Graham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Graham clear sky chart from 2005-11-06 to 2024-03-23.

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	22% 10% 4 7% 6% 3 4 4 5% 6% 29%	79098
	Night	24% 10% 4 7% 6% 3 4 4 4 6% 29%	80740
Transparency	Day	57% 4 13% 17% 7% 1	78819
	Night	58% 4 13% 16% 7% 1	80599
Seeing	Day	26% 10% 20% 25% 15% 4	14090
	Night	28% 8% 13% 26% 21% 4	22686

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	37% 15% 3 8% 6% 4 4 3 4 4 11%	2539
	11th to 20th	Day	26% 13% 3 7% 5% 2 4 4 4 6% 25%	2516
	21st to end	Day	38% 12% 4 8% 6% 3 3 4 4 4 12%	2902
February	1st to 10th	Day	31% 12% 4 7% 6% 3 5% 4 4 6% 18%	1710
	11th to 20th	Day	31% 12% 4 8% 6% 2 4 4 4 4 19%	1665
	21st to end	Day	29% 16% 5% 7% 5% 4 4 4 3 4 19%	1484
March	1st to 10th	Day	23% 14% 5% 8% 7% 4 5% 4 5% 6% 19%	1947
	11th to 20th	Day	28% 13% 4 7% 5% 4 5% 5% 4 5% 20%	2090
	21st to end	Day	25% 15% 6% 9% 8% 4 6% 6% 6% 5% 12%	2271
April	1st to 10th	Day	27% 12% 5% 8% 6% 4 5% 6% 5% 6% 15%	2160
	11th to 20th	Day	24% 12% 4 8% 7% 3 6% 5% 4 6% 22%	2140
	21st to end	Day	22% 13% 5% 8% 7% 4 5% 4 4 5% 22%	2366
May	1st to 10th	Day	19% 10% 4 6% 6% 3 5% 5% 5% 6% 30%	2520
	11th to 20th	Day	19% 9% 4 8% 6% 4 6% 4 6% 7% 25%	2477
	21st to end	Day	24% 9% 4 8% 6% 4 5% 5% 5% 6% 23%	2723
June	1st to 10th	Day	24% 9% 3 7% 6% 4 5% 5% 5% 6% 25%	2520
	11th to 20th	Day	22% 10% 4 8% 9% 5% 5% 5% 5% 7% 20%	2671
	21st to end	Day	12% 7% 3 7% 6% 4 5% 4 5% 7% 39%	2700
July	1st to 10th	Day	11% 6% 2 5% 5% 3 4 4 4 6% 49%	2700
	11th to 20th	Day	10% 5% 2 5% 5% 2 4 4 5% 7% 50%	2523
	21st to end	Day	8% 4 2 3 3 2 3 3 5% 6% 62%	2741
August	1st to 10th	Day	9% 5% 2 5% 5% 2 4 4 5% 7% 53%	2446
	11th to 20th	Day	7% 5% 2 4 3 2 3 3 5% 7% 58%	2298
	21st to end	Day	11% 6% 2 5% 5% 3 5% 4 5% 8% 45%	2458
September	1st to 10th	Day	10% 6% 2 5% 5% 2 4 3 4 7% 51%	2160
	11th to 20th	Day	14% 6% 3 7% 6% 3 5% 5% 5% 7% 39%	2160
	21st to end	Day	17% 9% 3 5% 5% 3 4 4 4 7% 39%	1904
October	1st to 10th	Day	21% 10% 4 7% 5% 3 4 4 5% 6% 32%	1800
	11th to 20th	Day	23% 9% 3 7% 5% 3 4 4 5% 8% 30%	1800
	21st to end	Day	30% 10% 4 6% 5% 3 3 5% 3 5% 26%	1882
November	1st to 10th	Day	30% 10% 5% 8% 7% 4 5% 5% 5% 5% 17%	1546
	11th to 20th	Day	30% 10% 4 7% 5% 4 6% 4 6% 5% 18%	1520
	21st to end	Day	29% 12% 4 7% 6% 3 4 4 5% 5% 22%	1520
December	1st to 10th	Day	27% 12% 4 6% 5% 3 4 5% 4 5% 26%	1520
	11th to 20th	Day	30% 12% 4 7% 7% 3 4 4 5% 9% 15%	1925
	21st to end	Day	28% 13% 4 9% 7% 4 5% 5% 4 5% 16%	2794
January	1st to 10th	Night	35% 15% 5% 8% 6% 4 4 4 3 4 11%	4406
	11th to 20th	Night	27% 12% 5% 8% 5% 3 4 3 5% 6% 23%	4382
	21st to end	Night	37% 11% 4 7% 6% 3 4 4 4 5% 15%	4755
February	1st to 10th	Night	30% 11% 5% 9% 6% 3 4 5% 3 6% 18%	2470
	11th to 20th	Night	28% 13% 4 7% 6% 3 4 4 4 5% 23%	2402
