Northdog Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northdog Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	9% 9% 3 9% 6% 5% 5% 5% 6% 8% 34%	105879
	Night	8% 11% 4 10% 6% 6% 6% 5% 7% 8% 31%	101729
Transparency	Day	45% 9% 16% 21% 9% 1	105603
	Night	48% 8% 15% 20% 8% 1	101651
Seeing	Day	14% 9% 23% 29% 19% 6%	16564
	Night	14% 8% 26% 34% 17% 2	24510

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	5% 16% 6% 15% 9% 6% 6% 5% 6% 6% 19%	1899
	11th to 20th	Day	8% 14% 4 9% 5% 8% 8% 6% 9% 7% 21%	1925
	21st to end	Day	8% 12% 4 8% 6% 7% 7% 5% 8% 8% 28%	2578
February	1st to 10th	Day	8% 13% 5% 13% 7% 6% 5% 4 8% 8% 23%	1520
	11th to 20th	Day	5% 15% 5% 12% 8% 5% 6% 4 6% 8% 25%	1537
	21st to end	Day	5% 15% 4 11% 9% 6% 6% 5% 6% 7% 26%	1491
March	1st to 10th	Day	3 8% 4 13% 10% 9% 7% 5% 7% 8% 26%	3138
	11th to 20th	Day	7% 13% 4 10% 5% 5% 5% 5% 6% 7% 33%	3270
	21st to end	Day	4 11% 5% 12% 6% 6% 6% 5% 7% 8% 28%	4165
April	1st to 10th	Day	4 11% 5% 10% 7% 6% 6% 5% 7% 8% 31%	2280
	11th to 20th	Day	7% 11% 3 10% 6% 5% 5% 5% 7% 8% 32%	2436
	21st to end	Day	9% 11% 3 8% 6% 6% 6% 6% 7% 8% 30%	2660
May	1st to 10th	Day	8% 10% 3 9% 6% 5% 6% 4 7% 8% 35%	4439
	11th to 20th	Day	9% 11% 3 9% 6% 6% 7% 5% 7% 8% 30%	4712
	21st to end	Day	13% 9% 4 9% 6% 5% 6% 5% 7% 8% 28%	5509
June	1st to 10th	Day	12% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	3040
	11th to 20th	Day	16% 10% 3 9% 6% 5% 5% 5% 6% 8% 28%	3040
	21st to end	Day	11% 7% 3 8% 5% 5% 5% 4 6% 9% 37%	3040
July	1st to 10th	Day	12% 4 2 5% 3 4 4 4 5% 8% 48%	5060
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 4 6% 7% 50%	5060
	21st to end	Day	12% 6% 2 6% 4 4 5% 5% 6% 8% 42%	5263
August	1st to 10th	Day	9% 6% 2 5% 4 4 4 3 5% 7% 50%	4401
	11th to 20th	Day	11% 5% 2 7% 5% 5% 4 4 7% 10% 40%	4330
	21st to end	Day	13% 6% 3 8% 5% 5% 6% 3 6% 7% 39%	4604
September	1st to 10th	Day	14% 6% 2 7% 4 4 4 4 6% 7% 43%	2280
	11th to 20th	Day	13% 7% 3 7% 5% 4 4 3 6% 8% 39%	2249
	21st to end	Day	11% 8% 3 8% 4 5% 5% 4 7% 8% 38%	2077
October	1st to 10th	Day	10% 9% 4 10% 5% 5% 6% 5% 6% 8% 33%	3180
	11th to 20th	Day	11% 10% 4 9% 5% 5% 6% 5% 7% 8% 29%	2890
	21st to end	Day	10% 12% 5% 8% 5% 4 5% 4 7% 8% 32%	3044
November	1st to 10th	Day	13% 15% 6% 12% 6% 6% 4 3 6% 7% 23%	1454
	11th to 20th	Day	10% 14% 5% 11% 6% 4 5% 4 6% 7% 29%	1330
	21st to end	Day	7% 15% 4 11% 8% 5% 6% 4 6% 8% 26%	1306
December	1st to 10th	Day	8% 14% 4 8% 8% 7% 5% 6% 5% 5% 29%	1140
	11th to 20th	Day	11% 13% 5% 10% 6% 6% 7% 5% 5% 8% 24%	1442
	21st to end	Day	5% 10% 3 7% 6% 5% 6% 5% 7% 10% 36%	2090
January	1st to 10th	Night	5% 15% 7% 17% 7% 5% 5% 4 5% 6% 23%	5046
	11th to 20th	Night	6% 12% 4 9% 7% 7% 6% 7% 10% 8% 24%	5002
	21st to end	Night	10% 15% 5% 8% 6% 5% 5% 5% 7% 8% 26%	5077
February	1st to 10th	Night	9% 15% 5% 11% 8% 6% 5% 4 6% 7% 25%	2660
	11th to 20th	Night	6% 14% 5% 13% 7% 5% 5% 5% 5% 6% 28%	2526
