18%	3719
April	1st to 10th	Night	20% 9% 3 9% 6% 5% 5% 4 7% 7% 25%	1900
	11th to 20th	Night	14% 9% 3 9% 6% 5% 5% 4 6% 8% 30%	1837
	21st to end	Night	16% 9% 3 9% 7% 5% 4 4 6% 7% 29%	1687
May	1st to 10th	Night	15% 8% 2 7% 5% 3 5% 3 5% 6% 41%	2540
	11th to 20th	Night	10% 6% 3 7% 5% 5% 4 4 6% 7% 42%	2516
	21st to end	Night	8% 8% 3 10% 6% 7% 7% 5% 8% 8% 31%	2762
June	1st to 10th	Night	13% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	1456
	11th to 20th	Night	11% 8% 3 6% 6% 7% 6% 5% 6% 9% 32%	1330
	21st to end	Night	5% 6% 2 8% 5% 4 5% 5% 6% 6% 48%	1330
July	1st to 10th	Night	5% 5% 1 5% 3 3 3 2 4 4 65%	2220
	11th to 20th	Night	3 2 1 3 2 2 3 3 3 5% 73%	2399
	21st to end	Night	5% 4 1 5% 3 3 3 3 5% 6% 61%	2762
August	1st to 10th	Night	2 2 1 4 2 2 3 2 4 6% 71%	2649
	11th to 20th	Night	9% 6% 2 6% 3 4 3 3 4 8% 53%	2812
	21st to end	Night	13% 6% 2 5% 4 3 4 3 4 6% 50%	3324
September	1st to 10th	Night	7% 4 2 4 3 3 4 3 4 6% 60%	1900
	11th to 20th	Night	12% 7% 3 6% 5% 4 4 3 5% 6% 44%	1900
	21st to end	Night	14% 6% 3 7% 4 5% 4 3 5% 7% 42%	2175
October	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 9% 43%	3800
	11th to 20th	Night	23% 8% 2 6% 4 4 5% 3 5% 7% 31%	3777
	21st to end	Night	27% 10% 3 8% 5% 4 4 3 5% 5% 27%	4273
November	1st to 10th	Night	30% 9% 3 7% 5% 4 4 3 4 5% 24%	2697
	11th to 20th	Night	29% 11% 3 8% 5% 5% 5% 3 4 4 24%	2800
	21st to end	Night	26% 11% 4 9% 6% 5% 4 4 5% 6% 22%	2800
December	1st to 10th	Night	25% 12% 4 8% 6% 4 4 4 4 5% 24%	2686
	11th to 20th	Night	27% 13% 5% 12% 7% 6% 4 4 4 4 14%	3280
	21st to end	Night	28% 15% 6% 11% 6% 5% 4 3 5% 4 13%	5159

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% 15% 6% 7% 2	2219
	11th to 20th	Day	61% 10% 12% 13% 3 1	2536
	21st to end	Day	76% 9% 8% 5% 2	2919
February	1st to 10th	Day	68% 11% 10% 7% 4	1710
	11th to 20th	Day	66% 9% 11% 9% 5%	1674
	21st to end	Day	61% 12% 13% 10% 5%	1464
March	1st to 10th	Day	54% 15% 18% 10% 3	3250
	11th to 20th	Day	47% 9% 16% 25% 3 1	3490
	21st to end	Day	60% 14% 14% 10% 1	3934
April	1st to 10th	Day	60% 13% 16% 10% 1	2280
	11th to 20th	Day	55% 10% 13% 14% 6% 2	2288
	21st to end	Day	55% 12% 15% 12% 5% 1	2491
May	1st to 10th	Day	40% 12% 18% 19% 9% 1	4420
	11th to 20th	Day	38% 13% 25% 19% 5%	4380
	21st to end	Day	56% 15% 18% 9% 2	4814
June	1st to 10th	Day	50% 12% 20% 12% 6% 1	2720
	11th to 20th	Day	56% 13% 17% 11% 3	2850
	21st to end	Day	40% 11% 21% 19% 7% 2	2850
July	1st to 10th	Day	26% 8% 9% 20% 23% 14%	4666
	11th to 20th	Day	20% 5% 10% 28% 33% 4	4478
	21st to end	Day	31% 5% 15% 33% 14% 2	4639
August	1st to 10th	Day	24% 5% 13% 36% 20% 2	4265
	11th to 20th	Day	40% 6% 21% 23% 8% 2	4065
	21st to end	Day	43% 6% 13% 26% 11% 1	4347
September	1st to 10th	Day	33% 9% 17% 24% 13% 5%	2280
	11th to 20th	Day	42% 11% 15% 15% 14% 2	2280
