Gillam
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gillam clear sky chart from 2005-11-06 to 2026-02-01.

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	12% 10% 4 10% 6% 6% 6% 5% 6% 7% 29%	132481
	Night	10% 12% 5% 12% 7% 7% 7% 5% 7% 6% 23%	128339
Transparency	Day	52% 6% 13% 22% 7% 1	132162
	Night	58% 6% 11% 18% 7%	128236
Seeing	Day	18% 9% 26% 28% 15% 5%	17607
	Night	17% 7% 27% 32% 15% 2	26599

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	6% 9% 6% 17% 10% 7% 9% 6% 8% 6% 15%	2808
	11th to 20th	Day	3 9% 5% 15% 10% 12% 12% 9% 9% 6% 11%	3229
	21st to end	Day	5% 13% 6% 15% 7% 6% 9% 6% 8% 9% 16%	3575
February	1st to 10th	Day	3 7% 4 10% 9% 9% 9% 8% 9% 9% 23%	1460
	11th to 20th	Day	4 10% 4 8% 9% 9% 9% 8% 8% 9% 21%	1754
	21st to end	Day	3 12% 4 11% 7% 6% 6% 7% 9% 10% 26%	1485
March	1st to 10th	Day	5% 10% 4 12% 6% 9% 6% 5% 7% 8% 27%	4419
	11th to 20th	Day	11% 12% 3 11% 8% 8% 7% 6% 6% 6% 22%	4978
	21st to end	Day	3 9% 4 9% 6% 8% 9% 8% 7% 7% 28%	5573
April	1st to 10th	Day	9% 10% 3 11% 7% 7% 7% 5% 6% 7% 27%	2507
	11th to 20th	Day	10% 11% 4 9% 7% 6% 5% 4 5% 7% 33%	2572
	21st to end	Day	6% 7% 3 9% 7% 5% 5% 4 7% 9% 38%	2778
May	1st to 10th	Day	15% 13% 5% 9% 4 4 4 3 4 7% 32%	6624
	11th to 20th	Day	20% 12% 6% 9% 5% 4 4 3 5% 6% 25%	6787
	21st to end	Day	15% 9% 3 7% 4 4 4 3 5% 7% 40%	7930
June	1st to 10th	Day	19% 11% 3 7% 5% 5% 5% 4 5% 6% 31%	3200
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 5% 7% 9% 36%	3390
	21st to end	Day	13% 8% 3 7% 6% 6% 5% 3 6% 7% 36%	3400
July	1st to 10th	Day	9% 5% 2 7% 4 4 4 4 7% 9% 45%	7578
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 8% 39%	7081
	21st to end	Day	9% 6% 2 6% 4 5% 5% 5% 8% 11% 40%	7400
August	1st to 10th	Day	12% 5% 2 6% 4 4 5% 5% 6% 8% 44%	4881
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 6% 9% 42%	4459
	21st to end	Day	21% 9% 3 8% 5% 4 5% 4 5% 7% 30%	4675
September	1st to 10th	Day	19% 10% 3 9% 6% 5% 5% 4 6% 7% 27%	2500
	11th to 20th	Day	20% 14% 3 7% 5% 4 4 3 6% 8% 26%	2345
	21st to end	Day	18% 13% 4 7% 4 5% 5% 4 6% 7% 27%	2129
October	1st to 10th	Day	17% 18% 5% 10% 6% 5% 4 3 5% 5% 21%	3260
	11th to 20th	Day	16% 14% 7% 11% 5% 5% 5% 4 6% 7% 21%	3104
	21st to end	Day	19% 18% 7% 13% 6% 4 4 3 4 4 16%	2832
November	1st to 10th	Day	15% 19% 8% 13% 10% 6% 5% 3 4 3 14%	1647
	11th to 20th	Day	14% 16% 6% 17% 10% 8% 6% 4 4 5% 10%	1467
	21st to end	Day	9% 21% 9% 17% 9% 6% 4 3 3 5% 14%	1260
December	1st to 10th	Day	6% 14% 8% 17% 10% 8% 8% 8% 7% 6% 10%	2020
	11th to 20th	Day	9% 12% 5% 18% 9% 7% 6% 6% 7% 6% 15%	2316
	21st to end	Day	7% 15% 7% 17% 8% 8% 8% 4 6% 5% 15%	3058
January	1st to 10th	Night	4 9% 4 18% 11% 10% 9% 6% 7% 5% 17%	7378
	11th to 20th	Night	3 8% 4 17% 12% 12% 14% 7% 10% 5% 7%	6909
	21st to end	Night	10% 13% 5% 14% 8% 7% 8% 7% 11% 6% 10%	7645
February	1st to 10th	Night	3 11% 5% 12% 9% 8% 10% 8% 8% 8% 19%	2937
