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 2024-11-28.

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	13% 11% 4 9% 6% 6% 5% 4 6% 7% 28%	106622
	Night	11% 12% 5% 12% 7% 6% 6% 5% 6% 6% 23%	101524
Transparency	Day	53% 6% 13% 21% 6%	106351
	Night	58% 6% 12% 18% 6%	101442
Seeing	Day	18% 8% 26% 28% 15% 5%	16088
	Night	17% 7% 26% 32% 15% 2	24963

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% 11% 7% 16% 11% 8% 10% 6% 7% 6% 12%	1913
	11th to 20th	Day	4 10% 5% 18% 11% 11% 9% 8% 9% 6% 11%	2199
	21st to end	Day	7% 15% 7% 17% 8% 8% 6% 5% 7% 7% 12%	2442
February	1st to 10th	Day	4 7% 4 11% 9% 8% 9% 8% 8% 9% 23%	1387
	11th to 20th	Day	4 11% 4 9% 9% 8% 9% 8% 8% 9% 21%	1664
	21st to end	Day	3 12% 4 11% 7% 6% 6% 7% 9% 9% 26%	1413
March	1st to 10th	Day	8% 10% 4 11% 5% 7% 5% 5% 6% 9% 31%	3081
	11th to 20th	Day	12% 13% 3 12% 10% 9% 6% 6% 6% 5% 18%	3478
	21st to end	Day	4 12% 5% 9% 7% 8% 9% 7% 6% 6% 27%	3895
April	1st to 10th	Day	9% 11% 3 10% 7% 7% 7% 5% 6% 7% 27%	2381
	11th to 20th	Day	9% 10% 4 9% 7% 6% 5% 4 5% 7% 34%	2442
	21st to end	Day	6% 7% 3 9% 6% 5% 5% 4 7% 9% 39%	2639
May	1st to 10th	Day	18% 15% 6% 11% 4 4 4 3 4 5% 26%	4626
	11th to 20th	Day	20% 12% 4 8% 6% 5% 5% 3 5% 7% 26%	4723
	21st to end	Day	20% 12% 4 9% 4 4 4 3 5% 7% 30%	5521
June	1st to 10th	Day	20% 11% 3 7% 5% 5% 5% 4 5% 6% 32%	3040
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 5% 6% 9% 35%	3220
	21st to end	Day	13% 8% 3 7% 6% 6% 5% 4 6% 7% 36%	3230
July	1st to 10th	Day	11% 7% 2 8% 5% 4 5% 5% 7% 9% 38%	5297
	11th to 20th	Day	14% 7% 2 7% 4 4 5% 4 7% 9% 38%	4947
	21st to end	Day	10% 7% 3 6% 5% 5% 6% 5% 8% 10% 36%	5150
August	1st to 10th	Day	11% 5% 2 6% 4 4 5% 5% 6% 8% 44%	4731
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 6% 9% 43%	4319
	21st to end	Day	21% 9% 3 8% 5% 4 5% 4 5% 7% 30%	4532
September	1st to 10th	Day	19% 10% 3 9% 6% 5% 5% 4 6% 7% 27%	2375
	11th to 20th	Day	21% 14% 3 7% 5% 4 4 3 6% 8% 26%	2228
	21st to end	Day	18% 13% 4 7% 4 5% 5% 4 6% 7% 26%	2023
October	1st to 10th	Day	17% 19% 5% 10% 6% 5% 4 3 5% 5% 21%	3160
	11th to 20th	Day	15% 14% 7% 11% 5% 5% 5% 4 6% 7% 21%	3009
	21st to end	Day	20% 17% 7% 13% 7% 4 4 3 4 4 17%	2744
November	1st to 10th	Day	15% 19% 8% 13% 10% 6% 5% 3 4 3 14%	1567
	11th to 20th	Day	13% 16% 6% 16% 10% 8% 7% 4 4 6% 11%	1398
	21st to end	Day	9% 21% 9% 16% 8% 6% 4 3 3 5% 15%	1182
December	1st to 10th	Day	9% 14% 5% 13% 10% 9% 6% 4 5% 8% 15%	1140
	11th to 20th	Day	7% 15% 6% 16% 9% 6% 6% 4 7% 7% 16%	1436
	21st to end	Day	5% 11% 7% 14% 8% 9% 11% 4 7% 5% 20%	2090
January	1st to 10th	Night	5% 9% 4 16% 12% 11% 10% 6% 9% 6% 12%	5043
	11th to 20th	Night	4 9% 4 19% 13% 11% 12% 7% 8% 6% 7%	4709
