Brandon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brandon clear sky chart from 2005-11-06 to 2025-03-12.

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	10% 8% 3 9% 6% 5% 5% 4 6% 8% 36%	112322
	Night	8% 9% 4 9% 6% 5% 5% 4 6% 8% 35%	115410
Transparency	Day	44% 7% 15% 25% 8%	112077
	Night	45% 7% 14% 24% 9% 1	115302
Seeing	Day	14% 9% 30% 29% 15% 4	16621
	Night	14% 7% 31% 33% 13% 2	26780

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 13% 6% 16% 10% 7% 8% 6% 7% 7% 16%	3179
	11th to 20th	Day	3 8% 5% 15% 9% 6% 9% 6% 8% 8% 22%	3496
	21st to end	Day	6% 12% 5% 14% 7% 5% 5% 4 6% 8% 28%	4004
February	1st to 10th	Day	10% 13% 4 13% 7% 6% 6% 5% 8% 7% 22%	1600
	11th to 20th	Day	6% 10% 4 10% 8% 7% 7% 5% 7% 6% 30%	1779
	21st to end	Day	7% 11% 5% 11% 6% 7% 8% 4 6% 8% 27%	1650
March	1st to 10th	Day	9% 13% 4 10% 7% 6% 5% 5% 7% 11% 22%	3230
	11th to 20th	Day	7% 12% 5% 12% 8% 6% 6% 4 7% 8% 25%	3395
	21st to end	Day	8% 9% 4 12% 8% 6% 6% 4 6% 7% 31%	3934
April	1st to 10th	Day	9% 10% 4 7% 5% 4 4 4 7% 9% 36%	2281
	11th to 20th	Day	13% 13% 4 9% 5% 5% 5% 3 5% 7% 31%	2442
	21st to end	Day	14% 9% 3 8% 6% 4 5% 4 6% 7% 35%	2470
May	1st to 10th	Day	21% 9% 3 6% 3 4 3 3 5% 7% 35%	4268
	11th to 20th	Day	21% 8% 3 7% 5% 5% 4 4 6% 7% 31%	4602
	21st to end	Day	17% 7% 2 6% 4 5% 5% 4 6% 8% 35%	5173
June	1st to 10th	Day	13% 7% 2 6% 6% 5% 5% 5% 7% 8% 36%	2850
	11th to 20th	Day	15% 7% 2 8% 5% 5% 5% 4 6% 7% 34%	2850
	21st to end	Day	11% 5% 2 6% 6% 6% 5% 5% 7% 9% 38%	2850
July	1st to 10th	Day	8% 4 1 6% 5% 4 6% 5% 8% 11% 42%	4750
	11th to 20th	Day	6% 4 1 5% 4 4 5% 5% 7% 9% 49%	4750
	21st to end	Day	4 3 2 5% 4 4 5% 5% 7% 11% 50%	4816
August	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 4 6% 8% 45%	4412
	11th to 20th	Day	9% 4 2 4 4 4 5% 4 7% 9% 47%	4084
	21st to end	Day	9% 5% 3 5% 4 4 4 4 5% 9% 48%	4532
September	1st to 10th	Day	9% 6% 2 6% 4 3 4 4 5% 7% 51%	2303
	11th to 20th	Day	14% 5% 3 6% 4 3 4 4 5% 7% 44%	2100
	21st to end	Day	14% 7% 2 5% 4 4 4 3 5% 5% 46%	2090
October	1st to 10th	Day	11% 8% 3 7% 3 4 3 3 4 6% 47%	3386
	11th to 20th	Day	7% 6% 2 7% 4 5% 4 3 5% 9% 48%	2863
	21st to end	Day	12% 12% 4 8% 5% 4 4 3 5% 7% 37%	3091
November	1st to 10th	Day	18% 12% 5% 9% 5% 5% 5% 3 5% 5% 28%	1732
	11th to 20th	Day	9% 17% 5% 9% 5% 5% 5% 3 6% 7% 29%	1533
	21st to end	Day	10% 15% 4 10% 6% 5% 5% 3 5% 7% 31%	1400
December	1st to 10th	Day	8% 12% 7% 16% 9% 9% 7% 5% 6% 5% 15%	2290
	11th to 20th	Day	8% 12% 5% 11% 7% 7% 7% 6% 6% 8% 24%	2639
	21st to end	Day	4 9% 4 16% 8% 6% 7% 3 6% 7% 30%	3498
January	1st to 10th	Night	4 11% 5% 16% 10% 8% 6% 5% 6% 7% 20%	6787
	11th to 20th	Night	5% 9% 5% 14% 9% 7% 9% 7% 8% 9% 19%	6418
	21st to end	Night	7% 14% 8% 15% 6% 5% 5% 4 7% 7% 23%	6974
February	1st to 10th	Night	8% 13% 5% 11% 7% 6% 5% 5% 7% 7% 28%	2800
	11th to 20th	Night	6% 9% 3 8% 6% 6% 5% 5% 6% 8% 37%	2524
