Northstar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northstar Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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	12% 9% 3 7% 4 4 4 3 5% 6% 43%	105373
	Night	14% 10% 3 7% 5% 4 4 4 5% 6% 39%	102727
Transparency	Day	41% 8% 12% 21% 14% 5%	105141
	Night	44% 8% 12% 19% 12% 5%	102605
Seeing	Day	17% 12% 24% 25% 17% 5%	14935
	Night	20% 7% 17% 32% 20% 3	24097

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	23% 25% 7% 9% 5% 5% 5% 3 5% 4 10%	2539
	11th to 20th	Day	22% 15% 4 9% 8% 5% 4 3 3 6% 20%	2516
	21st to end	Day	27% 19% 5% 10% 6% 5% 4 3 4 4 13%	2794
February	1st to 10th	Day	23% 16% 5% 9% 4 4 5% 3 5% 5% 20%	1520
	11th to 20th	Day	17% 17% 5% 9% 4 4 4 4 5% 5% 25%	1775
	21st to end	Day	18% 20% 6% 9% 6% 5% 4 3 4 4 22%	1570
March	1st to 10th	Day	12% 16% 3 8% 5% 5% 5% 5% 6% 7% 27%	3138
	11th to 20th	Day	11% 10% 3 7% 4 4 4 3 5% 6% 44%	3295
	21st to end	Day	18% 16% 4 10% 7% 6% 5% 4 5% 5% 20%	4093
April	1st to 10th	Day	16% 13% 3 9% 5% 6% 5% 5% 6% 6% 24%	2280
	11th to 20th	Day	13% 10% 4 9% 7% 5% 5% 4 6% 6% 32%	2335
	21st to end	Day	13% 10% 3 8% 6% 6% 6% 4 6% 7% 32%	2470
May	1st to 10th	Day	10% 7% 3 8% 5% 5% 6% 4 6% 8% 37%	4295
	11th to 20th	Day	7% 6% 2 6% 5% 4 5% 4 6% 10% 44%	4377
	21st to end	Day	10% 8% 2 8% 6% 6% 6% 4 7% 8% 34%	4904
June	1st to 10th	Day	17% 7% 2 7% 5% 5% 4 4 5% 7% 37%	2850
	11th to 20th	Day	14% 6% 2 7% 6% 4 5% 4 6% 8% 37%	2850
	21st to end	Day	7% 4 1 5% 4 4 4 4 6% 7% 55%	2850
July	1st to 10th	Day	2 2 1 2 2 3 2 2 3 4 76%	4527
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 79%	4420
	21st to end	Day	1 1 1 2 2 3 2 3 5% 5% 75%	4790
August	1st to 10th	Day	3 2 1 3 3 3 3 2 4 7% 69%	4401
	11th to 20th	Day	3 3 1 4 3 3 3 3 5% 7% 65%	4120
	21st to end	Day	3 3 1 4 3 3 4 4 6% 7% 63%	4305
September	1st to 10th	Day	5% 4 1 4 2 3 3 3 4 6% 65%	2280
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 5% 7% 50%	2157
	21st to end	Day	9% 6% 2 6% 4 5% 4 4 6% 8% 47%	2090
October	1st to 10th	Day	9% 5% 2 4 3 3 3 3 4 7% 56%	3402
	11th to 20th	Day	11% 7% 2 6% 5% 5% 5% 4 5% 8% 43%	2942
	21st to end	Day	17% 10% 4 8% 5% 5% 5% 4 6% 6% 30%	3101
November	1st to 10th	Day	26% 13% 5% 7% 5% 4 4 3 4 7% 24%	1764
	11th to 20th	Day	23% 15% 4 11% 4 4 4 4 4 4 23%	1724
	21st to end	Day	20% 19% 5% 10% 5% 5% 4 3 4 4 20%	1399
December	1st to 10th	Day	19% 17% 7% 9% 6% 4 5% 4 4 5% 19%	1330
	11th to 20th	Day	24% 23% 7% 12% 6% 4 4 3 3 3 10%	1682
	21st to end	Day	20% 21% 5% 11% 5% 5% 5% 3 5% 5% 16%	2488
January	1st to 10th	Night	25% 22% 4 9% 6% 5% 4 3 4 4 13%	4406
	11th to 20th	Night	19% 15% 5% 10% 5% 3 3 2 4 6% 26%	4382
	21st to end	Night	28% 19% 4 8% 5% 4 4 3 4 5% 17%	4862
February	1st to 10th	Night	20% 19% 4 7% 4 3 3 3 4 5% 25%	2660
	11th to 20th	Night	16% 15% 4 9% 5% 5% 4 4 5% 5% 29%	2289
