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However, the areal extent of each land surface tile type can vary in a rapid, interactive way during the model run, as rain falls then evaporates or snow accumulates then melts, etc.  The characteristics of the soil may also change (e.g. infiltration or runoff of rain, temperature structure of soil etc).  The slope and aspect of orography within each grid box (e.g. south-facing, steepness) is not taken into account and HTESSEL may consequently under- or over-estimate solar heating and runoff.

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• The average ground type within a grid box is not necessarily representative of an individual location.  Land surface characteristics can be very variable within a grid box.  Users and forecasters should take into account the peculiarities of a location when interpreting model output.
• Currently no representation of an urban or city surface where extensive concrete and buildings are likely to have very different characteristics from tiles shown above, and possibly also provide a source of heat (the heat island effect) and even moisture (from air-conditioning units).
• The slope and aspect of orography within each grid box (e.g. south-facing, steepness) is not taken into account and HTESSEL may consequently under- or over-estimate solar heating and runoff.

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