	21st to end	Night	6% 10% 4 10% 6% 5% 5% 5% 6% 9% 35%	1980
March	1st to 10th	Night	9% 12% 4 10% 6% 5% 5% 5% 7% 8% 28%	3854
	11th to 20th	Night	7% 11% 2 9% 5% 5% 4 4 6% 8% 39%	3580
	21st to end	Night	7% 8% 4 13% 8% 6% 5% 4 5% 7% 33%	3699
April	1st to 10th	Night	5% 9% 2 7% 5% 5% 5% 5% 7% 8% 41%	1898
	11th to 20th	Night	15% 13% 3 7% 5% 4 4 4 6% 8% 31%	1689
	21st to end	Night	13% 9% 2 6% 6% 5% 4 5% 6% 7% 37%	1710
May	1st to 10th	Night	19% 9% 3 7% 4 5% 4 4 5% 6% 35%	2700
	11th to 20th	Night	14% 7% 3 6% 5% 4 5% 4 5% 7% 40%	2302
	21st to end	Night	18% 8% 3 5% 4 4 4 3 5% 6% 41%	2414
June	1st to 10th	Night	12% 5% 2 5% 4 5% 5% 4 6% 9% 43%	1330
	11th to 20th	Night	13% 6% 2 6% 3 4 5% 4 6% 10% 42%	1330
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 7% 10% 44%	1330
July	1st to 10th	Night	9% 4 1 4 3 3 4 4 10% 10% 48%	2220
	11th to 20th	Night	6% 3 2 4 4 3 4 4 6% 8% 57%	2220
	21st to end	Night	3 4 1 4 4 4 4 4 7% 9% 58%	2744
August	1st to 10th	Night	7% 4 2 6% 3 3 4 4 6% 8% 54%	2528
	11th to 20th	Night	6% 3 2 6% 5% 4 5% 4 6% 10% 50%	2753
	21st to end	Night	9% 4 2 5% 4 3 4 4 5% 9% 52%	3135
September	1st to 10th	Night	8% 5% 2 4 3 3 3 3 6% 7% 55%	1875
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 6% 7% 49%	2080
	21st to end	Night	13% 7% 3 6% 4 4 4 3 5% 8% 43%	2090
October	1st to 10th	Night	11% 7% 2 6% 4 4 3 3 5% 7% 47%	3541
	11th to 20th	Night	10% 6% 2 5% 4 5% 5% 4 5% 8% 45%	4060
	21st to end	Night	8% 8% 3 7% 4 5% 5% 4 6% 7% 43%	4467
November	1st to 10th	Night	12% 12% 3 8% 6% 4 5% 4 6% 8% 32%	2565
	11th to 20th	Night	10% 13% 5% 11% 6% 5% 4 4 5% 7% 29%	2865
	21st to end	Night	10% 12% 4 11% 7% 4 4 4 6% 6% 32%	3000
December	1st to 10th	Night	7% 13% 4 14% 9% 8% 7% 5% 7% 8% 20%	4900
	11th to 20th	Night	9% 14% 5% 11% 6% 6% 6% 5% 6% 8% 24%	5568
	21st to end	Night	5% 9% 3 12% 9% 7% 7% 5% 6% 7% 29%	7480

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% 7% 11% 17% 5%	3179
	11th to 20th	Day	55% 7% 9% 20% 8%	3520
	21st to end	Day	52% 7% 14% 22% 4	4004
February	1st to 10th	Day	56% 7% 13% 17% 7%	1600
	11th to 20th	Day	49% 8% 12% 21% 10%	1788
	21st to end	Day	52% 7% 15% 18% 8%	1650
March	1st to 10th	Day	51% 9% 15% 20% 5%	3230
	11th to 20th	Day	52% 11% 13% 18% 6%	3397
	21st to end	Day	48% 9% 12% 21% 9%	3934
April	1st to 10th	Day	42% 7% 16% 25% 10% 1	2281
	11th to 20th	Day	53% 6% 11% 17% 12% 1	2442
	21st to end	Day	47% 7% 14% 23% 8% 1	2470
May	1st to 10th	Day	47% 6% 14% 23% 9% 1	4268
	11th to 20th	Day	50% 8% 10% 20% 11% 1	4606
	21st to end	Day	44% 8% 12% 28% 8%	5173
June	1st to 10th	Day	42% 9% 16% 20% 11% 1	2850
	11th to 20th	Day	47% 7% 15% 19% 10% 1	2850
	21st to end	Day	40% 10% 17% 25% 6% 1	2850
July	1st to 10th	Day	33% 10% 26% 26% 5%	4676
	11th to 20th	Day	28% 8% 19% 38% 7%	4669
	21st to end	Day	25% 8% 25% 32% 9%	4664
August	1st to 10th	Day	36% 7% 17% 29% 11%	4396
	11th to 20th	Day	31% 7% 22% 34% 6%	4123