	21st to end	Day	45% 10% 18% 16% 10% 2	2003
October	1st to 10th	Day	43% 6% 18% 27% 5%	3180
	11th to 20th	Day	57% 8% 13% 15% 7%	3161
	21st to end	Day	63% 6% 11% 11% 7% 1	3291
November	1st to 10th	Day	66% 8% 12% 10% 4	1597
	11th to 20th	Day	66% 9% 8% 11% 5% 1	1600
	21st to end	Day	64% 10% 9% 11% 6%	1600
December	1st to 10th	Day	63% 7% 10% 13% 6%	1528
	11th to 20th	Day	72% 13% 7% 6% 2 1	1928
	21st to end	Day	69% 13% 11% 5% 2	2492
January	1st to 10th	Night	71% 13% 8% 5% 2 1	4726
	11th to 20th	Night	55% 10% 13% 18% 3 1	4420
	21st to end	Night	72% 10% 8% 7% 2	4738
February	1st to 10th	Night	64% 10% 13% 9% 5%	2470
	11th to 20th	Night	58% 11% 13% 10% 6% 1	2404
	21st to end	Night	59% 11% 14% 11% 6%	1989
March	1st to 10th	Night	56% 16% 14% 10% 3	3622
	11th to 20th	Night	39% 12% 20% 26% 3 1	3490
	21st to end	Night	62% 17% 11% 10% 1	3719
April	1st to 10th	Night	55% 14% 17% 12% 2	1900
	11th to 20th	Night	48% 13% 16% 13% 8% 1	1837
	21st to end	Night	52% 12% 16% 13% 6% 1	1687
May	1st to 10th	Night	40% 13% 16% 21% 10%	2540
	11th to 20th	Night	34% 15% 25% 17% 8% 1	2516
	21st to end	Night	46% 23% 17% 11% 2	2762
June	1st to 10th	Night	44% 10% 20% 20% 5% 1	1456
	11th to 20th	Night	44% 15% 22% 16% 3	1330
	21st to end	Night	36% 9% 23% 21% 8% 3	1330
July	1st to 10th	Night	24% 6% 11% 22% 19% 17%	2185
	11th to 20th	Night	14% 6% 10% 29% 38% 3	2356
	21st to end	Night	25% 6% 15% 30% 19% 4	2674
August	1st to 10th	Night	17% 5% 12% 38% 26% 2	2641
	11th to 20th	Night	33% 5% 27% 22% 10% 2	2839
	21st to end	Night	36% 7% 16% 27% 13% 1	3324
September	1st to 10th	Night	25% 9% 18% 27% 16% 5%	1900
	11th to 20th	Night	40% 10% 13% 17% 15% 4	1900
	21st to end	Night	41% 9% 14% 23% 11% 2	2175
October	1st to 10th	Night	41% 6% 19% 29% 5%	3800
	11th to 20th	Night	53% 6% 15% 21% 5%	3777
	21st to end	Night	60% 5% 11% 15% 8% 1	4273
November	1st to 10th	Night	61% 10% 13% 12% 4 1	2697
	11th to 20th	Night	64% 8% 11% 10% 6% 2	2800
	21st to end	Night	60% 12% 12% 9% 7% 1	2800
December	1st to 10th	Night	62% 9% 10% 13% 6%	2686
	11th to 20th	Night	68% 13% 10% 5% 2	3276
	21st to end	Night	69% 12% 11% 7% 2	5159

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	36% 14% 17% 17% 12% 4	180
	11th to 20th	Day	32% 14% 20% 16% 14% 4	180
	21st to end	Day	34% 15% 20% 18% 9% 3	271
February	1st to 10th	Day	34% 16% 16% 21% 10% 3	370
	11th to 20th	Day	36% 17% 21% 16% 8% 2	365
	21st to end	Day	28% 18% 27% 18% 8% 2	332
March	1st to 10th	Day	25% 17% 26% 22% 9% 2	460
	11th to 20th	Day	26% 15% 21% 21% 12% 5%	460
	21st to end	Day	25% 18% 25% 20% 10% 2	506
April	1st to 10th	Day	28% 16% 23% 20% 10% 3	460
	11th to 20th	Day	22% 16% 29% 18% 8% 6%	452
	21st to end	Day	20% 15% 27% 24% 12% 2	460
May	1st to 10th	Day	14% 11% 26% 30% 15% 4	463
	11th to 20th	Day	22% 7% 21% 26% 17% 8%	488