	21st to end	Night	14% 17% 6% 14% 8% 6% 7% 6% 8% 6% 8%	5225
February	1st to 10th	Night	3 10% 5% 12% 8% 8% 9% 8% 8% 8% 19%	2791
	11th to 20th	Night	4 11% 4 9% 7% 7% 10% 7% 11% 7% 22%	2403
	21st to end	Night	4 10% 5% 12% 9% 6% 7% 7% 11% 7% 21%	2041
March	1st to 10th	Night	5% 9% 4 10% 6% 6% 7% 6% 8% 8% 31%	3808
	11th to 20th	Night	7% 15% 6% 17% 10% 7% 6% 6% 6% 4 17%	3480
	21st to end	Night	5% 12% 6% 10% 6% 7% 9% 8% 5% 5% 27%	3736
April	1st to 10th	Night	8% 10% 4 9% 6% 5% 6% 5% 6% 7% 33%	1796
	11th to 20th	Night	9% 10% 3 10% 7% 4 5% 4 6% 7% 34%	1689
	21st to end	Night	6% 9% 3 8% 7% 5% 6% 5% 6% 8% 37%	1540
May	1st to 10th	Night	18% 14% 6% 15% 6% 5% 5% 3 4 5% 20%	2329
	11th to 20th	Night	18% 12% 3 8% 4 4 4 3 5% 7% 30%	2182
	21st to end	Night	25% 12% 4 8% 4 4 3 2 4 6% 28%	2066
June	1st to 10th	Night	20% 10% 2 7% 5% 4 4 4 6% 6% 34%	1140
	11th to 20th	Night	17% 6% 3 6% 6% 5% 4 5% 6% 7% 36%	956
	21st to end	Night	15% 7% 3 7% 5% 5% 5% 5% 6% 6% 37%	950
July	1st to 10th	Night	12% 7% 2 8% 6% 5% 5% 5% 8% 10% 33%	1664
	11th to 20th	Night	13% 9% 2 6% 4 4 5% 4 7% 9% 38%	2021
	21st to end	Night	10% 5% 3 6% 5% 5% 5% 6% 8% 10% 37%	2414
August	1st to 10th	Night	10% 5% 2 7% 6% 5% 5% 3 6% 8% 42%	2209
	11th to 20th	Night	12% 5% 2 7% 3 3 4 3 5% 8% 47%	2517
	21st to end	Night	19% 7% 2 6% 4 4 5% 5% 7% 8% 34%	3135
September	1st to 10th	Night	20% 9% 3 9% 5% 5% 4 4 6% 7% 29%	1805
	11th to 20th	Night	21% 12% 3 7% 4 4 5% 4 5% 7% 28%	1951
	21st to end	Night	18% 13% 3 8% 4 4 5% 3 4 6% 31%	2138
October	1st to 10th	Night	19% 16% 5% 11% 6% 5% 5% 4 5% 6% 18%	3800
	11th to 20th	Night	17% 15% 5% 11% 5% 5% 4 3 5% 5% 24%	3912
	21st to end	Night	17% 18% 6% 12% 6% 5% 4 3 4 5% 21%	4814
November	1st to 10th	Night	15% 20% 7% 15% 8% 8% 5% 3 3 4 13%	2731
	11th to 20th	Night	13% 15% 8% 16% 9% 8% 6% 4 4 5% 13%	2998
	21st to end	Night	8% 23% 9% 14% 8% 6% 4 4 5% 6% 14%	3153
December	1st to 10th	Night	5% 17% 6% 15% 10% 7% 6% 5% 9% 6% 13%	3040
	11th to 20th	Night	6% 15% 5% 13% 9% 8% 9% 7% 9% 7% 13%	3761
	21st to end	Night	6% 12% 5% 14% 8% 8% 9% 6% 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% 7% 13% 11% 1	1913
	11th to 20th	Day	68% 6% 9% 14% 2	2220
	21st to end	Day	69% 5% 8% 12% 6%	2442
February	1st to 10th	Day	51% 7% 11% 23% 8%	1387
	11th to 20th	Day	53% 8% 12% 20% 7%	1673
	21st to end	Day	50% 5% 10% 19% 16%	1413
March	1st to 10th	Day	49% 4 13% 28% 5%	3081
	11th to 20th	Day	65% 5% 9% 14% 6% 1	3480
	21st to end	Day	56% 5% 8% 24% 7% 1	3895
April	1st to 10th	Day	53% 7% 10% 22% 7%	2381
	11th to 20th	Day	50% 6% 12% 21% 11% 1	2442
	21st to end	Day	41% 7% 13% 26% 14%	2639
May	1st to 10th	Day	59% 6% 14% 17% 4	4626
	11th to 20th	Day	57% 7% 7% 16% 13%	4727
	21st to end	Day	55% 5% 8% 25% 6%	5521