	21st to end	Night	3 14% 6% 11% 7% 6% 6% 4 7% 7% 28%	1963
March	1st to 10th	Night	3 8% 4 16% 10% 8% 7% 5% 6% 7% 26%	3736
	11th to 20th	Night	6% 13% 4 8% 5% 5% 5% 5% 7% 10% 31%	3705
	21st to end	Night	5% 10% 4 10% 6% 6% 6% 5% 7% 7% 35%	3488
April	1st to 10th	Night	5% 11% 4 10% 7% 7% 6% 5% 7% 9% 27%	1900
	11th to 20th	Night	8% 10% 5% 10% 7% 6% 4 4 7% 7% 32%	1692
	21st to end	Night	9% 11% 5% 9% 6% 5% 5% 4 6% 8% 32%	1520
May	1st to 10th	Night	8% 12% 3 10% 6% 5% 5% 4 6% 8% 33%	2520
	11th to 20th	Night	8% 10% 4 9% 5% 6% 7% 5% 8% 9% 30%	2177
	21st to end	Night	12% 10% 4 8% 6% 5% 5% 5% 9% 9% 27%	2066
June	1st to 10th	Night	11% 10% 3 7% 6% 5% 5% 4 6% 7% 35%	1140
	11th to 20th	Night	16% 10% 3 8% 6% 5% 6% 6% 7% 10% 26%	1140
	21st to end	Night	13% 6% 3 7% 5% 4 4 4 8% 9% 35%	1140
July	1st to 10th	Night	10% 4 2 5% 4 3 5% 4 6% 8% 48%	1900
	11th to 20th	Night	8% 4 2 6% 5% 6% 6% 5% 8% 9% 41%	1900
	21st to end	Night	10% 6% 2 7% 6% 6% 6% 5% 8% 10% 36%	2285
August	1st to 10th	Night	8% 5% 2 5% 3 4 5% 4 5% 8% 50%	2529
	11th to 20th	Night	10% 6% 3 5% 5% 5% 5% 4 6% 7% 44%	2497
	21st to end	Night	8% 5% 3 8% 6% 7% 6% 4 7% 9% 36%	3059
September	1st to 10th	Night	12% 6% 3 9% 5% 5% 4 4 7% 8% 37%	1900
	11th to 20th	Night	11% 7% 2 6% 4 5% 5% 3 6% 9% 40%	1930
	21st to end	Night	12% 8% 2 9% 5% 5% 5% 4 6% 8% 37%	2100
October	1st to 10th	Night	9% 11% 3 9% 5% 7% 6% 4 6% 8% 31%	3800
	11th to 20th	Night	7% 7% 3 9% 6% 6% 5% 4 7% 8% 38%	4039
	21st to end	Night	8% 12% 3 9% 6% 6% 7% 5% 7% 8% 29%	4508
November	1st to 10th	Night	13% 15% 6% 10% 6% 5% 5% 4 5% 7% 25%	2705
	11th to 20th	Night	9% 18% 5% 11% 6% 5% 5% 3 5% 6% 27%	2850
	21st to end	Night	10% 15% 5% 11% 7% 5% 4 4 5% 7% 27%	2873
December	1st to 10th	Night	8% 12% 5% 10% 8% 6% 4 4 6% 7% 29%	3040
	11th to 20th	Night	9% 16% 6% 11% 6% 6% 6% 5% 5% 6% 24%	3750
	21st to end	Night	4 10% 4 9% 7% 7% 6% 5% 6% 8% 33%	5566

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	60% 8% 13% 16% 3 1	1899
	11th to 20th	Day	55% 7% 11% 11% 15% 2	1943
	21st to end	Day	46% 13% 21% 14% 6%	2578
February	1st to 10th	Day	56% 7% 14% 13% 10%	1520
	11th to 20th	Day	55% 9% 14% 15% 7% 1	1545
	21st to end	Day	57% 6% 13% 16% 7% 1	1491
March	1st to 10th	Day	51% 9% 13% 18% 9%	3138
	11th to 20th	Day	46% 9% 13% 21% 12%	3270
	21st to end	Day	47% 11% 17% 17% 8%	4165
April	1st to 10th	Day	46% 10% 17% 18% 7% 1	2280
	11th to 20th	Day	47% 7% 16% 19% 10% 1	2436
	21st to end	Day	48% 9% 14% 21% 7% 1	2660
May	1st to 10th	Day	44% 9% 13% 22% 11% 1	4439
	11th to 20th	Day	45% 10% 14% 21% 9% 1	4716
	21st to end	Day	47% 13% 13% 16% 9% 2	5510
June	1st to 10th	Day	47% 8% 15% 19% 10% 1	3040
	11th to 20th	Day	53% 9% 12% 18% 8%	3040
	21st to end	Day	43% 9% 16% 22% 9% 1	3040
July	1st to 10th	Day	33% 7% 13% 26% 19% 2	4981
	11th to 20th	Day	31% 8% 15% 28% 15% 2	4973
	21st to end	Day	38% 8% 20% 26% 7% 1	5096
