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Blackfoot
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackfoot clear sky chart from 2005-11-06 to 2025-11-03.

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% 4 6% 8% 35%	131229
	Night	7% 9% 4 10% 6% 6% 5% 5% 6% 8% 34%	127507
Transparency	Day	45% 6% 17% 24% 8% 1	130927
	Night	47% 5% 16% 22% 9% 1	127389
Seeing	Day	12% 6% 25% 31% 19% 6%	16813
	Night	12% 6% 29% 35% 16% 2	25735

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	4 11% 5% 13% 8% 6% 6% 5% 7% 9% 27%	3179
	11th to 20th	Day	3 8% 3 10% 8% 7% 7% 6% 9% 11% 29%	3159
	21st to end	Day	8% 11% 3 9% 6% 6% 4 4 6% 9% 35%	3699
February	1st to 10th	Day	7% 13% 5% 12% 9% 7% 5% 4 7% 7% 24%	1600
	11th to 20th	Day	5% 12% 6% 12% 6% 6% 7% 5% 7% 8% 27%	1725
	21st to end	Day	4 12% 5% 11% 8% 6% 8% 5% 6% 8% 28%	1509
March	1st to 10th	Day	4 9% 5% 11% 7% 7% 6% 6% 7% 10% 30%	4518
	11th to 20th	Day	5% 12% 5% 11% 6% 6% 5% 3 5% 9% 32%	4999
	21st to end	Day	9% 13% 6% 16% 6% 5% 4 3 5% 7% 25%	5486
April	1st to 10th	Day	8% 13% 4 8% 5% 6% 6% 5% 6% 7% 31%	2525
	11th to 20th	Day	11% 12% 3 8% 5% 5% 5% 4 6% 7% 32%	2570
	21st to end	Day	12% 10% 3 8% 5% 5% 5% 4 6% 8% 32%	2600
May	1st to 10th	Day	11% 8% 3 8% 6% 5% 5% 4 5% 6% 39%	6632
	11th to 20th	Day	14% 11% 3 8% 6% 5% 6% 5% 6% 7% 28%	6734
	21st to end	Day	9% 8% 3 8% 5% 5% 5% 4 6% 8% 40%	7405
June	1st to 10th	Day	14% 8% 3 9% 5% 5% 6% 5% 5% 8% 32%	3000
	11th to 20th	Day	18% 10% 3 10% 6% 6% 5% 5% 6% 7% 23%	3000
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 4 8% 9% 36%	3000
July	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 7% 9% 46%	6780
	11th to 20th	Day	10% 5% 2 7% 5% 5% 4 4 7% 9% 43%	6780
	21st to end	Day	10% 6% 3 7% 4 5% 5% 4 6% 8% 41%	7376
August	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 4 7% 9% 40%	4717
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 5% 8% 10% 42%	4248
	21st to end	Day	10% 4 2 6% 4 4 4 4 7% 9% 45%	4664
September	1st to 10th	Day	11% 6% 1 6% 5% 4 5% 4 6% 8% 44%	2553
	11th to 20th	Day	13% 6% 3 6% 4 4 5% 3 5% 8% 42%	2200
	21st to end	Day	8% 7% 2 7% 5% 5% 5% 4 6% 9% 43%	2200
October	1st to 10th	Day	9% 7% 3 9% 6% 5% 6% 4 6% 7% 39%	3351
	11th to 20th	Day	10% 9% 3 7% 5% 5% 4 3 6% 9% 38%	3029
	21st to end	Day	7% 11% 4 8% 5% 5% 5% 4 6% 7% 38%	3109
November	1st to 10th	Day	8% 14% 5% 10% 6% 6% 5% 4 6% 8% 27%	1584
	11th to 20th	Day	6% 11% 5% 15% 7% 6% 6% 4 5% 7% 28%	1468
	21st to end	Day	4 12% 6% 13% 8% 5% 4 4 6% 8% 30%	1400
December	1st to 10th	Day	7% 14% 6% 11% 8% 7% 6% 5% 6% 7% 23%	2290
	11th to 20th	Day	9% 14% 4 11% 7% 7% 6% 5% 8% 8% 21%	2642
	21st to end	Day	3 7% 3 10% 7% 5% 6% 4 5% 8% 42%	3498
January	1st to 10th	Night	3 8% 4 14% 9% 8% 8% 6% 7% 8% 25%	6766
	11th to 20th	Night	2 8% 4 11% 8% 6% 5% 5% 6% 6% 38%	6739
	21st to end	Night	6% 12% 4 10% 6% 5% 5% 4 6% 8% 34%	7302
February	1st to 10th	Night	5% 10% 4 14% 8% 9% 7% 5% 6% 7% 26%	2800
