- Start by creating a new canvas the size of 400 by 300. Then create a gradient white on the bottom to blue on the top like in the picture using the Gradient tool.

- Create a new layer. Then select the bottom half of the image and add a yellow-green to a dark green gradient like in the picture.

- Then keep it selected and go to layer>Add layer mask>Hide selection. Then hit D to set default colors and choose the first gradient.

- Then create a gradient like in the picture.

- It should now look similar to this.

- Then select a dark green color. Now select the brush tool and click on the brush picker arrow so you can select the Grass brush. (If it is not there you will need to click on the right arrow and select reset brushes). Once you have selected the Grass brush change the size to around 100 and the opacity at 30%.

- Then create a new layer and hold down the left mouse button and brush from left to right starting from the middle and move down to the bottom of the image so it looks like the picture.

- Now if you want to add some clouds to give it a more realistic look hit D and  create a new layer above the blue gradient layer. Go to filter>render>clouds.

- Then right click on the clouds using the rectangular marquee tool and select free transform. Than right click on it again and select perspective. Then move the top right corner further to the right. Than move the bottom right corner closer to the image. Than right click on it again and select scale. Than drag the bottom middle corner to the middle so it looks similar to the picture.

- Than click on the clouds layer and change the layer to screen.

- And that's it. Here is what mine looks like.