Without context, your audience may not understand your message. Conversely, you might not understand your audience.
Situational context refers to the reason why you're speaking. Think of situational context as the event itself.
Environmental context refers to the physical space and time in which you speak. Think of environmental context as the time and venue of the event.
The key to understanding your context is to cultivate a habit of situational awareness. It's not something you'll learn overnight, but by being keenly aware of your surroundings, you'll learn to always think one step ahead should context change suddenly when speaking.
Just as you consider your audience when crafting your speech, you'll also want to consider the context in which your speech will be given. While context certainly includes your audience, it also encompasses many other factors that are important for you to consider as you craft your speech.
Consider for a moment when you hear just the tail end of a conversation in passing. It doesn't always make much sense. What you're missing, in this instance, is the context of that conversation. Just as you need it to understand the conversation you just missed, both you and your audience need to be on the same page about the context of your speech.
Situational context refers to the actual reason why you are speaking or presenting. If you're campaigning for office, you might deliver what's called a "stump speech" - a speech you repeat over and over on the campaign trail that gets at the main talking points and promises of your campaign. If you're at a funeral, you may be asked to deliver a eulogy. On a lighter note, you might be at your best friend's wedding and asked to give one of the first toasts.
The manner in which you deliver your speech, from the words you say to how you say them, relies on the situational context. For example, you wouldn't read a eulogy at a wedding?
Environmental context refers to the physical space in which you're speaking. Whether you're in a classroom presenting the findings from a lab report or in a stadium that seats thousands, environmental context can influence both your message and delivery. The audience will connect with you in different ways depending on the environmental context. You may need to work harder to build individual connections with your audience members the larger the audience you have.
The key then, to understanding your context is to develop a habit of situational awareness. Situational awareness refers to one's perception of their environment and situation around them on a moment by moment basis. In being situationally aware, you can anticipate changes to your environment. In this way, you're always thinking just one step ahead in any given situation or environment, and can be able to adapt accordingly. Cultivating this skill (and it does take time and a keen awareness of your surroundings) is especially helpful when your context may shift or change in subtle or major ways, or in an instant.
Source: Source: Boundless. "Situational and Environmental Context." Boundless Communications Boundless, 27 Feb. 2017. Retrieved 26 Jun. 2017 from https://www.boundless.com/communications/textbooks/boundless-communications-textbook/introduction-to-public-speaking-1/elements-of-speech-communication-21/situational-and-environmental-context-103-4186/
the surroundings, circumstances, environment, background, or settings that determine, specify, or clarify the meaning of an event or other occurrence.
the perception of environmental elements with respect to time and/or space, the comprehension of their meaning, and the projection of their status after some variable has changed, such as time, or some other variable, such as a predetermined event.