June	1st to 10th	Day	52% 7% 11% 20% 10%	3040
	11th to 20th	Day	44% 7% 13% 25% 11% 1	3220
	21st to end	Day	47% 8% 11% 25% 9% 1	3230
July	1st to 10th	Day	41% 7% 24% 24% 5%	5215
	11th to 20th	Day	42% 7% 20% 28% 3	4860
	21st to end	Day	40% 9% 21% 26% 4	4988
August	1st to 10th	Day	38% 7% 21% 30% 4	4714
	11th to 20th	Day	37% 7% 21% 28% 6%	4361
	21st to end	Day	54% 7% 14% 23% 3	4531
September	1st to 10th	Day	55% 9% 18% 16% 3	2375
	11th to 20th	Day	57% 6% 15% 20% 2	2228
	21st to end	Day	57% 7% 13% 18% 5%	2023
October	1st to 10th	Day	63% 6% 10% 16% 5%	3160
	11th to 20th	Day	62% 6% 9% 19% 5%	3009
	21st to end	Day	69% 6% 9% 12% 3	2744
November	1st to 10th	Day	75% 5% 8% 10% 2	1567
	11th to 20th	Day	72% 6% 12% 10%	1398
	21st to end	Day	73% 3 8% 14% 1	1182
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	66% 7% 12% 12% 3	5043
	11th to 20th	Night	71% 8% 8% 11% 2	4750
	21st to end	Night	71% 5% 7% 11% 6%	5225
February	1st to 10th	Night	55% 8% 12% 19% 7%	2791
	11th to 20th	Night	50% 8% 14% 21% 7%	2404
	21st to end	Night	54% 5% 10% 18% 13%	2041
March	1st to 10th	Night	46% 5% 16% 23% 9%	3808
	11th to 20th	Night	67% 5% 6% 15% 7% 1	3480
	21st to end	Night	54% 5% 9% 19% 12% 1	3736
April	1st to 10th	Night	48% 5% 11% 21% 14% 1	1796
	11th to 20th	Night	48% 6% 9% 22% 14% 1	1689
	21st to end	Night	42% 8% 10% 27% 12% 1	1540
May	1st to 10th	Night	63% 10% 10% 13% 3	2329
	11th to 20th	Night	53% 6% 8% 22% 11%	2182
	21st to end	Night	58% 5% 8% 22% 7%	2066
June	1st to 10th	Night	50% 6% 11% 21% 12%	1140
	11th to 20th	Night	46% 6% 9% 26% 13%	956
	21st to end	Night	46% 7% 10% 26% 10% 1	950
July	1st to 10th	Night	44% 8% 19% 24% 5%	1637
	11th to 20th	Night	41% 8% 21% 28% 2	1985
	21st to end	Night	38% 9% 20% 26% 6%	2337
August	1st to 10th	Night	39% 7% 20% 31% 3	2202
	11th to 20th	Night	36% 7% 17% 31% 10%	2541
	21st to end	Night	46% 7% 20% 21% 6%	3135
September	1st to 10th	Night	54% 8% 19% 16% 4	1805
	11th to 20th	Night	55% 7% 14% 21% 2	1951
	21st to end	Night	55% 4 15% 20% 5% 1	2138
October	1st to 10th	Night	65% 6% 10% 16% 3	3800
	11th to 20th	Night	62% 5% 10% 17% 7%	3912
	21st to end	Night	66% 5% 9% 15% 5%	4814
November	1st to 10th	Night	76% 5% 7% 10% 1	2731
	11th to 20th	Night	72% 6% 12% 10% 1	2998
	21st to end	Night	71% 5% 10% 12% 2	3153
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	9% 7% 29% 34% 19% 3	270
	21st to end	Day	14% 7% 27% 29% 18% 5%	297
February	1st to 10th	Day	6% 7% 27% 40% 16% 3	270
	11th to 20th	Day	7% 4 40% 29% 16% 4	270
	21st to end	Day	7% 10% 28% 30% 17% 8%	225
March	1st to 10th	Day	11% 7% 26% 23% 21% 13%	270
	11th to 20th	Day	10% 9% 33% 24% 16% 9%	270
	21st to end	Day	9% 9% 28% 30% 17% 6%	340
April	1st to 10th	Day	14% 6% 25% 31% 19% 6%	561
