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 2025-10-04.

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% 4 9% 6% 5% 6% 5% 7% 8% 34%	130919
	Night	8% 11% 4 10% 6% 6% 6% 5% 7% 8% 31%	127159
Transparency	Day	45% 8% 16% 21% 9% 1	130596
	Night	48% 8% 15% 20% 8% 1	127059
Seeing	Day	14% 9% 23% 29% 19% 7%	17829
	Night	14% 8% 26% 33% 16% 2	25702

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% 15% 6% 13% 8% 6% 6% 6% 9% 7% 20%	2719
	11th to 20th	Day	6% 14% 4 10% 6% 7% 8% 6% 9% 9% 23%	2773
	21st to end	Day	6% 11% 4 10% 6% 6% 6% 5% 7% 8% 31%	3704
February	1st to 10th	Day	8% 12% 5% 13% 7% 6% 5% 5% 8% 8% 23%	1600
	11th to 20th	Day	5% 15% 5% 12% 8% 5% 6% 5% 6% 8% 25%	1620
	21st to end	Day	5% 15% 4 11% 9% 6% 6% 5% 6% 7% 26%	1567
March	1st to 10th	Day	3 9% 4 14% 10% 9% 7% 5% 6% 8% 26%	4518
	11th to 20th	Day	6% 14% 5% 11% 5% 5% 5% 5% 7% 8% 29%	4676
	21st to end	Day	6% 11% 5% 11% 6% 6% 6% 5% 7% 8% 29%	5969
April	1st to 10th	Day	4 11% 4 10% 7% 6% 6% 5% 7% 8% 32%	2400
	11th to 20th	Day	7% 11% 3 10% 6% 5% 5% 5% 7% 8% 33%	2567
	21st to end	Day	9% 11% 3 8% 6% 6% 6% 6% 7% 8% 30%	2800
May	1st to 10th	Day	7% 8% 3 8% 6% 5% 6% 5% 7% 9% 35%	6353
	11th to 20th	Day	7% 10% 4 11% 7% 7% 7% 5% 8% 9% 26%	6766
	21st to end	Day	12% 8% 3 8% 5% 5% 6% 5% 8% 10% 30%	7907
June	1st to 10th	Day	12% 9% 3 8% 6% 5% 5% 4 6% 8% 34%	3200
	11th to 20th	Day	16% 10% 3 9% 6% 5% 5% 5% 6% 8% 27%	3200
	21st to end	Day	11% 7% 3 8% 5% 5% 5% 4 7% 9% 36%	3200
July	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 43%	7240
	11th to 20th	Day	7% 6% 3 7% 5% 5% 5% 4 6% 8% 45%	7240
	21st to end	Day	11% 6% 2 5% 4 4 4 4 6% 9% 44%	7556
August	1st to 10th	Day	9% 6% 2 5% 4 4 4 3 5% 7% 49%	4541
	11th to 20th	Day	11% 5% 2 8% 5% 5% 4 4 7% 10% 39%	4470
	21st to end	Day	13% 6% 3 7% 5% 5% 5% 3 6% 7% 40%	4749
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 6% 7% 44%	2400
	11th to 20th	Day	12% 7% 3 7% 5% 4 4 3 6% 8% 40%	2367
	21st to end	Day	11% 8% 3 8% 4 5% 5% 4 7% 8% 38%	2186
October	1st to 10th	Day	10% 9% 4 10% 6% 5% 6% 5% 6% 8% 33%	3211
	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	12% 15% 6% 11% 6% 6% 5% 3 6% 7% 23%	1498
	11th to 20th	Day	10% 15% 6% 11% 6% 4 5% 4 6% 7% 28%	1400
	21st to end	Day	7% 16% 5% 11% 8% 5% 6% 3 5% 8% 25%	1374
December	1st to 10th	Day	11% 19% 5% 11% 8% 6% 5% 5% 5% 5% 20%	1960
	11th to 20th	Day	9% 15% 6% 11% 7% 7% 7% 5% 5% 7% 21%	2262
	21st to end	Day	4 9% 4 9% 6% 5% 7% 5% 7% 9% 36%	2992
January	1st to 10th	Night	4 13% 7% 16% 8% 6% 5% 5% 6% 7% 22%	7226
	11th to 20th	Night	5% 11% 5% 11% 8% 7% 7% 6% 8% 8% 24%	7168
	21st to end	Night	8% 16% 5% 8% 5% 5% 5% 4 6% 7% 29%	7251
February	1st to 10th	Night	8% 15% 5% 11% 8% 6% 5% 4 6% 7% 25%	2800
	11th to 20th	Night	6% 14% 5% 13% 7% 5% 5% 5% 5% 7% 28%	2662