August	1st to 10th	Day	33% 9% 16% 35% 7%	4384
	11th to 20th	Day	37% 9% 18% 25% 11%	4372
	21st to end	Day	43% 7% 23% 23% 4	4603
September	1st to 10th	Day	40% 7% 21% 24% 7% 1	2280
	11th to 20th	Day	42% 8% 19% 24% 7%	2249
	21st to end	Day	41% 10% 17% 22% 10% 1	2077
October	1st to 10th	Day	47% 7% 21% 20% 6%	3180
	11th to 20th	Day	50% 7% 15% 24% 4	2890
	21st to end	Day	48% 7% 23% 19% 3	3044
November	1st to 10th	Day	60% 5% 13% 17% 5% 1	1454
	11th to 20th	Day	54% 7% 17% 16% 7%	1330
	21st to end	Day	56% 6% 14% 15% 7% 2	1306
December	1st to 10th	Day	54% 8% 12% 17% 9% 1	1140
	11th to 20th	Day	57% 7% 16% 14% 6%	1444
	21st to end	Day	39% 10% 16% 22% 11% 2	2090
January	1st to 10th	Night	60% 7% 11% 15% 6% 1	5046
	11th to 20th	Night	52% 7% 12% 13% 16% 1	5046
	21st to end	Night	51% 11% 19% 14% 5%	5077
February	1st to 10th	Night	57% 7% 14% 13% 9%	2660
	11th to 20th	Night	55% 7% 11% 19% 7% 1	2528
	21st to end	Night	53% 7% 16% 18% 6% 1	1963
March	1st to 10th	Night	53% 8% 13% 18% 9%	3736
	11th to 20th	Night	43% 10% 17% 20% 10%	3706
	21st to end	Night	41% 12% 15% 23% 9%	3488
April	1st to 10th	Night	49% 9% 15% 17% 10% 1	1900
	11th to 20th	Night	48% 8% 13% 19% 11% 1	1692
	21st to end	Night	48% 9% 12% 22% 9% 1	1520
May	1st to 10th	Night	46% 8% 11% 20% 13% 1	2520
	11th to 20th	Night	41% 13% 13% 22% 11% 1	2177
	21st to end	Night	45% 13% 16% 18% 8% 1	2066
June	1st to 10th	Night	46% 8% 13% 22% 11% 1	1140
	11th to 20th	Night	50% 10% 15% 20% 5%	1140
	21st to end	Night	42% 9% 18% 20% 11%	1140
July	1st to 10th	Night	32% 9% 12% 32% 15% 1	1870
	11th to 20th	Night	35% 9% 17% 20% 19%	1864
	21st to end	Night	37% 10% 24% 22% 7%	2213
August	1st to 10th	Night	30% 9% 19% 36% 6%	2521
	11th to 20th	Night	36% 9% 15% 29% 10% 1	2521
	21st to end	Night	42% 8% 26% 19% 4	3059
September	1st to 10th	Night	43% 8% 19% 22% 6% 1	1900
	11th to 20th	Night	39% 8% 19% 26% 8%	1930
	21st to end	Night	44% 8% 16% 20% 12% 1	2100
October	1st to 10th	Night	49% 7% 17% 18% 8%	3800
	11th to 20th	Night	40% 7% 16% 29% 7%	4039
	21st to end	Night	50% 8% 18% 19% 6%	4508
November	1st to 10th	Night	58% 6% 13% 18% 4 1	2705
	11th to 20th	Night	58% 8% 12% 15% 8%	2850
	21st to end	Night	56% 8% 13% 16% 5% 2	2873
December	1st to 10th	Night	52% 7% 13% 17% 10% 1	3040
	11th to 20th	Night	59% 8% 12% 14% 6%	3747
	21st to end	Night	45% 9% 14% 24% 7% 1	5566

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	11% 9% 17% 38% 19% 6%	180
	11th to 20th	Day	18% 8% 19% 32% 18% 4	180
	21st to end	Day	10% 12% 26% 34% 15% 4	198
February	1st to 10th	Day	12% 12% 18% 34% 18% 6%	180
	11th to 20th	Day	10% 13% 22% 31% 19% 6%	237
	21st to end	Day	10% 10% 21% 35% 21% 3	345
March	1st to 10th	Day	8% 8% 25% 34% 20% 6%	460
	11th to 20th	Day	9% 7% 23% 35% 19% 8%	460
	21st to end	Day	12% 7% 25% 30% 22% 4	506
April	1st to 10th	Day	7% 11% 27% 32% 16% 8%	460
	11th to 20th	Day	12% 10% 19% 35% 17% 7%	452
