Boise
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boise clear sky chart from 2005-11-06 to 2025-09-04.

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	7% 6% 2 5% 4 3 4 3 5% 6% 54%	134742
	Night	10% 8% 2 6% 4 4 4 3 5% 6% 49%	131293
Transparency	Day	31% 4 10% 21% 20% 14%	129527
	Night	36% 4 10% 21% 17% 11%	127520
Seeing	Day	11% 8% 23% 29% 22% 7%	15520
	Night	14% 4 18% 38% 22% 3	24775

By Month

Percentage Cloud Cover SW (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	16% 17% 5% 11% 5% 5% 5% 4 7% 6% 20%	3639
	11th to 20th	Day	10% 12% 4 9% 5% 4 5% 3 5% 8% 35%	3640
	21st to end	Day	14% 9% 4 9% 5% 4 4 2 4 6% 38%	4004
February	1st to 10th	Day	14% 12% 4 7% 6% 5% 5% 4 6% 7% 30%	1768
	11th to 20th	Day	11% 13% 3 8% 5% 4 5% 4 6% 7% 34%	1950
	21st to end	Day	11% 12% 4 6% 6% 5% 5% 4 5% 7% 35%	1650
March	1st to 10th	Day	7% 10% 4 9% 7% 6% 6% 4 6% 9% 32%	4518
	11th to 20th	Day	7% 9% 3 8% 4 4 4 4 5% 6% 47%	4814
	21st to end	Day	9% 11% 3 9% 6% 6% 5% 5% 6% 8% 31%	5739
April	1st to 10th	Day	10% 9% 3 6% 4 4 6% 5% 6% 9% 39%	2400
	11th to 20th	Day	7% 9% 3 5% 5% 5% 5% 4 6% 8% 45%	2415
	21st to end	Day	8% 6% 3 6% 4 4 4 5% 6% 8% 47%	2600
May	1st to 10th	Day	6% 7% 2 4 3 3 4 3 5% 6% 57%	5880
	11th to 20th	Day	6% 5% 2 5% 3 3 3 3 5% 7% 59%	6014
	21st to end	Day	4 4 1 3 3 3 4 3 5% 7% 62%	6902
June	1st to 10th	Day	8% 5% 2 6% 4 5% 4 5% 6% 8% 45%	2800
	11th to 20th	Day	6% 4 1 4 3 3 4 3 5% 8% 57%	2800
	21st to end	Day	3 2 1 3 2 2 3 3 4 5% 72%	2800
July	1st to 10th	Day	2 2 2 2 1 2 2 4 4 79%	6319
	11th to 20th	Day	2 2 2 1 2 2 2 3 4 82%	6320
	21st to end	Day	3 3 1 3 2 2 2 3 5% 7% 69%	6880
August	1st to 10th	Day	3 3 1 4 2 3 3 3 5% 7% 66%	5963
	11th to 20th	Day	1 1 3 1 2 2 2 3 4 81%	5798
	21st to end	Day	5% 3 1 4 3 3 3 3 4 5% 68%	5995
September	1st to 10th	Day	3 2 1 2 2 2 2 2 3 6% 74%	2317
	11th to 20th	Day	4 5% 2 3 3 3 3 2 5% 6% 64%	2104
	21st to end	Day	7% 4 2 3 2 2 2 3 3 6% 66%	2090
October	1st to 10th	Day	5% 4 2 4 3 2 2 2 3 5% 68%	3490
	11th to 20th	Day	8% 5% 2 4 3 3 4 4 5% 8% 55%	3046
	21st to end	Day	7% 7% 3 6% 3 4 3 4 5% 8% 49%	3101
November	1st to 10th	Day	14% 9% 2 5% 5% 4 4 4 5% 7% 41%	1764
	11th to 20th	Day	16% 9% 2 6% 5% 5% 6% 4 6% 7% 34%	1800