	21st to end	Day	18% 7% 23% 29% 16% 7%	529
June	1st to 10th	Day	20% 9% 23% 25% 17% 6%	550
	11th to 20th	Day	19% 13% 23% 28% 14% 3	680
	21st to end	Day	12% 7% 20% 34% 21% 6%	680
July	1st to 10th	Day	11% 4 18% 34% 25% 8%	671
	11th to 20th	Day	5% 4 15% 37% 30% 9%	565
	21st to end	Day	8% 3 10% 29% 38% 12%	531
August	1st to 10th	Day	8% 4 15% 29% 33% 10%	487
	11th to 20th	Day	9% 4 11% 30% 33% 13%	477
	21st to end	Day	14% 9% 17% 31% 22% 7%	539
September	1st to 10th	Day	14% 6% 22% 35% 18% 5%	490
	11th to 20th	Day	17% 8% 19% 27% 22% 7%	490
	21st to end	Day	17% 12% 22% 28% 17% 4	490
October	1st to 10th	Day	21% 10% 23% 21% 18% 7%	490
	11th to 20th	Day	26% 11% 18% 25% 15% 6%	490
	21st to end	Day	31% 14% 20% 19% 12% 4	444
November	1st to 10th	Day	33% 16% 23% 16% 12% 1	215
	11th to 20th	Day	33% 15% 24% 14% 10% 6%	200
	21st to end	Day	31% 12% 21% 17% 13% 8%	200
December	1st to 10th	Day	36% 15% 18% 14% 12% 5%	182
	11th to 20th	Day	34% 18% 23% 16% 8% 1	180
	21st to end	Day	36% 14% 22% 17% 11% 1	198
January	1st to 10th	Night	40% 13% 16% 16% 13% 2	850
	11th to 20th	Night	33% 11% 16% 22% 17% 2	760
	21st to end	Night	28% 11% 19% 25% 14% 2	836
February	1st to 10th	Night	32% 13% 19% 22% 12% 2	760
	11th to 20th	Night	26% 11% 22% 26% 14% 1	744
	21st to end	Night	27% 14% 23% 23% 11% 2	628
March	1st to 10th	Night	26% 13% 22% 24% 13% 1	760
	11th to 20th	Night	24% 15% 21% 25% 13% 2	760
	21st to end	Night	25% 15% 22% 27% 9% 1	836
April	1st to 10th	Night	23% 10% 26% 26% 13% 2	760
	11th to 20th	Night	17% 11% 26% 30% 13% 3	709
	21st to end	Night	19% 11% 22% 33% 13% 2	547
May	1st to 10th	Night	15% 11% 22% 34% 17% 1	380
	11th to 20th	Night	14% 5% 19% 34% 26% 2	374
	21st to end	Night	11% 6% 18% 38% 24% 3	411
June	1st to 10th	Night	12% 8% 22% 34% 22% 1	380
	11th to 20th	Night	13% 9% 26% 34% 16% 2	380
	21st to end	Night	9% 7% 19% 38% 25% 2	380
July	1st to 10th	Night	7% 3 15% 37% 36% 2	370
	11th to 20th	Night	5% 2 17% 41% 33% 2	368
	21st to end	Night	5% 2 11% 38% 39% 4	416
August	1st to 10th	Night	4 2 11% 32% 42% 9%	446
	11th to 20th	Night	6% 2 9% 29% 47% 7%	564
	21st to end	Night	10% 3 15% 35% 33% 4	727
September	1st to 10th	Night	9% 3 16% 39% 29% 4	760
	11th to 20th	Night	15% 4 15% 28% 33% 5%	760
	21st to end	Night	17% 7% 17% 34% 22% 3	760
October	1st to 10th	Night	17% 5% 20% 29% 26% 4	760
	11th to 20th	Night	25% 7% 17% 27% 20% 3	760
	21st to end	Night	27% 9% 21% 26% 15% 2	836
November	1st to 10th	Night	32% 11% 19% 26% 11%	780
	11th to 20th	Night	33% 12% 22% 20% 13% 1	800
	21st to end	Night	29% 10% 20% 23% 15% 2	800
December	1st to 10th	Night	28% 9% 17% 29% 16% 1	768
	11th to 20th	Night	34% 11% 22% 21% 10% 2	760
	21st to end	Night	39% 13% 19% 19% 9% 1	921

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