	21st to end	Night	3 14% 5% 11% 7% 6% 6% 4 7% 7% 29%	2063
March	1st to 10th	Night	3 7% 3 14% 9% 8% 7% 6% 7% 8% 28%	5376
	11th to 20th	Night	5% 13% 5% 10% 6% 6% 6% 5% 8% 10% 26%	5319
	21st to end	Night	7% 11% 4 9% 5% 5% 5% 4 6% 7% 37%	5006
April	1st to 10th	Night	5% 11% 4 10% 7% 7% 6% 5% 7% 9% 29%	2000
	11th to 20th	Night	8% 10% 4 10% 7% 6% 4 4 7% 7% 32%	1781
	21st to end	Night	9% 10% 5% 9% 6% 5% 6% 5% 7% 8% 31%	1600
May	1st to 10th	Night	7% 12% 3 9% 5% 5% 5% 4 5% 8% 36%	3606
	11th to 20th	Night	7% 9% 4 11% 6% 7% 7% 5% 8% 9% 26%	3123
	21st to end	Night	10% 8% 3 8% 6% 5% 5% 5% 9% 10% 32%	2968
June	1st to 10th	Night	11% 9% 3 8% 6% 6% 5% 4 7% 7% 35%	1200
	11th to 20th	Night	17% 10% 3 7% 6% 5% 6% 6% 6% 9% 26%	1200
	21st to end	Night	12% 6% 3 8% 6% 5% 5% 4 8% 9% 35%	1200
July	1st to 10th	Night	11% 6% 3 7% 5% 5% 6% 4 7% 8% 40%	2720
	11th to 20th	Night	9% 6% 3 7% 5% 6% 5% 5% 7% 8% 39%	2720
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	3283
August	1st to 10th	Night	8% 5% 2 5% 4 4 5% 4 5% 8% 49%	2609
	11th to 20th	Night	10% 6% 3 6% 5% 5% 5% 4 6% 7% 43%	2577
	21st to end	Night	8% 5% 3 8% 6% 6% 6% 4 7% 9% 38%	3156
September	1st to 10th	Night	12% 6% 3 9% 5% 5% 5% 4 7% 8% 39%	2000
	11th to 20th	Night	11% 7% 2 6% 4 5% 5% 3 6% 10% 41%	2032
	21st to end	Night	12% 9% 2 9% 5% 5% 5% 4 6% 8% 37%	2211
October	1st to 10th	Night	9% 11% 3 9% 5% 7% 6% 4 6% 8% 31%	3836
	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% 5% 10% 6% 5% 5% 4 5% 7% 25%	2780
	11th to 20th	Night	9% 18% 5% 11% 6% 5% 5% 3 5% 6% 27%	3000
	21st to end	Night	9% 17% 5% 11% 7% 5% 4 4 5% 7% 26%	3025
December	1st to 10th	Night	10% 14% 5% 11% 7% 6% 5% 4 6% 7% 25%	5220
	11th to 20th	Night	9% 18% 6% 14% 7% 5% 5% 4 5% 5% 21%	5930
	21st to end	Night	4 11% 5% 10% 7% 6% 6% 4 6% 8% 32%	7964

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	56% 7% 13% 14% 9% 1	2719
	11th to 20th	Day	53% 7% 13% 16% 11% 1	2791
	21st to end	Day	45% 11% 20% 16% 7%	3704
February	1st to 10th	Day	55% 7% 13% 13% 11%	1600
	11th to 20th	Day	55% 8% 13% 15% 7% 1	1628
	21st to end	Day	56% 6% 13% 16% 7% 1	1567
March	1st to 10th	Day	52% 8% 13% 19% 9%	4518
	11th to 20th	Day	49% 8% 13% 19% 11%	4676
	21st to end	Day	47% 9% 15% 18% 10%	5969
April	1st to 10th	Day	46% 10% 16% 20% 8% 1	2400
	11th to 20th	Day	46% 7% 16% 20% 11% 1	2567
	21st to end	Day	48% 9% 14% 22% 7% 1	2800
May	1st to 10th	Day	42% 8% 11% 23% 14% 2	6353
	11th to 20th	Day	47% 10% 15% 20% 7% 1	6770
	21st to end	Day	44% 11% 16% 18% 10% 1	7908
June	1st to 10th	Day	47% 8% 15% 20% 10% 1	3200
	11th to 20th	Day	54% 9% 12% 18% 7%	3200
	21st to end	Day	43% 9% 17% 22% 9% 1	3200
July	1st to 10th	Day	35% 8% 17% 22% 16% 2	7161
	11th to 20th	Day	36% 8% 15% 28% 12% 1	7153
	21st to end	Day	35% 7% 20% 31% 7%	7342