	21st to end	Day	33% 7% 19% 33% 8%	4531
September	1st to 10th	Day	32% 6% 15% 34% 11% 2	2303
	11th to 20th	Day	39% 7% 17% 27% 9% 1	2100
	21st to end	Day	40% 6% 12% 31% 11%	2090
October	1st to 10th	Day	39% 4 11% 38% 8%	3386
	11th to 20th	Day	35% 5% 12% 38% 10%	2863
	21st to end	Day	48% 5% 11% 28% 8%	3091
November	1st to 10th	Day	57% 6% 12% 17% 7% 1	1732
	11th to 20th	Day	54% 7% 16% 16% 6% 1	1533
	21st to end	Day	54% 6% 15% 20% 5%	1400
December	1st to 10th	Day	67% 8% 12% 9% 3	2290
	11th to 20th	Day	55% 8% 17% 16% 3	2640
	21st to end	Day	51% 7% 12% 19% 11% 1	3498
January	1st to 10th	Night	58% 8% 10% 19% 5%	6787
	11th to 20th	Night	56% 8% 10% 18% 7%	6456
	21st to end	Night	58% 7% 12% 19% 4	6974
February	1st to 10th	Night	54% 6% 13% 18% 9% 1	2800
	11th to 20th	Night	42% 9% 13% 22% 14%	2525
	21st to end	Night	44% 6% 14% 24% 12%	1980
March	1st to 10th	Night	49% 8% 15% 22% 6%	3854
	11th to 20th	Night	39% 9% 16% 24% 12% 1	3580
	21st to end	Night	48% 8% 12% 24% 7%	3699
April	1st to 10th	Night	39% 7% 10% 25% 17% 2	1898
	11th to 20th	Night	50% 6% 11% 20% 13% 1	1689
	21st to end	Night	45% 6% 15% 21% 11% 2	1710
May	1st to 10th	Night	47% 7% 10% 22% 13%	2700
	11th to 20th	Night	41% 7% 12% 25% 10% 5%	2302
	21st to end	Night	42% 6% 14% 27% 10%	2414
June	1st to 10th	Night	35% 8% 18% 25% 12% 2	1330
	11th to 20th	Night	37% 8% 18% 23% 12% 2	1330
	21st to end	Night	33% 9% 19% 30% 7% 1	1330
July	1st to 10th	Night	28% 10% 26% 27% 8% 1	2185
	11th to 20th	Night	24% 7% 18% 41% 9% 1	2178
	21st to end	Night	23% 6% 23% 34% 13% 1	2657
August	1st to 10th	Night	27% 6% 18% 32% 14% 2	2520
	11th to 20th	Night	29% 7% 20% 35% 8% 1	2780
	21st to end	Night	30% 7% 19% 34% 10% 1	3135
September	1st to 10th	Night	27% 5% 15% 34% 16% 2	1875
	11th to 20th	Night	33% 8% 18% 27% 13% 1	2080
	21st to end	Night	40% 5% 12% 30% 12% 2	2090
October	1st to 10th	Night	37% 5% 13% 28% 15% 2	3541
	11th to 20th	Night	36% 4 12% 35% 11% 1	4060
	21st to end	Night	40% 6% 13% 33% 8%	4467
November	1st to 10th	Night	50% 7% 14% 20% 7% 2	2565
	11th to 20th	Night	53% 8% 15% 17% 6% 1	2865
	21st to end	Night	51% 8% 16% 19% 6%	3000
December	1st to 10th	Night	60% 8% 14% 13% 4	4900
	11th to 20th	Night	56% 7% 15% 16% 5%	5566
	21st to end	Night	51% 8% 12% 18% 10% 1	7480

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	10% 7% 35% 29% 17% 2	300
	11th to 20th	Day	8% 7% 37% 34% 13% 1	300
	21st to end	Day	16% 9% 32% 29% 12% 3	330
February	1st to 10th	Day	15% 10% 40% 27% 7% 2	300
	11th to 20th	Day	8% 6% 37% 35% 13% 2	300
	21st to end	Day	4 8% 38% 33% 16% 2	249
March	1st to 10th	Day	13% 9% 30% 32% 14% 1	300
	11th to 20th	Day	11% 11% 30% 29% 16% 3	273
	21st to end	Day	15% 7% 33% 27% 14% 4	366
April	1st to 10th	Day	13% 11% 33% 31% 10% 2	461
	11th to 20th	Day	18% 12% 32% 24% 13% 2	640
	21st to end	Day	16% 8% 31% 30% 13% 1	650