	21st to end	Night	14% 15% 3 7% 6% 5% 5% 4 6% 6% 27%	1884
March	1st to 10th	Night	10% 12% 3 8% 5% 5% 5% 5% 8% 7% 34%	3736
	11th to 20th	Night	12% 9% 2 5% 3 3 4 4 4 5% 48%	3683
	21st to end	Night	14% 10% 3 9% 7% 6% 7% 5% 7% 7% 23%	3557
April	1st to 10th	Night	12% 10% 2 7% 6% 5% 6% 5% 7% 8% 33%	1900
	11th to 20th	Night	13% 10% 2 9% 5% 5% 4 4 4 5% 39%	1796
	21st to end	Night	11% 9% 3 8% 5% 5% 4 4 6% 6% 39%	1710
May	1st to 10th	Night	10% 6% 2 7% 6% 6% 6% 5% 6% 7% 36%	2654
	11th to 20th	Night	7% 5% 2 5% 4 5% 5% 4 6% 10% 47%	2516
	21st to end	Night	13% 6% 3 8% 6% 5% 5% 5% 7% 8% 34%	2666
June	1st to 10th	Night	13% 6% 2 7% 5% 4 5% 4 6% 8% 41%	1330
	11th to 20th	Night	11% 4 2 7% 5% 4 6% 5% 6% 9% 41%	1330
	21st to end	Night	5% 5% 2 5% 5% 4 5% 3 5% 7% 55%	1330
July	1st to 10th	Night	2 1 1 3 3 2 3 3 4 4 75%	2428
	11th to 20th	Night	2 1 1 2 2 2 2 1 3 4 82%	2540
	21st to end	Night	2 2 1 2 2 2 2 2 3 5% 77%	2762
August	1st to 10th	Night	3 3 1 3 2 2 2 2 5% 6% 72%	2529
	11th to 20th	Night	3 3 2 5% 3 3 4 3 5% 7% 62%	2715
	21st to end	Night	3 3 1 4 4 4 4 3 5% 6% 63%	3365
September	1st to 10th	Night	4 2 2 3 3 3 2 3 5% 7% 66%	1900
	11th to 20th	Night	6% 5% 2 6% 4 5% 4 3 5% 8% 52%	2023
	21st to end	Night	11% 5% 2 4 3 4 4 4 5% 8% 51%	2090
October	1st to 10th	Night	7% 3 2 4 3 4 3 3 5% 7% 59%	3575
	11th to 20th	Night	12% 5% 2 5% 4 5% 4 3 5% 6% 49%	3988
	21st to end	Night	13% 6% 2 7% 4 5% 5% 4 6% 7% 41%	4457
November	1st to 10th	Night	23% 12% 3 7% 4 4 4 4 5% 7% 26%	2535
	11th to 20th	Night	21% 14% 4 8% 5% 4 5% 4 4 5% 25%	2675
	21st to end	Night	22% 13% 3 9% 5% 5% 4 4 4 6% 25%	2912
December	1st to 10th	Night	19% 15% 5% 9% 4 4 4 3 5% 5% 25%	2850
	11th to 20th	Night	24% 20% 4 11% 6% 5% 4 4 4 4 13%	3510
	21st to end	Night	20% 16% 5% 11% 7% 6% 6% 5% 4 5% 14%	5182

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	75% 10% 9% 5% 2	2539
	11th to 20th	Day	63% 8% 11% 13% 3 2	2540
	21st to end	Day	73% 8% 11% 6% 1 1	2794
February	1st to 10th	Day	66% 8% 12% 9% 4 1	1520
	11th to 20th	Day	61% 9% 12% 11% 6%	1785
	21st to end	Day	64% 8% 9% 14% 5%	1570
March	1st to 10th	Day	51% 10% 12% 16% 9%	3138
	11th to 20th	Day	40% 8% 18% 31% 2	3295
	21st to end	Day	63% 12% 11% 9% 4	4093
April	1st to 10th	Day	57% 10% 16% 14% 3	2280
	11th to 20th	Day	49% 8% 15% 17% 9% 1	2335
	21st to end	Day	49% 11% 14% 18% 8% 1	2470
May	1st to 10th	Day	41% 11% 13% 28% 7%	4295
	11th to 20th	Day	31% 9% 19% 32% 9%	4380
	21st to end	Day	43% 14% 18% 22% 4	4905
June	1st to 10th	Day	45% 8% 13% 22% 11%	2850
	11th to 20th	Day	42% 8% 15% 22% 10% 2	2850
	21st to end	Day	27% 7% 15% 29% 19% 3	2850
July	1st to 10th	Day	13% 4 7% 20% 34% 23%	4458
	11th to 20th	Day	10% 3 6% 13% 28% 39%	4345
	21st to end	Day	12% 3 6% 20% 37% 22%	4639
August	1st to 10th	Day	16% 3 6% 24% 37% 13%	4384
	11th to 20th	Day	18% 4 10% 40% 24% 4	4161
	21st to end	Day	19% 5% 12% 35% 25% 4	4304
