I have a time series of daily snow cover area for certain years. How can i calculate daily,monthly and seasonal snow cover anomalies? I want to make histogram of snow cover anomalies vs number of occurrences.
DAILY DATA: Snow falls in season. You need to make the daily data into seasonal data. The rationale here is that normal snow fall in the winter may be larger than in the summer (if it is an area where snow falls in the summer or non-winter season). Find the mean for each season to be used as a reference value.
COVERAGE AREA BY FIX REFERENCE NUMBER: Define what is considered normal area of coverage? You past data would tell you: "normally, this month has this much snow coverage." If this is the case, this is your reference coverage area value. Any value above or below, you may designate as anomaly according to Z score:
Z = (Xobs - X^) / S
... where Z = critical value from the Z table; Xobs = observed area of snow covered; X^ = predetermine reference area of coverage; and S = standard deviation of observations.
ASSUME THERE IS NO REFERENCE NUMBER: If there is no reference number, use the mean observe value as the reference, thus in the equation X ^ = mean observed value. Here, group the data set by season (months).
REFERENCE: See attached table.
No matter what is your meteor-climatological variable, you can calculate anomalies just by following steps:
•A measure of distance, in standard units, between a data value and its mean.
•Removes influences of location and spread from data.
•Easier to discern normal vs. unusual values.
•Calculated by subtracting the mean from each observation, then dividing by the standard deviation.
•Have the following characteristics: mean=0 and standard deviation=1.
•Dimensionless
http://iridl.ldeo.columbia.edu/dochelp/StatTutorial/Climatologies/#Stand
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