APPARATUS AND METHOD FOR DETECTING SOLID WATER BUILD-UP BACKGROUND OF THE INVENTION [0001] The invention relates to wind turbines. More specifically, the invention relates to detection of ice and snow on wind turbine blades. [0002] Under some conditions, snow and ice can build up on wind turbine blades. The build-up of ice and/or snow on wind turbine blades adversely affects performance of the wind turbine due to the extra weight of the snow and/or ice and alteration of the aerodynamic profiles of the blades. [0003] A current technique for ice detection employs an indirect analysis using wind speed as measured by two anemometers, one of which is heated. Another technique compares actual power coefficient and actual power values with expected values. However, these techniques do not detect ice and/or snow in regimes in which such detection is desirable, resulting in a shorter operating season. BRIEF DESCRIPTION OF THE INVENTION [0004] An apparatus according to an embodiment comprises a wind turbine with a hub supporting a blade, and a sensor disposed and configured to monitor the blade and to produce a respective output signal. A computer processor disposed and configured to receive the output signal of the sensor executes a computer software application to facilitate comparing the output signal to a predetermined value to determine whether solid water is present at the blade and providing an indication of solid water build-up when solid water is present. [0005] According to an embodiment, a system is disclosed comprising a wind turbine comprising a hub supporting a blade and a sensor disposed and configured to monitor the blade and to produce a respective output signal. A computer is disposed and configured to receive data carried by the output signal from the sensor, to compare the data to a predetermined value to determine whether solid water is present at the blade, and to provide an indication of solid water build-up when solid water is present at the blade. BRIEF DESCRIPTION OF THE DRAWINGS [0006] FIG. 1 is a schematic representation of an apparatus according to an embodiment of the invention. [0007] FIG. 2 is a schematic isometric view of a wind turbine with various example installations according to an embodiment of the invention. [0008] FIG. 3 is a schematic flow chart of a method according to an embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION [0009] With reference to the accompanying Figures, examples of a solid water, i.e. snow and ice, build-up monitoring apparatus and method according to embodiments of the invention are disclosed. For purposes of explanation, numerous specific details are shown in the drawings and set forth in the detailed description that follows in order to provide a thorough understanding of embodiments of the invention. It will be apparent, however, that embodiments of the invention may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. [0010] A wind turbine blade and solid water, such as snow and ice, have characteristics that can be monitored to determine when solid water build-up is present. The characteristics include, but are not limited to, spectral reflectance, spectral albedo, temperature, and dielectric absorption. For example, the wind turbine blade with no build-up present has reflective characteristics that can serve as reference points or expected values. When build-up forms, the blade with build-up will have different reflective characteristics from the bare surface of the blade. Thus, by comparing the actual reflective characteristics of the vicinity of the blade surface to expected values for the bare surface, build-up can be detected. Additionally, the type and/or thickness of build-up can be determined from variations in the actual values since ice has characteristics that are different from those of snow, and the characteristics of ice and snow will vary with thickness, moisture content, density, and structure of the actual build-up. [0011] As seen schematically in FIG. 1, a solid water build-up detection apparatus 10 in an embodiment includes an electromagnetic radiation sensor 20 configured to monitor a wind turbine blade. The sensor 20 has a field of view cp expressed, for example, in degrees and produces an output signal 21 indicative of a radiation-related characteristic, such as, but not limited to, reflectivity or reflectance. A computer processor 30, such as a microprocessor, receives the output signal 21 of the sensor 20. The computer processor 30 compares the output signal 21 to a predetermined value to determine whether solid water build-up, such as an accumulation of ice and/or snow, is present in the field of view