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 2026-10-14.

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	17% 9% 3 7% 5% 4 4 3 5% 7% 35%	154909
	Night	20% 10% 3 8% 5% 4 4 3 4 6% 34%	145608
Transparency	Day	49% 4 15% 20% 10% 2	154628
	Night	52% 5% 14% 19% 8% 1	145471
Seeing	Day	24% 10% 20% 26% 15% 4	17502
	Night	27% 9% 13% 27% 21% 3	25744

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	33% 15% 4 11% 7% 6% 5% 3 5% 4 8%	4723
	11th to 20th	Day	18% 10% 3 9% 4 4 4 4 5% 7% 32%	4716
	21st to end	Day	26% 9% 3 8% 5% 4 3 3 4 4 30%	5405
February	1st to 10th	Day	29% 13% 4 8% 6% 5% 4 3 4 6% 17%	1890
	11th to 20th	Day	30% 14% 4 9% 5% 4 3 3 4 4 19%	1845
	21st to end	Day	29% 17% 4 8% 5% 4 4 3 4 4 18%	1636
March	1st to 10th	Day	17% 14% 4 10% 6% 6% 6% 5% 7% 7% 19%	4743
	11th to 20th	Day	20% 16% 4 9% 5% 5% 4 3 3 4 25%	5110
	21st to end	Day	23% 14% 4 11% 7% 6% 6% 4 6% 5% 13%	5784
April	1st to 10th	Day	26% 13% 4 8% 6% 6% 5% 4 6% 6% 17%	2520
	11th to 20th	Day	21% 13% 3 9% 6% 4 5% 4 5% 7% 25%	2502
	21st to end	Day	20% 13% 4 9% 6% 5% 4 4 5% 6% 23%	2761
May	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 7% 42%	8195
	11th to 20th	Day	16% 10% 3 9% 6% 6% 5% 4 6% 8% 25%	8160
	21st to end	Day	19% 8% 3 7% 4 5% 5% 4 6% 9% 32%	8993
June	1st to 10th	Day	22% 10% 2 8% 5% 5% 4 4 6% 7% 28%	2940
	11th to 20th	Day	20% 10% 3 9% 7% 6% 5% 4 5% 7% 24%	3117
	21st to end	Day	12% 7% 3 8% 6% 5% 4 3 6% 7% 38%	3150
July	1st to 10th	Day	7% 4 2 5% 3 3 3 3 4 6% 60%	8820
	11th to 20th	Day	4 4 1 4 3 2 3 2 3 5% 69%	8223
	21st to end	Day	7% 4 2 5% 3 3 3 3 5% 7% 59%	8916
August	1st to 10th	Day	4 3 1 4 2 3 3 3 4 7% 66%	6285
	11th to 20th	Day	6% 6% 3 6% 4 4 4 4 6% 8% 49%	5923
	21st to end	Day	13% 8% 2 6% 4 4 4 4 5% 8% 42%	6277
September	1st to 10th	Day	10% 6% 2 6% 4 4 4 3 4 7% 48%	2520
	11th to 20th	Day	13% 6% 2 7% 5% 4 5% 4 6% 7% 40%	2520
	21st to end	Day	17% 8% 3 6% 4 4 4 3 5% 8% 39%	2221
October	1st to 10th	Day	16% 8% 2 5% 4 4 3 3 5% 9% 40%	3380
	11th to 20th	Day	25% 9% 3 7% 4 4 4 3 5% 8% 28%	3292
	21st to end	Day	27% 10% 4 8% 5% 5% 4 4 5% 6% 23%	3373
November	1st to 10th	Day	29% 11% 4 9% 6% 5% 5% 4 5% 5% 17%	1712
	11th to 20th	Day	30% 11% 4 8% 6% 5% 5% 3 6% 5% 17%	1680
	21st to end	Day	28% 13% 3 7% 5% 4 4 3 6% 6% 21%	1680
December	1st to 10th	Day	22% 10% 2 6% 4 5% 5% 4 5% 6% 32%	2700
	11th to 20th	Day	31% 13% 4 9% 6% 5% 4 3 5% 7% 13%	3105
	21st to end	Day	31% 14% 4 10% 6% 5% 5% 3 4 5% 14%	4092
January	1st to 10th	Night	32% 16% 5% 11% 6% 5% 4 3 3 4 9%	8172
	11th to 20th	Night	18% 10% 5% 9% 4 4 4 2 4 7% 32%	8182
	21st to end	Night	24% 9% 3 8% 5% 4 3 3 4 5% 32%	8784
February	1st to 10th	Night	28% 13% 4 9% 6% 5% 4 4 3 6% 18%	2730
	11th to 20th	Night	27% 13% 4 8% 5% 4 4 3 4 5% 23%	2662
	21st to end	Night	27% 13% 3 7% 5% 4 5% 3 5% 5% 22%	2169