	11th to 20th	Night	4 10% 4 8% 7% 8% 11% 8% 11% 7% 21%	2533
	21st to end	Night	4 10% 5% 12% 9% 6% 7% 7% 11% 8% 21%	2145
March	1st to 10th	Night	4 10% 5% 13% 7% 6% 6% 7% 7% 7% 28%	5490
	11th to 20th	Night	7% 11% 4 14% 9% 8% 7% 6% 6% 5% 22%	4980
	21st to end	Night	3 9% 4 9% 6% 9% 13% 9% 7% 6% 25%	5358
April	1st to 10th	Night	8% 10% 4 10% 6% 5% 7% 5% 6% 7% 33%	1890
	11th to 20th	Night	9% 11% 3 10% 7% 5% 5% 4 6% 7% 33%	1779
	21st to end	Night	6% 9% 4 8% 7% 5% 6% 5% 6% 8% 36%	1621
May	1st to 10th	Night	15% 13% 5% 12% 5% 5% 5% 3 4 6% 27%	3341
	11th to 20th	Night	19% 14% 5% 9% 4 4 4 3 5% 6% 29%	3128
	21st to end	Night	17% 9% 3 6% 4 3 3 3 5% 5% 42%	2968
June	1st to 10th	Night	20% 10% 3 8% 5% 3 4 4 6% 5% 33%	1200
	11th to 20th	Night	16% 6% 3 6% 5% 5% 4 5% 6% 7% 37%	1006
	21st to end	Night	14% 7% 3 7% 5% 5% 6% 5% 6% 5% 37%	1000
July	1st to 10th	Night	10% 7% 2 8% 5% 5% 5% 5% 7% 10% 36%	2386
	11th to 20th	Night	10% 7% 2 6% 4 5% 6% 5% 7% 9% 40%	2897
	21st to end	Night	8% 5% 2 5% 5% 5% 5% 5% 7% 9% 45%	3466
August	1st to 10th	Night	10% 5% 2 7% 6% 5% 5% 3 6% 8% 42%	2279
	11th to 20th	Night	12% 6% 2 7% 3 3 4 3 5% 8% 46%	2597
	21st to end	Night	19% 7% 2 6% 4 4 5% 5% 7% 8% 34%	3234
September	1st to 10th	Night	20% 9% 3 9% 5% 5% 4 3 6% 7% 29%	1900
	11th to 20th	Night	21% 11% 3 7% 4 4 5% 4 5% 7% 29%	2054
	21st to end	Night	18% 12% 3 8% 4 4 5% 4 4 6% 32%	2251
October	1st to 10th	Night	19% 16% 5% 11% 6% 5% 5% 4 5% 6% 19%	3920
	11th to 20th	Night	18% 15% 5% 11% 5% 4 4 3 5% 5% 24%	4036
	21st to end	Night	17% 18% 7% 12% 6% 5% 4 3 4 5% 20%	4968
November	1st to 10th	Night	15% 20% 7% 15% 8% 8% 5% 3 3 4 14%	2871
	11th to 20th	Night	13% 15% 8% 16% 9% 7% 6% 4 4 5% 13%	3149
	21st to end	Night	8% 22% 8% 16% 9% 6% 4 3 4 5% 14%	3360
December	1st to 10th	Night	4 16% 7% 17% 10% 7% 7% 6% 8% 5% 12%	5390
	11th to 20th	Night	8% 13% 4 14% 10% 9% 8% 9% 9% 6% 11%	6111
	21st to end	Night	7% 16% 6% 17% 7% 6% 7% 5% 5% 5% 18%	8162

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	65% 5% 10% 13% 7%	2808
	11th to 20th	Day	67% 5% 9% 16% 3	3250
	21st to end	Day	62% 4 8% 19% 7%	3575
February	1st to 10th	Day	52% 7% 12% 22% 8%	1460
	11th to 20th	Day	52% 8% 12% 20% 7%	1763
	21st to end	Day	50% 5% 10% 19% 16%	1485
March	1st to 10th	Day	54% 3 11% 21% 9% 2	4419
	11th to 20th	Day	61% 4 7% 16% 11% 1	4980
	21st to end	Day	52% 5% 7% 23% 12% 1	5573
April	1st to 10th	Day	53% 7% 10% 22% 7%	2507
	11th to 20th	Day	51% 6% 12% 20% 11% 1	2572
	21st to end	Day	42% 7% 13% 26% 13%	2778
May	1st to 10th	Day	53% 5% 13% 21% 8%	6621
	11th to 20th	Day	59% 6% 6% 18% 10%	6791
	21st to end	Day	44% 4 6% 28% 14% 4	7930
June	1st to 10th	Day	53% 7% 10% 20% 10%	3200
	11th to 20th	Day	44% 7% 13% 25% 11% 1	3390
	21st to end	Day	47% 7% 12% 25% 9% 1	3400
July	1st to 10th	Day	35% 6% 23% 30% 6%	7496
	11th to 20th	Day	41% 6% 16% 33% 4	6994
	21st to end	Day	35% 8% 22% 32% 3	7193
