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 2024-01-14.

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	14% 10% 4 8% 7% 4 6% 6% 6% 7% 27%	74973
	Night	12% 12% 5% 10% 8% 4 7% 6% 7% 6% 23%	74812
Transparency	Day	54% 7% 12% 20% 6%	74639
	Night	58% 7% 12% 17% 5%	74665
Seeing	Day	18% 8% 25% 28% 16% 5%	14815
	Night	17% 6% 26% 32% 16% 2	23886

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	8% 13% 8% 12% 12% 6% 9% 7% 7% 6% 13%	1153
	11th to 20th	Day	7% 11% 5% 11% 8% 6% 8% 8% 11% 9% 17%	1281
	21st to end	Day	8% 11% 5% 12% 10% 5% 8% 8% 11% 9% 12%	1386
February	1st to 10th	Day	3 6% 3 9% 9% 6% 10% 11% 8% 10% 24%	1314
	11th to 20th	Day	5% 9% 5% 7% 9% 6% 9% 10% 9% 9% 22%	1574
	21st to end	Day	3 12% 5% 8% 9% 5% 6% 9% 9% 9% 25%	1332
March	1st to 10th	Day	8% 9% 5% 10% 7% 7% 8% 7% 7% 8% 24%	1765
	11th to 20th	Day	9% 9% 3 7% 9% 6% 8% 7% 7% 8% 27%	1978
	21st to end	Day	7% 9% 4 9% 8% 6% 8% 6% 6% 8% 30%	2218
April	1st to 10th	Day	10% 10% 3 9% 8% 5% 8% 7% 6% 7% 26%	2255
	11th to 20th	Day	8% 9% 5% 8% 8% 4 6% 6% 5% 7% 35%	2312
	21st to end	Day	6% 6% 4 7% 8% 4 5% 5% 7% 9% 39%	2500
May	1st to 10th	Day	14% 13% 7% 10% 6% 4 4 5% 4 5% 27%	2628
	11th to 20th	Day	15% 10% 5% 7% 6% 3 6% 5% 5% 7% 31%	2659
	21st to end	Day	19% 10% 4 7% 5% 3 4 5% 4 6% 33%	3112
June	1st to 10th	Day	19% 9% 3 6% 6% 4 5% 5% 5% 6% 32%	2880
	11th to 20th	Day	16% 6% 3 6% 6% 3 5% 6% 6% 9% 36%	3050
	21st to end	Day	13% 8% 3 6% 6% 4 6% 5% 6% 7% 35%	3060
July	1st to 10th	Day	17% 8% 3 7% 6% 4 5% 5% 6% 8% 31%	3016
	11th to 20th	Day	15% 6% 2 6% 6% 3 5% 5% 7% 8% 37%	2813
	21st to end	Day	15% 8% 3 6% 5% 3 5% 6% 6% 8% 35%	2939
August	1st to 10th	Day	17% 7% 2 6% 5% 3 4 5% 6% 8% 36%	2686
	11th to 20th	Day	18% 7% 4 7% 6% 3 4 4 6% 8% 33%	2472
	21st to end	Day	24% 9% 3 6% 5% 3 5% 5% 5% 7% 30%	2574
September	1st to 10th	Day	20% 10% 3 8% 7% 4 5% 5% 6% 7% 26%	2250
	11th to 20th	Day	22% 14% 4 6% 6% 3 3 4 5% 8% 24%	2111
	21st to end	Day	19% 13% 4 6% 5% 4 5% 5% 6% 7% 27%	1917
October	1st to 10th	Day	23% 17% 5% 8% 5% 2 3 4 5% 5% 23%	1800
	11th to 20th	Day	25% 18% 7% 10% 6% 3 5% 5% 3 4 14%	1732
	21st to end	Day	23% 21% 8% 12% 7% 3 3 3 3 4 13%	1584
November	1st to 10th	Day	16% 18% 8% 11% 12% 5% 6% 4 4 3 14%	1487
	11th to 20th	Day	13% 14% 7% 14% 12% 6% 7% 5% 4 6% 11%	1329
	21st to end	Day	9% 19% 10% 15% 10% 4 6% 4 3 5% 16%	1140
December	1st to 10th	Day	10% 13% 6% 13% 11% 6% 8% 5% 6% 8% 16%	1140
	11th to 20th	Day	7% 13% 7% 13% 13% 4 7% 6% 7% 7% 16%	1436
	21st to end	Day	5% 10% 7% 13% 9% 6% 11% 6% 6% 6% 20%	2090
January	1st to 10th	Night	8% 12% 6% 13% 13% 7% 9% 8% 8% 5% 12%	3013
	11th to 20th	Night	7% 11% 5% 11% 10% 6% 9% 10% 12% 9% 12%	2758
	21st to end	Night	9% 11% 4 11% 11% 6% 10% 11% 10% 7% 11%	2970
February	1st to 10th	Night	3 8% 4 11% 10% 6% 10% 11% 9% 9% 19%	2644
	11th to 20th	Night	4 10% 4 7% 8% 5% 10% 10% 11% 8% 23%	2273