September	1st to 10th	Day	21% 5% 11% 30% 23% 10%	2280
	11th to 20th	Day	32% 8% 12% 23% 21% 4	2157
	21st to end	Day	33% 10% 18% 24% 11% 4	2090
October	1st to 10th	Day	29% 5% 13% 37% 16% 1	3402
	11th to 20th	Day	41% 5% 19% 24% 11%	2942
	21st to end	Day	52% 8% 15% 18% 7%	3101
November	1st to 10th	Day	61% 8% 13% 14% 3	1764
	11th to 20th	Day	64% 9% 13% 8% 4 1	1724
	21st to end	Day	66% 8% 11% 11% 4	1399
December	1st to 10th	Day	66% 8% 10% 12% 3	1330
	11th to 20th	Day	77% 8% 8% 6% 1	1684
	21st to end	Day	66% 10% 11% 9% 4	2488
January	1st to 10th	Night	71% 11% 9% 8% 1 1	4406
	11th to 20th	Night	59% 8% 13% 14% 5% 2	4420
	21st to end	Night	68% 9% 12% 8% 2 1	4862
February	1st to 10th	Night	61% 8% 14% 12% 4 1	2660
	11th to 20th	Night	56% 8% 17% 12% 7%	2290
	21st to end	Night	55% 9% 11% 17% 7% 1	1884
March	1st to 10th	Night	43% 10% 13% 24% 9% 1	3736
	11th to 20th	Night	37% 7% 17% 32% 7%	3684
	21st to end	Night	52% 15% 14% 16% 3	3557
April	1st to 10th	Night	45% 10% 15% 22% 7%	1900
	11th to 20th	Night	46% 7% 12% 21% 13% 1	1796
	21st to end	Night	44% 8% 13% 23% 11% 1	1710
May	1st to 10th	Night	39% 11% 14% 23% 11% 1	2654
	11th to 20th	Night	27% 10% 13% 33% 17% 1	2516
	21st to end	Night	41% 15% 16% 20% 8%	2666
June	1st to 10th	Night	39% 9% 10% 22% 18% 2	1330
	11th to 20th	Night	38% 8% 13% 23% 13% 4	1330
	21st to end	Night	28% 6% 12% 26% 22% 6%	1330
July	1st to 10th	Night	12% 4 5% 16% 33% 30%	2388
	11th to 20th	Night	9% 2 4 12% 21% 51%	2492
	21st to end	Night	11% 2 4 16% 32% 34%	2674
August	1st to 10th	Night	15% 2 5% 19% 39% 20%	2521
	11th to 20th	Night	22% 3 8% 30% 29% 8%	2740
	21st to end	Night	20% 5% 10% 27% 29% 8%	3365
September	1st to 10th	Night	18% 5% 9% 28% 28% 13%	1900
	11th to 20th	Night	30% 7% 11% 22% 23% 8%	2023
	21st to end	Night	31% 6% 15% 27% 15% 6%	2090
October	1st to 10th	Night	25% 4 12% 40% 18% 1	3575
	11th to 20th	Night	36% 5% 14% 24% 20% 1	3988
	21st to end	Night	39% 9% 14% 28% 8% 2	4457
November	1st to 10th	Night	55% 9% 16% 14% 4 1	2535
	11th to 20th	Night	61% 8% 14% 11% 5% 2	2675
	21st to end	Night	59% 8% 15% 13% 5%	2912
December	1st to 10th	Night	61% 8% 13% 14% 4	2850
	11th to 20th	Night	71% 9% 10% 8% 1	3507
	21st to end	Night	63% 13% 12% 8% 3	5182

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	30% 19% 16% 19% 12% 4	180
	11th to 20th	Day	32% 16% 18% 20% 8% 7%	180
	21st to end	Day	33% 12% 26% 19% 11%	198
February	1st to 10th	Day	33% 18% 28% 12% 9%	180
	11th to 20th	Day	29% 19% 23% 20% 8% 1	408
	21st to end	Day	27% 20% 22% 21% 9% 1	382
March	1st to 10th	Day	25% 23% 22% 18% 10% 3	460
	11th to 20th	Day	22% 14% 21% 23% 18% 3	460
	21st to end	Day	26% 19% 25% 20% 9% 1	506
April	1st to 10th	Day	24% 16% 28% 21% 8% 3	460
	11th to 20th	Day	17% 21% 27% 18% 13% 4	452
	21st to end	Day	16% 17% 27% 26% 12% 2	460
May	1st to 10th	Day	13% 13% 26% 31% 14% 3	463