March	1st to 10th	Night	18% 15% 4 9% 5% 5% 5% 5% 6% 6% 21%	5366
	11th to 20th	Night	22% 12% 3 8% 5% 4 4 3 5% 6% 29%	5109
	21st to end	Night	29% 15% 4 10% 6% 5% 5% 4 5% 5% 13%	5432
April	1st to 10th	Night	22% 12% 3 8% 6% 5% 5% 5% 5% 7% 21%	2100
	11th to 20th	Night	19% 11% 4 7% 5% 5% 4 4 6% 6% 29%	2066
	21st to end	Night	21% 11% 4 8% 6% 4 4 4 4 7% 27%	1859
May	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 45%	4724
	11th to 20th	Night	19% 11% 3 10% 5% 5% 4 3 5% 7% 28%	4700
	21st to end	Night	13% 10% 2 8% 5% 5% 5% 4 6% 8% 33%	5178
June	1st to 10th	Night	17% 8% 3 7% 5% 5% 5% 3 5% 8% 34%	1680
	11th to 20th	Night	15% 9% 4 9% 5% 5% 3 4 5% 8% 33%	1496
	21st to end	Night	11% 7% 2 7% 5% 4 4 3 6% 5% 46%	1470
July	1st to 10th	Night	5% 3 1 3 2 3 3 3 5% 8% 65%	4140
	11th to 20th	Night	3 3 1 3 2 2 3 2 4 5% 73%	4733
	21st to end	Night	6% 3 1 4 3 3 4 3 5% 6% 64%	5152
August	1st to 10th	Night	3 2 1 2 2 2 2 2 3 5% 75%	3869
	11th to 20th	Night	8% 6% 2 6% 3 3 3 3 5% 8% 53%	4142
	21st to end	Night	14% 6% 2 5% 4 3 3 3 4 6% 51%	4884
September	1st to 10th	Night	9% 3 2 4 3 4 5% 4 5% 6% 55%	2100
	11th to 20th	Night	15% 8% 2 6% 4 3 3 3 4 5% 45%	2100
	21st to end	Night	15% 6% 2 6% 5% 4 4 2 4 6% 44%	2395
October	1st to 10th	Night	15% 7% 2 7% 4 4 4 3 4 8% 42%	4040
	11th to 20th	Night	25% 9% 3 6% 4 4 4 3 5% 6% 32%	3933
	21st to end	Night	23% 10% 3 9% 5% 4 4 3 5% 6% 26%	4431
November	1st to 10th	Night	32% 9% 3 7% 5% 5% 5% 4 4 5% 21%	2804
	11th to 20th	Night	31% 11% 4 8% 6% 4 4 3 4 4 21%	2940
	21st to end	Night	26% 12% 4 8% 5% 5% 4 4 5% 5% 22%	2940
December	1st to 10th	Night	21% 9% 3 7% 5% 4 4 4 5% 6% 30%	4700
	11th to 20th	Night	32% 13% 5% 10% 5% 5% 4 3 4 5% 14%	5320
	21st to end	Night	29% 14% 4 11% 6% 5% 4 3 4 5% 15%	7106

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	78% 4 8% 7% 2	4723
	11th to 20th	Day	49% 5% 13% 20% 11% 1	4740
	21st to end	Day	57% 2 6% 19% 14%	5406
February	1st to 10th	Day	68% 4 11% 10% 6%	1890
	11th to 20th	Day	70% 3 8% 12% 6% 1	1854
	21st to end	Day	71% 4 10% 11% 4	1636
March	1st to 10th	Day	62% 4 14% 15% 6%	4743
	11th to 20th	Day	64% 3 10% 21% 2	5110
	21st to end	Day	72% 4 13% 9% 1	5784
April	1st to 10th	Day	69% 5% 15% 10% 1	2520
	11th to 20th	Day	61% 5% 13% 14% 7% 1	2502
	21st to end	Day	64% 4 13% 14% 4 1	2761
May	1st to 10th	Day	43% 3 18% 27% 8%	8195
	11th to 20th	Day	56% 6% 20% 15% 2	8163
	21st to end	Day	51% 5% 25% 16% 3	8994
June	1st to 10th	Day	57% 5% 17% 15% 6%	2940
	11th to 20th	Day	61% 4 16% 15% 4	3117
	21st to end	Day	46% 5% 19% 20% 8% 1	3150
July	1st to 10th	Day	27% 5% 17% 23% 18% 10%	8746
	11th to 20th	Day	20% 3 10% 28% 33% 6%	8147
	21st to end	Day	25% 4 20% 36% 14% 1	8723
August	1st to 10th	Day	19% 3 12% 33% 27% 5%	6268
	11th to 20th	Day	33% 7% 24% 28% 6% 1	5962
	21st to end	Day	39% 6% 16% 28% 11% 1	6276