	21st to end	Day	16% 12% 22% 27% 18% 6%	460
May	1st to 10th	Day	16% 11% 25% 29% 17% 3	469
	11th to 20th	Day	14% 8% 19% 30% 21% 8%	680
	21st to end	Day	15% 6% 21% 27% 23% 8%	940
June	1st to 10th	Day	14% 10% 26% 30% 16% 4	870
	11th to 20th	Day	20% 8% 21% 26% 20% 6%	870
	21st to end	Day	14% 6% 19% 26% 24% 10%	870
July	1st to 10th	Day	15% 4 21% 30% 23% 7%	856
	11th to 20th	Day	12% 6% 25% 28% 21% 8%	852
	21st to end	Day	12% 8% 22% 32% 20% 7%	822
August	1st to 10th	Day	13% 8% 19% 27% 22% 12%	487
	11th to 20th	Day	9% 11% 28% 27% 17% 8%	477
	21st to end	Day	15% 8% 26% 28% 14% 8%	539
September	1st to 10th	Day	18% 11% 22% 24% 19% 6%	490
	11th to 20th	Day	18% 7% 24% 24% 19% 7%	490
	21st to end	Day	14% 12% 33% 20% 16% 5%	490
October	1st to 10th	Day	17% 13% 23% 26% 17% 4	490
	11th to 20th	Day	19% 12% 20% 29% 15% 4	335
	21st to end	Day	18% 12% 28% 21% 15% 6%	313
November	1st to 10th	Day	18% 8% 26% 26% 19% 4	188
	11th to 20th	Day	17% 8% 24% 34% 15% 2	180
	21st to end	Day	12% 9% 29% 34% 9% 5%	180
December	1st to 10th	Day	8% 7% 27% 37% 15% 6%	180
	11th to 20th	Day	12% 11% 24% 33% 17% 3	180
	21st to end	Day	15% 6% 16% 38% 17% 9%	198
January	1st to 10th	Night	14% 7% 24% 35% 17% 3	1040
	11th to 20th	Night	13% 9% 27% 32% 15% 2	1026
	21st to end	Night	16% 13% 28% 30% 12% 1	895
February	1st to 10th	Night	16% 10% 23% 29% 17% 5%	760
	11th to 20th	Night	13% 8% 25% 33% 19% 2	744
	21st to end	Night	8% 8% 23% 37% 19% 4	628
March	1st to 10th	Night	8% 8% 30% 32% 19% 3	760
	11th to 20th	Night	14% 7% 26% 38% 14% 2	760
	21st to end	Night	13% 6% 23% 34% 22% 2	836
April	1st to 10th	Night	10% 8% 29% 37% 15% 3	760
	11th to 20th	Night	13% 8% 23% 37% 18% 1	564
	21st to end	Night	12% 9% 32% 29% 18% 1	380
May	1st to 10th	Night	15% 7% 29% 35% 13% 2	380
	11th to 20th	Night	13% 7% 27% 35% 18% 1	374
	21st to end	Night	16% 6% 22% 33% 22% 2	411
June	1st to 10th	Night	16% 7% 25% 30% 22%	380
	11th to 20th	Night	21% 4 22% 29% 23% 2	380
	21st to end	Night	15% 4 24% 33% 21% 3	380
July	1st to 10th	Night	12% 3 21% 40% 24% 1	370
	11th to 20th	Night	14% 4 21% 35% 23% 3	368
	21st to end	Night	11% 4 23% 36% 22% 4	416
August	1st to 10th	Night	12% 3 18% 32% 31% 4	378
	11th to 20th	Night	8% 3 27% 37% 24% 2	376
	21st to end	Night	14% 4 26% 36% 19% 1	566
September	1st to 10th	Night	15% 6% 23% 36% 19% 2	760
	11th to 20th	Night	14% 6% 22% 35% 21% 3	760
	21st to end	Night	17% 11% 26% 31% 14% 2	760
October	1st to 10th	Night	17% 7% 30% 31% 14% 1	760
	11th to 20th	Night	13% 9% 28% 36% 12% 2	760
	21st to end	Night	17% 11% 32% 31% 9% 1	836
November	1st to 10th	Night	20% 6% 25% 32% 14% 3	807
	11th to 20th	Night	19% 10% 31% 29% 9% 1	950
	21st to end	Night	16% 10% 27% 35% 10% 2	961
December	1st to 10th	Night	13% 8% 28% 35% 13% 3	1040
	11th to 20th	Night	14% 8% 27% 32% 15% 3	1040
	21st to end	Night	14% 6% 25% 36% 18% 2	1144

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