August	1st to 10th	Day	38% 7% 21% 30% 4	4864
	11th to 20th	Day	38% 7% 21% 28% 6%	4501
	21st to end	Day	53% 7% 14% 23% 3	4674
September	1st to 10th	Day	54% 9% 18% 16% 3	2500
	11th to 20th	Day	57% 6% 16% 20% 1	2345
	21st to end	Day	56% 7% 14% 18% 5%	2129
October	1st to 10th	Day	63% 6% 10% 17% 4	3260
	11th to 20th	Day	62% 6% 9% 19% 4	3104
	21st to end	Day	70% 6% 9% 12% 3	2832
November	1st to 10th	Day	74% 5% 8% 11% 3	1647
	11th to 20th	Day	73% 6% 11% 10%	1467
	21st to end	Day	74% 3 8% 14% 1	1260
December	1st to 10th	Day	71% 5% 11% 11% 2	2020
	11th to 20th	Day	67% 4 10% 17% 2	2316
	21st to end	Day	69% 5% 8% 14% 4	3058
January	1st to 10th	Night	65% 5% 10% 11% 9%	7378
	11th to 20th	Night	70% 6% 10% 12% 2	6950
	21st to end	Night	66% 4 7% 15% 7%	7645
February	1st to 10th	Night	56% 7% 12% 18% 7%	2937
	11th to 20th	Night	51% 9% 14% 21% 6%	2534
	21st to end	Night	53% 5% 10% 18% 13%	2145
March	1st to 10th	Night	51% 4 12% 18% 15% 1	5490
	11th to 20th	Night	61% 4 6% 16% 12% 1	4980
	21st to end	Night	53% 5% 9% 18% 14% 1	5358
April	1st to 10th	Night	48% 5% 11% 22% 13% 1	1890
	11th to 20th	Night	49% 5% 9% 22% 14% 1	1779
	21st to end	Night	43% 8% 10% 26% 12% 1	1621
May	1st to 10th	Night	57% 8% 11% 16% 8%	3341
	11th to 20th	Night	57% 5% 8% 21% 9%	3128
	21st to end	Night	44% 4 6% 24% 15% 8%	2968
June	1st to 10th	Night	51% 6% 10% 21% 12%	1200
	11th to 20th	Night	45% 5% 10% 27% 13%	1006
	21st to end	Night	45% 7% 11% 26% 10% 1	1000
July	1st to 10th	Night	41% 7% 17% 30% 5%	2359
	11th to 20th	Night	39% 7% 18% 32% 4	2861
	21st to end	Night	33% 8% 18% 36% 5%	3368
August	1st to 10th	Night	39% 7% 20% 31% 3	2272
	11th to 20th	Night	36% 7% 17% 31% 10%	2621
	21st to end	Night	46% 7% 21% 21% 5%	3234
September	1st to 10th	Night	54% 8% 18% 16% 4	1900
	11th to 20th	Night	54% 7% 16% 21% 2	2054
	21st to end	Night	54% 4 17% 20% 5% 1	2251
October	1st to 10th	Night	64% 6% 11% 16% 3	3920
	11th to 20th	Night	62% 5% 10% 17% 7%	4036
	21st to end	Night	66% 5% 9% 15% 4	4968
November	1st to 10th	Night	75% 5% 7% 11% 2	2871
	11th to 20th	Night	73% 6% 11% 10% 1	3149
	21st to end	Night	72% 4 10% 11% 2	3360
December	1st to 10th	Night	69% 5% 12% 13% 2	5390
	11th to 20th	Night	67% 5% 9% 16% 3	6110
	21st to end	Night	65% 5% 11% 14% 3 1	8162

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	9% 4 34% 34% 14% 5%	228
	11th to 20th	Day	9% 7% 33% 33% 16% 2	330
	21st to end	Day	14% 10% 31% 27% 15% 4	363
February	1st to 10th	Day	6% 9% 27% 40% 14% 3	300
	11th to 20th	Day	7% 4 37% 31% 17% 4	300
	21st to end	Day	7% 9% 30% 31% 16% 8%	249
March	1st to 10th	Day	11% 7% 27% 23% 19% 12%	300
	11th to 20th	Day	10% 8% 36% 23% 15% 8%	300
	21st to end	Day	9% 9% 30% 30% 17% 6%	375
April	1st to 10th	Day	13% 7% 25% 32% 18% 5%	607
	11th to 20th	Day	15% 9% 27% 30% 16% 3	690
	21st to end	Day	10% 11% 28% 29% 18% 6%	700
May	1st to 10th	Day	17% 11% 30% 25% 13% 4	703