	21st to end	Day	13% 8% 3 8% 5% 6% 5% 5% 6% 9% 33%	1770
December	1st to 10th	Day	10% 6% 2 6% 4 3 3 3 4 7% 52%	2620
	11th to 20th	Day	18% 13% 4 10% 6% 6% 5% 4 6% 7% 20%	3028
	21st to end	Day	19% 12% 4 10% 5% 5% 4 3 6% 8% 25%	4004
January	1st to 10th	Night	18% 15% 4 10% 5% 5% 4 3 6% 7% 22%	6306
	11th to 20th	Night	15% 13% 3 7% 4 5% 4 3 4 7% 36%	6320
	21st to end	Night	12% 10% 3 8% 4 3 4 3 4 5% 43%	6952
February	1st to 10th	Night	17% 11% 3 8% 5% 5% 4 4 5% 7% 33%	2628
	11th to 20th	Night	12% 12% 3 5% 4 4 4 4 6% 8% 37%	2338
	21st to end	Night	13% 13% 4 6% 5% 5% 5% 4 5% 6% 33%	1980
March	1st to 10th	Night	9% 12% 2 7% 5% 4 3 3 5% 8% 41%	5376
	11th to 20th	Night	12% 13% 3 6% 4 3 3 3 4 5% 44%	5185
	21st to end	Night	11% 9% 3 7% 5% 5% 6% 4 7% 8% 35%	5232
April	1st to 10th	Night	11% 7% 3 5% 5% 4 5% 4 6% 6% 44%	2000
	11th to 20th	Night	10% 7% 2 6% 5% 4 4 4 5% 5% 48%	1931
	21st to end	Night	7% 6% 2 5% 4 4 3 3 7% 7% 52%	1800
May	1st to 10th	Night	10% 5% 1 3 2 3 3 3 5% 7% 57%	4100
	11th to 20th	Night	6% 5% 2 6% 4 4 4 3 5% 7% 55%	3873
	21st to end	Night	4 4 1 5% 4 3 3 3 4 5% 64%	3972
June	1st to 10th	Night	10% 5% 2 5% 5% 4 4 3 6% 9% 46%	1600
	11th to 20th	Night	7% 5% 1 4 2 3 4 3 4 7% 60%	1600
	21st to end	Night	3 2 1 4 3 3 4 3 5% 5% 67%	1600
July	1st to 10th	Night	2 1 1 2 2 2 2 2 3 5% 76%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 82%	3640
	21st to end	Night	2 2 1 4 4 4 4 3 6% 8% 60%	3970
August	1st to 10th	Night	3 3 2 4 3 3 4 3 5% 7% 63%	3841
	11th to 20th	Night	1 1 1 2 1 2 2 2 3 5% 79%	4051
	21st to end	Night	4 3 1 4 3 3 3 2 4 5% 68%	4984
September	1st to 10th	Night	3 3 1 2 2 2 2 2 3 5% 75%	1930
	11th to 20th	Night	5% 3 2 4 3 3 3 3 5% 6% 64%	2076
	21st to end	Night	7% 3 1 4 3 3 3 2 3 5% 67%	2090
October	1st to 10th	Night	4 3 1 4 3 2 3 2 3 5% 70%	3490
	11th to 20th	Night	7% 4 2 5% 3 3 4 2 4 6% 61%	3883
	21st to end	Night	10% 7% 2 5% 3 4 4 4 6% 7% 48%	4457
November	1st to 10th	Night	14% 7% 3 7% 4 4 4 3 5% 7% 44%	2535
	11th to 20th	Night	18% 8% 2 7% 4 4 4 4 5% 7% 39%	2600
	21st to end	Night	14% 9% 2 6% 4 4 3 3 4 7% 43%	2625
December	1st to 10th	Night	11% 7% 2 4 4 3 3 2 4 8% 52%	4560
	11th to 20th	Night	20% 13% 3 9% 5% 4 5% 4 6% 7% 22%	5176
	21st to end	Night	17% 11% 3 9% 5% 4 5% 3 6% 8% 28%	6952