August	1st to 10th	Day	34% 9% 16% 34% 7%	4524
	11th to 20th	Day	37% 9% 18% 24% 11%	4512
	21st to end	Day	42% 7% 22% 23% 5%	4748
September	1st to 10th	Day	39% 7% 20% 27% 7% 1	2400
	11th to 20th	Day	41% 8% 18% 25% 8%	2367
	21st to end	Day	42% 9% 17% 22% 9% 1	2186
October	1st to 10th	Day	47% 7% 20% 20% 6%	3211
	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% 4 1	1498
	11th to 20th	Day	55% 7% 16% 15% 7%	1400
	21st to end	Day	57% 6% 13% 15% 7% 2	1374
December	1st to 10th	Day	65% 7% 12% 10% 5%	1960
	11th to 20th	Day	62% 6% 16% 12% 4	2264
	21st to end	Day	40% 9% 14% 26% 9% 1	2992
January	1st to 10th	Night	58% 7% 11% 15% 9% 1	7226
	11th to 20th	Night	53% 6% 13% 15% 11% 1	7212
	21st to end	Night	50% 10% 18% 14% 8%	7251
February	1st to 10th	Night	57% 6% 14% 12% 10%	2800
	11th to 20th	Night	55% 7% 11% 19% 7% 1	2664
	21st to end	Night	52% 7% 15% 18% 7% 1	2063
March	1st to 10th	Night	48% 8% 14% 18% 12%	5376
	11th to 20th	Night	48% 9% 14% 20% 9%	5320
	21st to end	Night	41% 10% 12% 24% 13%	5006
April	1st to 10th	Night	47% 9% 14% 19% 11% 1	2000
	11th to 20th	Night	48% 8% 13% 19% 11% 1	1781
	21st to end	Night	47% 9% 12% 22% 9% 1	1600
May	1st to 10th	Night	44% 7% 10% 23% 14% 2	3603
	11th to 20th	Night	45% 11% 13% 21% 8%	3123
	21st to end	Night	40% 12% 19% 17% 8% 3	2968
June	1st to 10th	Night	46% 8% 13% 22% 11% 1	1200
	11th to 20th	Night	51% 10% 15% 20% 5%	1200
	21st to end	Night	42% 9% 18% 20% 10%	1200
July	1st to 10th	Night	39% 9% 15% 25% 11% 1	2690
	11th to 20th	Night	39% 8% 15% 24% 13%	2684
	21st to end	Night	35% 9% 20% 29% 7%	3192
August	1st to 10th	Night	31% 9% 19% 35% 6%	2601
	11th to 20th	Night	37% 9% 15% 28% 10% 1	2601
	21st to end	Night	41% 8% 25% 20% 6% 1	3156
September	1st to 10th	Night	42% 8% 19% 23% 7% 1	2000
	11th to 20th	Night	38% 7% 19% 26% 9%	2032
	21st to end	Night	44% 8% 15% 20% 11% 1	2211
October	1st to 10th	Night	49% 8% 18% 18% 8%	3836
	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% 7% 13% 18% 5% 1	2780
	11th to 20th	Night	58% 7% 12% 15% 7%	3000
	21st to end	Night	57% 8% 12% 16% 5% 2	3025
December	1st to 10th	Night	57% 8% 15% 14% 6%	5220
	11th to 20th	Night	64% 7% 14% 11% 4	5927
	21st to end	Night	47% 8% 13% 26% 6% 1	7964

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% 10% 16% 39% 20% 6%	200
	11th to 20th	Day	18% 10% 21% 32% 17% 4	200
	21st to end	Day	10% 14% 29% 31% 14% 3	220
February	1st to 10th	Day	11% 11% 18% 35% 20% 6%	200
	11th to 20th	Day	9% 12% 24% 31% 19% 6%	260
	21st to end	Day	10% 10% 21% 35% 20% 3	373
March	1st to 10th	Day	8% 8% 24% 35% 19% 6%	500
	11th to 20th	Day	9% 7% 24% 33% 19% 8%	500
	21st to end	Day	12% 8% 25% 29% 21% 5%	550
April	1st to 10th	Day	6% 10% 27% 33% 16% 8%	500
	11th to 20th	Day	12% 11% 21% 33% 16% 6%	492