	11th to 20th	Night	4 11% 4 10% 7% 6% 8% 5% 7% 8% 30%	2562
	21st to end	Night	3 9% 4 11% 7% 7% 7% 5% 7% 9% 31%	2120
March	1st to 10th	Night	4 10% 4 11% 6% 5% 5% 4 8% 10% 32%	5376
	11th to 20th	Night	4 9% 4 13% 7% 6% 6% 5% 6% 9% 31%	5000
	21st to end	Night	10% 14% 6% 15% 6% 5% 5% 4 5% 7% 24%	5474
April	1st to 10th	Night	7% 10% 3 8% 6% 7% 6% 5% 6% 7% 34%	1875
	11th to 20th	Night	9% 10% 5% 10% 7% 6% 6% 5% 6% 7% 29%	1781
	21st to end	Night	12% 10% 3 7% 5% 5% 4 5% 6% 7% 35%	1800
May	1st to 10th	Night	16% 7% 2 6% 5% 4 4 3 5% 5% 41%	3329
	11th to 20th	Night	13% 11% 3 9% 8% 6% 6% 5% 6% 6% 27%	3159
	21st to end	Night	11% 8% 3 7% 6% 6% 5% 4 7% 9% 35%	3470
June	1st to 10th	Night	14% 8% 4 8% 6% 5% 6% 5% 7% 6% 31%	1400
	11th to 20th	Night	16% 9% 3 9% 7% 7% 6% 5% 6% 8% 23%	1400
	21st to end	Night	9% 7% 3 7% 4 6% 5% 5% 7% 10% 38%	1400
July	1st to 10th	Night	6% 5% 2 7% 5% 5% 6% 5% 7% 8% 44%	3180
	11th to 20th	Night	11% 6% 3 7% 5% 5% 5% 4 6% 9% 38%	3180
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 43%	3467
August	1st to 10th	Night	13% 6% 2 5% 5% 4 5% 4 6% 7% 42%	2429
	11th to 20th	Night	10% 8% 2 6% 5% 5% 5% 4 6% 8% 40%	2807
	21st to end	Night	10% 6% 2 5% 4 4 4 4 7% 9% 45%	3245
September	1st to 10th	Night	10% 5% 2 7% 4 4 4 3 5% 8% 47%	1845
	11th to 20th	Night	11% 6% 2 6% 3 4 4 4 5% 8% 47%	2200
	21st to end	Night	8% 6% 2 7% 5% 4 4 4 6% 9% 44%	2200
October	1st to 10th	Night	7% 7% 3 8% 5% 5% 5% 4 6% 9% 41%	3840
	11th to 20th	Night	9% 7% 2 7% 4 5% 5% 5% 7% 9% 41%	4115
	21st to end	Night	6% 8% 3 9% 5% 6% 5% 4 7% 9% 37%	4659
November	1st to 10th	Night	11% 16% 5% 9% 6% 5% 4 4 5% 7% 28%	2759
	11th to 20th	Night	6% 14% 5% 11% 8% 6% 5% 4 5% 7% 27%	2930
	21st to end	Night	5% 12% 5% 12% 9% 5% 6% 5% 6% 7% 28%	3000
December	1st to 10th	Night	7% 16% 5% 12% 8% 6% 6% 5% 6% 7% 22%	4890
	11th to 20th	Night	7% 14% 5% 14% 8% 7% 6% 5% 7% 7% 20%	5550
	21st to end	Night	3 6% 3 8% 7% 6% 6% 5% 6% 8% 42%	7458

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	53% 5% 11% 21% 9%	3179
	11th to 20th	Day	47% 6% 13% 15% 19% 1	3180
	21st to end	Day	46% 5% 19% 24% 5%	3699
February	1st to 10th	Day	58% 5% 14% 14% 9% 1	1600
	11th to 20th	Day	52% 7% 16% 16% 9%	1734
	21st to end	Day	51% 7% 13% 21% 8%	1509
March	1st to 10th	Day	48% 5% 18% 20% 9%	4518
	11th to 20th	Day	49% 5% 15% 22% 9%	5000
	21st to end	Day	57% 6% 9% 22% 6%	5486
April	1st to 10th	Day	48% 7% 16% 19% 9% 1	2525
	11th to 20th	Day	48% 7% 14% 21% 10%	2570
	21st to end	Day	47% 7% 14% 23% 7% 1	2600
May	1st to 10th	Day	45% 5% 10% 25% 14% 1	6632
	11th to 20th	Day	52% 6% 12% 23% 7%	6738
	21st to end	Day	40% 6% 21% 21% 11% 1	7406
June	1st to 10th	Day	47% 7% 13% 21% 10% 1	3000
	11th to 20th	Day	56% 9% 15% 16% 4	3000
	21st to end	Day	40% 10% 21% 21% 8% 1	3000
July	1st to 10th	Day	33% 4 23% 30% 9% 1	6706
	11th to 20th	Day	38% 5% 17% 30% 10%	6699
	21st to end	Day	40% 4 22% 26% 8%	7169