	21st to end	Night	27% 13% 4 6% 5% 3 5% 4 5% 5% 22%	1969
March	1st to 10th	Night	24% 12% 5% 7% 5% 2 4 5% 5% 7% 22%	2228
	11th to 20th	Night	26% 11% 4 7% 7% 3 5% 4 5% 5% 24%	2089
	21st to end	Night	27% 14% 5% 9% 8% 3 5% 5% 5% 5% 15%	2129
April	1st to 10th	Night	24% 13% 3 7% 7% 3 6% 6% 4 7% 21%	1800
	11th to 20th	Night	21% 10% 5% 6% 5% 4 5% 5% 5% 6% 28%	1769
	21st to end	Night	22% 10% 4 7% 7% 3 5% 5% 4 8% 25%	1594
May	1st to 10th	Night	18% 9% 3 7% 5% 3 4 4 6% 7% 34%	1440
	11th to 20th	Night	18% 8% 3 7% 6% 3 6% 6% 5% 8% 30%	1416
	21st to end	Night	18% 8% 4 7% 6% 3 5% 6% 5% 6% 31%	1552
June	1st to 10th	Night	18% 7% 3 7% 6% 3 6% 4 5% 9% 33%	1440
	11th to 20th	Night	18% 9% 5% 9% 6% 4 4 5% 4 7% 29%	1283
	21st to end	Night	10% 6% 2 6% 5% 2 4 4 5% 6% 48%	1260
July	1st to 10th	Night	9% 4 2 4 4 3 3 4 4 7% 55%	1260
	11th to 20th	Night	7% 5% 2 5% 5% 2 3 3 3 6% 60%	1433
	21st to end	Night	4 3 2 3 3 1 3 3 4 5% 69%	1576
August	1st to 10th	Night	7% 4 2 3 4 2 3 3 4 8% 60%	1490
	11th to 20th	Night	7% 3 1 3 2 2 3 3 4 6% 65%	1602
	21st to end	Night	10% 4 2 4 5% 2 4 4 4 6% 55%	1898
September	1st to 10th	Night	10% 3 2 3 3 3 4 5% 5% 5% 58%	1800
	11th to 20th	Night	15% 7% 3 6% 5% 3 4 4 4 5% 45%	1800
	21st to end	Night	16% 6% 2 5% 6% 4 4 4 4 6% 44%	2052
October	1st to 10th	Night	19% 8% 3 6% 6% 3 4 4 4 6% 38%	2160
	11th to 20th	Night	23% 8% 3 6% 5% 3 4 4 4 5% 33%	2160
	21st to end	Night	26% 8% 3 6% 4 3 4 4 5% 6% 32%	2473
November	1st to 10th	Night	32% 9% 3 6% 5% 3 5% 5% 4 5% 22%	2530
	11th to 20th	Night	31% 10% 4 7% 6% 4 4 4 4 4 22%	2660
	21st to end	Night	27% 11% 4 7% 6% 3 5% 4 5% 5% 23%	2660
December	1st to 10th	Night	27% 10% 4 7% 6% 3 4 4 3 5% 25%	2660
	11th to 20th	Night	29% 12% 4 8% 5% 3 5% 5% 5% 5% 19%	3280
	21st to end	Night	28% 13% 5% 10% 7% 4 4 4 4 5% 16%	4862

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	79% 2 7% 8% 3 1	2539
	11th to 20th	Day	62% 4 15% 13% 4 2	2540
	21st to end	Day	75% 3 7% 10% 4	2902
February	1st to 10th	Day	69% 4 11% 10% 5%	1710
	11th to 20th	Day	70% 4 9% 12% 5% 1	1674
	21st to end	Day	71% 4 10% 10% 4	1484
March	1st to 10th	Day	68% 4 10% 15% 4	1947
	11th to 20th	Day	68% 3 9% 16% 3 1	2090
	21st to end	Day	75% 4 11% 9%	2271
April	1st to 10th	Day	71% 5% 13% 9% 1	2160
	11th to 20th	Day	65% 4 12% 13% 6% 1	2140
	21st to end	Day	67% 4 13% 12% 4 1	2366
May	1st to 10th	Day	57% 4 17% 17% 5%	2520
	11th to 20th	Day	59% 5% 18% 14% 4	2480
	21st to end	Day	63% 5% 16% 13% 2	2724
June	1st to 10th	Day	61% 4 17% 13% 5%	2520
	11th to 20th	Day	66% 4 17% 12% 2	2671
	21st to end	Day	46% 5% 20% 19% 9% 1	2700
July	1st to 10th	Day	39% 5% 14% 25% 14% 3	2626
	11th to 20th	Day	34% 5% 11% 31% 18% 1	2447
	21st to end	Day	24% 3 12% 39% 19% 4	2589
August	1st to 10th	Day	32% 5% 16% 30% 15% 2	2429
	11th to 20th	Day	26% 5% 19% 32% 14% 4	2298
	21st to end	Day	39% 4 15% 31% 10% 1	2458