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	61% 8% 15% 15% 1	3639
	11th to 20th	Day	47% 6% 18% 22% 6%	3640
	21st to end	Day	47% 6% 14% 22% 10% 1	4004
February	1st to 10th	Day	52% 7% 19% 17% 4 1	1768
	11th to 20th	Day	48% 8% 17% 20% 6%	1960
	21st to end	Day	48% 6% 16% 21% 9% 1	1650
March	1st to 10th	Day	46% 7% 12% 23% 10% 2	4518
	11th to 20th	Day	38% 3 11% 35% 13%	4814
	21st to end	Day	48% 5% 17% 21% 9% 1	5739
April	1st to 10th	Day	40% 5% 15% 24% 13% 2	2400
	11th to 20th	Day	36% 5% 10% 27% 17% 4	2418
	21st to end	Day	34% 4 14% 29% 18% 2	2600
May	1st to 10th	Day	28% 3 7% 27% 30% 6%	5880
	11th to 20th	Day	26% 3 6% 28% 29% 9%	6017
	21st to end	Day	22% 3 8% 30% 30% 7%	6904
June	1st to 10th	Day	35% 5% 11% 26% 20% 3	2800
	11th to 20th	Day	25% 3 8% 25% 28% 11%	2800
	21st to end	Day	16% 2 6% 20% 33% 24%	2800
July	1st to 10th	Day	11% 2 2 10% 23% 53%	6251
	11th to 20th	Day	10% 1 2 6% 21% 61%	6245
	21st to end	Day	16% 2 2 13% 35% 33%	6681
August	1st to 10th	Day	22% 2 4 13% 26% 33%	4343
	11th to 20th	Day	11% 2 2 14% 38% 34%	4197
	21st to end	Day	15% 1 3 17% 34% 29%	4325
September	1st to 10th	Day	13% 2 4 16% 33% 31%	2317
	11th to 20th	Day	22% 3 8% 24% 27% 16%	2104
	21st to end	Day	21% 3 6% 26% 33% 12%	2090
October	1st to 10th	Day	22% 2 7% 17% 40% 13%	3490
	11th to 20th	Day	29% 3 11% 27% 26% 4	3046
	21st to end	Day	34% 3 11% 29% 22% 1	3101
November	1st to 10th	Day	42% 6% 15% 24% 13%	1764
	11th to 20th	Day	47% 7% 19% 22% 5%	1800
	21st to end	Day	46% 8% 20% 21% 6%	1770
December	1st to 10th	Day	33% 6% 19% 38% 4	2620
	11th to 20th	Day	60% 9% 20% 10% 1	3028
	21st to end	Day	57% 7% 16% 18% 3	4004
January	1st to 10th	Night	60% 6% 17% 15% 2	6306
	11th to 20th	Night	48% 5% 14% 24% 8% 1	6320
	21st to end	Night	44% 6% 14% 24% 11% 3	6952
February	1st to 10th	Night	51% 7% 17% 20% 4 1	2628
	11th to 20th	Night	44% 6% 17% 22% 10% 1	2339
	21st to end	Night	50% 5% 13% 20% 11% 1	1980
March	1st to 10th	Night	41% 5% 9% 30% 13% 3	5376
	11th to 20th	Night	44% 2 6% 28% 19% 1	5185
	21st to end	Night	45% 5% 14% 26% 9%	5232
April	1st to 10th	Night	39% 4 11% 26% 17% 3	2000
	11th to 20th	Night	36% 5% 10% 26% 21% 3	1931
	21st to end	Night	31% 3 11% 28% 24% 2	1800
May	1st to 10th	Night	26% 3 7% 25% 30% 9%	4097
	11th to 20th	Night	29% 3 4 26% 31% 7%	3873
	21st to end	Night	22% 4 8% 26% 32% 8%	3972
June	1st to 10th	Night	35% 4 12% 25% 22% 3	1600
	11th to 20th	Night	25% 3 7% 23% 32% 10%	1600
	21st to end	Night	19% 3 5% 17% 32% 25%	1600
July	1st to 10th	Night	13% 1 2 9% 19% 56%	3600
	11th to 20th	Night	8% 1 1 6% 20% 63%	3592
	21st to end	Night	22% 2 2 10% 30% 32%	3857
August	1st to 10th	Night	24% 2 3 11% 24% 36%	2788
	11th to 20th	Night	11% 2 3 10% 28% 46%	2928
	21st to end	Night	16% 1 3 13% 32% 35%	3590
September	1st to 10th	Night	14% 2 3 12% 32% 37%	1930
	11th to 20th	Night	23% 2 6% 19% 26% 24%	2076
	21st to end	Night	21% 3 4 21% 32% 19%	2090
October	1st to 10th	Night	19% 2 5% 18% 31% 25%	3490
	11th to 20th	Night	27% 2 9% 26% 29% 7%	3883
	21st to end	Night	35% 3 10% 28% 21% 3	4457
November	1st to 10th	Night	41% 3 12% 22% 19% 1	2535
	11th to 20th	Night	46% 5% 13% 27% 10%	2600
	21st to end	Night	42% 4 16% 28% 9%	2625
December	1st to 10th	Night	31% 5% 23% 33% 7%	4560
	11th to 20th	Night	60% 6% 16% 16% 2	5176
	21st to end	Night	54% 5% 18% 18% 5%	6952