	21st to end	Night	5% 10% 6% 10% 10% 5% 8% 8% 10% 7% 21%	1924
March	1st to 10th	Night	5% 9% 5% 9% 9% 6% 7% 8% 8% 8% 25%	2212
	11th to 20th	Night	7% 11% 5% 10% 9% 4 8% 7% 6% 5% 27%	1980
	21st to end	Night	7% 9% 4 7% 8% 6% 9% 7% 5% 7% 32%	2138
April	1st to 10th	Night	9% 9% 4 8% 8% 4 6% 6% 6% 6% 33%	1702
	11th to 20th	Night	8% 8% 3 9% 9% 3 6% 5% 6% 8% 35%	1599
	21st to end	Night	6% 8% 4 7% 8% 4 7% 5% 5% 9% 37%	1459
May	1st to 10th	Night	17% 11% 6% 12% 7% 3 5% 4 4 5% 27%	1330
	11th to 20th	Night	14% 14% 5% 8% 6% 3 5% 4 5% 6% 30%	1236
	21st to end	Night	20% 9% 4 6% 4 2 4 5% 5% 7% 34%	1164
June	1st to 10th	Night	19% 9% 3 6% 6% 2 4 5% 6% 6% 35%	1080
	11th to 20th	Night	17% 5% 4 5% 6% 3 5% 6% 6% 7% 36%	906
	21st to end	Night	15% 6% 3 7% 6% 3 5% 5% 6% 6% 37%	900
July	1st to 10th	Night	18% 8% 2 5% 5% 3 5% 6% 5% 9% 34%	942
	11th to 20th	Night	15% 9% 2 5% 4 2 5% 6% 6% 8% 38%	1145
	21st to end	Night	14% 5% 2 5% 5% 4 6% 6% 7% 9% 36%	1379
August	1st to 10th	Night	17% 6% 2 5% 6% 3 4 4 6% 8% 38%	1253
	11th to 20th	Night	17% 6% 3 7% 6% 3 4 6% 6% 9% 34%	1426
	21st to end	Night	22% 8% 3 5% 5% 3 4 5% 6% 6% 34%	1782
September	1st to 10th	Night	21% 8% 4 8% 5% 4 5% 4 6% 7% 28%	1710
	11th to 20th	Night	22% 12% 3 6% 5% 3 5% 5% 5% 7% 27%	1848
	21st to end	Night	18% 12% 4 7% 5% 3 6% 4 4 5% 32%	2025
October	1st to 10th	Night	22% 15% 5% 8% 6% 3 4 5% 4 6% 22%	2160
	11th to 20th	Night	25% 20% 7% 10% 5% 3 4 3 4 4 15%	2226
	21st to end	Night	22% 22% 9% 10% 7% 3 4 3 3 4 13%	2772
November	1st to 10th	Night	16% 19% 7% 13% 10% 6% 6% 3 3 3 13%	2591
	11th to 20th	Night	12% 13% 8% 15% 10% 5% 8% 6% 4 5% 14%	2847
	21st to end	Night	8% 20% 9% 13% 10% 5% 5% 4 5% 6% 15%	3040
December	1st to 10th	Night	6% 15% 8% 14% 12% 4 7% 6% 9% 6% 13%	3040
	11th to 20th	Night	6% 13% 6% 11% 11% 6% 9% 9% 9% 8% 13%	3761
	21st to end	Night	6% 11% 5% 13% 9% 5% 9% 7% 6% 6% 22%	5577

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	68% 6% 12% 12% 1	1153
	11th to 20th	Day	56% 7% 11% 22% 4	1281
	21st to end	Day	60% 7% 9% 18% 7%	1386
February	1st to 10th	Day	49% 7% 12% 23% 9%	1314
	11th to 20th	Day	51% 8% 12% 21% 7%	1583
	21st to end	Day	52% 6% 10% 20% 13%	1332
March	1st to 10th	Day	55% 7% 13% 21% 4	1765
	11th to 20th	Day	53% 7% 10% 20% 9% 1	1980
	21st to end	Day	52% 6% 10% 22% 8% 2	2218
April	1st to 10th	Day	54% 7% 10% 22% 6%	2255
	11th to 20th	Day	48% 6% 12% 21% 12% 1	2312
	21st to end	Day	41% 7% 13% 25% 14%	2500
May	1st to 10th	Day	58% 6% 9% 21% 5%	2628
	11th to 20th	Day	52% 8% 9% 20% 11%	2663
	21st to end	Day	52% 7% 10% 26% 6%	3112
June	1st to 10th	Day	51% 7% 11% 20% 10%	2880
	11th to 20th	Day	44% 7% 13% 25% 11% 1	3050
	21st to end	Day	48% 8% 12% 23% 9% 1	3060
July	1st to 10th	Day	50% 7% 13% 25% 5%	2934
	11th to 20th	Day	44% 8% 16% 27% 5%	2726
	21st to end	Day	46% 10% 17% 24% 3	2776
