diff --git a/routes/weather.spec.ts b/routes/weather.spec.ts
index c566a70..3964910 100644
--- a/routes/weather.spec.ts
+++ b/routes/weather.spec.ts
@@ -4,6 +4,10 @@ import * as MockExpressRequest from 'mock-express-request';
 import * as MockExpressResponse from 'mock-express-response';
 import * as MockDate from 'mockdate';
 
+// The tests don't use OWM, but the WeatherProvider API key must be set to prevent an error from being thrown on startup.
+process.env.WEATHER_PROVIDER = "OWM";
+process.env.OWM_API_KEY = "NO_KEY";
+
 import { getWateringData } from './weather';
 import { GeoCoordinates, ZimmermanWateringData, WeatherData } from "../types";
 import { WeatherProvider } from "./weatherProviders/WeatherProvider";
diff --git a/routes/weatherProviders/DarkSky.ts b/routes/weatherProviders/DarkSky.ts
index 4ff6926..a7228aa 100644
--- a/routes/weatherProviders/DarkSky.ts
+++ b/routes/weatherProviders/DarkSky.ts
@@ -6,38 +6,36 @@ import { WeatherProvider } from "./WeatherProvider";
 
 export default class DarkSkyWeatherProvider extends WeatherProvider {
 
+	private readonly API_KEY: string;
+
+	public constructor() {
+		super();
+		this.API_KEY = process.env.DARKSKY_API_KEY;
+		if (!this.API_KEY) {
+			throw "DARKSKY_API_KEY environment variable is not defined.";
+		}
+	}
+
 	public async getWateringData( coordinates: GeoCoordinates ): Promise< ZimmermanWateringData > {
 		// The Unix timestamp of 24 hours ago.
 		const yesterdayTimestamp: number = moment().subtract( 1, "day" ).unix();
-		const todayTimestamp: number = moment().unix();
 
-		const DARKSKY_API_KEY = process.env.DARKSKY_API_KEY,
-			yesterdayUrl = `https://api.darksky.net/forecast/${ DARKSKY_API_KEY }/${ coordinates[ 0 ] },${ coordinates[ 1 ] },${ yesterdayTimestamp }?exclude=currently,minutely,daily,alerts,flags`,
-			todayUrl = `https://api.darksky.net/forecast/${ DARKSKY_API_KEY }/${ coordinates[ 0 ] },${ coordinates[ 1 ] },${ todayTimestamp }?exclude=currently,minutely,daily,alerts,flags`;
+		const yesterdayUrl = `https://api.darksky.net/forecast/${ this.API_KEY }/${ coordinates[ 0 ] },${ coordinates[ 1 ] },${ yesterdayTimestamp }?exclude=currently,minutely,daily,alerts,flags`;
 
-		let yesterdayData, todayData;
+		let yesterdayData;
 		try {
 			yesterdayData = await httpJSONRequest( yesterdayUrl );
-			todayData = await httpJSONRequest( todayUrl );
 		} catch ( err ) {
 			console.error( "Error retrieving weather information from Dark Sky:", err );
 			throw "An error occurred while retrieving weather information from Dark Sky."
 		}
 
-		if ( !todayData.hourly || !todayData.hourly.data || !yesterdayData.hourly || !yesterdayData.hourly.data ) {
+		if ( !yesterdayData.hourly || !yesterdayData.hourly.data ) {
 			throw "Necessary field(s) were missing from weather information returned by Dark Sky.";
 		}
 
-		/* The number of hourly forecasts to use from today's data. This will only include elements that contain historic
-			data (not forecast data). */
-		// Find the first element that contains forecast data.
-		const todayElements = Math.min( 24, todayData.hourly.data.findIndex( ( data ) => data.time > todayTimestamp - 60 * 60 ) );
-
-		/* Take as much data as possible from the first elements of today's data and take the remaining required data from
-			the remaining data from the last elements of yesterday's data. */
 		const samples = [
-			...yesterdayData.hourly.data.slice( todayElements - 24 ),
-			...todayData.hourly.data.slice( 0, todayElements )
+			...yesterdayData.hourly.data
 		];
 
 		// Fail if not enough data is available.
@@ -69,8 +67,7 @@ export default class DarkSkyWeatherProvider extends WeatherProvider {
 	}
 
 	public async getWeatherData( coordinates: GeoCoordinates ): Promise< WeatherData > {
-		const DARKSKY_API_KEY = process.env.DARKSKY_API_KEY,
-			forecastUrl = `https://api.darksky.net/forecast/${ DARKSKY_API_KEY }/${ coordinates[ 0 ] },${ coordinates[ 1 ] }?exclude=minutely,alerts,flags`;
+		const forecastUrl = `https://api.darksky.net/forecast/${ this.API_KEY }/${ coordinates[ 0 ] },${ coordinates[ 1 ] }?exclude=minutely,alerts,flags`;
 
 		let forecast;
 		try {
diff --git a/routes/weatherProviders/OWM.ts b/routes/weatherProviders/OWM.ts
index b35b4b8..939cfa0 100644
--- a/routes/weatherProviders/OWM.ts
+++ b/routes/weatherProviders/OWM.ts
@@ -4,9 +4,18 @@ import { WeatherProvider } from "./WeatherProvider";
 
 export default class OWMWeatherProvider extends WeatherProvider {
 
+	private readonly API_KEY: string;
+
+	public constructor() {
+		super();
+		this.API_KEY = process.env.OWM_API_KEY;
+		if (!this.API_KEY) {
+			throw "OWM_API_KEY environment variable is not defined.";
+		}
+	}
+
 	public async getWateringData( coordinates: GeoCoordinates ): Promise< ZimmermanWateringData > {
-		const OWM_API_KEY = process.env.OWM_API_KEY,
-			forecastUrl = "http://api.openweathermap.org/data/2.5/forecast?appid=" + OWM_API_KEY + "&units=imperial&lat=" + coordinates[ 0 ] + "&lon=" + coordinates[ 1 ];
+		const forecastUrl = `http://api.openweathermap.org/data/2.5/forecast?appid=${ this.API_KEY }&units=imperial&lat=${ coordinates[ 0 ] }&lon=${ coordinates[ 1 ] }`;
 
 		// Perform the HTTP request to retrieve the weather data
 		let forecast;
@@ -43,9 +52,8 @@ export default class OWMWeatherProvider extends WeatherProvider {
 	}
 
 	public async getWeatherData( coordinates: GeoCoordinates ): Promise< WeatherData > {
-		const OWM_API_KEY = process.env.OWM_API_KEY,
-			currentUrl = "http://api.openweathermap.org/data/2.5/weather?appid=" + OWM_API_KEY + "&units=imperial&lat=" + coordinates[ 0 ] + "&lon=" + coordinates[ 1 ],
-			forecastDailyUrl = "http://api.openweathermap.org/data/2.5/forecast/daily?appid=" + OWM_API_KEY + "&units=imperial&lat=" + coordinates[ 0 ] + "&lon=" + coordinates[ 1 ];
+		const currentUrl = `http://api.openweathermap.org/data/2.5/weather?appid=${ this.API_KEY }&units=imperial&lat=${ coordinates[ 0 ] }&lon=${ coordinates[ 1 ] }`,
+			forecastDailyUrl = `http://api.openweathermap.org/data/2.5/forecast/daily?appid=${ this.API_KEY }&units=imperial&lat=${ coordinates[ 0 ] }&lon=${ coordinates[ 1 ] }`;
 
 		let current, forecast;
 		try {