	11th to 20th	Day	21% 12% 27% 27% 9% 3	725
	21st to end	Day	22% 8% 25% 30% 13% 2	789
June	1st to 10th	Day	26% 10% 23% 22% 13% 5%	730
	11th to 20th	Day	18% 7% 25% 29% 15% 6%	730
	21st to end	Day	16% 9% 24% 28% 18% 5%	730
July	1st to 10th	Day	17% 8% 25% 31% 15% 4	721
	11th to 20th	Day	16% 6% 23% 30% 19% 6%	718
	21st to end	Day	16% 7% 21% 28% 19% 9%	794
August	1st to 10th	Day	20% 6% 22% 28% 18% 6%	726
	11th to 20th	Day	19% 6% 21% 27% 20% 7%	715
	21st to end	Day	27% 6% 21% 26% 15% 5%	803
September	1st to 10th	Day	25% 10% 26% 27% 11% 2	630
	11th to 20th	Day	24% 11% 23% 27% 13% 3	475
	21st to end	Day	22% 11% 33% 22% 10% 2	330
October	1st to 10th	Day	25% 10% 29% 24% 8% 3	330
	11th to 20th	Day	32% 10% 27% 17% 8% 6%	330
	21st to end	Day	27% 9% 28% 24% 10% 1	363
November	1st to 10th	Day	21% 14% 26% 25% 11% 4	330
	11th to 20th	Day	19% 10% 31% 24% 12% 4	291
	21st to end	Day	17% 12% 29% 30% 11% 1	220
December	1st to 10th	Day	15% 6% 25% 34% 19% 2	220
	11th to 20th	Day	11% 8% 33% 27% 17% 3	220
	21st to end	Day	7% 10% 31% 27% 19% 7%	242
January	1st to 10th	Night	12% 7% 29% 32% 18% 3	1050
	11th to 20th	Night	11% 8% 29% 33% 16% 4	1050
	21st to end	Night	14% 8% 28% 31% 17% 2	1155
February	1st to 10th	Night	7% 7% 31% 35% 17% 3	1001
	11th to 20th	Night	9% 5% 26% 36% 18% 6%	979
	21st to end	Night	8% 6% 24% 38% 19% 4	825
March	1st to 10th	Night	7% 5% 27% 36% 21% 4	857
	11th to 20th	Night	12% 9% 32% 31% 14% 3	600
	21st to end	Night	10% 5% 27% 35% 19% 4	660
April	1st to 10th	Night	12% 5% 26% 35% 20% 2	600
	11th to 20th	Night	14% 5% 28% 34% 16% 2	593
	21st to end	Night	10% 8% 28% 40% 14% 1	600
May	1st to 10th	Night	21% 9% 30% 30% 10% 1	600
	11th to 20th	Night	22% 10% 26% 30% 11% 1	584
	21st to end	Night	24% 8% 33% 27% 8%	433
June	1st to 10th	Night	28% 7% 24% 27% 12% 2	400
	11th to 20th	Night	18% 5% 23% 38% 15%	206
	21st to end	Night	14% 7% 30% 36% 13% 1	200
July	1st to 10th	Night	20% 5% 24% 36% 15% 1	241
	11th to 20th	Night	16% 5% 21% 39% 16% 3	461
	21st to end	Night	16% 5% 20% 35% 23% 2	657
August	1st to 10th	Night	18% 4 20% 37% 21% 1	597
	11th to 20th	Night	19% 4 20% 35% 20% 2	594
	21st to end	Night	24% 5% 21% 35% 13% 2	660
September	1st to 10th	Night	26% 5% 24% 32% 12% 2	600
	11th to 20th	Night	27% 8% 23% 29% 12% 1	600
	21st to end	Night	23% 8% 28% 29% 12% 1	651
October	1st to 10th	Night	29% 8% 29% 21% 13% 1	800
	11th to 20th	Night	32% 10% 21% 23% 12% 3	865
	21st to end	Night	31% 9% 27% 23% 10% 1	1100
November	1st to 10th	Night	24% 9% 29% 25% 11% 2	1025
	11th to 20th	Night	19% 9% 30% 27% 12% 2	1050
	21st to end	Night	19% 9% 30% 30% 10% 3	1050
December	1st to 10th	Night	10% 7% 30% 38% 14% 2	1050
	11th to 20th	Night	12% 7% 28% 33% 17% 3	1050
	21st to end	Night	13% 7% 24% 34% 18% 4	1155

Page generated at 2026-01-29 00:21:40. Version ac07

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