August	1st to 10th	Day	46% 9% 20% 22% 3	2669
	11th to 20th	Day	48% 9% 19% 21% 4	2472
	21st to end	Day	54% 8% 17% 16% 4	2574
September	1st to 10th	Day	56% 9% 17% 15% 3	2250
	11th to 20th	Day	59% 6% 15% 18% 1	2111
	21st to end	Day	57% 7% 13% 19% 4	1917
October	1st to 10th	Day	63% 6% 11% 15% 4	1800
	11th to 20th	Day	73% 7% 8% 10% 2	1732
	21st to end	Day	77% 6% 8% 6% 3	1584
November	1st to 10th	Day	75% 5% 7% 10% 3	1487
	11th to 20th	Day	72% 6% 12% 10%	1329
	21st to end	Day	72% 3 9% 15% 1	1140
December	1st to 10th	Day	67% 5% 10% 16% 3	1140
	11th to 20th	Day	65% 5% 11% 15% 3	1436
	21st to end	Day	62% 6% 9% 18% 5%	2090
January	1st to 10th	Night	69% 7% 11% 12% 2	3013
	11th to 20th	Night	60% 8% 10% 18% 3	2758
	21st to end	Night	64% 7% 7% 15% 7%	2970
February	1st to 10th	Night	54% 7% 12% 20% 7%	2644
	11th to 20th	Night	49% 8% 14% 22% 6%	2274
	21st to end	Night	55% 6% 10% 19% 11%	1924
March	1st to 10th	Night	53% 6% 15% 20% 6%	2212
	11th to 20th	Night	55% 6% 7% 21% 9% 1	1980
	21st to end	Night	49% 6% 10% 22% 11% 2	2138
April	1st to 10th	Night	49% 5% 11% 22% 13% 1	1702
	11th to 20th	Night	47% 6% 10% 23% 15% 1	1599
	21st to end	Night	42% 7% 10% 27% 13% 1	1459
May	1st to 10th	Night	60% 5% 10% 19% 5%	1330
	11th to 20th	Night	56% 5% 9% 18% 12% 1	1236
	21st to end	Night	50% 6% 10% 25% 9%	1164
June	1st to 10th	Night	49% 6% 11% 22% 12%	1080
	11th to 20th	Night	46% 6% 10% 26% 13%	906
	21st to end	Night	45% 7% 11% 26% 10% 1	900
July	1st to 10th	Night	46% 7% 16% 27% 5%	915
	11th to 20th	Night	41% 9% 17% 28% 4	1109
	21st to end	Night	40% 10% 20% 27% 3	1302
August	1st to 10th	Night	43% 9% 20% 26% 3	1246
	11th to 20th	Night	46% 9% 19% 22% 4	1426
	21st to end	Night	49% 8% 19% 20% 3	1782
September	1st to 10th	Night	55% 8% 19% 14% 4	1710
	11th to 20th	Night	56% 7% 14% 21% 2	1848
	21st to end	Night	55% 5% 15% 21% 4 1	2025
October	1st to 10th	Night	62% 7% 11% 15% 5%	2160
	11th to 20th	Night	74% 4 10% 9% 2	2226
	21st to end	Night	76% 5% 9% 7% 3	2772
November	1st to 10th	Night	76% 5% 8% 10% 1	2591
	11th to 20th	Night	71% 6% 12% 10% 1	2847
	21st to end	Night	70% 5% 11% 12% 2	3040
December	1st to 10th	Night	66% 6% 11% 14% 2	3040
	11th to 20th	Night	63% 7% 12% 13% 5%	3760
	21st to end	Night	60% 6% 12% 17% 5% 1	5577

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% 3 33% 34% 15% 5%	186
	11th to 20th	Day	10% 6% 29% 33% 19% 3	249
	21st to end	Day	15% 5% 24% 31% 19% 6%	264
February	1st to 10th	Day	3 8% 28% 41% 17% 3	240
	11th to 20th	Day	8% 4 40% 30% 16% 2	240
	21st to end	Day	8% 8% 27% 30% 18% 10%	198
March	1st to 10th	Day	11% 5% 25% 24% 22% 13%	240
	11th to 20th	Day	8% 8% 34% 24% 17% 9%	240
	21st to end	Day	10% 10% 28% 29% 17% 6%	304
April	1st to 10th	Day	15% 7% 26% 30% 17% 5%	515
	11th to 20th	Day	12% 8% 27% 31% 18% 4	590
	21st to end	Day	10% 10% 26% 29% 20% 6%	600
May	1st to 10th	Day	16% 11% 28% 27% 14% 4	603