September	1st to 10th	Day	36% 5% 16% 27% 13% 5%	2520
	11th to 20th	Day	43% 5% 15% 20% 14% 3	2520
	21st to end	Day	45% 4 17% 21% 11% 2	2221
October	1st to 10th	Day	41% 4 13% 31% 10% 1	3380
	11th to 20th	Day	55% 4 13% 18% 9% 1	3292
	21st to end	Day	62% 4 11% 12% 9% 1	3373
November	1st to 10th	Day	68% 5% 11% 12% 4 1	1712
	11th to 20th	Day	69% 5% 10% 9% 6% 2	1680
	21st to end	Day	65% 5% 7% 13% 9% 1	1680
December	1st to 10th	Day	53% 4 9% 19% 13% 1	2700
	11th to 20th	Day	70% 4 12% 11% 3	3108
	21st to end	Day	74% 3 11% 10% 2	4092
January	1st to 10th	Night	80% 4 7% 7% 2 1	8172
	11th to 20th	Night	50% 6% 11% 21% 11% 1	8220
	21st to end	Night	56% 3 8% 21% 11%	8784
February	1st to 10th	Night	68% 4 11% 9% 6% 1	2730
	11th to 20th	Night	64% 4 11% 11% 7% 2	2664
	21st to end	Night	66% 5% 11% 13% 5%	2169
March	1st to 10th	Night	62% 4 12% 16% 7%	5366
	11th to 20th	Night	57% 4 14% 22% 2 1	5110
	21st to end	Night	72% 5% 13% 10%	5432
April	1st to 10th	Night	63% 7% 16% 11% 2	2100
	11th to 20th	Night	57% 6% 15% 15% 7% 1	2066
	21st to end	Night	60% 5% 16% 14% 5%	1859
May	1st to 10th	Night	42% 4 16% 31% 7%	4721
	11th to 20th	Night	57% 6% 20% 14% 2	4700
	21st to end	Night	50% 8% 27% 14% 1	5182
June	1st to 10th	Night	52% 4 19% 20% 5%	1680
	11th to 20th	Night	52% 6% 19% 19% 4	1496
	21st to end	Night	40% 4 23% 22% 9% 2	1470
July	1st to 10th	Night	21% 7% 23% 25% 13% 12%	4105
	11th to 20th	Night	17% 3 11% 28% 37% 3	4686
	21st to end	Night	23% 5% 19% 37% 13% 2	5040
August	1st to 10th	Night	15% 3 10% 45% 23% 4	3861
	11th to 20th	Night	31% 7% 26% 27% 6% 2	4169
	21st to end	Night	36% 5% 20% 30% 8%	4884
September	1st to 10th	Night	31% 5% 18% 27% 15% 4	2100
	11th to 20th	Night	43% 4 13% 22% 14% 4	2100
	21st to end	Night	42% 5% 16% 23% 12% 3	2395
October	1st to 10th	Night	41% 5% 19% 30% 6% 1	4040
	11th to 20th	Night	53% 3 15% 21% 7% 1	3933
	21st to end	Night	58% 4 12% 16% 9%	4431
November	1st to 10th	Night	67% 5% 11% 13% 4	2804
	11th to 20th	Night	68% 4 10% 11% 5% 2	2940
	21st to end	Night	66% 4 9% 12% 9% 1	2940
December	1st to 10th	Night	53% 5% 11% 22% 9% 1	4700
	11th to 20th	Night	73% 4 11% 9% 3	5316
	21st to end	Night	72% 4 12% 10% 3	7106

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	35% 15% 16% 16% 12% 5%	220
	11th to 20th	Day	30% 13% 15% 21% 15% 5%	220
	21st to end	Day	35% 10% 18% 18% 14% 5%	319
February	1st to 10th	Day	40% 10% 17% 20% 11% 3	430
	11th to 20th	Day	43% 12% 19% 16% 8% 2	425
	21st to end	Day	38% 16% 21% 18% 7% 1	397
March	1st to 10th	Day	32% 16% 25% 20% 6% 1	540
	11th to 20th	Day	34% 13% 17% 21% 12% 2	540
	21st to end	Day	33% 15% 26% 18% 8% 1	594
April	1st to 10th	Day	33% 13% 20% 21% 11% 2	540
	11th to 20th	Day	26% 18% 25% 19% 8% 4	532
	21st to end	Day	25% 14% 24% 24% 9% 3	540
May	1st to 10th	Day	20% 13% 26% 24% 14% 3	543
	11th to 20th	Day	29% 11% 21% 23% 11% 4	568