August	1st to 10th	Day	40% 5% 20% 33% 2	4699
	11th to 20th	Day	35% 6% 24% 27% 8%	4289
	21st to end	Day	35% 5% 27% 28% 5% 1	4663
September	1st to 10th	Day	37% 7% 20% 27% 8% 2	2553
	11th to 20th	Day	39% 5% 19% 29% 7% 1	2200
	21st to end	Day	37% 8% 18% 27% 8% 2	2200
October	1st to 10th	Day	45% 3 22% 24% 6%	3351
	11th to 20th	Day	44% 4 14% 28% 10%	3029
	21st to end	Day	44% 5% 18% 28% 4	3109
November	1st to 10th	Day	53% 8% 16% 15% 6% 1	1584
	11th to 20th	Day	54% 6% 17% 19% 4	1468
	21st to end	Day	51% 7% 17% 19% 5% 1	1400
December	1st to 10th	Day	59% 6% 17% 13% 4 1	2290
	11th to 20th	Day	58% 5% 19% 15% 4	2644
	21st to end	Day	41% 4 12% 30% 12% 1	3498
January	1st to 10th	Night	55% 5% 13% 15% 10% 1	6766
	11th to 20th	Night	45% 6% 13% 17% 17% 1	6780
	21st to end	Night	47% 5% 20% 22% 6%	7302
February	1st to 10th	Night	55% 6% 13% 16% 9% 1	2800
	11th to 20th	Night	49% 7% 14% 18% 12% 1	2563
	21st to end	Night	47% 6% 14% 22% 10% 1	2120
March	1st to 10th	Night	45% 5% 18% 22% 10%	5376
	11th to 20th	Night	48% 4 16% 22% 10% 1	5000
	21st to end	Night	58% 6% 10% 17% 9%	5474
April	1st to 10th	Night	44% 8% 14% 19% 14% 1	1875
	11th to 20th	Night	51% 7% 11% 18% 10% 2	1781
	21st to end	Night	45% 7% 12% 22% 12% 2	1800
May	1st to 10th	Night	43% 4 9% 25% 16% 3	3329
	11th to 20th	Night	53% 6% 14% 19% 7%	3159
	21st to end	Night	44% 7% 18% 23% 8% 1	3470
June	1st to 10th	Night	48% 7% 13% 21% 10% 1	1400
	11th to 20th	Night	56% 8% 14% 16% 5%	1400
	21st to end	Night	39% 10% 23% 21% 6% 1	1400
July	1st to 10th	Night	37% 4 23% 21% 14% 1	3145
	11th to 20th	Night	42% 4 14% 32% 8% 1	3138
	21st to end	Night	37% 4 25% 28% 7%	3369
August	1st to 10th	Night	40% 6% 21% 31% 2	2422
	11th to 20th	Night	41% 4 22% 24% 8%	2832
	21st to end	Night	35% 5% 23% 30% 7%	3245
September	1st to 10th	Night	34% 6% 19% 28% 11% 2	1845
	11th to 20th	Night	35% 6% 17% 30% 10% 2	2200
	21st to end	Night	35% 8% 16% 27% 13% 2	2200
October	1st to 10th	Night	40% 3 21% 29% 7%	3840
	11th to 20th	Night	39% 4 14% 34% 8% 1	4115
	21st to end	Night	42% 4 22% 26% 5%	4659
November	1st to 10th	Night	55% 7% 12% 19% 7% 1	2759
	11th to 20th	Night	56% 7% 12% 18% 6% 1	2930
	21st to end	Night	53% 8% 16% 18% 5% 1	3000
December	1st to 10th	Night	60% 5% 17% 12% 5% 1	4890
	11th to 20th	Night	61% 5% 17% 11% 5%	5547
	21st to end	Night	39% 4 17% 22% 16% 2	7458

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% 4 29% 34% 20% 3	200
	11th to 20th	Day	10% 8% 31% 34% 14% 4	200
	21st to end	Day	10% 9% 35% 28% 13% 4	220
February	1st to 10th	Day	7% 12% 30% 37% 13% 3	200
	11th to 20th	Day	8% 6% 29% 36% 18% 3	285
	21st to end	Day	7% 5% 34% 30% 16% 8%	273
March	1st to 10th	Day	9% 7% 29% 35% 18% 3	500
	11th to 20th	Day	9% 4 32% 33% 16% 7%	500
	21st to end	Day	13% 5% 27% 34% 17% 4	550
April	1st to 10th	Day	10% 8% 30% 34% 15% 4	500
	11th to 20th	Day	14% 9% 21% 33% 17% 6%	492
	21st to end	Day	15% 7% 25% 32% 16% 5%	500