September	1st to 10th	Day	34% 4 15% 29% 14% 5%	2160
	11th to 20th	Day	44% 5% 14% 19% 15% 3	2160
	21st to end	Day	47% 4 16% 19% 13% 2	1904
October	1st to 10th	Day	52% 5% 12% 21% 10% 1	1800
	11th to 20th	Day	54% 3 16% 19% 6% 1	1800
	21st to end	Day	62% 2 10% 14% 10%	1882
November	1st to 10th	Day	69% 5% 10% 12% 4 1	1546
	11th to 20th	Day	68% 4 10% 9% 6% 2	1520
	21st to end	Day	65% 5% 6% 13% 9% 1	1520
December	1st to 10th	Day	62% 5% 10% 13% 8% 2	1520
	11th to 20th	Day	67% 4 12% 12% 4 1	1928
	21st to end	Day	72% 3 11% 10% 3	2794
January	1st to 10th	Night	80% 3 7% 6% 3 1	4406
	11th to 20th	Night	63% 4 11% 17% 4 1	4420
	21st to end	Night	73% 3 10% 9% 5% 1	4755
February	1st to 10th	Night	69% 4 11% 9% 5% 1	2470
	11th to 20th	Night	65% 4 11% 11% 7% 1	2404
	21st to end	Night	66% 5% 11% 13% 5% 1	1969
March	1st to 10th	Night	62% 4 13% 17% 5%	2228
	11th to 20th	Night	62% 4 12% 17% 4 1	2090
	21st to end	Night	71% 6% 12% 10%	2129
April	1st to 10th	Night	65% 8% 15% 11% 2	1800
	11th to 20th	Night	59% 6% 13% 15% 6% 1	1769
	21st to end	Night	62% 5% 15% 13% 5% 1	1594
May	1st to 10th	Night	52% 4 17% 18% 7%	1440
	11th to 20th	Night	53% 6% 20% 14% 5% 2	1416
	21st to end	Night	56% 8% 17% 16% 3	1552
June	1st to 10th	Night	53% 4 19% 19% 4	1440
	11th to 20th	Night	57% 6% 20% 14% 3	1283
	21st to end	Night	39% 4 24% 22% 9% 2	1260
July	1st to 10th	Night	32% 5% 17% 26% 16% 4	1225
	11th to 20th	Night	31% 3 13% 32% 19% 2	1386
	21st to end	Night	20% 5% 10% 40% 20% 6%	1488
August	1st to 10th	Night	27% 3 16% 32% 19% 3	1482
	11th to 20th	Night	23% 4 20% 35% 14% 4	1602
	21st to end	Night	32% 4 17% 33% 13% 1	1898
September	1st to 10th	Night	30% 5% 17% 28% 16% 4	1800
	11th to 20th	Night	44% 5% 12% 22% 14% 4	1800
	21st to end	Night	43% 4 14% 22% 13% 3	2052
October	1st to 10th	Night	49% 4 12% 24% 9% 1	2160
	11th to 20th	Night	53% 4 13% 21% 8% 1	2160
	21st to end	Night	55% 4 13% 17% 11%	2473
November	1st to 10th	Night	66% 4 12% 13% 4 1	2530
	11th to 20th	Night	66% 5% 11% 11% 5% 2	2660
	21st to end	Night	65% 4 8% 12% 10% 1	2660
December	1st to 10th	Night	63% 4 10% 14% 7% 2	2660
	11th to 20th	Night	66% 4 14% 10% 5% 1	3276
	21st to end	Night	72% 3 10% 11% 3	4862

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	37% 14% 14% 14% 14% 6%	180
	11th to 20th	Day	33% 16% 15% 19% 13% 3	180
	21st to end	Day	39% 11% 18% 17% 11% 4	267
February	1st to 10th	Day	42% 9% 17% 19% 11% 2	370
	11th to 20th	Day	44% 12% 17% 16% 8% 2	365
	21st to end	Day	38% 16% 21% 17% 8% 1	341
March	1st to 10th	Day	31% 16% 25% 21% 6% 1	460
	11th to 20th	Day	31% 13% 18% 21% 14% 3	460
	21st to end	Day	34% 15% 25% 17% 8% 1	471
April	1st to 10th	Day	35% 14% 19% 19% 10% 2	420
	11th to 20th	Day	28% 18% 25% 17% 7% 4	412
	21st to end	Day	28% 15% 24% 22% 9% 2	420
May	1st to 10th	Day	22% 12% 26% 24% 13% 3	423