May	1st to 10th	Day	17% 10% 26% 29% 17% 2	653
	11th to 20th	Day	20% 7% 28% 28% 14% 4	675
	21st to end	Day	19% 10% 24% 28% 17% 2	734
June	1st to 10th	Day	14% 12% 29% 25% 14% 6%	680
	11th to 20th	Day	18% 11% 32% 24% 12% 3	680
	21st to end	Day	13% 8% 27% 32% 17% 4	680
July	1st to 10th	Day	7% 5% 25% 32% 21% 9%	671
	11th to 20th	Day	9% 7% 31% 28% 19% 6%	668
	21st to end	Day	6% 7% 25% 31% 24% 7%	739
August	1st to 10th	Day	10% 7% 26% 32% 20% 6%	676
	11th to 20th	Day	11% 5% 22% 31% 22% 8%	665
	21st to end	Day	11% 8% 24% 33% 19% 5%	748
September	1st to 10th	Day	9% 11% 35% 26% 15% 4	513
	11th to 20th	Day	18% 8% 29% 32% 11% 2	310
	21st to end	Day	21% 11% 28% 28% 10% 3	300
October	1st to 10th	Day	23% 12% 25% 23% 14% 4	300
	11th to 20th	Day	16% 9% 36% 26% 11% 2	300
	21st to end	Day	21% 10% 37% 21% 8% 3	330
November	1st to 10th	Day	23% 10% 30% 22% 13% 2	300
	11th to 20th	Day	17% 9% 35% 29% 8% 1	300
	21st to end	Day	16% 11% 36% 28% 8% 1	300
December	1st to 10th	Day	13% 11% 35% 25% 13% 3	300
	11th to 20th	Day	14% 9% 41% 30% 5% 1	300
	21st to end	Day	10% 5% 35% 31% 16% 3	330
January	1st to 10th	Night	12% 8% 33% 33% 12% 2	1000
	11th to 20th	Night	16% 8% 35% 32% 9% 1	1000
	21st to end	Night	13% 8% 38% 31% 10%	1100
February	1st to 10th	Night	14% 9% 34% 31% 11% 2	1000
	11th to 20th	Night	10% 6% 36% 34% 11% 3	950
	21st to end	Night	10% 6% 30% 36% 15% 2	660
March	1st to 10th	Night	16% 6% 29% 34% 13% 3	800
	11th to 20th	Night	10% 7% 35% 34% 13% 2	632
	21st to end	Night	15% 6% 27% 35% 14% 2	627
April	1st to 10th	Night	9% 6% 34% 37% 13% 2	570
	11th to 20th	Night	19% 6% 31% 33% 10% 1	563
	21st to end	Night	16% 6% 34% 32% 11% 1	570
May	1st to 10th	Night	20% 7% 27% 32% 13%	570
	11th to 20th	Night	14% 8% 29% 33% 14%	561
	21st to end	Night	19% 7% 25% 35% 14%	617
June	1st to 10th	Night	14% 6% 34% 30% 15% 1	570
	11th to 20th	Night	16% 4 35% 31% 13% 1	570
	21st to end	Night	11% 4 26% 40% 18% 1	570
July	1st to 10th	Night	8% 4 23% 40% 24% 2	555
	11th to 20th	Night	6% 6% 26% 39% 21% 1	552
	21st to end	Night	6% 3 22% 46% 20% 2	624
August	1st to 10th	Night	8% 2 24% 44% 20% 1	567
	11th to 20th	Night	9% 4 21% 39% 23% 4	564
	21st to end	Night	12% 4 24% 40% 18% 1	627
September	1st to 10th	Night	10% 4 29% 34% 20% 3	570
	11th to 20th	Night	14% 8% 28% 38% 13%	570
	21st to end	Night	15% 9% 28% 35% 12% 1	570
October	1st to 10th	Night	16% 8% 33% 31% 11% 1	605
	11th to 20th	Night	17% 9% 30% 32% 12% 1	926
	21st to end	Night	16% 8% 35% 31% 9% 1	1045
November	1st to 10th	Night	17% 10% 34% 29% 9% 1	975
	11th to 20th	Night	16% 9% 34% 29% 10% 2	1000
	21st to end	Night	16% 11% 38% 27% 8% 1	1000
December	1st to 10th	Night	11% 7% 36% 31% 13% 2	1000
	11th to 20th	Night	16% 11% 33% 28% 10% 2	1000
	21st to end	Night	16% 6% 30% 33% 13% 2	1100

Page generated at 2025-03-09 07:08:48. Version ac07

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