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	27% 7% 21% 24% 18% 5%	200
	11th to 20th	Day	16% 10% 23% 25% 20% 7%	200
	21st to end	Day	18% 3 21% 31% 16% 10%	220
February	1st to 10th	Day	21% 11% 26% 23% 16% 3	315
	11th to 20th	Day	18% 11% 28% 27% 14% 3	495
	21st to end	Day	17% 12% 29% 26% 15% 1	414
March	1st to 10th	Day	16% 11% 31% 29% 12% 2	500
	11th to 20th	Day	15% 10% 30% 26% 15% 3	500
	21st to end	Day	16% 13% 33% 23% 14% 2	550
April	1st to 10th	Day	13% 17% 26% 23% 16% 5%	500
	11th to 20th	Day	11% 18% 29% 23% 14% 4	492
	21st to end	Day	10% 14% 32% 26% 14% 4	500
May	1st to 10th	Day	7% 14% 29% 28% 18% 4	503
	11th to 20th	Day	15% 10% 25% 26% 17% 7%	528
	21st to end	Day	8% 8% 20% 32% 19% 13%	573
June	1st to 10th	Day	9% 8% 22% 28% 25% 8%	530
	11th to 20th	Day	8% 12% 27% 33% 15% 5%	530
	21st to end	Day	4 7% 24% 33% 26% 6%	530
July	1st to 10th	Day	3 4 21% 38% 27% 7%	526
	11th to 20th	Day	2 3 22% 39% 27% 6%	524
	21st to end	Day	4 2 17% 38% 30% 9%	575
August	1st to 10th	Day	4 5% 22% 34% 27% 8%	527
	11th to 20th	Day	3 3 20% 31% 31% 13%	517
	21st to end	Day	4 5% 18% 29% 29% 14%	583
September	1st to 10th	Day	5% 4 14% 29% 36% 13%	503
	11th to 20th	Day	7% 5% 19% 29% 28% 13%	490
	21st to end	Day	6% 5% 19% 32% 30% 7%	490
October	1st to 10th	Day	7% 10% 18% 24% 30% 11%	490
	11th to 20th	Day	11% 7% 21% 26% 26% 10%	380
	21st to end	Day	14% 7% 25% 28% 22% 3	330
November	1st to 10th	Day	17% 9% 20% 33% 17% 3	300
	11th to 20th	Day	17% 9% 19% 33% 17% 5%	300
	21st to end	Day	16% 6% 16% 32% 24% 6%	285
December	1st to 10th	Day	21% 5% 20% 25% 21% 11%	200
	11th to 20th	Day	25% 5% 25% 29% 14% 4	200
	21st to end	Day	20% 10% 25% 26% 16% 3	220
January	1st to 10th	Night	23% 6% 19% 30% 19% 3	800
	11th to 20th	Night	20% 5% 19% 31% 22% 4	800
	21st to end	Night	18% 4 16% 33% 25% 4	880
February	1st to 10th	Night	23% 5% 21% 33% 16% 2	800
	11th to 20th	Night	18% 3 23% 36% 18% 2	784
	21st to end	Night	16% 7% 24% 37% 14% 2	660
March	1st to 10th	Night	18% 8% 24% 34% 15% 2	800
	11th to 20th	Night	18% 6% 20% 38% 16% 2	800
	21st to end	Night	17% 8% 24% 36% 12% 2	880
April	1st to 10th	Night	14% 7% 20% 39% 18% 2	800
	11th to 20th	Night	12% 8% 26% 36% 16% 3	743
	21st to end	Night	9% 8% 24% 43% 15% 2	600
May	1st to 10th	Night	10% 7% 23% 41% 18% 2	600
	11th to 20th	Night	10% 6% 20% 34% 26% 3	508
	21st to end	Night	8% 5% 17% 36% 29% 5%	433
June	1st to 10th	Night	11% 5% 15% 39% 27% 3	400
	11th to 20th	Night	9% 6% 20% 40% 24% 2	400
	21st to end	Night	3 2 18% 45% 30% 3	400
July	1st to 10th	Night	3 1 12% 52% 32% 2	390
	11th to 20th	Night	2 1 13% 56% 28% 1	388
	21st to end	Night	3 1 10% 48% 35% 3	438
August	1st to 10th	Night	4 3 12% 48% 32% 1	507
	11th to 20th	Night	1 2 12% 48% 36% 2	594
	21st to end	Night	4 2 13% 46% 31% 4	862
September	1st to 10th	Night	3 1 10% 44% 36% 5%	772
	11th to 20th	Night	6% 3 10% 39% 37% 6%	760
	21st to end	Night	9% 2 14% 37% 29% 9%	760
October	1st to 10th	Night	9% 3 15% 42% 27% 5%	760
	11th to 20th	Night	9% 4 15% 39% 28% 6%	760
	21st to end	Night	13% 4 18% 38% 22% 5%	836
November	1st to 10th	Night	15% 6% 17% 37% 23% 3	780
	11th to 20th	Night	22% 4 19% 35% 18% 3	800
	21st to end	Night	19% 2 16% 37% 21% 5%	800
December	1st to 10th	Night	20% 3 16% 36% 20% 4	800
	11th to 20th	Night	25% 4 16% 37% 15% 3	800
	21st to end	Night	28% 5% 22% 31% 13% 1	880

Page generated at 2025-08-31 23:51:06. Version ac07

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