Snow covered mountains in Alaska.
Photo: P. Slota/USGS National Wildlife Health Center (US)
Measuring snow-cover extent shallow snow that can be seen in visible band imagery. As a
result, time series from microwave and visible data sources
Snow-cover fluctuations in the hemispheres are monitored by can differ. Data sets from both sources show a similar range
satellite. Since 1966 the National Oceanic and Atmospheric for maximum Northern Hemisphere snow-cover extent that
Administration (NOAA) has produced snow-extent charts on exceeds 40 million km
2
consistently
1,10,11
. NOAA data, de-
at least a weekly basis
6,7
. Until 1999 the charts were primarily rived primarily from visible band sensors, show a significant
derived from the manual interpretation of satellite images tak- decreasing trend in mean monthly snow-cover extent (see
en within the visible band of the electromagnetic spectrum. text). Microwave data indicate a similar decreasing trend
Passive microwave data, available since 1978, and other data that is not significant at a 90% level. While NOAA data show
are now included in the source data for the charts
8,9
. decreasing trends in every month except for November and
December (see text), data from passive microwave sensors
Satellite passive microwave sensors can detect the snow is less clear. Both data sets indicate significant decreasing
surface through clouds and in darkness but may not detect trends during May to August (see text).
Regional trends in snow cover of water which would result from snow melt, has de-
creased by as much as 50–75 per cent
13
. This decrease is
Examination of regional trends in spring snow-cover attributed to an increase in temperature
14
; observations
duration from 1969–2003 using NOAA snow-cover data of temperatures in the western United States already
shows the western United States to be among the re- show warmer winters
15
. There is abundant evidence of
gions with the strongest decreases (Figure 4.4). This sup- earlier spring warm spells in the western United States
ports results from studies based on measurements on since 1950 at elevations below 2500 m, with impacts on
the ground
12,13
. Springtime snow cover shows a decline snow-cover duration as well as amount. There are more
particularly in the Pacific Northwest region of the west- frequent rain-on-snow events and snow melt begins ear-
ern United States, where snow water equivalent, a com- lier, with stream flows increasing in March and April
mon snow cover measurement equivalent to the depth and decreasing in May and June
16
.
CHAPTER 4 SNOW 43
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