	21st to end	Day	15% 12% 23% 27% 17% 6%	500
May	1st to 10th	Day	15% 12% 25% 28% 16% 3	510
	11th to 20th	Day	13% 8% 20% 30% 21% 8%	731
	21st to end	Day	15% 6% 22% 27% 22% 7%	1006
June	1st to 10th	Day	14% 10% 27% 30% 16% 4	930
	11th to 20th	Day	21% 8% 21% 25% 21% 6%	930
	21st to end	Day	14% 6% 20% 26% 24% 10%	930
July	1st to 10th	Day	15% 5% 22% 30% 22% 6%	916
	11th to 20th	Day	12% 6% 25% 29% 21% 8%	912
	21st to end	Day	12% 8% 21% 31% 20% 8%	881
August	1st to 10th	Day	13% 9% 18% 27% 22% 11%	527
	11th to 20th	Day	9% 11% 29% 27% 16% 7%	517
	21st to end	Day	14% 8% 25% 27% 16% 10%	583
September	1st to 10th	Day	17% 10% 22% 24% 19% 8%	530
	11th to 20th	Day	17% 7% 24% 24% 22% 7%	530
	21st to end	Day	15% 13% 33% 20% 14% 5%	530
October	1st to 10th	Day	17% 13% 23% 26% 17% 5%	503
	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	16% 9% 28% 27% 18% 4	200
	11th to 20th	Day	17% 9% 24% 34% 15% 3	200
	21st to end	Day	12% 12% 29% 33% 10% 5%	200
December	1st to 10th	Day	10% 8% 26% 37% 14% 6%	200
	11th to 20th	Day	12% 12% 23% 35% 16% 4	200
	21st to end	Day	13% 5% 17% 38% 17% 10%	220
January	1st to 10th	Night	13% 7% 24% 35% 17% 3	1100
	11th to 20th	Night	13% 10% 28% 32% 15% 2	1085
	21st to end	Night	16% 14% 29% 28% 11% 1	945
February	1st to 10th	Night	16% 10% 23% 29% 18% 5%	800
	11th to 20th	Night	13% 7% 26% 34% 19% 2	784
	21st to end	Night	8% 9% 25% 36% 18% 4	660
March	1st to 10th	Night	8% 8% 30% 32% 20% 3	800
	11th to 20th	Night	14% 7% 26% 38% 14% 2	800
	21st to end	Night	13% 6% 23% 34% 21% 2	880
April	1st to 10th	Night	9% 7% 29% 37% 15% 3	800
	11th to 20th	Night	13% 8% 24% 36% 17% 1	593
	21st to end	Night	12% 9% 32% 29% 18% 1	400
May	1st to 10th	Night	15% 7% 30% 35% 12% 2	400
	11th to 20th	Night	13% 7% 27% 35% 18% 1	394
	21st to end	Night	15% 6% 22% 32% 22% 2	433
June	1st to 10th	Night	15% 8% 25% 30% 22%	400
	11th to 20th	Night	21% 4 22% 28% 23% 2	400
	21st to end	Night	15% 4 25% 33% 21% 3	400
July	1st to 10th	Night	12% 4 21% 39% 23% 1	390
	11th to 20th	Night	14% 5% 21% 35% 22% 3	388
	21st to end	Night	10% 4 21% 36% 24% 4	438
August	1st to 10th	Night	12% 4 19% 32% 31% 4	398
	11th to 20th	Night	8% 4 27% 36% 23% 2	396
	21st to end	Night	13% 4 25% 36% 20% 1	597
September	1st to 10th	Night	14% 6% 23% 36% 20% 2	800
	11th to 20th	Night	14% 6% 22% 36% 21% 3	800
	21st to end	Night	17% 11% 27% 30% 13% 2	800
October	1st to 10th	Night	17% 7% 30% 31% 14% 1	772
	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% 7% 26% 31% 13% 3	830
	11th to 20th	Night	18% 11% 31% 30% 9% 1	1000
	21st to end	Night	16% 10% 27% 34% 11% 2	1013
December	1st to 10th	Night	14% 8% 28% 35% 12% 3	1100
	11th to 20th	Night	15% 8% 27% 32% 15% 3	1100
	21st to end	Night	13% 6% 25% 36% 18% 2	1210

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