Given the variable weather we have here in the UK it's worth understanding a bit about heat acclimatisation, how it works and how it can impact our training!
Our body works harder in weather conditions and temperatures that it's not as used to. In the UK with our relatively mild climate we are generally used to these milder temperatures and, taking myself as an example, excel in a range of 8 to 20 degrees (Celsius)! The challenge can come in colder or warmer conditions is that it takes out body time to adapt to and adjust its systems for these temperatures and we rarely get these temperatures for long enough to fully adapt.
In the heat our bodies need to sweat more, prioritise water usage differently and expend more energy to run the systems to keep our bodies cool. Over time our body learns and adapts to rising temperatures and learns what the right levels are in the different systems to keep our core body temperature at the right level; which is absolutely key for our health and functioning. With temperatures that vary over timescales of days or 1-2 weeks our bodies never get the time to completely adapt and adjust so will always find it a little harder.
The same applies to colder temperatures too; except here our body is using more energy to keep us warm and managing our circulation to keep core body temperature steady, sometimes at the expense of our extremities!
There is a theory that the brain acts as a central control unit for the body, constantly monitoring all the inputs and making decisions on what it will allow us to do, or not do. Things like levels of available energy, skin temperature, core temperature etc are all readying the brain will take to make these decisions. When we've trained for longer the brain learns that running in those sort of temperaturs is OK or safe, and will allow us to push on a bit more as it holds less back "just incase". So in both cold and hot temperatures this adjusting of comfort zone, again over a number of weeks, helps another aspect of our adaptation.
Another key aspect is the conscious learning we need to do as runners as to what to wear, eat, drink, and what routes to take etc in different conditions. Whilst advice and reading up can help there is nothing like learning from going out and running to find what hats work best for hot or cold weather or whether we need to wear gloves whilst running if the temperature is below a certain point. With more unfamiliar temperatures or sudden more dramatic changes in weather we are more likely to get caught out making decisions that won't help our training.
In summary, take it easy on your first few runs out at the start of a hot or cold weather span. Listen to your body and don't be surprised if pace is lower for the same effort level or heart rate for up to a few weeks; and don't worry about that the training effect will be the same! Make a note of what you wear, eat, drink and do in different weather conditions so you can help your body to adjust as quickly as possible.