	11th to 20th	Day	13% 9% 26% 31% 18% 3	640
	21st to end	Day	9% 10% 26% 30% 19% 6%	650
May	1st to 10th	Day	18% 11% 28% 26% 14% 4	653
	11th to 20th	Day	20% 10% 28% 29% 9% 3	675
	21st to end	Day	24% 8% 26% 29% 12% 1	734
June	1st to 10th	Day	26% 10% 22% 23% 14% 6%	680
	11th to 20th	Day	18% 7% 25% 29% 14% 6%	680
	21st to end	Day	16% 9% 24% 28% 18% 6%	680
July	1st to 10th	Day	18% 7% 25% 31% 15% 4	671
	11th to 20th	Day	16% 6% 23% 30% 18% 6%	668
	21st to end	Day	17% 6% 20% 28% 20% 9%	739
August	1st to 10th	Day	20% 6% 23% 28% 17% 6%	676
	11th to 20th	Day	17% 6% 22% 27% 20% 8%	665
	21st to end	Day	27% 6% 22% 25% 14% 5%	748
September	1st to 10th	Day	25% 10% 25% 26% 11% 2	585
	11th to 20th	Day	25% 11% 24% 26% 12% 3	438
	21st to end	Day	24% 10% 34% 21% 10% 2	300
October	1st to 10th	Day	26% 9% 29% 24% 9% 3	300
	11th to 20th	Day	31% 9% 28% 17% 8% 6%	300
	21st to end	Day	27% 8% 28% 24% 12% 2	330
November	1st to 10th	Day	22% 11% 25% 27% 12% 4	300
	11th to 20th	Day	18% 9% 30% 25% 13% 5%	265
	21st to end	Day	19% 11% 28% 28% 12% 2	194
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% 29% 33% 17% 4	950
	21st to end	Night	15% 8% 25% 32% 18% 2	1045
February	1st to 10th	Night	7% 6% 30% 36% 18% 3	950
	11th to 20th	Night	9% 5% 27% 36% 18% 5%	929
	21st to end	Night	8% 6% 23% 38% 20% 4	785
March	1st to 10th	Night	7% 5% 27% 36% 21% 4	814
	11th to 20th	Night	12% 9% 32% 31% 14% 3	570
	21st to end	Night	11% 6% 26% 34% 19% 4	627
April	1st to 10th	Night	12% 5% 26% 34% 20% 2	570
	11th to 20th	Night	13% 5% 28% 35% 17% 2	563
	21st to end	Night	10% 7% 27% 40% 14% 1	570
May	1st to 10th	Night	21% 8% 29% 31% 11% 1	570
	11th to 20th	Night	22% 9% 26% 30% 12% 1	555
	21st to end	Night	25% 8% 33% 26% 8%	411
June	1st to 10th	Night	27% 6% 25% 28% 12% 2	380
	11th to 20th	Night	19% 5% 23% 37% 16%	196
	21st to end	Night	15% 6% 30% 35% 13% 1	190
July	1st to 10th	Night	21% 4 23% 36% 14% 1	228
	11th to 20th	Night	17% 5% 21% 38% 16% 3	437
	21st to end	Night	16% 5% 18% 35% 23% 3	624
August	1st to 10th	Night	18% 3 20% 36% 21% 1	567
	11th to 20th	Night	20% 4 20% 34% 21% 2	564
	21st to end	Night	25% 4 21% 35% 13% 2	627
September	1st to 10th	Night	26% 5% 23% 32% 12% 2	570
	11th to 20th	Night	28% 8% 23% 28% 12% 1	570
	21st to end	Night	23% 8% 28% 28% 12% 1	618
October	1st to 10th	Night	30% 8% 27% 21% 13% 1	760
	11th to 20th	Night	32% 10% 21% 23% 12% 3	822
	21st to end	Night	32% 8% 27% 23% 10% 1	1045
November	1st to 10th	Night	25% 9% 27% 26% 11% 2	975
	11th to 20th	Night	19% 8% 31% 28% 13% 2	1000
	21st to end	Night	20% 9% 30% 29% 9% 3	986
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 2024-11-24 23:22:19. Version ac07

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