May	1st to 10th	Day	13% 10% 23% 31% 18% 5%	691
	11th to 20th	Day	12% 5% 25% 31% 21% 7%	725
	21st to end	Day	15% 6% 21% 30% 20% 8%	789
June	1st to 10th	Day	16% 9% 25% 30% 17% 4	730
	11th to 20th	Day	21% 7% 21% 26% 17% 8%	730
	21st to end	Day	13% 5% 20% 30% 22% 12%	730
July	1st to 10th	Day	10% 4 17% 33% 27% 9%	721
	11th to 20th	Day	11% 4 21% 30% 25% 9%	718
	21st to end	Day	9% 4 18% 32% 26% 11%	794
August	1st to 10th	Day	12% 3 19% 29% 25% 13%	638
	11th to 20th	Day	10% 4 25% 31% 23% 6%	517
	21st to end	Day	11% 7% 19% 30% 23% 10%	583
September	1st to 10th	Day	12% 7% 28% 27% 20% 6%	530
	11th to 20th	Day	17% 7% 22% 31% 20% 4	530
	21st to end	Day	10% 14% 32% 27% 14% 3	530
October	1st to 10th	Day	14% 6% 33% 26% 16% 5%	385
	11th to 20th	Day	15% 7% 30% 30% 15% 2	330
	21st to end	Day	14% 10% 38% 23% 11% 3	363
November	1st to 10th	Day	15% 5% 33% 33% 12% 2	306
	11th to 20th	Day	11% 6% 39% 30% 12% 1	233
	21st to end	Day	6% 6% 40% 31% 14% 4	200
December	1st to 10th	Day	12% 8% 32% 32% 12% 5%	200
	11th to 20th	Day	9% 9% 33% 32% 18% 2	200
	21st to end	Day	8% 5% 25% 40% 18% 5%	220
January	1st to 10th	Night	9% 6% 35% 35% 13% 2	1000
	11th to 20th	Night	9% 9% 36% 32% 13% 1	1000
	21st to end	Night	12% 10% 36% 30% 11% 2	1035
February	1st to 10th	Night	9% 8% 28% 35% 18% 3	800
	11th to 20th	Night	8% 5% 33% 37% 15% 1	784
	21st to end	Night	6% 5% 31% 36% 17% 5%	660
March	1st to 10th	Night	9% 6% 28% 41% 15% 3	800
	11th to 20th	Night	11% 4 30% 38% 15% 3	800
	21st to end	Night	13% 6% 26% 35% 17% 3	880
April	1st to 10th	Night	10% 6% 27% 35% 19% 3	675
	11th to 20th	Night	13% 6% 31% 33% 16% 1	593
	21st to end	Night	15% 7% 24% 37% 16% 1	600
May	1st to 10th	Night	15% 7% 23% 35% 18% 3	460
	11th to 20th	Night	12% 5% 24% 37% 19% 3	394
	21st to end	Night	20% 6% 18% 35% 18% 2	433
June	1st to 10th	Night	17% 7% 23% 38% 14% 2	400
	11th to 20th	Night	22% 5% 20% 31% 21% 1	400
	21st to end	Night	12% 5% 17% 37% 26% 4	400
July	1st to 10th	Night	12% 2 17% 42% 25% 1	390
	11th to 20th	Night	11% 3 19% 44% 22% 2	388
	21st to end	Night	10% 2 18% 39% 27% 3	438
August	1st to 10th	Night	11% 3 16% 33% 32% 5%	398
	11th to 20th	Night	12% 4 18% 40% 24% 3	536
	21st to end	Night	12% 3 20% 39% 25% 1	660
September	1st to 10th	Night	11% 4 21% 36% 26% 1	610
	11th to 20th	Night	13% 4 21% 42% 19% 1	800
	21st to end	Night	11% 6% 30% 38% 14% 2	800
October	1st to 10th	Night	14% 5% 28% 38% 14% 1	800
	11th to 20th	Night	14% 6% 31% 36% 12% 1	800
	21st to end	Night	12% 8% 36% 33% 11%	915
November	1st to 10th	Night	18% 7% 34% 31% 9% 2	986
	11th to 20th	Night	12% 8% 36% 31% 11% 3	1000
	21st to end	Night	9% 6% 38% 33% 12% 2	1000
December	1st to 10th	Night	12% 9% 35% 29% 13% 3	1000
	11th to 20th	Night	11% 7% 34% 35% 12% 2	1000
	21st to end	Night	9% 5% 31% 36% 16% 3	1100

Page generated at 2025-10-31 00:02:42. Version ac07

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