The best answer to this is to say that the person is talking about the weather. The fact that they are simply talking about the temperature on this particular day means that they are not talking about the climate. If they were talking about the climate, they should have said something like, “it is a hot day in Hertford just as most August days are.”
The major difference between weather and climate is the time frame. Weather refers to the conditions at a given time or over a given period of time that is relatively short. By contrast, the climate of a place has to do with the kind of weather that it gets over a relatively long time.
When the person says that this particular day in Hertford is hot, they are referring to the weather. If they talked about the typical weather in Hertford for a given time of year, they would be talking about climate because they would be referring to the type of weather that has long been common in Hertford at a given time of year. The heat in Hertford on this particular day may be entirely consistent with Hertford’s climate, but the fact that the person did not refer to any long-term trends or tendencies means that they are talking about weather and not climate.