	11th to 20th	Day	19% 10% 28% 30% 8% 3	625
	21st to end	Day	23% 8% 26% 30% 13% 1	679
June	1st to 10th	Day	25% 10% 22% 24% 14% 5%	630
	11th to 20th	Day	18% 6% 25% 30% 15% 6%	630
	21st to end	Day	17% 9% 24% 27% 18% 6%	630
July	1st to 10th	Day	19% 7% 25% 30% 15% 4	621
	11th to 20th	Day	17% 6% 23% 28% 19% 7%	618
	21st to end	Day	18% 6% 19% 29% 20% 9%	684
August	1st to 10th	Day	21% 6% 22% 27% 17% 7%	626
	11th to 20th	Day	19% 6% 22% 26% 20% 7%	615
	21st to end	Day	27% 6% 21% 26% 15% 5%	693
September	1st to 10th	Day	27% 10% 24% 27% 11% 2	540
	11th to 20th	Day	26% 10% 22% 27% 12% 2	401
	21st to end	Day	24% 7% 35% 21% 11% 2	270
October	1st to 10th	Day	25% 9% 28% 25% 9% 4	270
	11th to 20th	Day	33% 8% 27% 17% 9% 6%	270
	21st to end	Day	27% 6% 26% 27% 12% 2	297
November	1st to 10th	Day	20% 10% 24% 29% 12% 4	270
	11th to 20th	Day	15% 9% 31% 26% 14% 5%	239
	21st to end	Day	19% 11% 29% 28% 11% 2	180
December	1st to 10th	Day	17% 5% 24% 32% 19% 2	180
	11th to 20th	Day	11% 8% 28% 29% 20% 3	180
	21st to end	Day	5% 7% 29% 29% 23% 8%	198
January	1st to 10th	Night	12% 6% 29% 32% 18% 3	950
	11th to 20th	Night	11% 7% 28% 34% 16% 4	920
	21st to end	Night	14% 8% 25% 33% 18% 2	990
February	1st to 10th	Night	6% 6% 31% 35% 18% 4	900
	11th to 20th	Night	9% 5% 27% 35% 18% 5%	879
	21st to end	Night	9% 6% 22% 39% 20% 4	740
March	1st to 10th	Night	7% 4 26% 37% 22% 4	772
	11th to 20th	Night	12% 9% 32% 30% 14% 3	540
	21st to end	Night	11% 6% 26% 33% 20% 4	594
April	1st to 10th	Night	13% 5% 27% 34% 19% 2	540
	11th to 20th	Night	12% 5% 28% 36% 17% 2	533
	21st to end	Night	10% 7% 27% 40% 15% 1	540
May	1st to 10th	Night	21% 8% 29% 30% 11% 1	540
	11th to 20th	Night	22% 10% 26% 30% 12% 1	526
	21st to end	Night	25% 7% 33% 26% 9%	389
June	1st to 10th	Night	26% 6% 25% 29% 12% 2	360
	11th to 20th	Night	19% 4 22% 38% 16%	186
	21st to end	Night	15% 6% 30% 34% 14% 1	180
July	1st to 10th	Night	22% 4 22% 37% 15% 1	215
	11th to 20th	Night	17% 5% 21% 38% 17% 3	413
	21st to end	Night	16% 5% 18% 35% 23% 3	591
August	1st to 10th	Night	18% 3 20% 36% 21% 1	537
	11th to 20th	Night	20% 4 20% 35% 19% 2	534
	21st to end	Night	25% 4 20% 35% 13% 2	594
September	1st to 10th	Night	26% 5% 24% 31% 12% 1	540
	11th to 20th	Night	29% 8% 23% 29% 11% 1	540
	21st to end	Night	23% 8% 27% 29% 13% 1	585
October	1st to 10th	Night	30% 7% 27% 21% 14% 1	720
	11th to 20th	Night	33% 9% 19% 23% 12% 3	778
	21st to end	Night	32% 8% 25% 23% 11% 1	990
November	1st to 10th	Night	24% 8% 27% 27% 12% 2	925
	11th to 20th	Night	17% 8% 31% 28% 13% 2	950
	21st to end	Night	20% 8% 30% 29% 10% 3	950
December	1st to 10th	Night	11% 7% 29% 37% 15% 2	950
	11th to 20th	Night	12% 7% 26% 33% 19% 3	950
	21st to end	Night	13% 6% 22% 35% 19% 4	1045

Page generated at 2024-01-11 12:31:57. Version ac07

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