	21st to end	Day	27% 9% 21% 26% 14% 3	617
June	1st to 10th	Day	24% 10% 22% 26% 13% 5%	570
	11th to 20th	Day	22% 12% 27% 27% 10% 3	747
	21st to end	Day	14% 9% 21% 34% 18% 3	780
July	1st to 10th	Day	12% 6% 19% 38% 20% 3	771
	11th to 20th	Day	9% 7% 23% 37% 20% 4	566
	21st to end	Day	9% 3 19% 38% 25% 6%	619
August	1st to 10th	Day	11% 4 16% 39% 21% 8%	567
	11th to 20th	Day	8% 4 15% 41% 24% 8%	557
	21st to end	Day	13% 9% 22% 32% 17% 8%	627
September	1st to 10th	Day	14% 6% 21% 37% 18% 5%	570
	11th to 20th	Day	16% 9% 19% 29% 23% 4	570
	21st to end	Day	19% 9% 23% 29% 17% 3	570
October	1st to 10th	Day	24% 10% 20% 24% 18% 5%	570
	11th to 20th	Day	29% 11% 18% 20% 16% 5%	543
	21st to end	Day	33% 12% 20% 20% 10% 5%	473
November	1st to 10th	Day	35% 17% 15% 18% 14% 2	255
	11th to 20th	Day	39% 12% 13% 15% 12% 8%	220
	21st to end	Day	31% 12% 18% 17% 13% 9%	220
December	1st to 10th	Day	41% 11% 10% 13% 15% 11%	220
	11th to 20th	Day	42% 14% 18% 13% 12% 1	220
	21st to end	Day	39% 12% 15% 17% 15% 2	242
January	1st to 10th	Night	42% 11% 14% 19% 12% 2	840
	11th to 20th	Night	35% 9% 10% 24% 18% 3	840
	21st to end	Night	31% 9% 14% 25% 17% 4	924
February	1st to 10th	Night	36% 9% 15% 25% 13% 2	840
	11th to 20th	Night	34% 10% 12% 27% 15% 2	824
	21st to end	Night	35% 12% 15% 26% 11% 1	692
March	1st to 10th	Night	31% 12% 15% 25% 14% 2	840
	11th to 20th	Night	34% 12% 15% 23% 13% 2	840
	21st to end	Night	32% 13% 18% 26% 10% 2	924
April	1st to 10th	Night	30% 8% 18% 26% 16% 2	840
	11th to 20th	Night	25% 11% 19% 27% 16% 2	818
	21st to end	Night	27% 10% 12% 34% 14% 3	599
May	1st to 10th	Night	20% 9% 14% 36% 19% 2	420
	11th to 20th	Night	24% 8% 12% 27% 26% 3	414
	21st to end	Night	22% 7% 11% 29% 27% 4	455
June	1st to 10th	Night	20% 8% 14% 29% 27% 2	420
	11th to 20th	Night	20% 8% 15% 35% 20% 1	420
	21st to end	Night	13% 8% 13% 31% 32% 3	420
July	1st to 10th	Night	10% 4 8% 34% 39% 5%	410
	11th to 20th	Night	9% 3 9% 38% 38% 3	408
	21st to end	Night	5% 3 8% 32% 48% 4	460
August	1st to 10th	Night	10% 4 7% 26% 45% 9%	487
	11th to 20th	Night	10% 2 6% 26% 46% 10%	624
	21st to end	Night	13% 5% 9% 30% 37% 5%	829
September	1st to 10th	Night	11% 4 9% 34% 34% 7%	840
	11th to 20th	Night	17% 5% 11% 25% 34% 8%	840
	21st to end	Night	19% 6% 13% 30% 28% 4	840
October	1st to 10th	Night	21% 7% 11% 30% 26% 5%	840
	11th to 20th	Night	27% 8% 13% 26% 20% 5%	812
	21st to end	Night	31% 7% 16% 26% 16% 4	880
November	1st to 10th	Night	36% 10% 17% 26% 10% 1	820
	11th to 20th	Night	38% 10% 15% 21% 13% 3	840
	21st to end	Night	33% 10% 16% 22% 15% 4	840
December	1st to 10th	Night	32% 9% 11% 28% 17% 4	840
	11th to 20th	Night	40% 10% 14% 23% 11% 2	840
	21st to end	Night	41% 12% 16% 19% 11% 1	924

Page generated at 2026-10-11 02:06:29. Version ac07

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