	11th to 20th	Day	17% 9% 25% 25% 18% 5%	488
	21st to end	Day	12% 12% 21% 30% 19% 7%	581
June	1st to 10th	Day	18% 10% 22% 25% 20% 5%	680
	11th to 20th	Day	16% 14% 30% 22% 15% 3	680
	21st to end	Day	7% 8% 24% 29% 27% 6%	680
July	1st to 10th	Day	5% 5% 23% 35% 28% 4	554
	11th to 20th	Day	4 8% 26% 33% 24% 5%	484
	21st to end	Day	3 6% 23% 35% 25% 7%	531
August	1st to 10th	Day	6% 5% 23% 33% 25% 8%	487
	11th to 20th	Day	3 3 21% 35% 26% 11%	477
	21st to end	Day	6% 9% 26% 27% 21% 10%	539
September	1st to 10th	Day	8% 8% 26% 26% 22% 10%	490
	11th to 20th	Day	10% 13% 23% 25% 21% 7%	490
	21st to end	Day	12% 14% 25% 25% 20% 5%	490
October	1st to 10th	Day	15% 8% 26% 27% 17% 6%	490
	11th to 20th	Day	18% 13% 22% 24% 15% 9%	361
	21st to end	Day	31% 12% 22% 22% 10% 3	330
November	1st to 10th	Day	34% 12% 17% 25% 12%	300
	11th to 20th	Day	35% 12% 20% 16% 15% 2	262
	21st to end	Day	28% 13% 20% 23% 15% 1	194
December	1st to 10th	Day	28% 17% 21% 16% 12% 6%	180
	11th to 20th	Day	38% 11% 26% 17% 6% 2	180
	21st to end	Day	36% 14% 19% 20% 11% 2	198
January	1st to 10th	Night	39% 9% 17% 23% 9% 2	760
	11th to 20th	Night	29% 10% 17% 29% 13% 3	760
	21st to end	Night	27% 7% 19% 31% 14% 1	836
February	1st to 10th	Night	31% 9% 22% 27% 9% 2	760
	11th to 20th	Night	26% 9% 18% 31% 14% 2	744
	21st to end	Night	22% 11% 21% 28% 17% 2	628
March	1st to 10th	Night	22% 9% 20% 31% 17% 1	760
	11th to 20th	Night	22% 10% 16% 31% 19% 1	760
	21st to end	Night	21% 9% 22% 34% 12% 2	836
April	1st to 10th	Night	17% 8% 20% 36% 18% 1	760
	11th to 20th	Night	16% 9% 24% 34% 15% 2	668
	21st to end	Night	14% 9% 22% 34% 19% 2	570
May	1st to 10th	Night	10% 6% 22% 33% 25% 3	455
	11th to 20th	Night	12% 5% 17% 29% 30% 8%	374
	21st to end	Night	14% 5% 16% 31% 29% 5%	411
June	1st to 10th	Night	14% 7% 16% 36% 23% 4	380
	11th to 20th	Night	12% 8% 22% 33% 22% 3	380
	21st to end	Night	6% 4 13% 40% 32% 5%	380
July	1st to 10th	Night	4 2 12% 40% 40% 2	370
	11th to 20th	Night	2 2 14% 46% 32% 2	368
	21st to end	Night	2 2 9% 43% 40% 4	416
August	1st to 10th	Night	2 3 10% 38% 39% 7%	378
	11th to 20th	Night	4 1 9% 38% 39% 9%	504
	21st to end	Night	5% 4 14% 35% 38% 4	755
September	1st to 10th	Night	6% 2 14% 39% 33% 6%	760
	11th to 20th	Night	8% 5% 15% 30% 34% 8%	760
	21st to end	Night	13% 5% 17% 33% 26% 6%	760
October	1st to 10th	Night	14% 6% 13% 39% 25% 2	760
	11th to 20th	Night	18% 6% 15% 32% 26% 3	760
	21st to end	Night	21% 8% 20% 31% 16% 3	836
November	1st to 10th	Night	30% 8% 16% 33% 13%	780
	11th to 20th	Night	31% 10% 19% 27% 13% 1	800
	21st to end	Night	29% 9% 16% 28% 17% 2	942
December	1st to 10th	Night	27% 8% 18% 31% 15% 1	950
	11th to 20th	Night	35% 8% 17% 29% 9%	950
	21st to end	Night	36% 11% 17% 27% 8%	1026

Page generated at 2024-11-24 21:22:20. Version ac07

Copyright © 2024 Attilla Danko.