	11th to 20th	Day	31% 8% 21% 23% 13% 5%	448
	21st to end	Day	29% 9% 20% 24% 15% 3	485
June	1st to 10th	Day	27% 10% 22% 25% 12% 4	450
	11th to 20th	Day	25% 12% 26% 24% 10% 3	601
	21st to end	Day	13% 9% 22% 33% 20% 3	630
July	1st to 10th	Day	14% 6% 18% 36% 22% 3	621
	11th to 20th	Day	11% 6% 21% 37% 20% 4	446
	21st to end	Day	9% 2 18% 39% 26% 6%	487
August	1st to 10th	Day	12% 5% 15% 39% 21% 8%	447
	11th to 20th	Day	8% 3 13% 42% 25% 10%	437
	21st to end	Day	13% 8% 22% 34% 15% 7%	495
September	1st to 10th	Day	15% 6% 23% 36% 16% 4	450
	11th to 20th	Day	17% 8% 20% 29% 24% 3	450
	21st to end	Day	19% 9% 21% 29% 18% 4	450
October	1st to 10th	Day	27% 9% 19% 21% 18% 5%	450
	11th to 20th	Day	29% 10% 17% 20% 17% 6%	450
	21st to end	Day	33% 11% 18% 20% 11% 6%	397
November	1st to 10th	Day	35% 16% 15% 18% 14% 1	209
	11th to 20th	Day	39% 11% 12% 13% 14% 10%	180
	21st to end	Day	32% 13% 17% 14% 13% 11%	180
December	1st to 10th	Day	39% 13% 11% 13% 13% 11%	180
	11th to 20th	Day	39% 16% 17% 13% 14% 2	180
	21st to end	Day	38% 12% 16% 17% 16% 2	198
January	1st to 10th	Night	44% 11% 12% 18% 13% 2	760
	11th to 20th	Night	37% 10% 11% 23% 17% 3	760
	21st to end	Night	33% 9% 13% 25% 16% 4	836
February	1st to 10th	Night	37% 8% 13% 25% 14% 2	760
	11th to 20th	Night	35% 10% 12% 26% 16% 2	744
	21st to end	Night	36% 11% 15% 25% 12% 2	628
March	1st to 10th	Night	31% 12% 15% 25% 14% 3	760
	11th to 20th	Night	31% 12% 15% 25% 15% 2	760
	21st to end	Night	33% 13% 16% 25% 11% 2	800
April	1st to 10th	Night	32% 8% 17% 24% 16% 2	720
	11th to 20th	Night	27% 10% 17% 26% 17% 3	701
	21st to end	Night	28% 10% 11% 34% 14% 3	514
May	1st to 10th	Night	21% 8% 14% 34% 19% 3	360
	11th to 20th	Night	24% 6% 10% 28% 29% 3	354
	21st to end	Night	24% 7% 11% 26% 28% 4	389
June	1st to 10th	Night	20% 9% 13% 29% 27% 3	360
	11th to 20th	Night	23% 8% 15% 31% 22% 1	360
	21st to end	Night	12% 8% 13% 32% 33% 3	360
July	1st to 10th	Night	12% 3 7% 31% 40% 6%	350
	11th to 20th	Night	10% 3 8% 37% 39% 3	348
	21st to end	Night	5% 3 8% 30% 50% 4	394
August	1st to 10th	Night	12% 3 6% 25% 43% 11%	416
	11th to 20th	Night	9% 1 6% 25% 48% 11%	534
	21st to end	Night	13% 5% 9% 31% 37% 5%	710
September	1st to 10th	Night	12% 4 10% 35% 33% 6%	720
	11th to 20th	Night	17% 5% 10% 23% 36% 9%	720
	21st to end	Night	20% 6% 12% 29% 29% 4	720
October	1st to 10th	Night	23% 7% 10% 28% 27% 5%	720
	11th to 20th	Night	26% 8% 12% 26% 22% 6%	720
	21st to end	Night	31% 7% 16% 25% 17% 4	792
November	1st to 10th	Night	36% 10% 17% 26% 11% 1	740
	11th to 20th	Night	37% 10% 14% 22% 14% 3	760
	21st to end	Night	33% 10% 16% 21% 15% 4	760
December	1st to 10th	Night	31% 9% 12% 28% 16% 3	760
	11th to 20th	Night	39% 9% 14% 24% 12% 2	760
	21st to end	Night	42% 11% 16% 19% 11% 1	836

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